Information

What is the name of a gene whose effect comes from the dominant and recessive alleles


Thep53gene has a mutation which results in the cell having a cancerous phenotype. This effect is dominant as one allele being mutated is enough to result in this effect. However when both alleles are mutated then the phenotype changes / become more pronounced so it's not a simple dominant mutation. What is this kind of mutation called? Is it hemi-dominant or something?


Allelic effects

What is the name of a gene whose effect [… ]

The concept of dominance make sense for alleles. The gene has no name to refer to the allelic interaction.

Patterns of dominance

Patterns of dominance and recessivity are typically used for quantitative traits. Can you somehow put a a number into the phenotypes of all three genotypes? Making some kind of association table would help even if the phenotypes are not quantitative. See below.

Attempt to answer the question

It looks like you are considering a case where the phenotypes

aa -> healthy Aa -> sick AA -> very sick

Under this model, it is hard to tell whether you would considersickhalf way throughhealthyandvery sickor not. It feels like you want to considersickas being closer tovery sickthan tohealthyso if we put numbers we could get something like

aa -> 1 (healthy) Aa -> 2 (sick) AA -> 2.5 (very sick)

As such,Ashows partial dominance (aka incomplete dominance or semi-dominance).


What is the name of a gene whose effect comes from the dominant and recessive alleles - Biology

( If color name is highlighted at the beginning of its description, click on it to see a picture.)

DISCLAIMER: In recent years much has been discovered by the professional geneticist regarding inheritance. We now must cope with terms such as "copy number variance", which refers to the number of copies of a specific gene on a chromosome, "epigenetics", which refers to changes that are due to causes other than simple gene mutation, and "co-dominance", which I would hope is self-explanatory. What follows below is a treatment of "classical" or Mendelian genetics as applied to the domestic pigeon.

BARLESS: Barless is an autosomal recessive gene. "Autosomal" means it's not on the sex chromosome, and "recessive" means the bird must possess two of these genes (one from each parent) for the characteristic to be visible. A barless blue pigeon looks like an ordinary blue bar or blue check, except it does not possess the black markings (bars or checks) on the wingshields. Here is a photo of a pair of barless consisting of a true silver cock and a brown hen. Barless ash red (homer men call it "barless silver") looks like an ordinary silver red bar, but without the red bars. Such birds are often confused with spread ash, which looks very similar to barless ash. They are completely different genetically, however. If you have what you think is a barless ash red, you can test to see if it is really genetically barless by mating it to a blue of any pattern. If you ever obtain any black youngsters out of such a pairing, you can be sure that your ash is actually the spread factor rather than barless. (Black is the same as "spread blue.") If you obtain only birds that look like the ash parent, you can also be sure it is not barless because barless is recessive and if barless is mated to a bird that is barred or checkered and the latter does not carry the gene for barless, the young cannot be barless. Back to top of page

INDIGO: Indigo is an autosomal dominant gene. Since it is dominant, any bird that carries it will show it and therefore it is very easy to follow. Indigo, in its heterozygous state, gives a blue pigeon a purplish hue and changes the bars or checks from black to a plum color. It also washes out the tail bar so instead of being black the tail bar is lighter than the rest of the tail.
Indigo combined with black (blue plus the spread factor) yields the typical "Andalusian Blue" seen in many breeds. In the homozygous state, indigo on a blue base mimics ash red, and such a bird resembles an ash red with a slightly darker ground color. On a black base, homozygous indigo yields a bird with a near-white ground color and darker lacing. Indigo in combination with brown gives a bird that very closely resembles an ash red, but with richer wing markings and some tell-tale lacing on upper tail coverts. Indigo with ash red is virtually indistinguishable from ash red, although when combined with spread often gives a rich mahogany effect.
It has been noted by several fanciers and reported in the literature that indigo noticeably enriches the color of recessive red. Back to top of page

DOMINANT OPAL: Dominant opal is an autosomal dominant gene. It is extremely variable in its expression, ranging from slight washing out of the bar and check pattern all the way to white bars and checks. At the same time the tail bar may be washed out or it may be nearly black. One way to identify dominant opal is by the light-colored shaft of the tail feathers. The most common expression of dominant opal is a light greyish-brown in place of the normal black bars or checkering. Another common effect is that the normal blue groundcolor lightens up to a steely gray. The white bar or white check effect is more likely to occur if the gene is in combination with indigo, although that combination does not always yield the same results.
Dominant opal in combination with ash-red is often indistinguishable from ordinary ash red. When combined with black the results range from a very attractively laced bird with silvery groundcolor, to an overall gun-metal gray, all the way to what has sometimes been described as "dark mud" which is a washed out black with some lightening effects near the base of the feathers. Back to top of page

