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Why X-chromosome linked diseases happen due to recessive gene?

Why X-chromosome linked diseases happen due to recessive gene?


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All the X -chromosome linked genetic disease I have studied (Color-blindness, Haemophilia, Muscular dystrophy) happen due to a recessive gene.

At first, I thought it was because there could be a 'good' copy of that gene on the other X chromosome (in case of females). So that's why these genes are recessive. But this is a bogus reasoning, I soon realised.

So, why X-linked genetic diseases happen due to a recessive gene? Maybe not all. But why some are recessive?


First of all, most observed mutations are recessive (see the answer to this question; in brief: homozygous carriers are strongly selected against, heterozygous carriers are not affected only if the allele is recessive). So you wouldn't be surprised to find the same pattern on the sex chromosomes.

A good way to think about the specialty of recessive alleles on the X chromosome, is to think about why these diseases are most often visible in males (and you are on the right track there)… Most alleles causing heavy disease - be it a mutation on the autosomes or the sex chromosomes - are rare (it is for the same reason they are recessive which is described in more detail in the answer linked above; it is due homozygous carriers being affected by purifying selection again) and therefore female carriers are most likely heterozygous and therefore healthy. In males, however, a carrier is always affected by the disease because the carrier is hemizygous.


Why X-chromosome linked diseases happen due to recessive gene? - Biology

Most of the known genetic disorders are dominant gene-linked however, the vast majority of dominant gene linked disorders are not serious or debilitating. For example, the majority of those with Tourette’s Syndrome suffer only minor tics from time to time and can easily control their symptoms. Huntington’s Disease is a dominant gene linked disorder that affects the nervous system and is fatal, but does not appear until midlife. Recessive gene disorders, such as cystic fibrosis and sickle-cell anemia, are less common but may actually claim more lives because they are less likely to be detected as people are unaware that they are carriers of the disease. Some genetic disorders are sex-linked the defective gene is found on the X-chromosome. Males have only one X chromosome so are at greater risk for sex-linked disorders due to a recessive gene, such as hemophilia, color-blindness, and baldness. For females to be affected by the genetic defects, they need to inherit the recessive gene on both X-chromosomes, but if the defective gene is dominant, females can be equally at risk. Table 2.1 lists several genetic disorders.

Table 2.1 Genetic Disorders

Recessive Disorders (Homozygous): The individual inherits a gene change from both

parents. If the gene is inherited from just one parent, the person is a carrier and does not have the condition.

Cases per Birth

Sickle Cell Disease (SCD) is a condition in which the red blood cells in the body are shaped like a sickle (like the letter C) and affect the ability of the blood to transport oxygen. Carriers may experience some effects, but do not have the full condition.

1 in 36,000 Hispanic births

Cystic Fibrosis (CF) is a condition that affects breathing and digestion due to thick mucus building up in the body, especially the lungs and digestive system. In

CF, the mucus is thicker than normal and sticky.

Phenylketonuria (PKU) is a metabolic disorder in which the individual cannot metabolize phenylalanine, an amino acid. Left untreated intellectual deficits occur. PKU is easily detected and is treated with a special diet.

Tay Sachs Disease is caused by enzyme deficiency resulting in the accumulation of lipids in the nerve cells of the brain. This accumulation results in progressive damage to the cells and a decrease in cognitive and physical development. Death typically occurs by age five.

1in 30 American Jews is a carrier 1 in 20 French

Albinism is when the individual lacks melanin and possesses little to no pigment in the skin, hair, and eyes. Vision problems can also occur.

Fewer than 20,000 US cases per year

Autosomal Dominant Disorders (Heterozygous): In order to have the disorder, the

individual only needs to inherit the gene change from one parent.

Cases per Birth

Huntington’s Disease is a condition that affects the individual’s nervous system. Nerve cells become damaged, causing various parts of the brain to deteriorate. The disease affects movement, behavior and cognition. It is fatal, and occurs at

Tourette Syndrome is a tic disorder which results in uncontrollable motor and vocal tics as well as body jerking.

