PCAT PCAT Genetics 2 — Questions and Answers
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?
- 50% (Correct answer)
- 25%
- 75%
- 100%
Correct answer: 50%
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?
- Phenotype (Correct answer)
- Genotype
- Allele
- Locus
Correct answer: Phenotype
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?
- Locus (Correct answer)
- Allele
- Codon
- Promoter
Correct answer: Locus
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?
- Thymine (Correct answer)
- Cytosine
- Guanine
- Uracil
Correct answer: Thymine
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?
- Homozygous (Correct answer)
- Heterozygous
- Hemizygous
- Codominant
Correct answer: Homozygous
A homozygous organism carries two copies of the same allele (e.g., AA or aa) at a given locus.
Question 6: During which phase of meiosis do homologous chromosome pairs separate and move to opposite poles?
- Anaphase I (Correct answer)
- Anaphase II
- Metaphase I
- Telophase II
Correct answer: Anaphase I
In Anaphase I of meiosis, homologous pairs are pulled apart to opposite poles, halving the chromosome number.
PCAT PCAT Genetics 2
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?