PCAT PCAT Genetics 2

Question 1

If a carrier mother (X^A X^a) and a normal father (X^A Y) have sons, what percentage of sons will express the X-linked recessive trait?

Accepted Answer

50%

Answer

Sons inherit their single X chromosome only from the mother; half of her X chromosomes carry the recessive allele, so 50% of sons are affected.

Question 2

Which term describes the observable physical characteristics of an organism?

Accepted Answer

Phenotype

Answer

The phenotype is the physical expression of an organism's genetic information, influenced by both genotype and environment.

Question 3

What is the specific location of a gene on a chromosome called?

Accepted Answer

Locus

Answer

A locus is the fixed chromosomal position of a gene; alleles are the alternative versions of the gene found at that locus.

Question 4

Which nitrogenous base pairs with adenine in a DNA double helix?

Accepted Answer

Thymine

Answer

In DNA, adenine (A) pairs with thymine (T) via two hydrogen bonds, while guanine pairs with cytosine via three hydrogen bonds.

Question 5

What term describes an organism that carries two identical alleles at a particular gene locus?

Accepted Answer

Homozygous

Answer

A homozygous organism carries two copies of the same allele (e.g., AA or aa) at a given locus.

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