Achondroplasia is the most common form of disproportionate short stature. The

individual has abnormal bone growth resulting in short stature, disproportionately short arms and legs, short fingers, a large head, and specific facial features.

Sex-Linked Disorders: When the X chromosome carries the mutated gene, the disorder is referred to as an X-linked disorder. Males are more affected than females

because they possess only one X chromosome without an additional X chromosome to counter the harmful gene.

Cases per Birth

Fragile X Syndrome occurs when the body cannot make enough of a protein it needs for the brain to grow and problems with learning and behavior can

occur. Fragile X syndrome is caused from an abnormality in the X chromosome, which then breaks. If a female has fragile X, her second X chromosome usually is healthy, but males with fragile X don’t have a second healthy X chromosome.

This is why symptoms of fragile X syndrome usually are more serious in males.

Hemophilia occurs when there are problems in blood clotting causing both internal and external bleeding.

Duchenne Muscular Dystrophy is a weakening of the muscles resulting in an inability to move, wasting away, and possible death.


What are the different ways a genetic condition can be inherited?

Some genetic conditions are caused by variants (also known as mutations) in a single gene. These conditions are usually inherited in one of several patterns, depending on the gene involved:

One altered copy of the gene in each cell is sufficient for a person to be affected by an autosomal dominant disorder. In some cases, an affected person inherits the condition from an affected parent . In others, the condition may result from a new variant in the gene and occur in people with no history of the disorder in their family.

In autosomal recessive inheritance , variants occur in both copies of the gene in each cell. The parents of an individual with an autosomal recessive condition each carry one copy of the altered gene, but they typically do not show signs and symptoms of the condition. Autosomal recessive disorders are typically not seen in every generation of an affected family.

X-linked dominant disorders are caused by variants in genes on the X chromosome. In males (who have only one X chromosome), a variant in the only copy of the gene in each cell causes the disorder. In females (who have two X chromosomes), a variant in one of the two copies of the gene in each cell is sufficient to cause the disorder. Females may experience less severe symptoms of the disorder than males. A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons (no male-to-male transmission).

X-linked recessive disorders are also caused by variants in genes on the X chromosome. In males (who have only one X chromosome), one altered copy of the gene in each cell is sufficient to cause the condition. In females (who have two X chromosomes), a variant would have to occur in both copies of the gene to cause the disorder. Because it is unlikely that females will have two altered copies of this gene, males are affected by X-linked recessive disorders much more frequently than females. A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons (no male-to-male transmission).

Because the inheritance pattern of many X-linked disorders is not clearly dominant or recessive, some experts suggest that conditions be considered X-linked rather than X-linked dominant or X-linked recessive. X-linked disorders are caused by variants in genes on the X chromosome , one of the two sex chromosomes in each cell. In males (who have only one X chromosome), an alteration in the only copy of the gene in each cell is sufficient to cause the condition. In females (who have two X chromosomes), one altered copy of the gene usually leads to less severe health problems than those in affected males, or it may cause no signs or symptoms at all. A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons (no male-to-male transmission).

A condition is considered Y-linked if the altered gene that causes the disorder is located on the Y chromosome, one of the two sex chromosomes in each of a male's cells. Because only males have a Y chromosome, in Y-linked inheritance, a variant can only be passed from father to son.

In codominant inheritance , two different versions (alleles) of a gene are expressed, and each version makes a slightly different protein. Both alleles influence the genetic trait or determine the characteristics of the genetic condition.

Mitochondrial inheritance , also known as maternal inheritance, applies to genes in mitochondrial DNA. Mitochondria, which are structures in each cell that convert molecules into energy, each contain a small amount of DNA. Because only egg cells contribute mitochondria to the developing embryo, only females can pass on mitochondrial variants to their children. Conditions resulting from variants in mitochondrial DNA can appear in every generation of a family and can affect both males and females, but fathers do not pass these disorders to their daughters or sons.

