Massachusetts Comprehensive Assessment System (MCAS) Biology Practice Test

What is the probability that two heterozygous parents will have a child with galactosemia?

1. 100%

2. 50%

3. 75%

4. 25%

The correct answer is: 25%

The correct answer is based on the principles of Mendelian genetics, specifically looking at a genetic condition that follows a recessive inheritance pattern like galactosemia. For a trait that is recessively inherited, an individual must inherit two copies of the recessive allele to express the condition. If both parents are heterozygous for galactosemia, it means they each have one normal allele (dominant) and one allele for the recessive condition (galactosemia). We can denote the normal allele as "G" and the allele for galactosemia as "g." Therefore, each parent's genotype is Gg. When these parents have a child, we can determine the probability of the child inheriting the different combinations of alleles using a Punnett square, which shows all the possible genotypes that could result from their mating: - GG (normal) - Gg (normal carrier) - gG (normal carrier) - gg (affected by galactosemia) In the Punnett square: 1. The chance of getting GG is 1 out of 4 (25%). 2. The chance of getting Gg or gG is 2 out of 4 (50%). 3. The chance