General Genetics. Ayesha M. Khan Spring 2013. X-linked color blindness in humans Light detecting cells: Rods – See in shades of grey Cones – See in colors
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Ayesha M. Khan
The human eye actually detects only three colors—red, green, and blue—but the brain mixes the signals from different cone cells to create the wide spectrum of colors that we perceive.
a. Affected fathers transmit the recessive allele to all daughters (who are therefore carriers), and to none of their sons.
b. Father-to-son transmission of X-linked alleles generally does not occur.
c. Many more males than females exhibit the trait.
d. All sons of affected (homozygous recessive) mothers are expected to show the trait.
e. With a carrier mother, about 1⁄2 of her sons will show the trait and 1⁄2 will be free of the allele.
f. A carrier female crossed with a normal male will have 1⁄2 carrier and 1⁄2 normal daughters.Some characteristics of X-linked recessive inheritance:
Hemophilia (reduced blood clotting) is an X-linked recessive disease in humans. A woman with hemophilia mates with a man who exhibits normal blood clotting.
1) What is the probability that their child will have hemophilia?
2) What is the probability that their male child will have hemophilia?Concept check
Different numbers of X chromosomes in males and females -potential problem.
Equalizes the amount of protein produced by X-linked genes in the two sexes.Dosage Compensation
As a result of X inactivation, females are functionally hemizygousat the cellular level for X-linked genes.
Murray Barr (1949)
Observed condensed, darkly staining bodies in the nuclei of cells from female cats.Dosage Compensation
An interphase epithelial cell from a human female shows a dark-staining Barr body in the nucleus; representing an inactivatedX chromosome.A comparable cheek epithelial cell from a male does not show a Barr body. Thus, dosage compensation prevents excessive expression of X-linked genes in mammals.
Mary Lyon (1961)
Within each female cell, one of the two X chromosomes becomes inactive; which X chromosome is inactivated is random. If a cell contains more than two X chromosomes, all but one of them is inactivated.
Number of Barr bodies in human cells with different complements of sex chromosomes
In heterozygous females, proteins encoded by both alleles are produced, although not within the same cell.
Males are hemizygous and thus may be black (X+ Y) or orange (Xo Y) but not black and orange.
Females may be black (X+ X+ ), orange (XoXo), or tortoiseshell (X+ Xo ).Example: Tortoiseshell cats
Inheritance of Z-linked characteristics is the same as that of X-linked characteristics, except that the pattern of inheritance in males and females is reversed.
-Females are heterogametic sexZ-Linked Characteristics