7.1.2 Predicting Inheritance: Monohybrid Crosses

Key Example: Pea Pod Colour

• Green pod colour is controlled by a dominant allele (G).
• Yellow pod colour is controlled by a recessive allele (g).

The First Cross ($F₁$ Generation)

If a pure breeding green-podded plant (GG) is crossed with a yellow-podded plant (gg):

• All offspring ($F₁$ generation) will be heterozygous (Gg) and have green pods, as the dominant allele masks the recessive allele.

The Second Cross ($F₂$ Generation)

When two heterozygous (Gg) individuals are crossed, the offspring phenotypes will follow a 3:1 ratio:

• 3 green-podded plants (GG or Gg).

• 1 yellow-podded plant (gg). Explanation:

• Gamete production is random, and when fertilisation occurs, the combinations are governed by probability.

• A Punnett square can illustrate this: | Parent Gametes | G | g | |---|---|---| | G | GG | Gg | | g | Gg | gg |

• The resulting genotypes are:

  • 1 GG (homozygous dominant).
  • 2 Gg (heterozygous).
  • 1 gg (homozygous recessive).

Practical Considerations

• The 3:1 ratio is an expected ratio based on probability.
• In real experiments, actual results may vary slightly due to chance in gamete fertilisation.