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Old 10-02-2011
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Genetics Color-Blindness Inheritance

A patient presents to the physician's office to ask questions about color blindness. The patient is color-blind, as is one of his brothers. His maternal grandfather was color-blind, but his mother, father, daughter, and another brother are not. His daughter is now pregnant. What is the risk that her child will be color-blind?
A. 100%
B. 50%
C. 25%
D. 12.5%
E. Virtually 0%
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Old 10-02-2011
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looks like
c) 25 percent
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Old 10-02-2011
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The answer is: C
Males always transmit their single X chromosome to their daughters. Therefore, a daughter of a male affected with an X-linked disorder is an obligate carrier for that disorder. When the condition is X-linked recessive, as with most forms of color-blindness, the daughter is unlikely to show any phenotypic evidence that she is carry-ing this abnormal gene. Offspring of female carriers are of four types: (1) female carrier with
one normal and one mutant allele, (2) normal female with two normal alleles, (3) affected male with a single mutant allele, and (4) normal male with a single normal allele. The chance of having an affected child is thus 1/4 or 25%. If the obligate carrier female gives birth to a son, the chance of the son being color-blind is 50%.
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Old 10-02-2011
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Simply, p1=1/2= probability of daughter transmitting mutant gene,
p2= 1/2 = probability of child having male sex.

p=p1 x p2 = 1/4
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Old 10-02-2011
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probability of a child to be color blind is 25 %
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