Sex Linked Inheritance: Meaning and Characteristics | Genetics | Biotechnology

In this article we will discuss about the meaning and characteristics of sex linked inheritance.

Meaning of Sex Linked Inheritance:

Besides carrying genes for determining sex the sex chromosomes also bear genes for other characters. These genes are carried from generation to generation and are called sex linked genes and the characters controlled by these genes are called sex linked characters. The inheritance of sex linked genes or a trait is known as sex linked inheritance.

Sex linkage was first discovered by Morgan in 1910 in fruit fly, Drosophila melanogaster. The fruit fly has XX and XY sex chromo­somes in the female and male respectively. The gene for eye colour is located in the X chromosome and there is no corresponding allele in the Y chromosome.

The male will therefore express the sex linked trait, even if it has a single gene, dominant or recessive. This is known as hemizygous condition. Hemizygous is when an organism possesses only a single allele for a trait. In contrast, in the homozygous and heterozygous conditions, two alleles are present for a trait.

Morgan conducted the following crosses to prove that the gene for eye colour is located in the X chromosome. The normal eye colour is red and is dominant over the mutant white eye colour.

Cross I – Red Eyed Female x White Eyed Male:

If a homozygous red eyed female (WW) is mated with a hemizygous white eyed male (wY), all the flies, irrespective of their sex are red eyed. When the red eyed males and females of F₁ are inter crossed, the F₂ flies are in the ratio of 2 red eyed females, 1 red eyed male and one white eyed male. Thus, red and white eyed flies are in the ratio of 3:1 in F₂ generation (Fig. 12).

Cross II – Heterozygous F₁ Female x White Eyed Male:

When the heterozygous female (Ww) of F₁ obtained from the cross I is crossed with the white eyed male (wY), the results obtained are illustrated in the Fig. 13. This shows that the white colour is not limited to the males, but can occur in either of the two sexes.

Cross III – White Eyed Female x Red Eyed Male:

The cross between the white eyed female and the red eyed male is a reciprocal cross of the first cross. The results obtained in not the same as expected from the normal Mendelian ratio (Fig. 14). All the males are white eyed and all the females are red eyed. This shows that the pattern of inheritance of eye colour parallels the behaviour of the X chromosome. Therefore it was concluded that the gene for eye colour is located on the X chromosome.

Non-Disjunction:

Non-disjunction is the failure of separation of homologous chromosomes during meiosis. It was first reported in Drosophila by Bridges where the X chromosomes fail to separate and results in the formation a gamete with a XX condition and another with no X chromosome (Fig. 15). Union of the XX gamete with a Y sperm resulted in a XXY condition, which is a female. Non-disjunction can also occur in autosomes.

Characteristics of Sex Linked Inheritance:

The characteristics for sex linked inheritance are as follows:

a. The pattern of inheritance of sex linked trait is criss-cross. The father passes the X linked allele of a trait to the daughters who pass it on to the grandsons. The father cannot pass a sex linked allele to a son directly.

b. The mother can pass the allele of a trait to both daughter and son.

c. Only homozygous females can express a recessive trait, while heterozygous female are carriers and do not express the trait.

d. Males express the trait immediately because of the absence of a corresponding allele. This is the reason why males suffer from sex linked disorders more than females.

e. Most of the sex linked traits are recessive. Some examples of sex linked traits include Haemophilia or Bleeder’s disease, Daltinism or Colour blindness (Fig. 24 and 25).