sex linked inheritance - Sex Limited Genes, Sex Influenced Genes, Sex Linkage, Holandric Genes, Criss-Cross Inheritance, Hemizygous, Sex Linked Lethal Genes

 Sex Limited Genes:

Sex limited genes express characters in only one sex. The sex limited genes may be located on any chromosome (sex linked genes are located only on the sex chromosome). Their expression in the vertebrates is governed by the sex-hormones. Sex limited genes are responsible for the secondary sexual characteristics s as well as primary sexual characters. Examples

1. In man, the beard is produced by sex limited genes. A woman, normally, does not have a beard, yet she surely carries all the genes necessary to produce a beard. But the expression of that particular e in ladies is prevented by the absence of a particular hormone. In rare cases, abnormalities in hormone secretion may occur in a woman gene which allow these genes to express themselves and the result is a bearded lady.

2. Breast development is normally limited to woman but hormone unbalance may cause breast development in a man.

3. The genes for the deep masculine voice and masculine musculature in man will express themselves only when the male hormone is present. Genes for the feminine musculature, express themselves in the absence of male hormone. They do not require the presence of the female hormone.

4. In cattles, the milk production is controlled by sex limited genes. The bulls also carry the gene for milk production. But it is not expressed in the male sex. In the cows, the genes for milk production makes its expression in the presence of female hormones. The brilliant plumage of peacock is also due to some sex limited genes.

5. An excellent illustration of sex limited inheritance is provided by the plumage pattern in birds. In domestic fowl of Leghorn, males have long curved, fringed feathers on tail and neck, but feathers of female are shorter, straighter and without fringe. Thus the males are cock feathered and females are hen feathered. Results of various crosses show that hen feather is due to a dominant gene H and cock feathering is due to it's recessive allele "h". It has been found that the expression of the genes H and h, de- pends upon the sex hormone. Any male that receives atleast one dominant gene will be hen feathered, but those males which are homozygous for the recessive gene (hh), will show the cock feathered condition. Females are all hen feathered regardless of genotype. It shows that a particular type of feathering depends upon specific combination of genotype and sex hormones. The dominant gene H produces hen feathering in the presence of male or female sex hormone. The recessive gene, "h" produces cock feathering in the absence of female hormone and hen feathering in the presence of female hormone.

Sex Influenced Genes:

The sex influenced genes are influenced by the sex of the bearer. They are located on the autosomes. The sex influenced genes express more frequently in one sex than in the other.

1.Baldness in Man:

 The baldness in man is a very good example for sex influenced character. This particular character is dominant in men and recessive in women. This is because the gene for baldness (B) in heterozygous condition (Bb) expresses itself in male but the heterozygous fe males are normal even though they carry the gene for baldness. It shows that only one gene is enough to produce a baldman, whereas a woman must require two such genes to be bald. In man, the single gene for baldness can operate only in the presence of male hormone.

2. Index Finger:

There is another interesting sex influenced gene in man which affects the length of the index finger. The short index finger is due to a gene which is dominant in the male and recessive in the female.

3. Horns in Sheep:

In sheep, the Dorset breed have horns while Suffolk breeds are hornless. The horned condition is dominant (HH) and the hornless condition is recessive (hh). A pure horned Dorset breed is crossed with Suffolk hornless breed (hh). The F, offspring are heterozygous (Hh). The males are horned and females are hornless.

HH genotype produces horns in both sexes.

Hh genotype produces horns in male but not in female.

hh genotype produces hornless condition in both sexes.

In males, one dominant gene, in cooperation with male hormones, develops horns. But in females, one dominant gene cannot develop horns because of the absence of male hormones. When the F, hybrids are crossed, In the F, the males having one or two dominant alleles, develop horns. The females having two dominant alleles alone develop horns. Thus horn in sheep is influenced by male sex.

Sex Linkage:

The genes located on the sex chromosomes are inherited along with sex. This is called sex linkage. It is also called sex linked inheritance. Eg. Haemophilia, colour blindness.

Holandric Genes:

The Y linked genes are called holandric genes because they are present only in the male sex. (Holos=whole; Andros = male). The holandric genes are located on the non homologyous  region of the Y chromosome. The holandric genes are transmitted from father to son and never to daughters. Eg. Hypertrichosis - hair on the pinna. Ichthyosis hystrix - scales on the body. Genes for histocompatibility antigen. Genes for spermatogenesis.

Criss-Cross Inheritance:

The transmission of a character from father to grandson through his daughter is called criss-cross inheritance. It is also called zig-zag inheritance. In criss-cross inheritance, the character appears in alternate generations only. The sex linked characters exhibit criss-cross inheritance. Eg. Haemophilia, Colour blindness.

Hemizygous:

When only one gene is present for a character, the condition is called hemizygous. The organism containing only one gene for a character is called a hemizygote. Haemophilia is controlled by the recessive gene h. It is present on the X chromosome. This gene is absent from the Y chromosome. So the male sex has only one gene for haemophilia. So man is a hemizygote.

Sex Linked Lethal Genes:

The sex chromosome linked genes which kill the possessor and called sex-linked lethal genes. The characters controlled by sex linked lethals are called sex linked lethal characters. Their inheritance is called sex linked lethal gene inheritance. Eg. Haemophilia. Haemophilia is a hereditary blood disease. It is a recessive char- acter (hh). In the homozygous, recessive persons, the blood never clots in injuries. Whenever there is bleeding, the person bleeds con- tinuously and the loss of blood leads to death (Ref. Haemophilia).