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Table of contents

The age of onset of Huntington disease. The graph shows that people carrying the allele generally do not express the disease until after child-bearing age. Some other rare dominant conditions are polydactyly extra digits and brachydactyly short digits , shown in Figure , and piebald spotting, shown in Figure Some rare dominant phenotypes of the human hand. The numbers in the accompanying pedigree left give the number of fingers in the more Piebald spotting, a rare dominant human phenotype.

Although the phenotype is encountered sporadically in all races, the patterns show up best in those with dark skin. Pedigrees of Mendelian autosomal dominant disorders show affected males and females in each generation; they also show that affected men and women transmit the condition to equal proportions of their sons and daughters. Phenotypes with X-linked recessive inheritance typically show the following patterns in pedigrees:.

Many more males than females show the phenotype under study. If the recessive allele is very rare, almost all persons showing the phenotype are male. Half of the sons of these carrier daughters are affected Figure Note that, in common X-linked phenotypes, this pattern might be obscured by inheritance of the recessive allele from a heterozygous mother as well as the father. None of the sons of an affected male show the phenotype under study, nor will they pass the condition to their offspring. Pedigree showing that X-linked recessive alleles expressed in males are then carried unexpressed by their daughters in the next generation, to be expressed again in their sons.

In the pedigree analysis of rare X-linked recessives, a normal female of unknown genotype is assumed to be homo-zygous unless there is evidence to the contrary.

3. 1 History and background of X chromosome inactivation

Perhaps the most familiar example of X-linked recessive inheritance is red-green colorblindness. People with this condition are unable to distinguish red from green and see them as the same. The genes for color vision have been characterized at the molecular level. Color vision is based on three different kinds of cone cells in the retina, each sensitive to red, green, or blue wavelengths. The genetic determinants for the red and green cone cells are on the X chromosome. As with any X-linked recessive, there are many more males with the phenotype than females. Another familiar example is hemophilia , the failure of blood to clot.

Many proteins must interact in sequence to make blood clot. The most common type of hemophilia is caused by the absence or malfunction of one of these proteins, called Factor VIII.

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The most well known cases of hemophilia are found in the pedigree of interrelated royal families in Europe Figure The son of the last czar of Russia, Alexis, inherited the allele ultimately from Queen Victoria, who was the grandmother of his mother Alexandra. Nowadays, hemophilia can be treated medically, but it was formerly a potentially fatal condition.

It is interesting to note that, in the Jewish Talmud, there are rules about exemptions to male circumcision that show clearly that the mode of transmission of the disease through unaffected carrier females was well understood in ancient times. The inheritance of the X-linked recessive condition hemophilia in the royal families of Europe.

Learn more about Intragenic Recombination

A recessive allele causing hemophilia failure of blood clotting arose in the reproductive cells of Queen Victoria, or one of her parents, through mutation. Duchenne muscular dystrophy is a fatal X-linked recessive disease. The phenotype is a wasting and atrophy of muscles. Generally the onset is before the age of 6, with confinement to a wheelchair by 12, and death by The gene for Duchenne muscular dystrophy has now been isolated and shown to encode the muscle protein dystrophin. This discovery holds out hope for a better understanding of the physiology of this condition and, ultimately, a therapy.

A rare X-linked recessive phenotype that is interesting from the point of view of sexual differentiation is a condition called testicular feminization syndrome , which has a frequency of about 1 in 65, male births. People afflicted with this syndrome are chromosomally males, having 44 autosomes plus an X and a Y, but they develop as females Figure They have female external genitalia, a blind vagina, and no uterus.

The chromosomal basis of inheritance (article) | Khan Academy

Testes may be present either in the labia or in the abdomen. Although many such persons marry, they are sterile. The condition is not reversed by treatment with the male hormone androgen, so it is sometimes called androgen insensitivity syndrome. The reason for the insensitivity is that the androgen receptor malfunctions, so the male hormone can have no effect on the target organs that contribute to maleness. In humans, femaleness results when the male-determining system is not functional.