RECESSIVE OPAL: Recessive opal is another autosomal recessive gene. It causes the bars and checks, as well as the tail bar, on an otherwise blue pigeon to become much lighter. That lightening effect is extremely variable, and the wing markings can range anywhere from a medium grey rather than black, to a greyish-tan, to a "bronzy" black, to a pink or reddish hue. The last often mimics the color of ash red, including the absence of a tail bar. When this happens, some fanciers are led to believe that they have produced a red check out of a pair of blue checks, which is genetically impossible. In such a case the checkering on the wingshield is often edged in gray. If such a psuedo-red check (or pseudo-silver) is mated to a blue that does not carry the gene for recessive opal, all young will be blues. It is not uncommon for the inexperienced fancier to confuse recessive opal with dominant opal. Click here to see a photo of the extreme red phase of recessive opal. Click here to see two of his 2007 offspring, still in their juvenile plumage, and note how similar they look to "silver red bar" (ash red bar). Here is one of those same young birds as a 2-year-old. The extreme version of red phase recessive opal is also referred to as "Cherry" and it is believed to be a recessive allele of opal. Others think it might be recessive opal with some additional modifier. Several years ago Steve Souza of California reported on it and his limited results pointed to it as being the former. Back to top of page

SMOKEY: Smokey is the gene responsible for what most Racing Homer flyers call "slate". It is a simple autosomal recessive that is very common in homers. This gene causes the normal blue pigment to wash out to the point that the bird's ground color is a duller gray than the normal blue-gray of a blue bar or blue check. It also causes the pattern on the wingshield, as well as the tail bar, to be less well-defined. The gene got its name when someone commented that it looked as if the pigeon was being viewed through a thick layer of smoke. One of the tell-tale signs of smokey is that the albescent (white) strips along the outer edges of the two outer tail feathers are not there. Also, smokey pigeons will usually have a much lighter beak than that of an ordinary blue pigeon. Smokey in the presence of ash red is believed to be responsible for some of the "plum" colored reds seen in some lofts. The smokey gene is believed to be very common in the Janssen and Trenton strains of Racing Homers. It is also common in Russian Tumblers and several other breeds. Back to top of page

REDUCED: Reduced is a sex-linked recessive gene. Since it is sex-linked, it need occur just once in hens (hens have only one "active" sex chromosome), but in cocks it must occur twice. Reduced causes blue pigeons to have a light, frosty ground color - often with a silvery crescent on the crop area - with pinkish-colored bars and checks. Reduced checkers are the most attractive. Reduced barless are the least attractive, because they have no bars or checks to show off the effects of this gene. Reduced ash red in either bar or check pattern gives the closest thing I have seen to a pink pigeon. Back to top of page

DILUTE: Dilute is also a sex-linked recessive gene. It changes a blue pigeon to a "true" silver (as opposed to the silver of the racing homer, which is actually ash red bar), or as many US racing homer men call it, "dun bar" or "dun check". An ash red dilute is a yellow check or "cream" or yellow bar, and a brown dilute is a khaki, which in the bar and check patterns is often confused with dilute ash red. The former, however, will always show a dark tail bar while the latter will have a lighter tail bar or no tail bar at all. Back to top of page

PALE: Pale is a sex-linked recessive gene that is allelic (at the same location on the chromosome) to dilute. The effect of pale is to lighten the overall color of the pigeon, with the effect being somewhat lighter than the intense color but not quite as light as the dilute. Pale is seen frequently in gold Modenas and the Gimpel (Archangel) breed. It has been moved to other breeds as well. Back to top of page

