IGCSE Biology 0610: Inheritance and Punnett Squares - Study Guide PDF with Answers

Use letters consistently

A gene can have alternative forms called alleles. In a simple diploid example, each parent contributes one allele to an offspring. Genotype is the allele combination, while phenotype is the observable feature under the stated model. Let A be a dominant allele and a a recessive allele for a hypothetical trait. AA and Aa show the dominant phenotype; aa shows the recessive phenotype. These symbols describe a teaching model, not a rule that every real trait has only two alleles or shows complete dominance.

Make gametes before boxes

A homozygous AA parent supplies A gametes; aa supplies a gametes. A heterozygous Aa parent can supply A or a. Put one parent's possibilities along one edge of a two-by-two Punnett square and the other's along the other edge. Combine one allele from each parent in every cell. The four cells represent equally likely combinations under this simple model; repeating an allele along an edge keeps the square complete and makes ratios easy to read.

Genotype ratio differs from phenotype ratio

Crossing Aa with Aa yields AA, Aa, Aa and aa. The genotype ratio is 1 AA : 2 Aa : 1 aa, while the phenotype ratio is 3 dominant : 1 recessive if A has complete dominance. Crossing Aa with aa yields Aa, aa, Aa and aa, giving a 1:1 phenotype ratio. A ratio describes expected probabilities for each offspring, not a guarantee that exactly that pattern will appear in four births or seeds.

Limits of a simple prediction

An observed sample may differ from a calculated ratio because fertilisation is random, especially in a small sample. A recessive phenotype in this model establishes aa, but a dominant phenotype could be AA or Aa. A cross with aa can help distinguish those genotypes if enough offspring are observed, though a small group is not conclusive. Real traits can involve multiple genes, environment, codominance or linkage; choose this simple model only when the question states it.

1. A heterozygote cross

In a hypothetical plant, A is completely dominant to a. Predict a cross Aa x Aa.

1. Each parent can give an A or an a gamete.
2. Four equally likely boxes give AA, Aa, Aa and aa.
3. Genotypes are 1:2:1; phenotypes are 3 dominant : 1 recessive. The recessive probability is 1/4 per offspring.

2. A test-style cross

Cross a dominant-looking heterozygote Aa with recessive aa in the same simple model.

1. Gametes are A or a from Aa, and a only from aa.
2. Boxes give Aa, aa, Aa and aa.
3. Half the expected offspring show the dominant phenotype and half the recessive phenotype, a 1:1 ratio.

3. Interpret a family observation carefully

Two Aa parents have four offspring. None show the recessive phenotype. Is the 3:1 prediction false?

1. Each offspring independently has probability 3/4 of the dominant phenotype in this model.
2. All four could happen to be dominant-looking: (3/4)^4 = 81/256, about 31.6%.
3. The 3:1 ratio is an expectation over many offspring, not a promise of one recessive in every group of four.