Genetic Variation

Genetic Variation

Genetic variation results from the crossing over and exchange of chromosomal parts, and occurs during meiosis II. Every individual has a unique genotype and phenotype. This is based on three events: chromo-some segregation and independent assortment, cross of homologues and gene recombination, and random fertilization.

Chromosome Segregation and Independent Assortment

Each pair of replicated homologous chromosomes synapses during meiosis I. A tetrad is formed during both spermatogenesis and oogenesis. The tetrads are randomly aligned on the meiosis I metaphase spin-dle. Therefore, maternal and paternal chromosomes become randomly distributed to the daughter nuclei. Also, gametes vary widely in their design. The two most important points about the metaphase of meiosis I are:

■■ Two alleles determine each trait and these are segregated to different gametes. When errors occur in this process, they may result in can cer progression, Down syndrome, or infertility. Down syndrome is the most common cause of trisomy 21 and occurs as maternal disjunction at meiosis 1.

■■ With homologous chromosomes, alleles on the different pairs are distributed independently. This results in each gamete having a single allele for each trait. That allele represents just one of four possible parental alleles.

There is a method for calculating the number of ­different gamete types that result from the independent­ assortment of homologues during meiosis I. The ­formula is known as 2n. The letter “n” refers to the num-ber of homologous pairs. Therefore, if there are three homologous pairs, the formula would mean:

23 = (2 × 2 × 2). This equals eight different gamete types.

The number of gamete types increases greatly as the chromosome number increases. If a cell has six pairs of homologues, it would produce 2⁶ or 64 types of gametes. Another example is the amount of gam-etes able to be produced within a male’s testes, based only on independent assortment. This is 2²³ = about 8.5 ­million! However, in a female’s ovaries, the number­ of gamete types is much less. This is due to the ovaries completing no more than 500 reduction divisions over her reproductive lifetime. However, every ovulated oocyte is usually unique in genetic terms due to inde-pendent assortment.

Crossover of Homologues and Gene Recombination

Crossover is the result of recombination of genes on homologous pairs of chromosomes during meiosis. During meiosis I, genes are linearly arranged down the length of each chromosome. Genes on the same chro-mosome are called linked, since they are transmitted as a unit to daughter cells. Chromosomes can break, however, precisely exchanging gene segments with their homologues. This leads to recombinant chromosomes, which have mixed components from each parent. Gene recombination results in chromo-somes that have mixed contributions from each parent.

In a chiasma (crossover), a break occurs between linked genes (FIGURE 27-2). An example of this may be when one gamete has alleles for brown eyes and blond hair, while another gamete has alleles for blue eyes and brown hair. Because of this crossover, two of the four chromatids found in the tetrad will have a mixed set of alleles. Some of these are maternal and some are paternal. Therefore, when the chromatids become seg-regated, each gamete will have a unique combination of parental genes. Since humans have 23 tetrads and crossovers occur in all of them during meiosis I, the possible variations are significant.

Random Fertilization

A single human egg is fertilized by a single sperm on a completely random basis. Therefore, from ­random fertilization and independent assortment alone, there may be one of approximately 72 trillion different pos-sible zygotes. Additionally, further variations occur because of crossover, making this number much larger. This helps to explain both the vast differences and also the similarities between siblings.

1. Why is there such a large difference in the amount of gametes that can be produced by a male compared to a female?

2. How can the term “chiasma” be defined?