In a monohybrid cross between two heterozygous parents (Aa × Aa), what fraction of offspring are expected to be homozygous recessive (aa)?
A1/2
B1/4
C3/4
D0 — two dominant-phenotype parents cannot produce recessive offspring
From the Punnett square for Aa × Aa, the four equally probable outcomes are AA, Aa, Aa, aa — giving a 1:2:1 genotypic ratio. Only 1 in 4 offspring is expected to be aa (homozygous recessive). Option D is a common misconception: heterozygous parents show the dominant phenotype but carry one recessive allele, which can be passed to offspring.
Question 2 True / False
Two genes located on the same chromosome generally follow Mendel's Law of Independent Assortment.
TTrue
FFalse
Answer: False
Independent assortment holds when genes are on different chromosomes (or very far apart on the same chromosome). Genes physically close together on the same chromosome are linked and tend to be inherited together, violating independent assortment. Mendel's original results worked because the seven traits he chose happen to be on different chromosomes or far enough apart to behave independently — he was lucky in his choice of traits.
Question 3 Short Answer
A family has four children, all showing the dominant phenotype. Both parents are heterozygous (Aa). Is this result surprising? Why or why not?
Think about your answer, then reveal below.
Model answer: No, this is not surprising. Each child independently has a 3/4 probability of showing the dominant phenotype. The probability that all four show it is (3/4)⁴ ≈ 0.32, or about 32% — quite common. The 3:1 ratio is an expectation over many offspring, not a guarantee for any specific family.
Mendel's ratios are probabilistic: each offspring is an independent event with fixed probabilities derived from the parents' genotypes. Small sample sizes (like a family of four) frequently deviate from expected ratios. This is why Mendel needed hundreds of plants per cross to reliably observe the 3:1 ratio, and why chi-square tests are used to assess whether observed deviations are within expected random variation.