Quick questions on Mendelian inheritance, Punnett squares and test crosses (QCE Biology Unit 4)

Law of segregation. Each individual has two alleles for each gene; the two alleles separate during meiosis so that each gamete carries only one. This is the chromosomal behaviour of homologous chromosomes separating in anaphase I.

<!-- Diagram: Punnett Tt x Tt | reviewed 2026-05-21 --> <svg class="fig" viewBox="0 0 320 240" role="img" aria-labelledby="qcepun-t qcepun-d"> <title id="qcepun-t">Punnett square Tt crossed with Tt</title> <desc id="qcepun-d">Two by two Punnett square. Parent gametes T and t along the top and side. Offspring cells T T, T t, T t, and t t giving genotype ratio one to two to one and phenotype ratio three dominant to one recessive.</desc> <g font-size="14" font-weight="700" text-anchor="middle" class="var"> <text x="140" y="40">T</text> <text x="220" y="40">t</text> <text x="60" y="110">T</text> <text x="60" y="190">t</text> </g> <g fill="var(--paper)" stroke="var(--ink)" stroke-width="1.4"> <rect x="100" y="60" width="80" height="60"/> <rect x="180" y="60" width="80" height="60"/> <rect x="100" y="120" width="80" height="60"/> <rect x="180" y="120" width="80" height="60"/> </g> <g font-size="13" font-weight="700" text-anchor="middle" class="var"> <text x="140" y="98">TT</text> <text x="220" y="98">Tt</text> <text x="140" y="158">Tt</text> <text x="220" y="158">tt</text> </g> <text x="160" y="216" text-anchor="middle" font-size="11" class="muted">Genotype 1:2:1 (TT:Tt:tt); phenotype 3:1 (tall:short).</text> </svg>

A dihybrid cross follows two genes simultaneously. The dihybrid ratio is the product of the two monohybrid ratios.

Each individual has two alleles for each gene; the two alleles separate during meiosis so that each gamete carries only one. This is the chromosomal behaviour of homologous chromosomes separating in anaphase I.

Alleles for different genes segregate into gametes independently of one another, provided the genes are on different chromosomes (or far enough apart on the same chromosome to recombine freely). This is the chromosomal behaviour of independent metaphase I alignment.

Cross the unknown with a homozygous recessive individual (tt). The recessive partner contributes only t gametes, so any t in the offspring must come from the unknown parent.

State Mendel's law of segregation and law of independent assortment, and explain the chromosomal basis for each. [3 marks]

A heterozygous Rr (round, dominant) pea plant is crossed with a homozygous rr (wrinkled). Predict the offspring genotype and phenotype ratios and identify this as a test cross. [3 marks]

Refer to a dihybrid cross AaBb $\times$ AaBb. (a) List the gametes each parent produces. (b) Predict the phenotypic ratio assuming complete dominance and independent assortment.