Biology Tutorials > Genetics and Evolution > Independent Assortment and Crossing Over

Independent Assortment and Crossing Over

A schematic diagram implying independent assortment

The previous tutorial investigates the process of meiosis, where 4 haploid gametes are created from the parent cell. Half the genetic information from a parent is present in these haploids, which fuse with gametes of the opposite sex to create a zygote, with a complete chromosome complement that will create offspring after prolonged growth.

The process of meiosis increases genetic diversity in a species. The sex organs which produce the haploid gametes are the site of many occurrences where genetic information is exchanged or manipulated.

Independent Assortment of Chromosomes

Alleles for a particular phenotype determine what characteristic an organism will express, as with the following example where

Chromosome 1 contains an allele for blonde hair
Chromosome 2 contains an allele for brown hair
Chromosome 3 contains an allele for blue eyes
Chromosome 4 contains an allele for brown eyes

The top assortment to the left produces 2 blonde hair/blue eyes gametes while the below produces 2 brown hair/brown eyes gametes

The top assortment on the right produces 2 blonde hair/brown eyes gametes while the below produces 2 brown hair/blue eyes gametes

The above indicates that even though the two homologous chromosomes contain the same genetic information, the assortment of the chromosomes (the order they lie in) can determine what genetic information is present in each of the 4 gametes produced. With 23 chromosomes in a human gamete, there are 2²³ combinations (8388608 combinations)

Crossing Over

During meiosis, when homologous chromosomes are paired together, there are points along the chromosomes that make contact with the other pair. This point of contact is deemed the chiasmata and can allow the exchange of genetic information between chromosomes. This further increases genetic variation.

There are also many other ways in which genetic variation is increased in a species gene pool, all of which are described in the following pages.

The next tutorial investigates the work of Gregor Mendel, an Austrian monk famous for his work involving monohybrid and dihybrid crossing, alongside the continuation into looking at genetic diversity through meiosis and genetics in general.