The inheritance of a trait (phenotype) that is determined by a gene located on one of the sex chromosomes is called sex linked inheritance. The expectations of sex-linked inheritance in any species depend on how the chromosomes determine sex. For example, in humans, males are heterogametic. It has one X chromosome and one Y chromosome. But females are homogametic. They two X chromosomes. In human males, the entire X chromosome is active. But one of a female’s X chromosomes is largely inactive. Random inactivation of one X chromosome occurs during the early stages of female embryogenesis. Therefore, every cell that forms from a particular embryonic cell has the same X chromosome inactivated. This pattern of sex determination occurs in most vertebrates, but in birds and many insects and fish the male is the homogametic sex.
In general terms, traits determined by genes on sex chromosomes are not different from traits determined by autosomal genes. Sex-linked traits are distinguishable by their mode of transmission through successive generations of a family. In humans it is called X-linked or Y-linked inheritance.
I. X-linked recessive Traits: These are expressed in all
heterogametic and homogametic which are homozygous for the recessive allele. An example is the sex-linked recessive is horns in sheep that appear only in males. The recessive phenotypes of such genes are more common in males than in females. The examples of X-linked recessive trait in human are Color blindness, Duchenne muscular dystrophy, Hemophilia. Kennedy disease.
Summary of X-Linked Recessive Inheritance
- Males are affected almost exclusively
- Transmission occurs through carrier females to their sons
- Male-to-male transmission does not occur
- Affected males are at risk of transmitting the disorder to their grandsons through their carrier daughters
X-linked recessive, carrier mother
- X — linked dominant inheritance: Pattern of X- linked dominant inheritance is different from X- linked recessive. It is more common in females than males. All daughters are affected by their father. But none of his sons are affected. A heterozygous affected mother passes the trait equally to half of her sons and half of her daughters. Hypophosphatemic rickets is an X — linked dominant trait. It is a rare hereditary disease. It is different from common dietary rickets. Other examples of X-linked dominant traits are Retinitis pigmentosa and Rett syndrome.
- V- linked trait: The genes present on the Y chromosomes are called Y-linked genes and their traits are called Y-linked traits. Y chromosome is not completely inert it carries a few genes. These genes have no counterpart on X chromosome.
Example: $RY gene is present on Y chromosome of man. It determines maleness. Y — linked traits are found only in males. These traits directly pass through Y chromosome from father to son only. Females do not inherit Y chromosome. So such traits can not pass to them.
- X and V linked Traits: The genes present on both X and Y chromosomes are called X and Y linked genes. For example, the bobbed gene in Drosophila are present on X and Y both. The pattern of inheritance of X and Y linked gene is like autosomal genes. So these are also called pseudoautosomal genes.