Many health conditions are caused by the combined effects of multiple genes (described as polygenic) or by interactions between genes and the environment. Such disorders usually do not follow the patterns of inheritance listed above. Examples of conditions caused by variants in multiple genes or gene/environment interactions include heart disease, type 2 diabetes, schizophrenia, and certain types of cancer. For more information, please see What are complex or multifactorial disorders?

Disorders caused by changes in the number or structure of chromosomes also do not follow the straightforward patterns of inheritance listed above. To read about how chromosomal conditions occur, please see Are chromosomal disorders inherited?

Other genetic factors sometimes influence how a disorder is inherited. For an example, please see What are genomic imprinting and uniparental disomy?


Basic Concepts in Human Molecular Genetics

Christine M. Koellner MS, CGC , . W. Edward Highsmith Jr. PhD , in Molecular Pathology (Second Edition) , 2018

X-Linked Recessive Inheritance

X-linked recessive inheritance is designated when phenotypic expression is observed predominantly in males of unaffected, heterozygous mothers. All female offspring of affected males are obligate carriers. Visually, the pedigree typically shows a horizontal pattern of affected individuals with no instance of direct male-to-male transmission. However, males may transmit the disorder indirectly to a grandson, through a carrier female daughter.

It is not uncommon for X-linked recessive disorders to appear in a family such that before a certain generation the disease is not apparent, but is observed to be segregating in the family after that generation. This phenomenon is due to new mutations appearing de novo in an individual. This was explained by the American geneticist Haldane, and his theory is referred to as the Haldane hypothesis. If the reproductive fitness of a male affected with an X-linked recessive disorder is low or nil, then in a population one-third of all affected X chromosomes will be removed from the gene pool every generation. If the incidence of the disease is constant, then one-third of cases must be due to mutations arising de novo in a family. An example of decreased reproductive fitness among males is Duchenne muscular dystrophy.

No doubt the most famous family to be afflicted with an X-linked recessive condition is the House of Saxe-Coburg and Gotha, the British Royal family. Queen Victoria, apparently a carrier of a new hemophilia A mutation, had one affected son, Prince Leopold, and two daughters who were carriers of the disease. The daughter of Princess Alice, Princess Alix, was married to Tsar Nicholas II of Russia and the mother of the affected Tsarovich Alexei. The current royal family, the House of Windsor, is descended from Queen Victoria through an unaffected male, King Edward VII, so that branch of the family does not carry the hemophilia A mutation.


Why are X-linked traits exhibited more frequently by males than females?

because females have a pair of X chromosomes so there are lot of chances for them to be heterozygous i.e. two different alleles and dominant allele will be expressed. They are prone to get affected only if they are homozygous. Where as males have only one X chromosome so they are prone to X linked disorders more than females

Human beings have two sex chromosomes – X and Y . Males have XY and females have XX. Y chromosome have some male specific genes ….but the x chromosome have copies of many genes which the Y chromosome is lacking and a major portion of Y chromosome is considered genetically INERT.

Thus females have two gene loci available on each X chromosome and males have only one gene locus on one X chromosome ( as the other one is Y chromosome) .

In females , the expression of sex linked trait is when it is Dominant …or if it is Recessive …then female needs to be Homozygous for it .

Whereas in the case of Males… the allele present on X chromosome will be directly expressed whether it is Recessive or Dominant ..because males have only one X chromosome .

Thus X linked traits are more frequently expressed in males than in the females.


Inheritance Inheritance

Lesch Nyhan syndrome is inherited in an X-linked recessive manner. [3] A condition is X-linked if the changed ( mutated ) gene responsible for the condition is located on the X chromosome . The X chromosome is one of the two sex chromosomes females have two X chromosomes, and males have one X and one Y chromosome . Females who have one mutated copy of the responsible gene (on one of their X chromosomes) usually do not have the condition and are referred to as carriers . This is because they still have a working copy of the responsible gene on their other X chromosome. Males with one mutated copy of the responsible gene have signs and symptoms of the condition (they are affected) because they do not have another X chromosome with a working copy of the gene. This is why X-linked recessive disorders, including Lesch Nyhan syndrome, occur much more frequently in males.

