X -Linked Recessive Inheritance
Humans have many X – linked traits of which some like hemophilia and colour blindness are recessive while others like hypophosphatemia or vitamin D resistant rickets are dominant. X – linked dominant is a trait that is identified by an X linked dominant gene, while X – linked recessive is a trait that is identified by an X – linked recessive gene. Their patterns of inheritance are very different from each other.
Experimental matings are not virtually possible in humans. The mode of inheritance of human traits can be traced through pedigrees.
Genetics of Haemophilia:
Haemophilia is an unusual X– linked recessive trait. Haemophiliac’s blood fails to clot properly after an injury, due to the fact that it has either a reduction or malfunction or complete absence of blood clotting factors. It is a severe hereditary disease due to the fact that a haemophiliac may bleed to death even from small 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 quality. 80% of haemophiliacs, suffer from haemophilia A due to the irregularity 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 similarly because it is autosomal. Possibilities 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 just when she is homozygous for the recessive allele, but a male 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 directly from father to son. Gene for normal is H. Gene for haemophilia A is h. In generation I of this pedigree a man (I – 2) suffering from haemophilia A marries a normal woman (I – 1). He passes the haemophilia gene to his daughter (II – 2) through his X chromosome. He cannot pass this gene to his son (II – 3) because the son receives just the Y chromosome from him. His daughter (II – 2) likewise gets another X but with a regular dominant allele from her mother (I – 1).
The daughter looks phenotypically regular, but she is heterozygous and a carrier for the recessive gene. When she marries a normal man (II – 1) she passes her father’s characteristic to one of her two boys (HI – 4) who acquires grandfather’s X from her.
The single recessive allele for haemophilia reveals successfully in the hemizygous son due to the fact that his Y chromosome does not bring its equivalent. The other boy (III – 3) is normal as he inherits grandmother’s X with a normal gene, one daughter (III – 1) with both normal X is normal, but the other daughter (III – 2) is a carrier like her mother.
Lots of X – linked traits in man are likewise found X – linked in other mammals like mouse, rabbit, canine, sheep, horse, donkey, livestock, kangaroo and chimpanzee.
Genetics of colour-blindness
Normal trichromatic colour vision is based upon three different kinds of cone cells in the retina, each sensitive to only one of the 3 primary colours, red, green or blue. Each kind of cone cell has specific light-absorbing proteins called opsins. The genes for red and green opsins are on the X chromosome, while the gene for blue opsin is present on autosome 7. Mutations in opsin genes cause 3 kinds of colour-blindness.
A dichromat can view 2 main colours but is not able 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 individuals can identify red and green but with a transformed understanding of the relative shades of these colours. They have irregular however 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 likewise 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 child to a grandson. It never passes directly from father to son.
This type of colourblindness is more typical in males than females because opportunities for a male to be affected by it are much more than a woman.
Testicular feminization syndrome is a rare X-linked recessive trait. Although the persons affected by this trait have a male set of XY chromosomes, yet tfn geneon their X chromosome establishes them physically into females. They have breasts, female genitalia, a blind vagina but no uterus.
The deteriorated testis is also present in the abdomen. Such individuals are gladly married as women however are sterile. It is an androgen insensitivity syndrome. Male sex hormone testosterone has no effect on them.