D3.2.20 Recombinants in crosses involving two linked or unlinked genes (HL) Notes

Recombinants Show the Complexities of Genetic Crosses

1. When crossing plants with different traits, some offspring display unexpected combinations not seen in either parent.
2. These are recombinants.
3. They arise due to:
   1. Independent Assortment: When genes are located on different chromosomes or far apart on the same chromosome.
   2. Crossing Over: When genes are linked (close together on the same chromosome), recombination occurs during meiosis through crossing over.

Independent Assortment Produces Predictable Ratios

1. For unlinked genes, independent assortment ensures all allele combinations have an equal chance of appearing in gametes.
2. This leads to predictable phenotypic ratios:
   1. A test cross involving heterozygous and homozygous recessive individuals typically produces a 1:1:1:1 ratio of offspring phenotypes.
3. This outcome confirms the genes are unlinked.

Linked Genes Deviate From Expected Ratios

1. For linked genes, independent assortment does not occur because the genes are inherited together.
2. However, crossing over during meiosis can create recombinants.
   1. Parental Types: More frequent.
   2. Recombinants: Less frequent, reflecting the rarity of crossing over.
3. These frequencies suggest linkage because recombinants appear at lower rates.

How Recombination Frequency Maps Genes

1. The percentage of recombinants indicates the physical distance between genes on a chromosome:
   1. Higher Frequency: Genes are further apart (e.g., 20% recombinants).
   2. Lower Frequency: Genes are closer together (e.g., 5% recombinants).

Why Are Recombinants Important?

1. Genetic Diversity: Recombinants increase variation in populations.
2. Mapping Genes: The frequency of recombinants helps determine the distance between genes on a chromosome.