Fig. 2.5. Process of meiosis

Fig. 2.6. Process of meiosis (continuation)

two sex cells unite, the new organism has the normal number of chromosomes for that species and the especial individual make-up.

Significance of meiosis: prior to the mixing of one individual’s genotype with that of another at fertilization, meiosis provides the opportunity for new combinations of the existing alleles of genes to arise as follows.

Crossing over takes place when breaks occur in chromatids at the beginning of meiosis, when the chromosomes are paired, and the broken end of each chromatid joins with the chromatid of an homologous chromosome. By this means, alleles of linked genes can become separated. This can result in the formation of new combinations of alleles and give four genetically different chromatids each of which ends up in a different gamete. This may lead to the formation of new phenotypes in the next generation.Alleles are exchanged between chromosomes.

Sex-Linked Traits

The human Y chromosome is unusual in that it carries very few genes. The X chromosome, however, carries many genes. A trait controlled by genes on the sex chromosomes is called a sex-linked trait.

One sex-linked gene determines red-green color-blindness and has trouble telling red colors from green colors.

Females are seldom color-blind. Each of their two X chromosomes has one gene that influences the trait of color vision. The gene for normal color vision is dominant over the gene for color-blindness.

More males than females tend to be color-blind. The Y chromosome has no genes for color vision. If the male’s only X chromosome has the gene for color-blindness, then here will be color-blind. This is because his Y chromosome has no color vision to mask the color-blind gene.

Another sex-linked gene causes the disease hemophilia. A person with hemophilia has blood that does not clot. Hemophilia is caused by a recessive gene on the X chromosome, so it occurs generally in males. Females, however, may be carriers. These females have one normal gene that dominates the recessive gene for the trait. Carriers do not have hemophilia, but they can pass the recessive gene to their offspring.