RECESSIVE RED AND RECESSIVE YELLOW: Recessive red is an autosomal recessive gene which masks the underlying basic color (blue-black, ash-red, or brown) and pattern. It causes the pigeon to take on a fairly uniform rust or red-brown coloration. Ideal recessive red coloration is uniform throughout, with the flight feathers and tail the same shade as the rest of the bird. (A good example is the red Carneau or the red schietti Modena.) However, such ideal coloration requires some additional modifying genes, not all of which have been positively identified. It is claimed by many fanciers that indigo quite noticeably improves the color, and the spread factor does likewise. (The best colored recessive red Racing Homer I ever owned was also spread and indigo.) Many recessive reds possess a smuttiness that dulls the color, and the tail is more brown than red. This is particularly true in recessive red Racing Homers, where the emphasis is on performance rather than on color. Recessive yellow is the combination of the recessive red gene with the gene for dilution, which is also recessive (see above). Recessive yellows are not really yellow, but more of a rich tan similar to the "buff" found in chickens. Many recessive reds and yellows, particularly among Racing Homers, exhibit a white mottling or "splash" effect on the wingshields and head, and the cause of such mottling has not been firmly established. Click here to access an article I wrote concerning the inheritance of the mottling effect seen in many recessive red homers. Back to top of page

WHITE: White is a complicated subject for the simple reason that a white pigeon may be the result of several genetic factors. For example, homozygous grizzle causes a blue pigeon to be stork-marked (white body with some coloring on the flights and tail feathers), but a homozygous grizzle ash red is almost pure white, with perhaps a few very light red feathers visible upon close inspection. Such whites can usually be distinguished from other genetic combinations because they will have orange eyes rather than the "bull" eyes of many whites. Other whites are the result of one or more of the genes that cause a bird to be pied, and the particulars of some of these genes have not yet been worked out. Examples would be the saddle, whiteside, baldhead, Lahore, and other patterns.

It has been established, however, that many whites are the result of a specific gene for "recessive white". Such a gene has been identified in the various "homer" breeds: Racing Homers, American Giant Homers, German Beauty Homers, Dragoons, etc. Recessive white is believed by some to be at the same locus as the gene for the gazzi pattern, which has been shown to be a simple recessive, and recessive white is epistatic (see "epistasis" below) to all other colors and patterns, a possible exception being recessive red. (I am unaware of any research concerning the combination of those two genes.) The action of recessive white is identical to that of recessive red in that if a pair of recessive whites are mated together, all young are recessive white. If a recessive white is mated to a self-colored bird that does not carry the recessive white gene, all young are colored. If two colored birds carrying the recessive white gene are mated, the offspsring exhibit the classic 1:2:1 Mendelian ratio, which is: 1 white, 2 colored birds carrying white, and 1 colored bird that does not carry white. Most, if not all, recessive whites have dark or "bull" eyes. Back to top of page

"SEX-LINKED" MATINGS: A sex-linked mating is a mating of two pigeons whose sex-linked color characteristics (dilution, reduced, almond, faded, Quinn-mutant, along with the three "basic" colors of blue-black, brown, and ash-red) are such that the gene for the hen's particular sex-linked characteristic is dominant to the cock's corresponding gene for that characteristic. When this happens, all youngsters having the mother's color will be cocks and all young having the sire's color will be hens. In other words, in a sex-linked mating the young are just the opposite color of the parents. The reason for this is that the cock has two active sex chromosomes (one from each parent) and the hen has only one that is active. She receives her sex chromosome from her sire only hence her color must be inherited strictly from him. Since the young cock receives one chromosome from each parent, and since in a sex-linked mating the mother's color is the dominant one, the son must appear the color of his mother. (He will carry the sire's color on the other chromosome and is capable of passing it on to later generations.) Examples of sex-linked matings in which the young have just the opposite colors of their parents include, but are not limited to: Blue cock x ash-red hen Brown cock x ash-red or blue hen Non-almond cock x almond hen dilute cock (dun, yellow, "true" silver or silver-dun-bar, cream, etc.) x intense (blue-black, ash-red, brown) hen. Indigo, dominant and recessive opal, toy stencil, etc. are not sex-linked characteristics and therefore these principles don't apply to them. Click here for a more detailed discussion of sex linkage. Back to top of page

THE PATTERN SERIES: The pattern series consists of T-pattern (sometimes called "velvet"), checker, bar, and barless, in descending order of dominance. That is, anything later in this list is recessive to anything listed ahead of it. Pattern is an autosomal characteristic and therefore is controlled by 2 genes, one from each parent. A bird that appears to be a T-pattern can carry any of the four pattern genes (T-pattern, checker, bar or barless) on the second chromosome but it appears T-pattern because T-pattern is dominant to all the rest. A bird that is checker can carry checker, bar or barless on the other chromosome, but it appears checker because checker is dominant to bar and barless. It cannot carry T-pattern because if it did it would appear T-pattern rather than checker. Likewise a bar can carry bar or barless on the other chromosome, but it cannot carry T-pattern or checker because if it did it would not be a bar it would be T-pattern or checker, due to the dominance of the latter. A barless must be barless on both chromosomes because the gene for barless is recessive to all the other pattern genes.

