Useful Notes on Sex-Linked Inheritance in Human (with diagram)

In dealing with sex-linked inheritance, we are mainly concerned with those genes, which are carried by sex chromosomes or allosomes.

Sexual reproduction is of paramount significance in the biological world as an effective way of generating genetic variability. It involves the fusion of two specialized haploid cells called gametes— a male gamete and a female gamete. In most of the cases, the male and female individuals differ in many characteristic features, which may be grouped into two categories: Primary and Secondary sex characters.

Primary sex characters are related to the gamete producing organs of male and female individuals and depend on the genes present in the zygote.

Secondary sex characters are all those characters that show consistent differences between the male and female individuals of a species, such as, genital ducts, genitalia and many other characteristic signs on the body.


In human, these secondary sex characters are related to facial hair, mammary gland development, pitch of voice, degree of muscle development, subcutaneous fat deposition etc. At the same time, external administration of human male and female hormones (testosterone and estrogen, respectively) helps to modify the intensity of the features.

We have already known that a genotype is me actual constitution of an organism while a phenotype IS the expression of the genotype in the organism. The genotypes are usually described in terms of alleles at a single locus. The genes, found in several alternative forms are known as allelomorphs or alleles. Naturally, as the chromosomes occur in pairs, an allele is found to be present at the same locus on each chromosome of a homologous pair. This arrangement gives rise to either homozygous or heterozygous conditions according to the nature of allele.

As aforesaid, the homozygous individual shows two identical forms of alleles at a particular genetic locus, on each chromosome of a homologous pair. But heterozygous individual always possesses alleles of alternate character at the locus of two homologous chromosomes.

In dealing with sex-linked inheritance, we are mainly concerned with those genes, which are carried by sex chromosomes or allosomes. It may be assumed that each of the sex chromosomes (X and Y chromosome) consists of two segments where one of the segments remains homologous between the two chromosomes – X and Y.


Schematic Representation of Homologous Chromosomes (A) Crossing Over (B) Recombination (C)

The genetic materials on homologous segments though have a fair chance to be exchanged during the cell-division, but genes located on this part of X and Y chromosome seem to have a very little significance and so they are called partially sex-linked. Such traits are sometimes transmitted as simply as autosomal traits, either through X chromosome or Y chromosome. On the other hand, the non-homologous segments of X and Y chromosomes are of great significance because true sex- linked genes are located on them.

Completely Y-linked genes are called holandric and this type inheritance is extremely rare. In some cases they are considered doubtful also. Rather, the genes on the non-homologous segment of X chromosome are the most common types of sex-linked gene and the term sex-linked is usually referred as the X-linked traits.

Since these genes are mainly carried by the X-chromosome, they are commonly called sex-linked genes and the peculiar pattern of inheritance followed by them is termed as sex-linked inheritance. Therefore, it is a particular allele, that being posited in one of the sex determining chromosomes (usually the X-chromosome) determine the sex- linked traits.

Sex Linkage in Drosophila:

In 1910, when T.H. Morgan was working on the genetics of the fruit fly, Drosophila melanogaster, in his laboratory at Columbia University, he came across a male fly having white eye colour. In Drosophila, the normal eye colour is red; the white colour arose due to mutation. The red eye colour is therefore the wild type and is dominant over its allele for white eye colour. Morgan searched for the pattern of inheritance and discovered the presence of sex-linked gene.


There are four pairs of chromosomes (2n = 8) in Drosophila. Of these, a pair of heteromorphic chromosomes is concerned with sex determination. The male is heterogamatic (XY) and the female is homogametic (XX).

The X-chromosome contains large number of genes and mainly consists of euchromatin. The Y- chromosome is largely inert and has no alleles of any genes, as such, it is composed of heterochromatin. Therefore, in the male fly (XY), the phenotype is determined by the genes present in the X- chromosome.

For the presence of a single X-chromosome, male fly can be termed as hemizygous. This means, that the male possesses only one copy of a gene in the place of normal two. In contrast to this, the female fly carries two sets of sex-linked genes in their two X-chromosomes (XX).


Since there are two sex-linked genes (alleles) in the female fly, the dominant gene expresses itself. If both the genes are recessive, recessive genes get the scope to express themselves. But the situation is different in male flies because he by nature possesses either a dominant or a recessive gene (allele) and in either case, the gene expresses itself. Therefore, all male flies are capable to show the trait, whether dominant or recessive.

When a red eyed female fly (XX) is crossed with a white eyed male fly (X Y), in the F1 generation, all the male and female individuals are red eyed. In the F2 generation, all females are red eyed but all males are not red-eyed. 50% of the males show white eye. The phenomenon has been diagrammatically represented in Fig: 6.11.

