Sex Linked Inheritance in Man

The sex linked inheritance in man may be X-linked or Y-linked. The X-linked traits are of two types X-linked recessive e.g. hemophilia, colorblindness, X-linked dominant e.g. brown teeth, vitamin D resistant ricket etc.

X-Linked Recessive Inheritance

The mode of inheritance of human trait can be traced through pedigree.

Pedigree

It is a chart to show the pattern of inheritance in the generation of the family. Male is indicated by square and the female by circle. The affected male is shown by Colored Square and the females by colored circle and the carrier by a half colored square and half colored circle respectively.

Pedigree

Hemophilia

Hemophilia is a rare X-linked recessive trait with a long well documented history. Hemophiliacs bleed excessively when injured because they have inherited an abnormal allele for a factor involved in blood clotting. The most seriously affected person may bleed to death.

Types of Hemophilia

It is of three types: A, B and C. Hemophilia A and B are non-allelic recessive sex linked, but hemophilia C is an autosomal recessive trait. Being X-linked recessive hemophilia A and B affect men more than women, but hemophilia C affects both the sexes equally because it is autosomal.

A simplified pedigree showing the X-linked inheritance of haemophilia in European royal family

Pattern of Inheritance of A and B Hemophilia

These never pass directly from father to son. The mode of inheritance is from maternal grandfather through a carrier daughter to a grandson.

Color Blindness

Each of the three kinds of cone cells in retina is sensitive to only one of the three primary colors, red, green or blue. The light absorbing proteins are called opsin and each type of cone cells have specific opsin. The genes for red and green opsin are on X- chromosome. The gene for blue opsin is present on autosome 7. Mutations in opsin genes cause three types of color blindness. A dichromate can perceive two primary colors but is unable to perceive the opsins which are missing due to mutation. The red blindness is protanopia, green blindness is deuteranopia and blue blindness is tritanopia. Some people can detect red and green but with altered perception of the relative shades of these colors. They have abnormal but still partially functional opsins. They are protanomalous for red and deuteranomalous for green. A monochromat can perceive only one color. A trichromat can perceive all the three colors. Monochromacy is true color blindness. Blue cone monochromacy is an X-linked recessive trait in which both red and green cone cells are absent, so it is also known as red green color blindness. It is a common hereditary disease.

Problem: More men are color blind than the women, why?

Solution: The color blindness is due to the X linked recessive gene. The women are color blind only in homozygous condition. The Y chromosome has no dominant allele for the color blindness. The men are affected in the hemizygous condition. Thus men are more color blind than women.

Hairy Pinnae

Dominant X-Linked Inheritance

Brown teeth are an example of dominant X-linked inheritance. The gene is dominant, so it will appear in both males and females either it is homozygous or heterozygous. If it is homozygous all the offspring will be affected. If it is heterozygous then half will be affected and half will be unaffected.

Y-Linked Inheritance

Y-linked trait passes through Y-chromosome from father to son only. All sons of an affected father are affected by a Y-linked trait. SRY gene on Y chromosome determines maleness in man. For example the hairs on the ears of male are due to the Y linked genes.

Sex Limited Trait

The traits which are limited to only males or females are called sex limited traits. These traits may be controlled by sex linked or autosomal genes. Genes for milk production can affect only in milk producing mammals for example: cows, human females etc. Beard growth is limited to men. A woman can pass the genes specifying heavy beard growth to her sons.

Sex Influenced Traits

Some genes not located on X or Y chromosomes are expressed differentially in the two sexes, and therefore the traits they control are referred as sex influenced traits.

Pattern baldness is caused by an autosomal allele that is dominant in male due to presence of testosterone, the male sex hormone. Baldness in females is due to an autosomal recessive trait. A heterozygous male is bald but a heterozygous female is not. A woman can be bald when she is homozygous recessive. Heterozygous women with adrenal tumor develop pattern baldness, but hair returns when the tumor is removed.