Genetic Variations and Sexual Life Cycles

Inheritable genetic variations can result from:
- New genetic combinations through meiosis.
- Viable errors occurring during replication.
- Mutations caused by environmental factors.

Genetic variation produced in sexual life cycles contributes to evolution.
Mutations, which are changes in an organism's DNA, are the original source of genetic diversity.
Mutations create different versions of genes called alleles.
Reshuffling of alleles during sexual reproduction produces genetic variation.

The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation.
Three mechanisms contribute to genetic variation:
- Crossing over in Prophase I
- Independent assortment of chromosomes in Metaphase I
- Random fertilization (after meiosis)

Homologous pairs of chromosomes orient randomly at metaphase I of meiosis I.
In independent assortment, each pair of chromosomes sorts maternal and paternal homologs into daughter cells independently of the other pairs.
The number of combinations possible when chromosomes assort independently into gametes is $2^n$ , where n is the haploid number.
For humans ( $n = 23$ ), there are more than 8 million ( $2^{23}$ ) possible combinations of chromosomes.

Crossing over produces recombinant chromosomes, which combine DNA inherited from each parent.
Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome.
In humans, an average of one to three crossover events occurs per chromosome.

Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg).
The fusion of two gametes (each with 8.4 million possible chromosome combinations from independent assortment) produces a zygote with any of about 70 trillion diploid combinations.
Crossing over adds even more variation.
Each zygote has a unique genetic identity.

Natural selection results in the accumulation of genetic variations favored by the environment.
Sexual reproduction contributes to the genetic variation in a population, which originates from mutations.
Animals that always reproduce asexually are quite rare.

Prophase I: Each homologous pair undergoes synapsis and crossing over between nonsister chromatids with the subsequent appearance of chiasmata.
Metaphase I: Chromosomes line up as homologous pairs on the metaphase plate.
Anaphase I: Homologs separate from each other; sister chromatids remain joined at the centromere.