Inheritance of Eye Colour in Drosophilla

Besides, a reciprocal cross may be arranged between a white eyed female (XX) and a red eyed male (XY). In the F1 generation, all females flies will be red eyed and all the males will be white eyed. Now if the flies of the F1 generation are intercrossed, in the F2 generation 50% of the female individuals will come out as red eyed. Similarly 50% of the male flies will show red eyes. This means, 50% of both males and females of this generation will be white eyed. (Fig: 6.12)

Inheritance of Eye Colour in Drosophilla

The inheritance of red-white eye colour pattern in Drosophila thus reveals the peculiar pattern of transmission of sex-linked genes from generation to generation. It is very interesting in the sense that these genes are located in X-chromosome instead of autosomes. Further, the eye colour has nothing to do with the expression of sex; the only criterion of sex linkage is that the gene for eye colour is carried by the X-chromosome.

The characteristic feature of X-linked (sex-linked) inheritance is that, here no transmission of traits (characters) is possible from male to male i.e. father to son. Because, X-chromosome of the male (father) is not transmitted to his sons, rather it passes to daughters. But the same father can transmit his sex-linked traits to grandsons, through his daughters. Therefore, the phenomenon has been referred as criss-cross inheritance in genetics. This criss-cross pattern of inheritance is possible only when the mother is a homozygous recessive (XX) and the father bears the dominant (XY) gene.

The geneticists usually perform their experiments on standardized organisms, which can be reared under controlled environment. Much of the modern theory of genetics is based on studies of the small fly, Drosophila. Because this animal can be bred in the laboratory in large numbers with relatively little trouble and expense.

A sexual generation from egg to egg, may be completed in ten days; a single pair produces hundreds of offspring’s. Moreover, the species can be collected from anywhere of the world. The temperate or tropical climate does not impose much variation. Again, as the species exhibit a small number of chromosomes, it is quite easy to observe them.

Sex Linkage in Man:

The genes present in the X-chromosome of man follow the same pattern of inheritance as in the sex-linked traits of Drosophila. But the linkage study in man is not at all easy because, in case of man, such study demands some known pedigree to note the inheritance of characters, down through the generations.

A close examination of X and Y chromosomes in man reveals that the majority of the genes for X linked traits are found on the portion of X-chromosome, for which there is no corresponding homologous region in the Y chromosome. The Y chromosome is comparatively small in size and contains very few genes.

Moreover, just like Drosophila, man is hemizygous. He has only one X- chromosome. Since there is no allele in the Y chromosome corresponding to the genes contained in the X-chromosome, a single sex-linked recessive gene carried by the male (through the X chromosome) expresses its effect in the phenotype.

However, the characteristic features of inheritance for a sex- linked trait can be summarized as follows:

1. Ordinarily, genes governing sex-linked traits are not transmitted from male parents directly to their male progeny. Because individuals receive their X chromosome from their mothers, while their male parents contribute the Y-chromosome.

2. A male transmits his sex-linked genes to all his daughters, since females receive one of their two X-chromosomes from father. These daughters, in turn, transmit this gene to half of their male progeny. Thus, a sex-linked recessive gene is transmitted from a male to its female progeny, and thereafter to half of the male progeny of such females. All sex-linked genes therefore pass from male to female and then come back to a male of F2 generation (grand-children generation). This criss-cross pattern of inheritance is same as found in Drosophila.

3. Since sex-linked genes are not located in the Y chromosome, the heterogametic sex (human male) is hemizygous for such genes i.e. it possess only one allele of sex-linked genes. Therefore, recessive alleles of sex-linked genes express themselves in the hemizygous condition, while they have to be present in homozygous state in order to express themselves in the homogametic sex. This is the reason why sex-linked recessive traits are more frequent in the heterogametic than in the homogametic sex.

The cause of sex-linkage can therefore be shown with two reasons:

(i) The location of a gene in X chromosome

(ii) The absence of its allele in the Y chromosome.

For example, in human, the long arm of Y chromosome is homologous to the short arm of X chromosome. Genes located in that region of X chromosome does not show sex linked inheritance. Rather such genes undergo normal autosomal pattern of inheritance, if their allele is located in Y chromosome.

A few genes have been discovered in the non-homologous region of the Y-chromosome, which have no allele in X chromosome. Such genes follow an entirely different pattern of inheritance. For example, the incidence of hypertrichosis (long hair growth in the ears) is found due to Y-linked genes.

