Question

Meiosis Essay Question:

Three events in sexual reproduction contribute to genetic variation in a population: independent assortment, crossing over, and random fertilization.

Explain how the three processes of sexual reproduction are advantageous to organisms and generate genetic variation within a population.

- State which phase of meiosis is occurring during each of the three events.  

- Also, explain what is happing during these phases that results in the genetic diversity of offspring.

- How is this advantageous to the population of a species, and how does this contribute to evolution?

Answer 1

1)

Stages of meiosis in which each of the three events occurs:

- independent assortment: Anaphase I

- Crossing over: Pachytene of Prophase I

- Random fertilization: Anaphase II (formation of final haploid genome) and after Telophase II (during fertilization)

2)

Independent assortment: This states that two genes assort into gametes independently of each other. Any two genes that are on differently numbered chromosomes assort completely independent of each other. In addition, there is equal chance of a particular allele for a gene to go into either daughter cells after meiosis I (since homologous chromosomes separate). However, genes on the same chromosome are linked and are more likely to be inherited together. This is where crossing over comes in. However, independent assortment itself allows different combinations of gamete genetic make-up. In humans, four gametes are produced from one meiotic division. There are 23 chromosomes in the haploid gamete. There are two possibilities for each chromosome (mother or father). Hence, any gamete can have [1÷ (2^23)] possible combinations. And this excluding the other two factors.

Crossing over: Crossing over further increases genetic variation by allowing independent assortment of linked genes. Hence, for example, if rolling of tongue and blue eyes are on the same chromosome and a person is heterozygous for these genes, his/her gametes can end up having blue eyes but the recessive allele for tongue rolling. Hence this further adds to the allelic combinations possible for a gamete that are not even parental.

Random fertilization: This means that the ability of a sperm to fertilize an egg is not dependent on the genes it carries. It also means that the female egg's ability to be fertilized is not dependent on it's genes. This means that every gametic genome produced has an equal probability of being passed on to the next generation. This gives all genomes a chance to be passed on, thus increasing variation.

3)

High genetic variation contributes to the population of a species as a whole, even though it means an individual may end up with few deleterious genes. The major reason for genetic variation is that it confers resistance to parasites and other infections and having a wide range of immunity increases fitness of the individual. This is evident in how MHC genes participate in sexual selection through smell. Further, it also helps accommodate other stresses like environmental stress and gives dietary variation.