Four siblings with testicular feminization syndrome congenital insensitivity to androgens. All four subjects in this photograph have 44 autosomes plus an X and a Y chromosome, but they have inherited the recessive X-linked allele conferring insensitivity to more Affected males pass the condition to all their daughters but to none of their sons Figure Affected heterozygous females married to unaffected males pass the condition to half their sons and daughters Figure Pedigree showing that all the daughters of a male expressing an X-linked dominant phenotype will show the phenotype.

Pedigree showing that females affected by an X-linked dominant condition are usually heterozygous and pass the condition to half their sons and daughters.


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There are few examples of X-linked dominant phenotypes in humans. One example is hypophosphatemia, a type of vitamin D-resistant rickets. Early in the development of female mammals, one of the X chromosomes in each cell becomes inactivated. The inactivated X chromosome becomes highly condensed and is visible as a darkly staining spot called a Barr body Figure Surprisingly, this chromosomal inactivation persists through all the subsequent mitotic divisions that produce the mature body of the animal.

The inactivation process is random, affecting either of the X chromosomes. As a result of this inactivation, the adult female body is a mixture, or mosaic , of cells with either of the two different X chromosome genotypes Figure During the growth and development of tissues, the mitotic descendants of a progenitor cell often stay next to each other, forming a cluster; so, if a female is heterozygous for an X-linked gene that has its effect in that tissue, the two alleles of the heterozygote are expressed in patches, or sectors.

A mosaic phenotype familiar to most of us is the coat pigmentation pattern of tortoiseshell and calico cats Figure Such cats are females heterozygous for the alleles O which causes fur to be orange and o which causes it to be black. Inactivation of the O -bearing X chromosome produces a black patch expressing o , and inactivation of the o -bearing X chromosome produces an orange patch expressing O. A Barr body, a condensed inactivated X chromosome, in the nucleus of a cell of a normal woman. Men have no Barr bodies.

Tissue expression of OPN1LW - Summary - The Human Protein Atlas

The number of Barr bodies in a cell is always equal to the total number of X chromosomes minus one. Karen Dyer Montgomery. X-chromosome inactivation in mammals. The zygote of a female mammal heterozygous for an X-linked gene becomes a mosaic adult composed of two cell lines expressing one or the other of the alleles of the heterozygous gene because one or the other X chromosome more A calico cat.

Both calico and tortoiseshell cats are females heterozygous for two alleles of an X-linked coat-color gene, O orange and o black. The orange and black sectors are caused by X-chromosome inactivation. The white areas are caused by a more Although all human females have one of their X chromosomes inactivated in every cell, this inactivation is detectable only when a female is heterozygous for an X-linked gene. This is particularly striking when, as in tortoiseshell cats, the phenotype is expressed on the exterior of the body.

Such a condition is anhidrotic ectodermal dysplasia. Males carrying the responsible allele let us call it d in its hemizygous condition have no sweat glands.

On The Same Wavelength - An X-Chromosome Story (The X-Chromosome Sequence)

Somatic mosaicism in three generations of females heterozygous for sex-linked anhidrotic ectodermal dysplasia absence of sweat glands. Areas without sweat glands are shown in green. The extent and location of the different tissues is determined by chance, more Interestingly, the X- chromosome location of the gene causing testicular feminization was confirmed when it was shown microscopically that, in females heterozygous for the gene, half their fibroblast cells bind androgen but the other half do not.

It should be noted that X inactivation is canceled in the female germinal tissue, so both X chromosomes are passed into the eggs.

Genes on the differential region of the human Y chromosome are inherited only by males, with fathers transmitting the region to their sons. The gene that plays a primary role in maleness is the TDF gene, which codes for testis-determining factor. The TDF gene has been located and mapped on the differential region of the Y chromosome see Chapter However, other than maleness itself, no human phenotype has been conclusively proved to be Y linked.

Hairy ear rims Figure has been proposed as a possibility.

The phenotype is extremely rare among the populations of most countries but more common among the populations of India. An Indian geneticist, K. Dronamraju, studied the phenotype in his own family. Every male in the family descended from a certain male ancestor showed the phenotype. In other Indian families, however, males seem to transmit the phenotype to only some of their sons, which is part of the reason that the evidence for Y-linked inheritance is considered to be inconclusive.

Amanda Melin of Washington University in St.