2. Galactosemia is a recessive human genetic disorder that results in the inabil
ID: 282100 • Letter: 2
Question
2. Galactosemia is a recessive human genetic disorder that results in the inability to digest the monosaccharide galactose, which is a metabolic by product of lactose metabolism. This disorder results from the inability of the liver to produce the enzyme, galactose-1-phosphate uridyltransferase. A normal women marries a normal man and together the couple has one daughter with Galactosemia. a. What is the probability that this couple will have a female child with Galactosemia? Read and think carefully. b. If the couple has 7 children, what is the probability that the first two will be galactosemic, and then among the remaining 5, 1 will have normal galactose metabolism and 4 will be galactosemic?Explanation / Answer
Galactosemia is an autosomal recessive disorder, which means one copy of each gene is present in teach of the parents body. As it is recessive, it doesnt show any symptoms in the parents, so they are termed as carrier.
lets consider that one of the X' chromosomes contains the gene for galactosemia, and one X' chromosome will have the female have the gene for the same.
Now if we cross the parents-
(X'Y) x (X'X) = X'X', X'Y, X'X, XY
which means out of the four children ,one female is affected , one female and one male is carrier , and one male is unaffected.
we have to consider the probability if the Y is affected in themale parent ,
In such a cross , XY' x X'X = X'X, X'Y',XY, XY'-One male is affected , one male and one female is a carrier, and one male is affected.
But here we can see that affected girls are none, so the probability here is 0.
total probability of a girl child to have galactosemia is 1/4.
2.From the above example, the total probabilty of the children who will have galactosemia is 1/4.
so for seven children , probabiliyt of galactosemic pateint is 7/4.
for the first two children , one of them can be a carrier , so the probability os 1/2.
total probabilty tat the first two child will hvae galactosemia is 7/4 x 1/2 = 7/8 .
In the remaning population the probability of an affected child is 5/7.
Normal galactose metabolism is in the ratio 1/4 for four kids, so for five kids it is 5/4.
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