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Is there a dominant gene for right-handedness?

Is there a dominant gene for right-handedness?


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Has there been any definitive research about handedness being genetic? Also, why is right-handedness clearly dominant in humans? I'm interested in evolutionary theories, as well as any molecular explanation (if known).


There is a lot of information at OMIM - too much to summarize here - regarding the genetics of handedness and links to which hemisphere of the brain dominates (in an individual), to schizophrenia (slight association with non-right-handedness), and to hair whorl patterns on the scalp. References are provided at the above link. Basically, hand skill appears to be a complex multifactorial phenotype with a heterogeneous background.


OMIM (online mendelian inheritance in man) is a good example to explain how complicated heritability usually is. Simple Mendelian traits, where smooth or wrinkly peas will have wrinkly offspring, allow us to segregate individuals with pure dominant / heterologous and pure recessive traits. When we have done so the chance of inheriting the trait can be predicted precisely (i.e. 25/50/25%).

The number of simple mendelian traits which are observed in an individual are pretty doggone rare. Even things that are dominant like dark hair are usually more complex. I'm of 100% japanese descent, but my son was born with blonde hair and it looks like it won't go darker than mousy brown - black hair is not really dominant, he will have a color between my wife's and mine.

I also need to make a distinction between mendelian traits, which are observed in the individual organsim as opposed to specific DNA variants like a single site mutation (A->T, C->T etc) which are usually mendelian.

The rarity of mendelian inheritance is because there are so many checks and balances in our gene networks that no single gene is solely responsible for any function. Medical research has discovered many single mutation (Mendelian) diseases, but they are very rare in practice. There are single mutations that cause diabetes like systems (called MODY diabetes) for instance, but more than 99% of diabetes are not caused by these mutations. A single mutation advantage is quickly adapted so that single mutations cannot nullify the advantage.

Common mutations that cause diseases or other disadvantages are likewise relatively quick to disappear through adaptations.

I once saw the old text bound copy of "Mendelian Inheritance in Man" It was not a small book, but it seemed to me that when I paged through it, when you look at traits like 'handedness'. there were only a few hundred simple mendelian traits identified. Most of what we are is the result of the interplay of several or even many of our ~30,000 genes.

For an example of how this works, see this article about what happens to new fly genes.


Inheritance of a specific disease, condition, or trait depends on the type of chromosome that is affected. It can be either an autosomal chromosome or a sex chromosome. It also depends on whether the trait is dominant or recessive. Sex-linked diseases are inherited through one of the sex chromosomes, which are the X and Y chromosomes.

Dominant inheritance occurs when an abnormal gene from one parent can cause a disease, even though a matching gene from the other parent is normal. The abnormal gene dominates the gene pair.

For an X-linked dominant disorder: If the father carries the abnormal X gene, all of his daughters will inherit the disease and none of his sons will have the disease. That is because daughters always inherit their father's X chromosome. If the mother carries the abnormal X gene, half of all their children (daughters and sons) will inherit the disease tendency.

For example, if there are four children (two boys and two girls) and the mother is affected (she has one abnormal X and has the disease) but the father does not have the abnormal X gene, the expected odds are:

  • Two children (one girl and one boy) will have the disease
  • Two children (one girl and one boy) will not have the disease

If there are four children (two boys and two girls) and the father is affected (he has one abnormal X and has the disease) but the mother is not, the expected odds are:

These odds do not mean that the children who inherit the abnormal X will show severe symptoms of the disease. The chance of inheritance is new with each conception, so these expected odds may not be what actually occurs in a family. Some X-linked dominant disorders are so severe that males with the genetic disorder may die before birth. Therefore, there may be an increased rate of miscarriages in the family or fewer male children than expected.


Womb Wars

As anyone who took Biology 101 remembers, we’re all composites of our parents. Mom gives us 50 percent of our DNA and our dad fills in the other half. But only the students who were really paying attention are likely to recall that not all genes are expressed equally. In many mammals, the scales seem to be tipped toward fathers, whose genes often win the war underway in the womb.

This is due in part to the perplexing puzzle known as epigenetics. Basically, epigenetics influence the way your DNA is actually expressed. This can alter your dad’s sperm, which in turn may affect you. It can also affect the way the genes you have are read — and the proteins they produce — across your lifetime.

Take, for example, a 2015 study in Nature Genetics that showed the expression of thousands of different genes in mice varied depending on whether they came from a mom or a dad. While each parent technically contributed half of an offspring’s genome, approximately 60 percent of the dad’s genes were more expressive than the mom’s.

These epigenetic factors can play a role in numerous parts of your life, but they aren’t just about quirks like eye color or whether or not you can roll your tongue. Researchers think differential expression can also change your mental and physical wellbeing. If mom has a predisposition toward a given disease, you may still inherit it. But if your dad passes on genes that pass on an illness or a mutation of some kind, you may be more likely to be sick yourself, simply because his genes are more likely to be expressed.


Dominant Traits for Selective Breeding

Just like in Mendel’s experiments, humans have been using genetics for selective breeding in animals, as well as fruit and vegetables, for thousands of years.

