Chromosomes Color Blindness

Red-green color blindness is explained by unequal intragenic recombination between a pair of X chromosomes 814... 

Color blindness is X-chromosome linked. Bearing in mind the phenomenon of X-chromosome inactivation, suggest an explanation for the observation that females who are heterozygous for the defective gene show no signs of color blindness. 

The answer is e. (Murray, pp 812-828. Scriver, pp 3-45. Sack, pp 97—158. Wilson, pp 23-39.) The common forms of color blindness are X-linked recessive, as indicated by the initial 3 of the McKusick number (304000). The couple s daughters will be obligate carriers—that is, carriers implied by the pedigree. Using a lowercase c to represent the recessive color blindness allele, the woman is X X , while her husband is X hf. The Punnett square below indicates that all daughters will be carriers (X X ), while sons will be normal (X Y). Note again that loci on the X chromosome cannot be transmitted from father to son, since the son receives the father s Y chromosome. 

Sex linked traits - the Y chromosome found only in males (XY) carries very little genetic information, whereas the X chromosome found in females (XX) carries very important information. Since men have no second X chromosome to cover up a recessive gene, the recessive trait is expressed more often in men. Women need the recessive gene on both X chromosomes to show the trait. Examples of sex linked traits include hemophilia and color-blindness. 

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