Genetic diseases are diseases caused by genetic mutations that lead to cellular functions or proteins becoming abnormal. They can be inherited by offspring if such mutations occur in cells that give rise to gametes. Gametes are cells involved in forming offspring through sexual reproduction. In humans, male gametes are sperms and female gametes are eggs. An egg and sperm fuse in fertilization to form a zygote, which develops into an embryo in the mother’s womb.
Interestingly, some real-life data and observations have found that certain genetic diseases, such as hemophilia and red-green colorblindness, are more common in males than females! Why is this so? Well, in such diseases, the mutations happen to occur in genes located on sex chromosomes. Males and females have different sex chromosomes – XX in females and XY in males! Thus, males can only have one copy of a gene located on the X chromosome, while females can have two.
Why is this important? Well, this is the very reason that makes females less susceptible to such sex-linked genetic diseases! If a female has one faulty copy of the gene on one of her X chromosomes, she may not necessarily develop the disease if she has a healthy copy on the other X chromosome. On the other hand, a male will always develop the disease so long as he has one faulty copy of the gene because he would not have any other copy of the gene that could potentially be healthy to mask the effect of the faulty one.
Genes that behave this way are called sex-linked genes, meaning that the characteristic coded for by the genes is associated with the sex of an individual, as such genes are located on sex chromosomes. Most sex-linked genes are located on the X chromosome, simply due to the fact that X chromosomes are longer and thus have more genes than Y chromosomes.
How was sex linkage discovered? Thomas Hunt Morgan, an American geneticist, is responsible for this important discovery. In 1910, Morgan carried out breeding experiments using fruit flies (Drosophila). He investigated eye colour where red is the dominant colour and white is recessive and noticed unusual results, where all the white-eyed flies were male. This led to his idea on sex-linkage, which proved to apply to humans too!