Apart from 22 paired chromosomes in humans which are known as autosomes there is one set of chromosomes in both males and females that are known as sex chromosome. This is called so because it determines sex of the person. In females all chromosomes are in homologous pair-XX and in male‘s heterozygous pair –XY of chromosome is there. At the time of gamete production female produces two X gametes whereas males produces one X gamete and one Y gamete. Thus this type of sex mechanism is known as XX-XY or only XY type.
Sex linkage or sex linked inheritance is the transmission of characters and their determining genes along with sex determining genes which are borne on the sex chromosome and therefore are inherited together from one generation to the next generation. Sex linkage was observed by Morgan when he studied the inheritance of red white eye colour trait in drosophila. In Drosophila the eyes are red, but there appeared a white-eyed mutant and it freely mated with the red-eyed females to give rise to red-eyed F1individuals. In F2 the ratio between the red-eyed and white-eyed individuals was 3:1 and white-eyed individuals were all the males. These results were in accordance with the Mendelian monohybrid ratio, where one of the genes is dominant. Normally about half of the individuals in every group of characters must be males and other half females, as such among the white-eyed individuals half must be males and half females, but in this case all the white-eyed individuals were found males. This peculiar behaviour of genes can be explained on the basis of the hypothesis that they are carried on the X-chromosome. The genes which are carried on X-chromosome e.g. the gene for red-eyed and white-eyed characters in drosophila are known as sex linked gene. Sex linked genes are present in human beings also, e.g., green color blindness or deuteranopia and red color blindness or protanopia both these disorders are hereditary and the genes responsible for them are sex-linked and recessive. Similarly haemophilia is also due to a sex-linked recessive gene.
Characteristics of sex linked inheritance
It is criss-cross inheritance. Father does not pass the sex-linked allele of a trait to his son. The same is passed to the daughter. It is because the males have only one X-chromosome which is transferred to the female offspring.
- Only Y-chromosome of the father is transferred to the male offspring but this sex chromosome does not carry any alleles.
- Mother passes the alleles of sex-linked traits to both son and daughter.
- Majority of the sex linked traits are recessive.
- Females generally function as carriers of sex-linked disorders because recessive genes can express themselves in females only in the homozygous state.
Traits governed by sex-linked recessive genes:
(a) Produce disorders in males more often than in females.
(b) Express themselves in males even when represented by a single allele because Y-chromosome does not carry any corresponding alleles.
(c) Seldom appear in both father and son.
(d) Fail to appear in females unless their father also possesses the same and the mother is a carrier.
(e) Females heterozygous for the trait function as carrier.
(f) Female homozygous for the recessive trait transfers the trait to all the sons.
Traits governed by sex-linked dominant genes:
(a) Produce disorders in females more often than in males.
(b) All the female offspring will exhibit them if father possesses the same.
(c) Do not get transmitted to son if mother does not exhibit them.
In this type of recessive sex linked gene, the eye fails to distinguish red and green colors. The gene for the normal vision is dominant. The normal gene and its recessive allele are carried by X- chromosomes. In females, color blindness appears only when both the sex chromosomes carry the recessive gene (XcXc). The females have normal vision but function as carrier if a single recessive gene for color blindness is present. In human males, the defects appear in the presence of single recessive gene because Y chromosome of male does not carry any gene for color vision.
Sex influenced and Sex limited genes
Sex-influenced genes : There are only two possible relationships between alleles: the dominance or blending. In the former case presence of the dominant gene masks the expression of its recessive allele and it makes no difference whether the individual is a male or a female. In latter case where dominance is lacking the heterozygous individuals exhibit a blending effect. In human beings baldness, for example, is due to a gene which is dominant in men. If we let, H, the gene for baldness which is dominant in men and its allele, h dominant in women, the genotypes will be like HH condition will leads to baldness in both men and women, Hh will leads to non-bald women and bald men and hh will leads to non-bald women and non-bald men.
Sex limited genes: In many species there are certain differences between males and female individuals. In human beings, for example, men possess beard where as women develop no beard, such differences are due to hormones secreted by the endocrine and other glands, among which testes and ovaries are very important. Some of these traits which are also referred to as secondary sexual characters include not only physical and physiological differences but also sometimes there is differential behaviour of nervous system. All such traits are due to certain genes, whose expression in sex limited. It means these genes are capable of their expression in either males or in females only. The expression of these genes seems to depend on the presence or absence of the male or female sex hormones. There are many sex-limited characters, found in mammals and birds. In poultry for example the male birds (cocks) usually have sickle shaped tail feathers and the female birds (hens) have straight tail feathers. The hen-feathering (straight-tail feathers) is due to a dominant gene and cock-feathering is due to its recessive allele. Ordinary poultry birds are homozygous recessive for tail feathering. Thus the males have sickle-shaped tail feathers and females are straight-tail feathered and the homozygous dominant genes produce hen-feathered males. Heterozygous male and females both are hen-feathered. Thus, the genotypes and their corresponding phenotypes will be like SS, will lead to hen- feathered female and hen feathered male. Ss will lead to hen-feathered female and hen-feathered males and ss will lead to hen- feathered female and cock-feathered males. Genes, S, seems to inhibit cock-feathering in the presence of female as well as male hormones. Thus, SS and Ss male produce hen-feathered tail and its recessive allele does not inhibit the action of sex hormones. Thus, ss male is cock-feathered and the female is hen-feathered.