Dominant traits that are favorable, such as white wool in sheep, smooth coats in horses, and short legs in dachshunds, can be increased in a population by breeding individuals who have the dominant alleles. By consistently breeding individuals with the desired dominant trait, the dominant allele becomes more common in the population.

However, selective breeding does have downsides. When efforts are made to breed from a small founding population which is homozygous for the desired dominant trait, variation within the gene pool is low. Recessive genes that may cause health problems increase in frequency within the population, and are expressed when they end up homozygous. This is known as inbreeding, and can cause issues such as increased rate of cancers, heart disease and vision or hearing disorders.


Is there a dominant gene for right-handedness? - Biology

BIOLOGY 100 - Human Biology

GENETICS PRACTICE PROBLEMS

Use the Punnett square to determine all of the offspring genotypes (and their relative frequencies) from the following crosses (assume independent assortment):

In the problem above, the "R" allele is a dominant allele specifying for round seeds (in peas), while the "r" allele is the recessive allele specifying for wrinkled seeds in addition, the "Y" allele specifies for the dominant yellow seed color trait, while "y" specifies for green seeds (recessive). Give the expected frequencies (as percentages or ratios) for the phenotypes of the offspring resulting from each of the crosses above.

In turkeys a dominant gene R produces the familiar bronze color its recessive allele r results in red. Another dominant gene H results in normal feathers its recessive allele h produces feathers without webbing, so that they resemble tufts of hair. Two bronze turkeys with normal feathers were mated, and their offspring consisted of 8 bronze with normal feathers, three bronze with hairy feathers, two red with normal feathers, and one red with hairy feathers. What were the genotypes of the parents?

In horses black is dependent upon a dominant gene, B, and chestnut upon its recessive allele, b. The trotting gait is due to a dominant allele T, and the pacing gait to its recessive allele, t. If a homozygous black pacer is mated to a homozygous chestnut trotter, what will be the appearance of the F1 generation?

Referring to the previous question (Question 4), what would be the genotypes and phenotypes (and their expected ratios or percentages) of the offspring produced by a mating between an F1 generation individual and a chestnut pacer?

Assume right-handedness (R) dominates over left-handedness (r) in humans, and that brown eyes (B) are dominant over blue (b). A right-handed, blue-eyed man marries a right-handed, brown-eyed woman. One of their two children is right-handed/blue-eyed, while the other is left-handed/brown-eyed. The man marries again, and this time the woman is right-handed and brown-eyed. They have 10 children, all right-handed and brown-eyed. What are the genotypes of the husband and two wives?


Laterality of Basic Motor Control Mechanisms

Robert L. Sainburg , in Laterality in Sports , 2016

Motor Asymmetries in Left-Handers

The rationale for studying right-handers to understand motor lateralization is that right-handedness reflects a fairly homogenous population, with respect to both behavioral and neurophysiological measures ( Dassonville, Zhu, Ugurbil, Kim, & Ashe, 1997 Perelle & Ehrman, 1982, 1994 ). In contrast, left-handers represent a much more diverse and heterogeneous group. Thus, left-handedness should not be considered an opposite condition to right-handedness, nor should one predict that left-handers should show opposite patterns of behavior and neural activity. As stated by Perelle and Ehrman (1982, p. 1256) , “Left handed individuals do not fit into a single, neat, phenotypic or genotypic classification. They are different from righthanders, but more important, they are different from other left-handers and should not be lumped into a single category.” The degree of lateralization of left-handers, measured as a laterality quotient, is on average significantly lower than that of right-handers, while the variability of that measure is substantially higher than that of right-handers ( Chatagny et al., 2013 ). Thus left-handedness seems to represent individuals with a range between those that show completely mixed patterns of hand use, with no particular bias for one hand, and those that appear mirror imaged to right-handers, behaviorally ( Chatagny et al., 2013 Perelle & Ehrman, 1982 ). In light of these differences, the fact that left-handers also show substantially different patterns of neural activation during unimanual tasks should not be surprising. In fact, left-handers tend to show greater activation of ipsilateral cortex during unilateral unimanual tasks when compared to right-handers performing the same tasks ( Dassonville et al., 1997 Kim et al., 1993 ). This pattern of activation suggests that the tendency for left-handers to show a greater degree of behavioral symmetry is not due to a more symmetric (less lateralized) brain, but rather to a difference in recruitment of a lateralized brain. That is, left-handed individuals display motor behavior that is more symmetric because of bilateral hemisphere recruitment, which allows each hand to benefit from the specialization of each hemisphere. If, on the other hand, left-hander’s brains were more symmetric, each hemisphere would reflect a mirror image in structure and function to the other hemisphere. Symmetry in behavior would be associated with greater contralateral hemisphere recruitment, regardless of which arm was used. The fact that recruitment of ipsilateral cortex is greater in left-handers than right-handers is consistent with the requirement for left-handers to adapt to a right-handed environment, and thus use the nondominant right arm in dominant arm tasks, such as opening doors and using scissors.