  • 50% (1 in 2) chance of having an unaffected son or daughter
  • 25% (1 in 4) chance of having an affected son
  • 25% chance of having a carrier daughter [3]

What are some of the different types of X-linked recessive conditions?

Examples of X-linked recessive conditions include red-green color blindness and hemophilia A:

Red-green color blindness. Red-green color blindness simply means that a person cannot distinguish shades of red and green (usually blue-green). Their visual acuity (ability to see) is normal. There are no serious complications however, affected individuals may not be considered for certain occupations involving transportation or the Armed Forces where color recognition is required. Males are affected more often than females, because the gene is located on the X chromosome.

Hemophilia A. Hemophilia A is a disorder where the blood cannot clot properly due to a deficiency of a clotting factor called Factor VIII. This results in abnormally heavy bleeding that will not stop, even from a small cut. People with hemophilia A bruise easily and can have internal bleeding into their joints and muscles. The occurrence of hemophilia A (Factor VIII deficiency) is around 1 in 4500 live male births. The occurrence of hemophilia B (Factor IX deficiency) is one in 20,000 live male births. Hemophilia A accounts for most cases. Treatment is available by infusion of Factor VIII (blood transfusion). Female carriers of the gene may show some mild signs of Factor VIII deficiency, such as bruising easily or taking longer than usual to stop bleeding when cut. However, not all female carriers present these symptoms. One-third of all cases are thought to be new mutations in the family (not inherited from the mother).


What causes genetic diseases?

Genetic diseases are all caused by one or more errors in the parts of DNA that make proteins. If you have the genes that produce a genetic disease, it’s as much a part of you as your hair color or how many fingers you have.

There are many different ways that genetic diseases can get passed down through generations. Here are some of the more common ways that genetic diseases can be inherited:

Recessive Genetic Disorders

You inherit two alleles (a copy of a gene) from your parents—one from your mom, and one from your dad.

Some genetic diseases are passed down as autosomal recessive alleles. Autosomal simply refers to the fact that these genes “live” in autosomes (non-sex chromosomes) and can affect anyone regardless of gender. Recessive alleles will be ignored by your body if the other allele is normal but if you have two recessive alleles, you will have the disease. Your body won’t have a backup normal allele to read the code from.

People who have one normal copy of the allele and one disease-causing allele are called carriers. They will only know that they have the disease-causing allele if they get tested for it, yet they still “carry” the disease in the population. If they have children with another carrier, their kids will have a 25% chance of having the disease.

On the flip side, if you inherit two copies of the disease-causing allele, you will have the disease. This is because the body doesn’t have a normal gene to reference. The only thing it “knows” is how to make the incorrect protein.

Have a look at this Punnett square. Can you predict what percent of a couple’s children will be normal, carriers, or have the disease if 1) the parents both have the disease, 2) one parent has the disease and the other is normal, or 3) one parent is a carrier while the other parent is normal?

Most genetic diseases are recessive disorders. Some common ones you may have heard of include cystic fibrosis, Tay-Sachs disease, and sickle-cell anemia.

Dominant Genetic Disorders

Genetic disorders are sometimes passed down as dominant alleles. These diseases are a bit scarier because you only need a single copy of the disease-causing allele in order to have the disease. It’s not possible to be only a carrier because if you have the disease-causing allele at all, you have the disease.

Fortunately, these diseases are much more rare than other types of genetic diseases due to the lack of carriers. It’s easier for recessive genetic diseases to “hide out” in a population inside of carriers. With dominant genetic diseases, they’re out in the open for all to see, for better or for worse. It’s certainly not fun for the person who has the disease, but if the disease makes a person less able to reproduce because of it, the disease will naturally be weeded out of the population.

