Haploid gametes, diploid zygotes and genetic crosses in plants

Haploid gametes, diploid zygotes and genetic crosses in plantsSpec D3.2.1, D3.2.2

Each parent produces haploid gametes by meiosis, carrying one copy of each gene. At fertilization two gametes fuse to give a diploid zygote with two copies of each autosomal gene, one from each parent. This pattern of inheritance is common to all eukaryotes with a sexual life cycle.

Carrying out a cross in flowering plants

Pollen contains the male gametes and the female gametes are in the ovules inside the ovary, so pollination is needed to carry out a cross. Plants such as peas produce male and female gametes in the same flower, so they normally self-pollinate and self-fertilize.

1. Choose two pure-breeding (homozygous) parents with contrasting traits: the P generation.
2. Remove the immature anthers from flowers of one parent so it cannot self-pollinate.
3. Transfer pollen from the other parent to its stigmas with a small brush, then cover the flowers with a bag to stop other pollen arriving.
4. Collect the seeds and grow them: these plants are the F1 generation. Record their phenotypes.
5. Allow the F1 plants to self-pollinate (or cross them with each other). The offspring are the F2 generation. Count each phenotype.
6. Use a Punnett grid to predict the expected ratios and compare them with the counts.

Genetic crosses like these are widely used to breed new varieties of crop and ornamental plants, combining useful traits such as yield, disease resistance or flower colour.