Evidence that ipsilateral hemisphere recruitment during nondominant arm use can be increased with experience has been demonstrated by Philip and Frey (2014) . In this study, right-handed individuals with partial or complete dominant hand amputations were studied, after the individuals had largely switched dominance functions to the unimpaired left hand. The improved performance of the previously nondominant arm in activities, such as rapid tracing tasks, was associated with greater activation of the ipsilateral (left) hemisphere. Thus the experience-dependent improvement in left arm function, associated with dominance retraining, occurred through recruitment of the lateralized functions of the left, ipsilateral hemisphere, rather than by rewriting the lateralized functionality of the contralateral-right hemisphere. Taken together, these findings suggest that left-handers are a heterogeneous group of individuals who do not represent the neural or behavioral mirror image of right-handers.


Contents

  • Right-handedness is by far the most common type. Right-handed people are more skillful with their right hands. Studies suggest that approximately 90% of people are right-handed. [5][9]
  • Left-handedness is far less common than right-handedness. Studies suggest that approximately 10% of people are left-handed. [5][10]
  • Mixed-handedness or cross-dominance is the change of hand preference between different tasks. This is very uncommon in the population with about a 1% prevalence. [11]
  • Ambidexterity refers to having equal ability in both hands. Those who learn it still tend to favor their originally dominant hand. This is very uncommon, with about a 1% prevalence. [citation needed]

Handedness may be measured behaviourally (performance measures) or through questionnaires (preference measures). The Edinburgh Handedness Inventory has been used since 1971 but contains many dated questions and is hard to score. The longer Waterloo Handedness Questionnaire is not widely accessible. More recently, the Flinders Handedness Survey (FLANDERS) has been developed. [12]

There are several theories of how handedness develops. Occurrences during prenatal development may be important researchers studied fetuses in utero and determined that handedness in the womb was a very accurate predictor of handedness after birth. [13] In a 2013 study, 39% of infants (6 to 14 months) and 97% of toddlers (18 to 24 months) demonstrated a hand preference. [14]

Language dominance Edit

One common handedness theory is the brain hemisphere division of labor. In most people, the left side of the brain controls speaking. The theory suggests it is more efficient for the brain to divide major tasks between the hemispheres—thus most people may use the non-speaking (right) hemisphere for perception and gross motor skills. As speech is a very complex motor control task, the specialised fine motor areas controlling speech are most efficiently used to also control fine motor movement in the dominant hand. As the right hand is controlled by the left hemisphere (and the left hand is controlled by the right hemisphere) most people are, therefore right-handed. The theory implies that left-handed people have a reversed organisation. [15]

However, this theory does not address the fact that the majority of left-handers have left-hemisphere language dominance—just like right-handers. [16] [17] Only around 30% of left-handers are not left-hemisphere dominant for language. Some of those have reversed brain organisation, where the verbal processing takes place in the right-hemisphere and visuospatial processing is dominant to the left hemisphere. [18] Others have more ambiguous bilateral organisation, where both hemispheres do parts of typically lateralised functions. When tasks investigating lateralisation are averaged across a group of left-handers, the overall effect is that left-handers show the same pattern of data as right-handers, but with a reduced asymmetry. [19] This finding is likely due to the small proportion of left-handers who have atypical brain organisation.

Genetic factors Edit

Handedness displays a complex inheritance pattern. For example, if both parents of a child are left-handed, there is a 26% chance of that child being left-handed. [20] A large study of twins from 25,732 families by Medland et al. (2006) indicates that the heritability of handedness is roughly 24%. [21]

Two theoretical single-gene models have been proposed to explain the patterns of inheritance of handedness, by Marian Annett [22] of the University of Leicester, and by Chris McManus [20] of UCL.

However, growing evidence from linkage and genome-wide association studies suggests that genetic variance in handedness cannot be explained by a single genetic locus. [23] [24] [25] [26] [27] [28] [29] [30] From these studies, McManus et al. now conclude that handedness is polygenic and estimate that at least 40 loci contribute to the trait. [31]

Brandler et al. performed a genome-wide association study for a measure of relative hand skill and found that genes involved in the determination of left/right asymmetry in the body play a key role in handedness. [32] Brandler and Paracchini suggest the same mechanisms that determine left/right asymmetry in the body (e.g. nodal signaling and ciliogenesis) also play a role in the development of brain asymmetry (handedness being a reflection of brain asymmetry for motor function). [33]

In 2019, Wiberg et al. performed a genome-wide association study and found that handedness was significantly associated with four loci, three of them in genes encoding proteins involved in brain development. [34]

Epigenetic factors Edit

Twin studies indicate that genetic factors explain 25% of the variance in handedness, and environmental factors the remaining 75%. [35] While the molecular basis of handedness epigenetics is largely unclear, Ocklenburg et al. (2017) found that asymmetric methylation of CpG sites plays a key role for gene expression asymmetries related to handedness. [36] [37]