Marfan syndrome and Huntington’s disease are two examples of dominant genetic disorders.

Sex-Linked Genetic Disorders

Do you know any balding men? If so, they may be suffering from male pattern baldness—a sex-linked genetic disease that’s probably the most common of any genetic disorder

Sex-linked genetic disorders affect men more often than women. To see how this works, we need to briefly review how chromosomes differ between men and women.

Women possess two sex chromosomes, X and X, which they got from their mother and father. If a woman has a child, she will only be able to pass on an X chromosome to her offspring, since that’s the only type of chromosome she has to give.

Men, on the other hand, have two different sex chromosomes—X and Y. Males get their X chromosome from their mothers (since that’s the only type of chromosome they can give), and their Y chromosomes come from their fathers.

Women have an advantage because if one allele on the X chromosome is defective, they’ve got a backup copy on the other X chromosome. Men have no such advantage—if they have a defective allele on the X chromosome, they have no backup copy. They will have the disease.

This Punnett square describes how this works. Can you think of a situation in which a female might have a sex-linked disease?

One of the most fateful cases of sex-linked inheritance occurred in the Romanov family, the last dynastic rulers of Russia. Tsarina Alexandra inherited one copy of the gene for the blood-clotting deficiency called hemophilia on her X chromosome from her grandmother, Queen Victoria. She was a carrier for the disease, but did not have it herself.

Tsarina Alexandra married Tsar Nicholas II and began having children. Her first four children were daughters who were totally healthy. The kids may have been carriers for the disease if they received one of Alexandra’s disease-causing alleles, but they were safe because they also received a backup copy of the normal allele from their dad’s X chromosome.

Then Alexandra gave birth to a son—finally—but he was unlucky enough to receive a copy of Alexandra’s disease-causing allele. He had no second X chromosome to protect him, so he got the disease. Small bruises and cuts became life-threatening situations because he could have bled to death.

In a desperate attempt to protect him, Alexandra fell into the bad company of Rasputin, a mystic who promised to save her son but instead took advantage of a position of power. This ultimately contributed to the demise and murder of the entire family by Russian revolutionaries.

Chromosomal Disorders

Some genetic diseases aren’t caused by one allele at all, but rather abnormalities in entire chromosomes. It is possible to have either too much or not enough genetic material.

A person can have too much genetic material through a process called nondisjunction. When a parent’s cells are forming either eggs or sperm, they halve their number of chromosomes from 46 (what’s in their own body cells) to 23 (what’s in their sperm or egg cells). That way, a person’s children will have the normal number of chromosomes when their DNA is combined with the other parent’s DNA.

When nondisjunction happens, an extra copy of a chromosome accidently gets put into a sperm or egg cell. That child will have three copies of a chromosome—one more than they should have.

Down’s syndrome is the most famous example of this disorder. Usually, a mother’s egg cell will accidentally end up with two copies of the 21 st chromosome. When that egg cell combines with a sperm cell carrying one copy of that chromosome, the resulting child will have three copies of the 21 st chromosome—hence the disease’s alternate name, Trisomy 21.

Chromosomal disorders can also happen through deletions—when a portion of an entire chromosome is accidentally broken off. In cri-du-chat (cry-of-the-cat) syndrome, a short portion of the 5 th chromosome is completely missing, resulting in its hallmark symptom: an odd cat-like cry from infants.

You may notice that the chromosomal abnormalities resulting in diseases are pretty specific. There’s a good reason: people with these specific chromosome abnormalities can typically still survive with the genetic material they have.

There are many more possible chromosomal abnormalities, but it’s pretty rare for people to survive them. Chromosomal abnormalities, unlike errors in the DNA of a single gene, can affect hundreds or thousands of genes in one fell swoop. Most of the time, when chromosomal abnormalities happen, they affect a critical gene that a person can’t live without, and the embryo may die in the womb before it even starts forming.


