Y Linked Inheritance

Y linked inheritance is known as the inheritance of genes on the Y chromosome. As only males mainly have a Y chromosome, Y-linked genes can only be transmitted from father to son. Y-linked inheritance is also known as the holandric inheritance. Humans and other different mammalian males have the sex chromosomes such as the X and Y chromosomes. The y chromosome is generally smaller than the X chromosome and therefore, carries relatively fewer genes. Besides the genes for sperm development, specific hormonal output, and other traits characterizing males, the Y chromosome also carries genes other than for sex determination.

Hypertrichosis of the ears, webbed toes and porcupine man are some of the examples of Y-linked inheritance in humans. Hypertrichosis of the ears (or hairy ears) is a condition in which there is a remarkable growth of hair on the outer rim of the ear. Webbed toes condition is characterized by having a web-like connection between the second and the third toes. Porcupine man represents a condition when the skin thickens and slowly becomes darker, scaly, rough and bristle-like outgrowths. Since Y-linked inheritance makes the involvement of the Y chromosome, Y-linked inheritance is passed on from father to son.

Overview of Y Linked Inheritance

Y linkage might be difficult to detect. This reason is partly correct because the Y chromosome is small and possesses fewer genes than the autosomal chromosomes or the X chromosome. It is found to contain about 200 genes. Previously, the human Y chromosome was believed to have little importance. However the Y-chromosome is mainly the sex-determining chromosome in humans and some other species, not all genes that have a role in sex determination are Y-linked. The Y-chromosome, usually do not undergo genetic recombination and only small regions known as pseudoautosomal regions exhibit recombination. The majority of the Y-chromosome genes that do not play any role in recombination are situated in the “non-recombining region”.

Characteristics of Y Linked Inheritance

• It occurs only in males

• It usually appears in all sons of males who are exhibiting that trait

• Is absent from daughters of trait carriers; whereas the daughters are normal phenotypically and do not have affected offspring

Moreover, this trait occurs rarely and has not been wholly resolved. Y-chromosome deletions are a frequent genetic cause of male infertility.

Y linked Disorders

Hypertrichosis

Hypertrichosis is an atypical form of hair growth in excess over the body Trichosis is mainly an archaic term that is defined as a disease or abnormality generalized by an overgrowth of hair. The term hypertrichosis is the most widespread medical term that is used to refer to a condition that is characterized by the occurrence of growth of too much hair all over the body. Hypertrichosis may be localized, i.e. the hair growth over a certain body part, and therefore, is known as localized hypertrichosis. A typical example is a synophrys, which is hypertrichosis occurring in between the eyebrows. This condition results in an appearance of a single long eyebrow (or unibrow). When hypertrichosis involves the entire body then this condition is known as generalized hypertrichosis, just like in the case of werewolf syndrome (or terminal hypertrichosis). Hypertrichosis may be congenital or it can be later acquired in life. Congenital hypertrichosis is generally caused by genetic mutations.

Webbed Toes

Webbing of toes or fingers in man is mainly due to a local arrest of development, leading to retention of the normal embryonic webbing. This form of digital fusion includes only the skin, and the skeleton remains unaffected. The extensor tendons of the toes might sometimes be fused.

Webbed digits are formed normally in some marsupials, rodents, and insectivores. They might also occur in varying degrees in some Primates, such as Hylobates. Research studies based on five new pedigrees along with those that were already published demonstrate that webbing of toes in man may be inherited in either a mendelian or sex-linked manner. In another case, this character may follow the course of the Y-chromosome.

Porcupine Man

Ichthyosis hystrix or the porcupine man refers to a group of rare skin disorders in the ichthyosis family of skin disorders that is generally characterized via massive hyperkeratosis along with an appearance like spiny scales. This term is also used to refer to a form of epidermal nevi having extensive bilateral distribution.

This is mainly a rare condition of autosomal dominant inheritance, however, few sporadic cases do occur. It was once believed to be a rare example of a Y-linked condition but this has been disproved and there is an equal sex incidence. The degree of penetrance of the condition varies greatly within families.

Pedigree Analysis

After completing this pedigree with Y linked inheritance, the trait is further carried on to the Y chromosome and is then transmitted from father to son only. While writing genotypes we mainly use X and Y as symbols for the chromosomes passed on from the previous generation, but only the Y chromosome will have an allele for the gene. For example, all females will possess the genotype XX (no alleles). Males having the trait will have the genotype XYR and males without the trait will have the genotype XY (with the allele on the Y).  

Conclusion

The Y chromosome is mainly quite smaller and comprises relatively fewer genes which nonetheless are still very important. The majority of the genes are linked to male infertility and spermatogenesis and are been identified on the long arm (q-arm) of the Y chromosome. Here we come to an end of this topic, we hope that you were able to grasp a clear concept of linked inheritance.