Prenatal hormone exposure Edit

Four studies have indicated that individuals who have had in-utero exposure to diethylstilbestrol (a synthetic estrogen based medication used between 1940 and 1971) were more likely to be left-handed over the clinical control group. Diethylstilbestrol animal studies "suggest that estrogen affects the developing brain, including the part that governs sexual behavior and right and left dominance". [38] [39] [40] [41]

Prenatal vestibular asymmetry Edit

Previc, after reviewing a large number of studies, found evidence that the position of the fetus in the final trimester and a baby's subsequent birth position can affect handedness. About two-thirds of fetuses present with their left occiput (back of the head) at birth. This partly explains why prematurity results in a decrease in right-handedness. Previc argues that asymmetric prenatal positioning creates asymmetric stimulation of the vestibular system, which is involved in the development of handedness. In fact, every major disorder in which patients show reduced right-handedness is associated with either vestibular abnormalities or delay, [42] and asymmetry of the vestibular cortex is strongly correlated with the direction of handedness. [43]

Ultrasound Edit

Another theory is that ultrasound may sometimes affect the brains of unborn children, causing higher rates of left-handedness in children whose mothers receive ultrasound during pregnancy. Research suggests there may be a weak association between ultrasound screening (sonography used to check the healthy development of the fetus and mother) and left-handedness. [44]

Infants have been observed to fluctuate heavily when choosing a hand to lead in grasping and object manipulation tasks, especially in one- versus two-handed grasping. Between 36 and 48 months, there is a significant decline in variability between handedness in one-handed grasping it can be seen earlier in two-handed manipulation. Children of 18–36 months showed more hand preference when performing bi-manipulation tasks than with simple grasping. [45]

The decrease in handedness variability in children of 36–48 months may be attributable to preschool or kindergarten attendance due to increased single-hand activities such as writing and coloring. [45] Scharoun and Bryden noted that right-handed preference increases with age up to the teenage years. [4]

Intelligence Edit

In his book Right-Hand, Left-Hand, Chris McManus of University College London argues that the proportion of left-handers is increasing, and that an above-average quota of high achievers have been left-handed. He says that left-handers' brains are structured in a way that increases their range of abilities, and that the genes that determine left-handedness also govern development of the brain's language centers. [46]

Writing in Scientific American, he states:

Studies in the U.K., U.S. and Australia have revealed that left-handed people differ from right-handers by only one IQ point, which is not noteworthy . Left-handers' brains are structured differently from right-handers' in ways that can allow them to process language, spatial relations and emotions in more diverse and potentially creative ways. Also, a slightly larger number of left-handers than right-handers are especially gifted in music and math. A study of musicians in professional orchestras found a significantly greater proportion of talented left-handers, even among those who played instruments that seem designed for right-handers, such as violins. Similarly, studies of adolescents who took tests to assess mathematical giftedness found many more left-handers in the population. [47]

Conversely, Joshua Goodman found that evidence for left-handers was overrepresented amongst those with higher cognitive skills, such as Mensa members and higher-performing takers of SAT and MCAT tests, due to methodological and sampling issues in studies. He also found that left-handers were overrepresented among those with lower cognitive skills and mental impairments, with those with intellectual disability (ID) being roughly twice as likely to be left-handed, as well as generally lower cognitive and non-cognitive abilities amongst left-handed children. [48] In a systematic review and meta-analysis, Ntolka and Papadatou-Pastou found that right-handers had higher IQ scores, but that difference was negligible (about 1.5 points). [49]

Early childhood intelligence Edit

Nelson, Campbell, and Michel studied infants and whether developing handedness during infancy correlated with language abilities in toddlers. In the article they assessed 38 infants and followed them through to 12 months and then again once they became toddlers from 18 to 24 months. What they discovered was that when a child developed a consistent use of their right or left hand during infancy (such as using the right hand to put the pacifier back in, or grasping random objects with the left hand), they were more likely to have superior language skills as a toddler. Children who became lateral later than infancy (i.e., when they were toddlers) showed normal development of language and had typical language scores. [50] The researchers used Bayley scales of infant and toddler development to assess all the subjects.

Music Edit

In two studies, Diana Deutsch found that left-handers, particularly those with mixed hand preference, performed significantly better than right-handers in musical memory tasks. [51] [52] There are also handedness differences in perception of musical patterns. Left-handers as a group differ from right-handers, and are more heterogeneous than right-handers, in perception of certain stereo illusions, such as the octave illusion, the scale illusion, and the glissando illusion. [53]

Health Edit

Left-handed people are much more likely to have several specific physical and mental disorders and health problems. For example:

Lower-birth-weight and complications at birth are positively correlated with left-handedness. [54]

A variety of neuropsychiatric and developmental disorders like autism spectrum disorders, depression, bipolar disorder, anxiety disorders, schizophrenia, and alcoholism has been associated with left- and mixed-handedness. [37] [55]

A 2012 study showed that nearly 40% of children with cerebral palsy were left-handed, [56] while another study demonstrated that left-handedness was associated with a 62 percent increased risk of Parkinson's disease in women, but not in men. [57] Another study suggests that the risk of developing multiple sclerosis increases for left-handed women, but the effect is unknown for men at this point. [58]

