How many different genotypes could be produced from a single mating event?
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Answer:
In 1911, while studying the chromosome theory of heredity, biologist Thomas Hunt Morgan had a major breakthrough. Morgan occasionally noticed that "linked" traits would separate. Meanwhile, other traits on the same chromosome showed little detectable linkage. Morgan considered the evidence and proposed that a process of crossing over, or recombination, might explain his results. Specifically, he proposed that the two paired chromosomes could "cross over" to exchange information. Today, we know that recombination does indeed occur during prophase of meiosis (Figure 1), and it creates different combinations of alleles in the gametes that result (i.e., the F1 generation; Figure 2).
A two-part schematic shows allele combinations on chromosomes following meiosis two. Crossing over does not occur between the chromosomes shown on the left side of the diagram. Crossing over does occur between the chromosomes shown on the right side of the diagram. In both scenarios, two sets of homologous chromosomes are shown: the first set is orange, and the second set is green. The chromosomes are shown before and after meiosis two has occurred, illustrating how each is affected by the presence or absence of a cross-over event.
Figure 1: Recombination and gamete production.
A comparison of nonrecombination (left) with recombination (right), shows how recombination affects the way chromosomes are passed into gametes in Meiosis II. On the right, a single crossover event produces half nonrecombinant gametes and half recombinant gametes.
© 2014 Nature Education Adapted from Pierce, Benjamin. Genetics: A Conceptual Approach, 2nd ed. All rights reserved. View Terms of Use
Figure Detail
When proposing the idea of crossing over, Morgan also hypothesized that the frequency of recombination was related to the distance between the genes on a chromosome, and that the interchange of genetic information broke the linkage between genes.
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