In Drosophila, about 150 genes are sex-linked. In human, over 200 genes exhibit sex linkage; most of these cause genetic diseases. Some well known examples of sex-linked human traits are, hemophilia (inability of blood to clot on exposure to air), colour blindness (inability to perceive one or the other colour, e.g. green colour), optic atrophy (degeneration of the optic nerve), juvenile glaucoma (hardening of eye -ball), juvenile glaucoma (hardening of eye-ball), myopia (near sightedness), defective iris, epidermal cysts, distichiasis (double eyelashes), white occipital lock of hair, mitral stenosis (abnormality of mitral valve in the heart), cystic fibrosis etc.

The situations can be explained in the following way: If a male with any of the above mentioned defects marry a normal homozygous female, all their sons will be normal. But their daughters will be the carriers (heterozygous) for the particular disease or defect, although they look almost normal.

This condition is produced due to a sex-linked recessive gene. When the daughters (carriers) get married to normal males, 50% of their sons will be perfectly normal while remaining 50% will get the defect. A few concrete instances will help in understanding the situation.

Red-Green Colour-blindness in Man:

Perhaps the best known example of sex-linkage in man is the colour blindness. Several types of colour-blindness are known, but the most common type is red-green blindness, which is an X-linked recessive trait. Persons suffering from this colour blindness cannot differentiate between the red colour and the green colour. Individually, the red colour blindness is referred as protanopia whereas the green colour blindness is known as deuteranopia.

The genotype for normal vision may be symbolized by (XX), and colour blindness by (XX’). x indicates the sex-linked recessive gene for colour blindness. If a colour blind man 0(Y) marries a normal woman (XX), in the F1 generation all male progeny (sons) will be normal (XY). The female progeny (daughters) though will show normal phenotype, but genetically they will be heterozygous (XX). Since these daughters bear the recessive gene of colour blindness, they are the carriers of the trait.

Inheritance Pattern of Colour Blindness in Human (Situation A)

If such a carrier woman with normal vision (heterozygous for colour blindness) marries a normal man (XY), the following progeny may be expected in the F2 generation: Among the daughters, 50% are normal and 50% are carriers for the diseases; among sons, 50% are colour blind and 50% are with normal vision.

Again, if a carrier woman (XX”) marries a colour blind man (X Y), the following progeny will be expected in the F1 generation. Among daughters, 50% are colour blind and 50% are the carriers of this disease; and among the sons, 50% are colour blind and another 50% are with normal vision.

If the colour blind woman (XX) marries a normal man (XY), the resulting male progeny (sons) in F2 generation will be all colour blind, and the female progeny (daughters) will be all carriers of colour-blindness. Thus, the occurrence of colour blind man will be more frequent than the colour­-blind woman.

Inheritance Pattern of Colour Blindness in Human (Situation B)

Hemophilia in Man:

Another sex-linked recessive gene in man is that for Hemophilia or bleeding. In this type, blood fails to clot easily on exposure to air, so that even minor injuries may cause prolonged bleeding leading to death. The trait was first observed in a European Royal Family; Queen Victoria (1819 – 1901) herself was a carrier of this recessive gene.

One of her sons, Leopold, Duke of Albany, died of hemophilia, when he was only 31 years. The pedigree record of Queen Victoria reveals that she has transmitted the disease to two of her daughters, as some of their (daughters’) male descendants were also hemophiliacs.

A woman may be carrying a gene for hemophilia and in the heterozygous condition (XX), she does not exhibit any visible effect of the disease. But she is capable of transmitting the disease to 50% of her sons. Hemophilia is very rare in women, because, to have this disease the woman must be monozygous (XX) with me recessive genes. A hemophilic woman normally dies before adolescence due to severe bleeding.

Among the children of a hemophilic man, all the daughters are carriers of hemophilia but none of his sons are affected by the disease (provided the mother of the children is neither hemophilic nor a heterozygous carrier). This is one of the characteristic features of X-linked inheritance that the father gives X-chromosome to his daughters, but the sons receive no X-chromosome. Hence, there is no scope of transmission of X-linked traits from father to son, but the grandsons often receive these traits through daughters. Therefore, this trait also follows a criss-cross pattern of inheritance.

If a hemophilia carrier woman marries a normal man, 50% of her daughters will be normal; other 50% will be the carriers. Among the sons, 50% will be normal and other 50% will be hemophiliacs. Usual frequency of hemophilic male birth is about 1 in 10,000 while hemophilic female birth occurs once in 100,000,000 births.Inheritance of Hemophilia in Man

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