Left-handed women have a higher risk of breast cancer than right-handed women and the effect is greater in post-menopausal women. [59]

At least one study maintains that left-handers are more likely to suffer from heart disease, and are more likely to have reduced longevity from cardiovascular causes. [60]

Left-handers are more likely to suffer bone fractures. [61]

One systematic review concluded: "Left-handers showed no systematic tendency to suffer from disorders of the immune system". [62]

As handedness is a highly heritable trait associated with various medical conditions, and because many of these conditions could have presented a Darwinian fitness challenge in ancestral populations, this indicates left-handedness may have previously been rarer than it currently is, due to natural selection. However, on average, left-handers have been found to have an advantage in fighting and competitive, interactive sports, which could have increased their reproductive success in ancestral populations. [63]

Income Edit

In a 2006 U.S. study, researchers from Lafayette College and Johns Hopkins University concluded that there was no statistically significant correlation between handedness and earnings for the general population, but among college-educated people, left-handers earned 10 to 15% more than their right-handed counterparts. [64]

However, more recently, in a 2014 study published by the National Bureau of Economic Research, Harvard economist Joshua Goodman finds that left-handed people earn 10 to 12 percent less over the course of their lives than right-handed people. Goodman attributes this disparity to higher rates of emotional and behavioral problems in left-handed people. [48]

Left-handedness and sports Edit

Interactive sports such as table tennis, badminton and cricket have an overrepresentation of left-handedness, while non-interactive sports such as swimming show no overrepresentation. Smaller physical distance between participants increases the overrepresentation. In fencing, about half the participants are left-handed. [65] The term southpaw is sometimes used to refer to a left-handed individual, especially in baseball and boxing. [66]

Other, sports-specific factors may increase or decrease the advantage left-handers usually hold in one-on-one situations:

  • In cricket, the overall advantage of a bowler's left-handedness exceeds that resulting from experience alone: even disregarding the experience factor (i.e., even for a batsman whose experience against left-handed bowlers equals his experience against right-handed bowlers), a left-handed bowler challenges the average (i.e., right-handed) batsman more than a right-handed bowler does, because the angle of a bowler's delivery to an opposite-handed batsman is much more penetrating than that of a bowler to a same-handed batsman (see Wasim Akram). [citation needed]
  • In baseball, a right-handed pitcher's curve ball will break away from a right-handed batter and towards a left-handed batter. While studies of handedness show that only 10% of the general population is left-handed, the proportion of left-handed MLB players is closer to 39% of hitters and 28% of pitchers, according to 2012 data. [67] Historical batting averages show that left-handed batters have a slight advantage over right-handed batters when facing right-handed pitchers. [68] Because there are fewer left-handed pitchers than right-handed pitchers, left-handed batters have more opportunities to face right-handed pitchers than their right-handed counterparts have against left-handed pitchers. [69] Fourteen of the top twenty career batting averages in Major League Baseball history have been posted by left-handed batters. [70] Left-handed batters have a slightly shorter run from the batter's box to first base than right-handers. This gives left-handers a slight advantage in beating throws to first base on infield ground balls. [citation needed]
    • Because a left-handed pitcher faces first base when he is in position to throw to the batter, whereas a right-handed pitcher has his back to first base, a left-handed pitcher has an advantage when attempting to pick off baserunners at first base. [71]
    • Defensively in baseball, left-handedness is considered an advantage for first basemen because they are better suited to fielding balls hit in the gap between first and second base, and because they do not have to pivot their body around before throwing the ball to another infielder. [72] For the same reason, the other infielder's positions are seen as being advantageous to right-handed throwers. Historically, there have been few left-handed catchers because of the perceived disadvantage a left-handed catcher would have in making the throw to third base, especially with a right-handed hitter at the plate. [73] A left-handed catcher would have a potentially more dangerous time tagging out a baserunner trying to score. [73] With the ball in the glove on the right hand, a left-handed catcher would have to turn his body to the left to tag a runner. In doing so, he can lose the opportunity to brace himself for an impending collision. [73] On the other hand, the Encyclopedia of Baseball Catchers states: [73]

    One advantage is a left-handed catcher's ability to frame a right-handed pitcher's breaking balls. A right-handed catcher catches a right-hander's breaking ball across his body, with his glove moving out of the strike zone. A left-handed catcher would be able to catch the pitch moving into the strike zone and create a better target for the umpire.

