Question

Part B - Identify the mode of inheritance

Look over the pedigree you constructed in Part A.

Based on the inheritance pattern, which mode of inheritance must be the cause of galactosemia?

Based on the inheritance pattern, which mode of inheritance must be the cause of galactosemia?

sex-linked dominant

autosomal dominant

sex-linked recessive

autosomal recessive

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Part C - Predicting the probability of inheritance

Jane and John are considering having another child. Given the pedigree you constructed and the mode of inheritance for galactosemia, what is the risk that their next child will have the disorder?

Jane and John are considering having another child. Given the pedigree you constructed and the mode of inheritance for galactosemia, what is the risk that their next child will have the disorder?

3/4 (because they are both heterozygotes)

0 (because they already have one child with the disorder)

1/16 (because they each have 1/4 chance of passing on the allele)

1/4 (because they are both heterozygotes)

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Part D - Genetic testing

If Jane and John want to have another child, they plan to see a genetic counselor to find out when it would be best to test for galactosemia. A newborn with galactosemia must be put on a lactose- and galactose-free diet as soon as possible after birth. Even on this diet, affected individuals may still suffer from learning disabilities, ovarian failure (in young women), late-onset cataracts, and early death.

Which of the following tests would be most useful for Jane and John to have?

Which of the following tests would be most useful for Jane and John to have?

no testing

fetal chromosome analysis (examining fetal chromosomes for abnormalities as early as 11 weeks into the pregnancy)

carrier screening of Jane and John (to identify whether either or both of them carry the allele for galactosemia)

newborn screening (either assaying for the GALT enzyme or measuring excess galactose in the newborn’s blood)

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Answer 1

1) Autosomal recessive

Jane nor John both has not the same condition as their daughter, and also it is not example of sex-linkage, galactosemia must be an autosomal recessive trait.

2) 1/4 (because they are both heterozygotes)
Punnett square shows two heterozygous (Gg) parents
offspring genotype probabilities:
1/4 GG
1/2 Gg
1/4 gg

3) newborn screening (either assaying for the GALT enzyme or measuring excess galactose in the newborn’s blood)

Newborn screening will give information early enough to begin dietary restrictions. Fetal testing is not option because it carries greater risk than newborn screening, and there is no treatment before birth.