How does crossing over change the combination of alleles in gametes?
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Crossing over, also known as genetic recombination, is an important process that occurs during meiosis, specifically during prophase I. It involves the exchange of genetic material between homologous chromosomes, resulting in a recombination of alleles.
During crossing over, the non-sister chromatids of homologous chromosomes pair up and form a structure called a chiasma. The chiasma facilitates the exchange of segments of DNA between the chromatids through a process called crossing over.
As a result of crossing over, the combination of alleles in gametes is altered. Let's take a hypothetical example to understand this better:
Suppose two homologous chromosomes, one inherited from the mother and the other from the father, have the following alleles for a specific gene:
- Chromosome 1 (maternal): A, B, C, D
- Chromosome 1 (paternal): a, b, c, d
During crossing over, segments of genetic material between the maternal and paternal chromosomes are exchanged. This can result in new combinations of alleles, such as:
- Chromosome 1 (maternal): A, b, C, d
- Chromosome 1 (paternal): a, B, c, D
These modified chromosomes will then be separated during meiosis, and each will be included in separate gametes. So, the resulting gametes will contain different combinations of alleles compared to the original chromosomes.
For example, two possible gametes resulting from the above example could be:
- Gamete 1: A, b, C, D
- Gamete 2: a, B, c, d
Thus, crossing over increases genetic diversity by creating new combinations of alleles in gametes, contributing to the variation observed in offspring.