    • In four wall handball, typical strategy is to play along the left wall forcing the opponent to use their left hand to counter the attack and playing into the strength of a left-handed competitor.
    • In handball, left-handed players have an advantage on the right side of the field when attacking, getting a better angle, and that defenders might be unused to them. Since few people are left-handed, there is a demand for such players.
    • In water polo, the centre forward position has an advantage in turning to shoot on net when rotating the reverse direction as expected by the centre of the opposition defence and gain an improved position to score. Left-handed drivers are usually on the right side of the field, because they can get better angles to pass the ball or shoot for goal. typically uses a strategy in which a defence pairing includes one left-handed and one right-handed defender. A disproportionately large number of ice hockey players of all positions, 62 percent, shoot left, though this does not necessarily indicate left-handedness. [74]
    • In American football, the handedness of a quarterback affects blocking patterns on the offensive line. Tight ends, when only one is used, typically line up on the same side as the throwing hand of the quarterback, while the offensive tackle on the opposite hand, which protects the quarterback's "blind side," is typically the most valued member of the offensive line. Receivers also have to adapt to the opposite spin. [75] While uncommon, there have been several notable left-handed quarterbacks.
    • In bowling, the oil pattern used on the bowling lane breaks down faster the more times a ball is rolled down the lane. Bowlers must continually adjust their shots to compensate for the ball's change in rotation as the game or series is played and the oil is altered from its original pattern. A left-handed bowler competes on the opposite side of the lane from the right-handed bowler and therefore deals with less breakdown of the original oil placement. This means left-handed bowlers have to adjust their shot less frequently than right-handed bowlers in team events or qualifying rounds where there are possibly 4-10 people per set of two lanes. This can allow them to stay more consistent. However, this advantage is not present in bracket rounds and tournament finals where matches are 1v1 on a pair of lanes.

    Sex Edit

    According to a meta-analysis of 144 studies, totaling 1,787,629 participants, the best estimate for the male to female odds ratio was 1.23, indicating that men are 23% likelier to be left-handed. In terms of proportions this odds ratio implies that if the incidence of left-handedness for females was 10%, then the incidence of male left-handedness would be 12%. [76] [ clarification needed ]

    Sexuality and gender identity Edit

    A number of studies examining the relationship between handedness and sexual orientation have reported that a disproportionate minority of homosexual people exhibit left-handedness, [77] though findings are mixed. [78] [79] [80]

    A 2001 study also found that children who were assigned male at birth and whose gender identity is not male were more than twice as likely to be left-handed than a clinical control group (19.5% vs. 8.3%, respectively). [81]

    Paraphilias (atypical sexual interests) have also been linked to higher rates of left-handedness. A 2008 study analyzing the sexual fantasies of 200 males found "elevated paraphilic interests were correlated with elevated non-right handedness". [82] Greater rates of left-handedness has also been documented among pedophiles. [83] [84] [85] [86]

    A 2014 study attempting to analyze the biological markers of asexuality asserts that non-sexual men and women were 2.4 and 2.5 times, respectively, more likely to be left-handed than their heterosexual counterparts. [87]

    Mortality rates in combat Edit

    A study at Durham University — which examined mortality data for cricketers whose handedness was a matter of public record — found that left-handed men were almost twice as likely to die in war as their right-handed contemporaries. [88] The study theorised that this was because weapons and other equipment was designed for the right-handed. “I can sympathise with all those left-handed cricketers who have gone to an early grave trying desperately to shoot straight with a right-handed Lee Enfield .303,” wrote a journalist reviewing the study in the cricket press. [89] The findings echo those of previous American studies, which found that left-handed US sailors were 34% more likely to have a serious accident than their right-handed counterparts. [90]

    Episodic memory etc Edit

    A high level of handedness (whether strongly favoring right or left) is associated with poorer episodic memory, [91] [92] and with poorer communication between brain hemispheres, [93] which may give poorer emotional processing, although bilateral stimulation may reduce such effects. [94] [95]

    Corpus callosum Edit

    A high level of handedness is associated with a smaller corpus callosum whereas low handedness with a larger one. [96]

    Many tools and procedures are designed to facilitate use by right-handed people, often without realizing the difficulties incurred by the left-handed. John W. Santrock has written, "For centuries, left-handers have suffered unfair discrimination in a world designed for right-handers." [6]

    As a child British King George VI (1895-1952) was naturally left-handed. He was forced to write with his right hand, as was common practice at the time. He was not expected to become king, so that was not a factor. [97] McManus noted that, as the Industrial Revolution spread across Western Europe and the United States in the 19th century, workers needed to operate complex machines that were designed with right-handers in mind. This would have made left-handers more visible and at the same time appear less capable and more clumsy. During this era, children were taught to write with a dip pen. While a right-hander could smoothly drag the pen across paper from left to right, a dip pen could not easily be pushed across by the left hand without digging into the paper and making blots and stains. [98]

    Negative connotations and discrimination Edit

    Moreover, apart from inconvenience, left-handed people have historically been considered unlucky or even malicious for their difference by the right-handed majority. In many European languages, including English, the word for the direction "right" also means "correct" or "proper". Throughout history, being left-handed was considered negative, or evil even into the 20th century, left-handed children were beaten by schoolteachers for writing with their left hand.

    The Latin adjective sinister or sinistra (as applied to male or female nouns ⁠— ⁠Latin nouns are gender specific) means "left" as well as "unlucky", and this double meaning survives in European derivatives of Latin, including the English words "sinister" (meaning both 'evil' and 'on the bearer's left on a coat of arms') and "ambisinister" meaning 'awkward or clumsy with both or either hand'.