Patterns of Inheritance

The various patterns of inheritance are attributed to the Austrian scientist Gregor Mendel, who discovered them while working with garden pea hybrids in the 1800s. Mendel sometimes is referred to as the father of modern genetics likewise, the patterns of inheritance for single-gene diseases are often described as Mendelian.  

According to Mendel's work, there are five distinct patterns of inheritance: autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive, and mitochondrial.

Two primary factors influence the likelihood a person will inherit a genetic disorder:

  • Whether one copy of the mutated gene (from either parent) is passed down or whether two copies (one from both parents) are passed down
  • Whether the mutation is on one of the sex chromosomes (X or Y) or on one of 22 other pairs of non-sex chromosomes (called autosomes)

Autosomal Dominant

In autosomal dominant disorders, only one copy of a mutated gene is necessary and males and females are equally likely to be affected. Children who have a parent who has an autosomal dominant disorder have a 50% risk of inheriting the disorder. Sometimes, however, these disorders result from a new mutation and happen in people with no family history.   Examples of autosomal dominant disorders include Huntington's disease and Marfan syndrome.

Autosomal Recessive

In autosomal recessive disorders, both copies of a mutated gene—one from each parent—are present. A person with only one copy will be a carrier. Carriers will not have any signs or symptoms of the disorder. They can, however, pass the mutation to their children.

If families in which both parents carry the mutation for an autosomal recessive disorder, the odds of the children having the disorder are as follows:  

  • 25% risk of inheriting both mutations and having the disorder
  • 50% risk of inheriting only one copy and becoming a carrier
  • 25% risk of not inheriting the mutation at all

Examples of autosomal recessive disorders include cystic fibrosis, sickle cell disease, Tay-Sachs disease, and phenylketonuria (PKU).

X-Linked Dominant

X-linked dominant disorders are caused by mutations in genes on the X (female) chromosome. In females, who have two X chromosomes, it takes a mutation in only one of the two copies of the gene for a disorder to manifest. In males (who have one X chromosome and one Y chromosome), a mutation in only one copy of the gene in each cell is enough to cause the disorder.

Most times, males have more severe symptoms of an X-link disorder than females. However, one feature of X-linked inheritance is that fathers cannot pass on these traits to their sons. Fragile X syndrome is an example of an X-linked dominant disorder.

X-Linked Recessive

In X-linked recessive disorders, the mutated gene occurs on the X chromosome. Because males have one X chromosome and one Y chromosome, a mutated gene on the X chromosome is enough to cause an X-linked recessive disorder.

Females, by contrast, have two X chromosomes, so a mutated gene on one X chromosome usually has less effect on a female because the non-mutated copy on the other largely cancels out the effect.

However, a female with the genetic mutation on one X chromosome is a carrier of that disorder. From a statistical standpoint, this means 50% of her sons will inherit the mutation and develop the disorder, while 50% of her daughters will inherit the mutation and become a carrier. Examples of X-linked recessive disorders are hemophilia and red-green color blindness.

Mitochondrial

Mitochondria are structures called organelles that exist in each cell of the body where they convert molecules into energy. Each mitochrondrion contains a small amount of DNA: A mutation of that DNA is responsible for mitochondrial disorders.

Mitochondrial disorders are passed down from mothers: Only females can share mitochondrial mutations with their offspring because egg cells contribute mitochondria to the developing embryo sperm cells do not.  

Conditions resulting from mutations in mitochondrial DNA can appear in every generation of a family and can affect both males and females. An example of a mitochondrial inherited disorder is Leber hereditary optic neuropathy, a form of sudden vision loss.  


Understanding Genetics: A New York, Mid-Atlantic Guide for Patients and Health Professionals.

It is important to understand the basic laws of inheritance to appreciate how conditions are passed on in a family. An accurate family health history is a valuable tool to illustrate how conditions are passed down through generations.