If a bird has the same pattern gene on both chromosomes we say the bird is "homozygous" for that pattern. If a bird is homozygous for a given pattern and is mated to a bird of another pattern that is recessive to it, all the young will have the same pattern as the dominant parent, but will carry the gene for the pattern of the recessive parent. Such youngsters are said to be "heterozgous" for each pattern. If two heterozygotes are mated together we get the classical 1:2:1 Mendelian pattern, which means 25% of those young will be homozygous for the dominant gene, 50% heterozygous, and 25% homozygous for the recessive gene. In appearance they will be 3:1, since only the one that is homozygous for the recessive gene will show it.

Caution! The gene for spread, which makes a blue pigeon black, is NOT part of the pattern series. Back to top of page

EPISTASIS: Sometimes you hear a statement such as "Recessive red is epistatic to all other colors and to the pattern series." This simply means that recessive red covers or hides all other colors or patterns. For example, a recessive red may be genetically a bar or a checker, but one cannot tell by looking at the bird because the pattern is hidden by the recessive red gene. Recessive red also hides ("is epistatic to") the underlying sex-linked color of blue-black, ash-red, or brown. The spread factor is epistatic to the pattern on an otherwise blue pigeon, making it appear solid black. The combination of both recessive red and spread often makes for a much richer and more even shade of red. Back to top of page

"SILVER": I have placed the word "Silver" in quotes because within the pigeon fancy it has two separate meanings.

To a geneticist and to breeders of most fancy breeds, it means the same as dilute blue bar. This is an ordinary "wild type" blue bar pigeon with the single recessive sex-linked gene for dilute (homozygous, or two copies of the gene, in cocks one copy of the gene in hens). Such a bird has a light silvery ground color with bars that are almost black. Racing Homer breeders often refer to this as a "dun bar" or "silver dun bar". (See notes on "dilute", above.) Sometimes a brown bar is called silver, but the bars of a brown are more chocolate colored. Also, a brown will fade considerably with continued exposure to sunlight, and usually will have "false pearl" or pinkish tinted eyes. A true silver (dilute blue) will fade with heavy exposure to sunlight, but not nearly as much as a brown.

The term "silver", when used by Racing Homer breeders in the United States, or breeders of many of the show strains of the Racing Homer or the American Show Racer, means the same as "silver red bar", which is simply an ash red bar pigeon with clear ash wingshields and often with a "frosty" or silvery sheen to the feathers on the neck. Note that in this case we are dealing with a sex-linked dominant. If such a bird shows any smuttiness or dirtiness in the wingshield area, it is usually referred to as a "mealy". Back to top of page

ALMOND: "Classical" almond coloration is the color combination most often seen in the English Shortface Tumbler: golden ground color with haphazard flecking of black and lighter grey throughout. The color got its name from the fact that the ground color is supposed to be the same color as that of the inside of the shell of the almond nut. Unfortunately that classical color is actually the result of several genes: the almond gene plus T-pattern, kite bronze, and recessive red. The almond gene alone, which is sex-linked, simply is a "lightening" gene that cancels the effects of most pigments, making an otherwise blue pigeon look like a dirty white with a few dark flecks. Such birds are often mistaken for homozygous grizzle or some other mutation. For a much more thorough treatment of the almond complex, see the link to Frank Mosca's website below. Back to top of page

CREST: Crest is another autosomal recessive gene that causes the feathers on the back of the head to be reversed. One gene appears to be responsible for the reversal of feathers, while additional "modifier" genes determine whether the reversal appears as a "peak" crest as in Oriental Frills and Archangels, or a "shell" crest as seen in Helmets, Nuns and English Trumpeters. UPDATE: Recent studies would indicate that there may be more than one gene for crest, and that peak and shell crest may not be the same gene, so the reference to "modifier" genes above may be an oversimplification. For the beginning fancier, however, it will suffice to think of crest as a recessive gene. Back to top of page

