In this article we will discuss about:- 1. Examples of Sex-Linked Genetic Diseases 2. General Rules of the Sex-Linked Genetics 3. Y-Linked Genes.
Examples of Sex-Linked Genetic Diseases:
i. X-Iinked Recessive Diseases:
Hemophilia A and B, color blindness, Duchenne muscular dystrophy.
ii. X-linked Dominant Diseases:
Vitamin D resistant rickets with hypophosphatemia, some types of ectodermal anidrotic dysplasia, genetic defects of the enzyme glucose-6-phosphate dehydrogenase, or skin disease with additional symptoms such as missing teeth, oro-facio-digital syndrome (like cleft palate with other cleavages in the oral area), syndactyly, the defective enamel of the teeth, and one blood group Xg.
General Rules of the Sex-Linked Genetics:
(a) Male patients pass on the defective gene to all their daughters. On the average. these will suffer a lighter case of the anomaly.
(b) Female patients will pass the defective gene to half their children, regardless of sex. As a rule, the sons will be more severely affected than the daughters.
(c) In case of complete dominance, individuals who are themselves phenotypically normal, regardless of their sex, will — with a probability approaching 100% — be free of the defective gene and have normal children.
(d) In very rare cases, a very careful examination is required in order to ensure that the symptoms, and the results of the biochemical tests, are identical with the symptoms of the ascertained genetic cases. This would exclude the possibility of a phenocopy. So, one would have to verify that the parents are really free of micro-symptoms.
(e) Some of the X-chromosomal dominant diseases are prenatally lethal to all hemizygotes and heterozygotes manifest phenotypical anomalies.
(f) The death of the hemizygous embryos results in considerable natural selection against the defective gene.
The Y-chromosome has no corresponding locus in the X-chromosome and the mode of transmission of a Y-linked gene is very simple. The female has no Y-chromosome, so women cannot exhibit the trait. The normal male has only one Y-chromosome and so the gene is necessarily unpaired and, if present, it must be expressed.
So, the question of dominant or recessiveness cannot arise. The gene simply follows the path of the Y-chromosome and so is handed on by a affected male to all his sons.
The best known example of Y-linked human gene is the growth of hair on the outer rim of the ear (trichosis) and porcupine man. The gene, present in Y-chromosome, is also known as holandric gene.
A histocompatibility gene (H-Y), however, has recently been mapped on the short arm of the human Y-chromosome. However, one conclusion is that very few genes are located on the human Y-chromosome and most part of the Y-chromosomc is permanently hetero- chromatic.