A person has two copies of almost every gene, one copy from mom and one copy from dad. Scientists have studied human genes to learn how they normally work and how changes in genes can change how they work. Some changes are very minor and do not affect the way a gene works. These changes are often called single nucleotide polymorphisms (SNPs, pronounced “snips”) or gene variants. Other changes, called mutations, affect how a gene works and can lead to disease.

For some conditions, family members with the same mutation may not have the same symptoms. For other conditions, individuals with different mutations can have similar characteristics. This is because gene expression is influenced by genes, as well as by the environment.

Diseases caused by mutations in a single gene are usually inherited in a simple pattern, depending on the location of the gene and whether one or two normal copies of the gene are needed. This is often referred to as Mendelian inheritance because Gregor Mendel first observed these patterns in garden pea plants. Most single gene disorders are rare but, in total, they affect millions of people in the United States.

Several basic modes of inheritance exist for single-gene disorders: autosomal dominant, autosomal recessive, X-linked dominant, and X-linked recessive. However, not all genetic conditions will follow these patterns, and other rare forms of inheritance such as mitochondrial inheritance exist. (See table at the end of this section.)

Inheritance PatternCharacteristicsDisease Examples
Autosomal DominantEach affected person usually has an affected parent occurs in every generationHuntington’s disease, neurofibromatosis, achondroplasia, familial hypercholesterolemia
Autosomal RecessiveBoth parents of an affected person are carriers not typically seen in every generationTay-Sachs disease, sickle cell anemia, cystic fibrosis, phenylketonuria (PKU)
X-linked DominantFemales are more frequently affected because all daughters and no sons of an affected man will be affected can have affected males and females in same generation if the mother is affectedHypophatemic rickets (vitamin Dresistant rickets), ornithine transcarbamylase deficiency
X-linked RecessiveMales are more frequently affected affected males often present in each generationHemophilia A, Duchenne muscular dystrophy
MitochondrialCan affect both males and females, but only passed on by females because all mitochondria of all children come from the mother can appear in every generationLeber’s hereditary optic neuropathy, Kearns-Sayre syndrome

Dominant mutations are expressed when only one copy of that mutation is present. Therefore, anyone who inherits one dominant disease mutation such as the mutation for Huntington’s disease will have that disease. Dominantly inherited genetic diseases tend to occur in every generation of a family. Each affected person usually has one affected parent. However, dominant mutations can also happen in an individual for the first time, with no family history of the condition (spontaneous mutation).

Recessive mutations require two mutated copies for disease to develop. Recessive genetic diseases are typically not seen in every generation of an affected family. The parents of an affected person are generally carriers: unaffected people who have a copy of a mutated gene. If both parents are carriers of the same mutated gene and both pass it to the child, the child will be affected.

Inheritance patterns differ for genes on sex chromosomes (chromosomes X and Y) compared to genes located on autosomes, non-sex chromosomes (chromosomes numbers 1-22). This is due to the fact that, in general, females carry two X chromosomes (XX), while males carry one X and one Y chromosome (XY). Therefore, females carry two copies of each X-linked gene, but males carry only one copy each of X-linked and Y-linked genes. Females carry no copies of Y-linked genes.

Diseases caused by mutated genes located on the X chromosome can be inherited in either a dominant or recessive manner. Since males only have one X chromosome, any mutated gene on the X chromosome, dominant or recessive, will result in disease. Because females have two copies of X-linked genes, they will not be affected by inheriting of a single recessive mutation on an X-linked gene. For X-linked recessive diseases to occur in females, both copies of the gene must be mutated. Families with an X-linked recessive disorder often have affected males, but rarely affected females, in each generation.

For X-linked dominant diseases, however, a mutation in one copy of an X-linked gene will result in disease for both males and females. Families with an X-linked dominant disorder often have both affected males and affected females in each generation.

A striking characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons fathers only pass X chromosomes to their daughters and Y chromosomes to their sons. In contrast, mothers pass X-linked genes to both sons and daughters.


Watch the video: Genetic Alterations (September 2022).


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