Questions: Monohybrid Crosses and Mendel's Law of Segregation

Question 1

An Aa × Aa cross produces only 75 living offspring instead of the expected 100, and the phenotypic ratio in living offspring is 2:1 (dominant:recessive) rather than 3:1. What is the most likely explanation?

• A. The dominant allele A is incompletely dominant, producing an intermediate phenotype in heterozygotes
• B. The homozygous dominant genotype (AA) is lethal, causing those offspring to die before scoring
• C. The cross was actually Aa × aa, not Aa × Aa
• D. Environmental conditions favored recessive phenotype survival

Answer: (B)

When AA is lethal, the expected genotypic output of Aa × Aa is still 1 AA : 2 Aa : 1 aa — but the AA class dies. The surviving offspring are 2 Aa (dominant phenotype) : 1 aa (recessive phenotype), producing a 2:1 phenotypic ratio. This also explains the reduced total offspring count. Incomplete dominance would give a 1:2:1 phenotypic ratio with three distinct phenotype classes, not a 2:1. The 2:1 ratio in living offspring with reduced counts is a hallmark of a recessive lethal in combination with a dominant phenotype.

Question 2

From an F2 generation produced by Aa × Aa, which offspring genotypes will breed true (produce uniform offspring) if self-crossed?

• A. Only the recessive homozygotes (aa)
• B. Both homozygous genotypes (AA and aa), but not the heterozygotes (Aa)
• C. All three genotypes breed true because F2 represents a stable generation
• D. Only the dominant phenotype individuals, since they carry the dominant allele

Answer: (B)

Only homozygous individuals breed true. AA × AA produces all AA offspring; aa × aa produces all aa offspring — in both cases, all offspring show one uniform phenotype. Heterozygotes (Aa) do not breed true: Aa × Aa again produces 1 AA : 2 Aa : 1 aa, with both phenotypes present. The common error is to assume all 'dominant phenotype' individuals breed true — but 2/3 of dominant-phenotype F2 offspring are Aa heterozygotes, which will not.

Question 3

The Law of Segregation is grounded in the physical separation of homologous chromosomes during meiosis I, not just a statistical rule about probability.

• T. True
• F. False

Answer: True

Mendel derived the law statistically from pea plant crosses, but its physical basis was discovered later: homologous chromosomes (carrying the two alleles of a gene) line up and separate during meiosis I, each going to a different daughter cell. This physical separation is not probabilistic — it is a mechanical process that can be observed microscopically. The 1:1 allele ratio in gametes is a direct consequence of this physical separation, not an assumption.

Question 4

In a standard Aa × Aa cross, 75% of F2 offspring show the dominant phenotype, which means 75% of F2 offspring carry at least one dominant allele.

• T. True
• F. False

Answer: True

Both statements are true and consistent. The F2 genotypic ratio is 1 AA : 2 Aa : 1 aa. Of the four equally likely outcomes, three (AA, Aa, aA) carry at least one A allele and show the dominant phenotype, giving 75%. The fourth (aa) carries no A allele and shows the recessive phenotype. This is not a misconception — it is correct, unlike the common error of thinking that 75% are homozygous dominant (only 25% are).

Question 5

Explain why a 1:2:1 phenotypic ratio (rather than 3:1) from an Aa × Aa cross indicates incomplete dominance rather than complete dominance.

Model answer: With complete dominance, AA and Aa are phenotypically identical because A fully masks a. So three genotype classes (1 AA : 2 Aa : 1 aa) collapse into two phenotype classes (3 dominant : 1 recessive). With incomplete dominance, A does not fully mask a, so heterozygotes (Aa) display an intermediate phenotype distinct from both homozygotes. The three genotype classes now produce three distinct phenotypes — one for AA, one for Aa (intermediate), and one for aa — giving a 1:2:1 phenotypic ratio that directly mirrors the genotypic ratio.

The 3:1 ratio depends entirely on AA and Aa being phenotypically indistinguishable. Incomplete dominance breaks this equivalence: the heterozygote is visibly different from both homozygotes, so you see all three genotypic classes expressed as distinct phenotypes. Classic examples include snapdragon flower color (red × white → pink heterozygotes) and familial hypercholesterolemia (intermediate cholesterol levels in heterozygotes). The phenotypic ratio thus directly reveals the dominance relationship.