WILD TYPE: The phrase "wild type", when used in genetics, simply means the total gene package of an individual which has no mutations. It is a reference point from which geneticists work. In the case of the pigeon, it represents a blue bar bird of normal size, with clean legs, no crest or frill, orange eyes, etc. In other words, it represents a bird resembling the "wild type" rock dove, columba livia. "Wild type" in reference to the pattern series means barred "wild type" in reference to foot feathering means clean legged "wild type" in reference to basic color means blue etc. In discussing "wild type", many pigeon geneticists envision a blue bar racing homer, although the racing homer is admittedly not "wild type" when the homing instinct is considered. Back to top of page

Neck Frill: Neck frill (sometime called "cravat" or "zipper") is a simple autosomal recessive. It is a staple of certain owl breeds, as well as the Oriental Frill and the Turbit. Back to top of page

BROWN: Brown is a sex-linked recessive, which causes the black areas on a "wild type" pigeon to become a chocolate brown. It is often confused with the dilute of blue, or what many refer to as "silver dun bar" or "true silver". However, brown is intense, and brown youngsters will have normal down when in the nest, unlike the short down of a true silver. The dilute of brown is Khaki. Khaki is often confused with cream, or ash yellow, which is the dilute of ash red. Cream, however, does not show a tail bar, and khaki always does. Both brown and khaki are extremely susceptible to bleaching by sunlight, whereas true silver and cream may show some bleaching but nothing near the effect on brown. (See notes above, under "dilute" and "silver".) Back to top of page

DRIZZLE: Drizzle is apparently an autosomal dominant discovered in the early 2000's by the late Larry Long of Iowa. It causes a blue pigeon to be more grayish than ordinary and the bars and checks are more charcoal gray than the black of a normal blue. A spread blue is more of a solid charcoal than a black as well. Drizzle on ash red washes out the red pigment extensively. The trait showed up again in 2007 in the loft of Tom Barnhart of Ohio. Upon checking pedigrees, it was found that the Barnhart source as well as the original bird discovered by Larry Long both traced back to the same source loft of Pete Hogan in Buffalo, NY. Long gave the trait the name "drizzle" because he thought it looked as if the bird was being viewed through a drizzle or mist. James Gratz is currently (2010) doing breeding tests on this gene to see if there is any significant difference in the phenotypes of the heterozygous and homozygous forms.

Update: Recent research by Gratz and others seem to support the thesis that the drizzle gene is lethal in the homozygous form. Back to top of page


Gene Interactions: Meaning and Characteristics | Genetics

When expression of one gene depends on the presence or absence of another gene in an individual, it is known as gene interaction. The interaction of genes at different loci that affect the same character is called epistasis.

The term epistasis was first used by Bateson in 1909 to describe two different genes which affect the same character, one of which masks the expression of other gene. The gene that masks another gene is called epistatic gene, and the gene whose expression is masked is termed as hypostatic gene. Epistasis is also referred to as inter-genic or inter-allelic gene interaction.

Characteristics of Gene Interaction:

The interaction of genes has several characteristics.

The important features of gene interaction are briefly described below:

The epistatic gene interaction always involves two or more genes. This is an essential feature of gene interaction.

ii. Affect same Character:

The epistatic genes always affect the expression of one and the same character of an individual.

The phenotypic expression of one gene usually depends on the presence or absence of epistatic gene. The gene which has masking effect is called epistatic gene and the gene whose effect is masked is known as hypostatic gene.

iv. Modification of Dihybrid Segregation Ratio:

Epistasis leads to the modification of normal dihybrid or tri-hybrid segregation ratio in F2 generation.

Epistasis is usually governed by dominant gene, but now cases of recessive epistasis are also known.

Gene Interaction for Comb Shape in Poultry:

In gene interaction, sometimes two dominant genes controlling the same character produce a new phenotype in F1 when they come together from two different parents. Such case of gene interaction was observed by Bateson and Punnett for comb shape in poultry.

There are three types of comb shape in poultry, viz., rose, pea and single. The comb shape is controlled by two pairs of alleles. The rose comb is governed by a dominant gene R and pea comb by a dominant gene P. The single comb is governed by two recessive genes (rrpp).

When a cross was made between rose (RRpp) and pea (rrPP), a new phenotype called walnut developed in F1. The walnut comb developed as a consequence of combining two dominant alleles R and P together in F1. Inter-mating of F1 birds produced four types of combs, viz., walnut, rose pea and single in 9 : 3 : 3 : 1 ratio in F2 generation.

