Figure 8 shows some characteristics of pea plants - Edexcel - GCSE Biology Combined Science - Question 4 - 2023 - Paper 1

The completed Punnett square is as follows:

    R   r
  ---------
R | Rr | Rr |
  ---------
r | rr | rr |

This square represents the cross between a pea plant with round seeds (Rr) and a plant with wrinkled seeds (rr).

From the Punnett square, we see that 2 out of 4 offspring (or 50%) will produce round seeds. Therefore, the percentage is 50%.

To calculate the ratio, we first need to find the numbers of each type of flower:

• Purple flowers: 133
• White flowers: 46

The ratio is calculated as:

$\text{Ratio} = \frac{133}{46} \approx 2.89 \text{ (which rounds to 3)}$

Thus, the ratio of offspring with purple flowers to offspring with white flowers is 3:1.

This cross can produce offspring with white flowers because both parent plants are heterozygous (Pp), meaning each parent has one allele for purple flowers and one for white flowers. When these alleles segregate during gamete formation, there is a possibility of the offspring inheriting two recessive alleles (pp), leading to white flowers.

Pea plants are diploid organisms, which means they have two sets of chromosomes (14 total). Gametes, however, are haploid, containing only one set of chromosomes. Thus, pea plant gametes have seven chromosomes (half of 14).

During fertilisation, the male gamete (pollen) fuses with the female gamete (ovule) to form a zygote. This process restores the diploid chromosome number, combining the genetic material from both parents. The zygote will then undergo cell division and development to form a new organism.