    There are many negative connotations associated with the phrase "left-handed": clumsy, awkward, unlucky, insincere, sinister, malicious, and so on. A "left-handed compliment" is one that has two meanings, one of which is unflattering to the recipient. In French, gauche means both "left" and "awkward" or "clumsy", while droit(e) (cognate to English direct and related to "adroit") means both "right" and "straight", as well as "law" and the legal sense of "right". The name "Dexter" derives from the Latin for "right", as does the word "dexterity" meaning manual skill. As these are all very old words, they would tend to support theories indicating that the predominance of right-handedness is an extremely old phenomenon.

    Black magic is sometimes referred to as the "left-hand path".

    Until very recently in Taiwan (and still in Mainland China, Japan and both North and South Korea), left-handed people were forced to switch to being right-handed, or at least switch to writing with the right hand. Due to the importance of stroke order, developed for the comfortable use of right-handed people, it is considered more difficult to write legible Chinese characters with the left hand than it is to write Latin letters, though difficulty is subjective and depends on the writer. [99] Because writing when moving one's hand away from its side towards the other side of the body can cause smudging if the outward side of the hand is allowed to drag across the writing, writing in the Latin alphabet might possibly be less feasible with the left hand than the right under certain circumstances. Conversely, right-to-left alphabets, such as the Arabic and Hebrew, are generally considered easier to write with the left hand in general. [ citation needed ] Depending on the position and inclination of the writing paper, and the writing method, the left-handed writer can write as neatly and efficiently or as messily and slowly as right-handed writers. Usually the left-handed child needs to be taught how to write correctly with the left hand, since discovering a comfortable left-handed writing method on one's own may not be straightforward. [100] [101]

    In the Soviet Union, all left-handed children were forced to write with their right hand in the Soviet school system. [102] [103]

    International Left-Handers Day Edit

    International Left-Handers Day is held annually every August 13. [104] It was founded by the Left-Handers Club in 1992, with the club itself having been founded in 1990. [104] International Left-Handers Day is, according to the club, "an annual event when left-handers everywhere can celebrate their sinistrality (left-handedness) and increase public awareness of the advantages and disadvantages of being left-handed." [104] It celebrates their uniqueness and differences, who are from seven to ten percent of the world's population. Thousands of left-handed people in today's society have to adapt to use right-handed tools and objects. Again according to the club, "in the U.K. alone there were over 20 regional events to mark the day in 2001 – including left-v-right sports matches, a left-handed tea party, pubs using left-handed corkscrews where patrons drank and played pub games with the left hand only, and nationwide 'Lefty Zones' where left-handers' creativity, adaptability and sporting prowess were celebrated, whilst right-handers were encouraged to try out everyday left-handed objects to see just how awkward it can feel using the wrong equipment!" [104]

    Kangaroos and other macropod marsupials show a left-hand preference for everyday tasks in the wild. 'True' handedness is unexpected in marsupials however, because unlike placental mammals, they lack a corpus callosum. Left-handedness was particularly apparent in the red kangaroo (Macropus rufus) and the eastern gray kangaroo (Macropus giganteus). Red-necked (Bennett's) wallabies (Macropus rufogriseus) preferentially use their left hand for behaviours that involve fine manipulation, but the right for behaviours that require more physical strength. There was less evidence for handedness in arboreal species. [105] Studies of dogs, horses, and domestic cats have shown that females of those species tend to be right-handed, while males tend to be left-handed. [106]


    Introduction

    The most obvious sign that our brains function asymmetrically is the near-universal preference for the right hand, which goes back at least as far as the historical record takes us, and has long been a powerful source of symbolism, with the dexterous right associated with positive values and the sinister left with negative ones [1]. This has often led to stigmatization of left-handed individuals, sometimes forcing them to switch hand use, occasionally with grievous consequences. Superstitions about left and right were compounded by the discovery, in the 1860s, that speech was based predominantly in the left hemisphere of the brain [2]. Since language itself is uniquely human, this reinforced the idea that brain asymmetry more generally is a distinctive mark of being human [3]. Because the left hemisphere also controls the dominant right hand, it came to be widely regarded as the dominant or major hemisphere, and the right as nondominant or minor. Nevertheless, further evidence that the right hemisphere was the more specialized for perception and emotion also led to speculation, some of it far-fetched, about the complementary roles of the two sides of the brain in maintaining psychological equilibrium [4].

    Interest flagged for a while, but was revived a century later, in the 1960s, with the study of patients who had undergone split-brain surgery, in which the main commissures connecting the two hemispheres were cut as a means of controlling intractable epilepsy. Testing of each disconnected hemisphere again revealed the left to be specialized for language and the right for emotional and nonverbal functions [5],[6]. This work won Roger W. Sperry the Nobel Prize for Physiology and Medicine in 1981, but again led to speculation, most of it exaggerated or ill-founded, about the complementary functions of the two sides of the brain.

