Q.When a cross is made between tall plant with yellow seeds (TtYy) and tall plant with green seed (Tt yy), what proportions of phenotype in the offspring could be expected to be

This is a dihybrid cross where one parent is heterozygous for both traits (TtYy) and the other is heterozygous for height but homozygous recessive for seed colour (Tt yy). The key is to treat each trait separately using Mendel's law of independent assortment, then combine the probabilities.

For height: Tt × Tt gives 3 tall : 1 dwarf.

For seed colour: Yy × yy gives 1 yellow : 1 green.

Now multiply the probabilities for each required phenotype:

• Tall and green: Probability of tall = 3/4; probability of green = 1/2. Combined = 3/4 × 1/2 = 3/8. …

This is a dihybrid cross, but with a twist — one parent is heterozygous for both traits (TtYy) while the other is heterozygous for height but homozygous recessive for seed colour (Tt yy). The classic 9:3:3:1 ratio of Mendel’s dihybrid cross only appears when both parents are heterozygous for both genes. Here, because the second parent carries only recessive alleles for seed colour, the pattern simplifies.

Let’s break it down trait by trait. Height is controlled by the T gene: T (tall) is dominant over t (dwarf). Both parents are Tt, so the offspring for height follow a monohybrid ratio — 3 tall : 1 dwarf. Seed colour is controlled by the Y gene: Y (yellow) is dominant over y (green). The first parent is Yy, the second is yy. This is a test cross for colour, giving a 1:1 ratio of yellow to green.

Since the two genes assort independently (they are on different chromosomes, as per Mendel’s law of independent assortment), we can multiply the probabilities. For any offspring, the chance of being tall is 3/4 and dwarf is 1/4. The chance of being yellow is 1/2 and green is 1/2.

Now, to answer the specific questions:

1. Tall and green — This requires the offspring to be tall (probability 3/4) and green (probability 1/2). Multiplying: 3/4 × 1/2 = 3/8. So three-eighths of the offspring are expected to be tall with green seeds.
2. Dwarf and green — This requires dwarf (probability 1/4) and green (probability 1/2). Multiplying: 1/4 × 1/2 = 1/8. So one-eighth of the offspring are expected to be dwarf with green seeds. …

For a cross like this, the fork-line (branch) method is often faster than a full 8-cell Punnett square: draw one branch point for height (3/4 tall, 1/4 dwarf) and, off each of those branches, a second branch point for colour (1/2 yellow, 1/2 green), then multiply along each path. Tall→green is 3/4 × 1/2 = 3/8, and dwarf→ …