AGB 121: Principles of Animal Genetics and Population Genetics (2+1)

X-LINKED DOMINANT

• In X-linked dominance, both males and females can display the trait or disorder by having only one copy of the allele.
  • Example : Incontinentia Pigmenti (IP), X-linked hypophosphatemia, Fragile X syndrome, Aicardi syndrome, Congenital hemidysplasia with ichthyosiform erythroderma and limb defects (also known as "CHILD syndrome"), Lujan–Fryns syndrome (LFS) also referred to as X-linked mental retardation with Marfanoid habitus and Lujan syndrome.
• In 1910 Thomas Hunt Morgan observed a male Drosophila melanogaster which was white eyed. He mated this fly with a red eyed female and got the following result.

  
  

• Morgan interpreted the above results assuming that the genes for eye colour must be located on the sex chromosome. He postulated that females must be homogametic carrying two X chromosomes (XX) and males with one X chromosome and one Y chromosome (XY) therefore heterogametic.

• In F2, equal number of red and white eyed individuals with normal sex ratio appeared.
• This is a significant deviation from normal autosomal gene inheritance wherein, when crosses are made between two pure breeding varieties which differ with reference to one or two characters there is no difference between reciprocal crosses.
• Thus this peculiar pattern of genetic behaviour shown by the white-eye colour indicates that its gene is carried on the X-chromosome.