    One popular example is Betty Edwards' Drawing on the Right Side of the Brain, first published in 1979 but now in its fourth edition [7], which epitomizes the popular view that the right hemisphere is responsible for creativity. Brain imaging shows, though, that creative thought activates a widespread network, favoring neither hemisphere [8]. A more recent example is Iain McGilchrist's 2009 book The Master and His Emissary, which draws on cerebral asymmetry in a sweeping account of the forces that shaped Western culture, and provocatively declares the right hemisphere to be the dominant one (“the master”) [9]. Although widely acclaimed, this book goes far beyond the neurological facts. Polarities of left and right brain are broadly invoked in art, business, education, literary theory, and culture, but owe more to the power of myth than to the scientific evidence [10].


    Dominant and Recessive Genes

    Have you ever wondered why some people have blue or brown eyes? The coloring of the blue and brown eyes is an example of different versions of a gene. Different versions of a gene are called alleles . Alleles can be considered dominant or recessive, with dominant being the trait that is observed or shown and recessive being the trait is not seen.

    Dominant alleles are seen as an uppercase of a letter for example, B. Recessive alleles are seen as a lower case of a letter b. In order for a person to show the dominant trait, one of the person’s parents must have the dominant trait (which is an uppercase letter). Remember that human cells carry 2 copies of each chromosome, one from the biological mother’s genes and one from the biological father’s genes. With that being said, there are 2 sets of alleles that can be dominant or recessive. If a person carries a heterozygous set of alleles (both uppercase and lower case letter of the gene) then the person will show the dominant trait (being that there is an uppercase letter present). For example, the brown eye allele is dominant, B. You would need at least one copy of the brown eye allele (B) to have brown eyes. When you have two copies of the alleles that are both dominant, this is called codominance . For example, if the dominant trait is red for flowers and another dominant trait is white, then the flower will have both red and white as the dominant traits are expressed equally. If a person carries two copies of the brown eye allele, since they are codominant, the person would have brown eyes. Recessive alleles are the genes that do not show the trait. If a person has one copy of the brown eye allele (dominant) and one copy of the blue eye allele (recessive) then that person is considered to be a carrier of the blue eye allele, since they would have brown eyes but still have the blue eye trait that is not shown. Recessive alleles only show the traits if the person has 2 copies of the same alleles. This is considered being homozygous , having the same 2 copies of alleles. If a person has 2 copies of the blue eye allele (both recessive) then the person would have blue eyes.

    Fig. 1 Illustration to show the inheritance of dominant and recessive alleles for eye colour.
    Image credit: Genome Research Limited

    Sex-linked genes are genes that are inhererited through the X chromosome. Remember that a biological female carries 2 sets of X chromosomes (XX) and a biological male carries one set of the X and one set of Y chromosomes (XY). If the offspring is a boy, the X chromosome comes from the mother and the Y comes from the father. If the offspring is a girl, one of the X chromosomes comes from the mother and the other X chromosome comes from the father. In some genetic diseases that are caused by sex-linked genes, for example haemophila , a color blindness trait, the allele for the disease is recessive. You can recall that recessive traits are only shown if they are homozygous (both copies of the alleles are recessive). For a female to have the disease, both of her X chromosomes must carry the recessive diseased copies of alleles. For a male to have a sex-linked gene, only one copy of the recessive sex-linked gene is needed for the male to have the disease. Dominance does not matter in sex-linked genes for XY males. If the mother is a carrier (unaffected but still have the affected trait), her offspring could be affected. Males are more likely to inherit a sex-linked gene as only one chromosome of a diseased trait is needed, whether the disease trait is dominant or recessive. You can see that sex-linked genes are by chance. Even though the father is affected with a dominant trait, only half of their offspring is affected, especially from the girls because they have to inherit a chromosome from the father. The male offspring was unaffected because they had already received a Y chromosome from the father so they got the non affected X chromosome form the mother. In this photo, the mother is affected with a dominant trait but only half of their offspring was able to be affected. The offsprings had a 50% chance of getting the affected trait. With an unaffected mother whose carrier, meaning the disease trait is recessive, only one of the offspring was affected and one is unaffected but a carrier. This is an example of how dominance genes does not matter as it depends on which X chromosome you can get and whether or not the set chromosomes you inherited contain the diseased trait being dominant or recessive. With males especially, they would only get a 50/50 chance of inheriting a non diseased trait, as they can only get the X chromosome from the mother. With females, they have a lower chance of getting a diseased trait as it depends on what chromosome she inherited from the mother whether its dominant or recessive and what X chromosome she inherited from her father.

    Fig. 2 X-linked gene inheritance. The expression of recessive X-linked genes is more common in boys who only have one X gene.


    WHO WAS THE SHOOTER?

    THINGS TO BE LEARNED FROM THIS CASE

    Bloodstain pattern evidence can leave strong images with the jury that can help resolve a case long after the expert has left the stand. Right- and left-handedness can be important even when investigators feel that it is insignificant. Standard interview questions should include requests of all interviewees, witnesses as well as suspects, regarding whether they are right- or left-handed. Sometimes unsuspected witnesses interviewed become suspects later. Many times the issue of left-handed versus right-handed enters the case much later. It saves time to have that information available if and when the investigation focus shifts.

    Whenever you have two defendants, be aware that retrial after appellate review may be unreliable. The best bet is to do it right the first time.