Describe the characteristics of sex-linked inheritance and write about Y-linked traits in humans beings. Name any two disorders that occur due to sex-linked inheritance.

Explanation

Characteristics of Sex-linked Inheritance
(i) It is a criss-cross inheritance, i.e. fathers pass the X-linked allele trait to their daughters, who pass it on to the grandsons.
(ii) Mothers pass the allele of the trait to both sons and daughters.
(iii) Only .homozygous females express a X-linked recessive trait. For this, she should receive the mutant allele from both her parents.
(iv) Heterozygous females are carriers of X-linked recessive trait and themselves do not express the trait. (v) Any male receiving X-linked recessive allele from his mother will express the trait as his Y-chromosome has no corresponding allele, (vi) If a recessive X-linked gene causes a disorder, then more males than females have the disorder, because the disorder is caused by a single allele in males. Their Y-chromosome does not carry any, corresponding allele. In females, the disorder appears only when her mother is a carrier and father also passes the allele of the disorder.
Y-lmked Traits in Humans Any gene that occurs exclusively on the Y-chromosome is said to be holandric and it is not expressed in females. Such a Y-linked gene normally occurs from father to son holandric inheritance. One is the histocompatibility gene, known as the H-Y gene which is present on the short arm of the Y chromosome. Another important Y-Iinked gene is the TDF gene that codes for testis determining factor. This locus plays an important role in primary sex-determination. Another phenotype known to be associated with the Y-chromosome is the condition hypertrichosis. The gene concerned with this condition leads to the development of hairy pinna. Sex-linked disorders in humans are haemophilia and colour blindness. Such first cell-like structures are termed as protobionts, protocells or eobionts. However, first cellular forms of life could have originated close to 2 billion years back.