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 have two X chromosomes. This pattern of sex determination occurs in most vertebrates, but in birds and many insects and fish the male is 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 inheritance or Y-linked inheritance.
The X-linked inheritance is X-linked recessive inheritance and X-linked dominant inheritance.
1. 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 daughter. Hypophosphatemic rickets is an X-linked dominant trait. It is a rare hereditary disease. It is different from common dietary rickets which could be cured by taking vitamin D. It does not result from vitamin D deficiency but its cause is a genetic communication failure at molecular level. The gene encoding bone proteins never receive vitamin D’s message to function.
2. X-linked recessive inheritance:
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 Colour blindness, Duchenne muscular dystrophy, Haemophilia.
Haemophilia
Haemophilia is a rare X-linked recessive trait. Haemophiliac’s blood fails to clot properly after an injury, because it has either a reduction or malfunction or complete absence of blood clotting factors. It is a serious hereditary disease because a haemophiliac may bleed to death even from minor cuts. Haemophilia is of three types: A, B and C. Haemophilia A and B are non- allelic recessive sex linked, but haemophilia C is an autosomal recessive trait. 80% - haemophiliacs, suffer from haemophilia A due to abnormality of factor VIII, about 20% suffer from haemophilia B due to disturbance in factor IX, but less than 1% suffer from haemophilia C due to reduction in factor XI. Being X-linked recessives, haemophilia A and B affect men more than women, but haemophilia C affects both the sexes equally because it is autosomal. Chances for a man to be affected by haemophilia A and B are greater than a woman. A woman can suffer from haemophilia A or B only when she is homozygous for the recessive allele, but a man with just one recessive allele will display the trait. Haemophilia A and B zigzag from maternal grandfather through a carrier daughter to a grandson. It never passes direct from father to son. Gene for normal is ‘H’. Gene for haemophilia A is ‘h’.
Colour Blindness:
Normal trichromatic colour vision is based on three different kinds of cone cells in the retina, each sensitive to only one of the three primary colours, red, green or blue. Each type of cone cell has specific light absorbing proteins called opsins. The genes for red and green opsins are on X chromosome, while the gene for blue opsin is present on autosome 7. Mutations in opsin genes cause three types of colour-blindness. A dichromat can perceive two primary colours but is unable to perceive the one whose opsins are missing due to mutation. Protanopia is red blindness, deuteranopia is green blindness, while tritanopia is blue blindness. Some people can detect red and green but with altered perception of the relative shades of these colours. They have abnormal but still partially functional opsins. They are protanomalous and deuteranomalous for red and green weakness respectively. A monochromat can perceive one colour. Monochromacy is true colour-blindness. Blue cone monochromacy is an X-linked recessive trait in which both red and green cone cells are absent. That is why it is also called red-green colour blindness. It is a common hereditary disease. Like any sex-linked recessive trait, it also zigzags from maternal grandfather through a carrier daughter to a grandson. It never passes direct from father to son. This type of colour blindness is more common in men than women, because chances for a male to be affected by it are much more than a female.
Muscular Dystrophy
It is a group of muscle diseases. It results in increasing weakening and breakdown of skeletal muscles over time. Many people will eventually become unable to walk. The muscular dystrophy group contains thirty different genetic disorders. These disorders are classified into nine main categories or types. The most common type is Duchenne muscular dystrophy (DMD). It typically affects males beginning around the age of four. The disorder is X-linked recessive. About two thirds of cases are inherited from a person's mother. But one third of cases are due to a new mutation. It is caused by a mutation in the gene for the protein dystrophin. Dystrophin is important to maintain the muscle fiber's cell membrane.
3. Y-linked inheritance:
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. SRY 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 cannot pass to daughters.
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