Here individuals with R-P-(9/16) genotypes produce walnut comb, because two dominant genes together produce walnut comb. Individuals with R-pp (3/16) will give rise to rose comb, and those with rrP-(3/16) genotypes will produce pea comb. The single comb (1/16) will develop from a double recessive, genotype (Fig. 8.1).


How are genes and alleles inherited?

When humans procreate, the child receives 23 chromosomes (long strands of DNA) from each parent. Each matching chromosome pair contains the same set of genes, with unique genes located at certain spots known as the gene locus.

This inheritance means that individuals have two gene copies for a given trait, one inherited from their mother and the other from their father. These are known as maternal alleles and paternal alleles. It is how these alleles interact that is responsible for unique characteristics.

What is a dominant gene and what is a recessive gene?

The totality of genes encoded on a human’s 46 chromosomes is known as their genotype. But not all gene variations will be expressed. For instance, you might have one allele for brown eyes and another for blue eyes, but you will not therefore have one blue and one brown eye.

Individuals do not display the characteristics encoded on each matching pair of genes. Instead, the genes that are expressed result in the phenotype, which is how genes are expressed in observable characteristics.

How does the body know which alleles to express? This comes down to the properties of alleles that are paired.

Every individual has two copies, or alleles, or a single gene. When the alleles are the same, they are known as homozygotes. When they are different, they are called heterozygotes.

Homozygotes code for the same trait, for instance, blue eyes. If you have two blue eye alleles, your eyes will be blue. But if you have one allele for blue eyes and another for brown eyes, your eye color will be dictated by whichever allele is dominant.

A dominant allele is one that always determines the phenotype when present. On the other hand, a recessive allele is one that is not expressed when its paired allele is dominant.

With eye color, the brown eye allele is dominant to the blue eye allele. This means that a child with a blue allele from their mom and a brown allele from their dad will end up with brown eyes. But a child with two blue alleles will display the blue eye phenotype.


Main Differences Between Dominant and Recessive Allele

  • Dominant alleles have the ability to override the effect of other alleles and maintain phenotypic expression, while recessive alleles are the ones that are unable to express themselves in the presence of other genes.
  • Dominant genes have the ability to mask the effect of other alleles, while recessive alleles cannot mask the effect of other genetic alleles.
  • The representation of dominant alleles is through upper case alphabets while recessive alleles are repres
  • ented by recessive alleles.A dominant allele can express itself in both homozygous and heterozygous pair, while a recessive allege expresses itself only in homozygous paring conditions.
  • A dominant allele is likely to be inherited, while a recessive allele is less likely to express, even if it gets inherited.
  • A dominant allele usually encodes functional proteins while a recessive allele encoded non-functional protein.

Punnett Square: Dominant and Recessive Traits

Every human on earth is a combination of two sets of genes: your mother&rsquos and your father&rsquos. These genes created a blueprint for you, and they make you unique. Genes lead to different traits, or characteristics, such as brown eyes or blue eyes. Parents passing on their genes to their offspring is called heredity. In genetic terms, you are a hybrid of both of your parents: a combination of their many different genes.

What&rsquos the probability of receiving a particular gene? The Punnett Square is a tool that allows you to see the different gene combinations that are possible when two parents of any species create offspring.

When looking at the model of inheritance which the Punnett Square illustrates (referred to as Mendelian inheritance), you are observing combinations of dominant alleles and recessive alleles. An allele is a version of a gene (the eye color gene can consist of blue, brown, green, gray, and hazel alleles). Dominant genes mask recessive genes. For example, brown eyes are the dominant gene for eye color, and blue eyes are recessive, so when the genes for brown and blue eyes are combined in offspring, there is a 75% chance that the offspring will have brown eyes. This is why the majority of people in the world have brown eyes.

Problem

How does a Punnett Square work?

