The first human genetic trait to be identified and linked to a specific chromosome was colour blindness.
Through my research I have discovered that one pair of chromosomes is involved in the determination of sex– the XX chromosomes of the female and XY hromosomes of the male. The traits determined by the genes carried in the X chromosomes will show a special kind of inheritance, called sex-linked inheritance. One of the traits in question is colour blindness, a condition in which a person is unable to distinguish colours, particularly red and green, easily distinguished by a person with normal vision.
Colour blindness is a recessive trait with respect to normal vision, which is the dominant trait. I will identify the gene for normal vision as C and the gene for colour blindness as c. Consider now a marriage in which the mother is colour-blind and the father has normal vision. The colour-blind mother carries two X chromosomes, both with color blind (c) genes, and all the eggs she produces will also carry c. The father, with normal vision, has an X chromosome with normal colour (C) gene, and a Y chromosome that does not carry a gene for colour vision at all. Half of his sperm will have an X chromosome and the other half will have a Y chromosome.
The X-bearing sperm fertilizing the egg cells will give rise to daughters, who will carry one X chromosome with C and another with c gene: that is Cc. Since normal vision is dominant over colour blindness, these girls will have normal vision, but will harbor the gene for colour blindness, c.
If such a woman is married to a normal man (C), all of the daughters will have normal vision; but half of them will harbor the gene for colour blindness. Half of all the sons will inherit the X chromosome with the gene for colour blindness from their mother, and will consequently be colour-blind (c); the other half of the male offspring will inherit the gene for normal vision (C).
A colour-blind man (c) married to a normal woman not carrying the gene for colour blindness (CC) produces children with normal vision only. But while his sons (C) are free from the gene for colour blindness, all his daughters carry this gene in a hidden condition (Cc) and will transmit it to half of their children.
The punnett square is a device (chart) used to predict the probability of offspring having certain genetic characteristics. I will now use the punnett square to further explore the probability as it relates to colour blindness. A normal colour vision man is identified by XC Y.
A normal colour vision women who carries the gene for colour blindness is
identified by XC Xc.
A normal colour vision women who carries the gene for colour blindness is
identified by XC Xc.
| Xc | Y | |
| XC | XC Xc | XC Y |
| Xc | Xc Xc | Xc Y |
A colour-blind man is identified by Xc Y.
A colour-blind women is identified by Xc Xc.
| Xc | Y | |
| Xc | Xc Xc | Xc Y |
| Xc | Xc Xc | Xc Y |
This is a very rare occurrence.
