Crossing over occurs during prophase I, and it is the exchange of genetic material between non-sister chromatids of homologous chromosomes. Recall during prophase 1, homologous chromosomes line up in pairs, gene-for-gene down their entire length, forming a configuration with four chromatids, known as a tetrad. At this point, the chromatids are very close to each other and some material from two chromatids switch chromosomes, that is, the material breaks off and reattaches at the same position on the homologous chromosome. This exchange of genetic material can happen many times within the same pair of homologous chromosomes, creating unique combinations of genes, as shown in the animation below. This process is also known as recombination.
Independent Assortment and Random Fertilization
In humans, there are over 8 million configurations in which the chromosomes can line up during metaphase I of meiosis. It is the specific processes of meiosis, resulting in four unique haploid cells, that result in these many combinations. This independent assortment, in which the chromosome inherited from either the father or mother can sort into any gamete, produces the potential for tremendous genetic variation. Together with random fertilization, more possibilities for genetic variation exist between any two people than the number of individuals alive today.
Sexual reproduction is the random fertilization of a gamete from the female using a gamete from the male. In humans, over 8 million (223) chromosome combinations exist in the production of gametes in both the male and female. A sperm cell. with over 8 million chromosome combinations, fertilizes an egg cell, which also has over 8 million chromosome combinations.
That is over 64 trillion unique combinations, not counting the unique combinations produced by crossing over. In other words, each human couple could produce a child with over 64 trillion unique chromosome combinations!
How does crossing over, independent assortment, and random fertilization lead to genetic variation.
Crossing over, independent assortment, and random fertilization all contribute to genetic variation in different ways:
1. Crossing over during prophase I of meiosis allows for the exchange of genetic material between non-sister chromatids of homologous chromosomes. This results in the creation of unique combinations of genes, as parts of the chromatids break off and reattach to the homologous chromosome. This exchange of genetic material can happen multiple times within the same pair of homologous chromosomes, further increasing genetic variation.
2. Independent assortment occurs during metaphase I of meiosis, when homologous pairs of chromosomes line up randomly along the cell's equator. This means that the chromosome inherited from either the father or mother can sort into any gamete. As there are over 8 million possible configurations of chromosome alignment, independent assortment further increases genetic variation by randomly determining which chromosomes will end up in each gamete.
3. Random fertilization is the process in which a gamete from the female is fertilized by a gamete from the male. Each gamete contains over 8 million chromosome combinations, resulting from independent assortment and crossing over. When a sperm cell with its unique chromosome combination fertilizes an egg cell with its unique chromosome combination, the resulting zygote will have a completely new and unique set of chromosomes. This further increases genetic variation as each individual gamete can unite with any other individual gamete.
Overall, crossing over, independent assortment, and random fertilization introduce a high degree of variability in the genetic makeup of offspring. This genetic variation is essential for evolution, as it allows for the potential for individuals with beneficial combinations of traits to arise and survive in changing environments.