Materials

  • Ruler
  • Coin
  • Tape
  • Piece of paper
  • Pencil
  • Paper
  • Colored pencils

Procedure

  1. Invent two parents, one male and one female, that have different characteristics. For example, make one parent have blonde hair and blue eyes and the other have brown hair and brown eyes. You can make a drawing with your colored pencils to help you keep track of all the characteristics you&rsquore combining.
  2. Let&rsquos make the Punnett Square for eye color. If the father is heterozygous for brown eyes, this means he has one brown eye allele and one blue eye allele. Write Bb at the top of the square. The capital B is for brown eyes and the lower case b is for blue eyes. Write bb (a homozygous blue eye allele) on the side of the square &ndash these are the mother&rsquos genes.
  3. Now tape a capital and lower case B on each side of two different pennies. These represent the gene that the child gets from each parent.
  4. Flip each penny. What do you get? If you get two capital B&rsquos, the child will have brown eyes. If you get two lower case b&rsquos, the child will have blue eyes. Do you remember what happens if you have a capital B (brown) gene and a lower case b (blue) gene?
  5. How likely is it that the child will have blue eyes?
  6. Try the experiment with other characteristics as well. You can create a whole fictitious family with different gene combinations.

Results

The Punnett Square shows you how different gene combinations lead to different characteristics like eye color. In the given scenario (one parent is heterozygous for brown and blue eye alleles and one is homozygous for blue eyes), their child has a 50% chance of having blue eyes. If both parents had one blue eye allele and one brown eye allele, their child would have a 25% chance of having blue eyes.

The Punnett Square, named after British Geneticist Reginald C. Punnett, is a good tool for thinking about dominant and recessive alleles, but it isn&rsquot a perfect scientific model. It only works if the genes are independent of one another (situations where having a certain gene doesn&rsquot change the probability of having another). There are also many different genes that combine to produce a characteristic like eye color, not just one. That&rsquos why there are many different patterns and shades of brown, blue, green, hazel, and gray eyes.

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What is Codominance?

Codominance is the expression of the effects of both alleles independently in one phenotype. It is a kind of a dominance relationship between alleles of a gene. Moreover, it is a type of non-Mendelian inheritance. In heterozygous state, both alleles are expressed fully and show the effect of the allele in the offspring independently. Neither allele suppresses the effect of the other allele in codominance. Hence, the final phenotype is neither dominant nor recessive. Instead, it comprises the combination of both traits. Both alleles manifest the phenotype with its effect without mixing the individual effects. In the final phenotype, the effects of both alleles can be distinguished clearly when the codominance situation. Furthermore, there is no quantitative effect in codominance.

Figure 01: Codominance

ABO blood group system is an example for codominance. Allele A and allele B are codominant to each other. Hence, blood group AB is neither A nor B. It serves as a separate blood group because of the codominance between A and B. Another classic example of codominance is the tabby cat. When pure black cats and brown cats mate with each other, the 1 st filial generation will consist of kittens (tabby cats) which are black and having brown stripes or spots or vice versa. Codominance can also be observed among the Shorthorn cattle.


What Is A Dominant Gene/Factor/Trait/Allele?

A dominant gene or a dominant version of a gene, is a particular variant of a gene which for a variety of factors, expresses itself more strongly all by itself than the other version of the gene which the person is carrying. The other version of the gene is recessive. They are masked by the dominant phenotype and you need to get two copies of that trait to see it in your phenotype.

Dominant alleles show their effect even if the individual only has one copy of the allele also referred to as being heterozygous. For example, the allele for brown eyes is dominant, therefore you only need one copy of the brown-eye allele to have brown eyes (although, with two copies you will still have brown eyes).

However, in cases where both alleles are dominant, it is referred to as codominance. The resulting characteristic is due to both alleles being expressed equally. An example of this is the blood group AB which is the result of codominance of the A and B dominant alleles.

Most dominant traits are due to genes located on the autosomal (the no-sex chromosomes).an autosomal dominant trait typically affects males and females with equal likelihood and with similar severity. Examples of dominant disorders include:

  • Polycystic kidney disease
  • Achondroplasia (a common form of dwarfism with short arms and legs).
  • Familial hypercholesterolemia (high blood cholesterol leading to premature coronary artery disease).
  • Hunington disease (a form of progressive dementia)
  • Neurofibromatosis (a neurologic disorder with an increased risk of malignant tumors).

What You Need To Know About Dominant Genes

  • A dominant allele is a gene that produces its effect (expresses itself) in the presence of the other (recessive) allele.
  • Recessive genes always express the dominant trait.
  • It does not require another similar allele to produce its effect on the phenotype e.g Tt is tall.
  • Dominant allele or trait can form complete polypeptide or enzyme for expressing its effects e.g red color of flower in Pea.
  • The dominant genes mask the effect of the recessive genes.
  • Example of dominant trait is Brown eyes, A and B blood type.
  • It is written in Uppercase Letter (T).
  • Dominant gene is more likely to be inherited.
  • The dominant gene is more prone to produce diseases.

Other Inheritance Patterns

Incomplete Dominance

Not all genetic disorders are inherited in a dominant–recessive pattern. In incomplete dominance, the offspring express a heterozygous phenotype that is intermediate between one parent’s homozygous dominant trait and the other parent’s homozygous recessive trait. An example of this can be seen in snapdragons when red-flowered plants and white-flowered plants are crossed to produce pink-flowered plants. In humans, incomplete dominance occurs with one of the genes for hair texture. When one parent passes a curly hair allele (the incompletely dominant allele) and the other parent passes a straight-hair allele, the effect on the offspring will be intermediate, resulting in hair that is wavy.

Codominance

Codominance is characterized by the equal, distinct, and simultaneous expression of both parents’ different alleles. This pattern differs from the intermediate, blended features seen in incomplete dominance. A classic example of codominance in humans is ABO blood type. People are blood type A if they have an allele for an enzyme that facilitates the production of surface antigen A on their erythrocytes. This allele is designated I A . In the same manner, people are blood type B if they express an enzyme for the production of surface antigen B. People who have alleles for both enzymes (I A and I B ) produce both surface antigens A and B. As a result, they are blood type AB. Because the effect of both alleles (or enzymes) is observed, we say that the I A and I B alleles are codominant. There is also a third allele that determines blood type. This allele (i) produces a nonfunctional enzyme. People who have two i alleles do not produce either A or B surface antigens: they have type O blood. If a person has I A and i alleles, the person will have blood type A. Notice that it does not make any difference whether a person has two I A alleles or one I A and one i allele. In both cases, the person is blood type A. Because I A masks i, we say that I A is dominant to i. The following table summarizes the expression of blood type.

Table 1. Expression of Blood Types
Blood type Genotype Pattern of inheritance
A I A I A or I A i I A is dominant to i
B I B I B orI B i I B is dominant to i
AB I A I B I A is co-dominant to I B
O ii Two recessive alleles

Lethal Alleles

Certain combinations of alleles can be lethal, meaning they prevent the individual from developing in utero, or cause a shortened life span. In recessive lethal inheritance patterns, a child who is born to two heterozygous (carrier) parents and who inherited the faulty allele from both would not survive. An example of this is Tay–Sachs, a fatal disorder of the nervous system. In this disorder, parents with one copy of the allele for the disorder are carriers. If they both transmit their abnormal allele, their offspring will develop the disease and will die in childhood, usually before age 5.

Dominant lethal inheritance patterns are much more rare because neither heterozygotes nor homozygotes survive. Of course, dominant lethal alleles that arise naturally through mutation and cause miscarriages or stillbirths are never transmitted to subsequent generations. However, some dominant lethal alleles, such as the allele for Huntington’s disease, cause a shortened life span but may not be identified until after the person reaches reproductive age and has children. Huntington’s disease causes irreversible nerve cell degeneration and death in 100 percent of affected individuals, but it may not be expressed until the individual reaches middle age. In this way, dominant lethal alleles can be maintained in the human population. Individuals with a family history of Huntington’s disease are typically offered genetic counseling, which can help them decide whether or not they wish to be tested for the faulty gene.


What is an example of Overdominance?

An example of epistasis is pigmentation in mice. The wild-type coat color, agouti (AA), is dominant to solid-colored fur (aa). The recessive c allele does not produce pigment, and a mouse with the homozygous recessive cc enotype is albino regardless of the allele present at the A locus.

Secondly, what is incomplete dominance give an example? Examples of Incomplete Dominance Pink roses are often the result of incomplete dominance. When red roses, which contain the dominant red allele, are mated with white roses, which is recessive, the offspring will be heterozygotes and will express a pink phenotype.

Hereof, what is Overdominance hypothesis?

o·ver·dom·i·nance (ō'vĕr-dom'i-năns), That state in which the heterozygote has greater phenotype value and perhaps is more fit than the homozygous state for either of the alleles that it comprises. Compare: balanced polymorphism.

What is Codominance in biology?

Codominance is a relationship between two versions of a gene. Individuals receive one version of a gene, called an allele, from each parent. If the alleles are different, the dominant allele usually will be expressed, while the effect of the other allele, called recessive, is masked.