What is hemizygosis?

Diploid organisms are those that have two copies of each chromosome, as opposed to haploid organisms that have only one copy. Furthermore, diploid organisms generally reproduce by sexual reproduction through haploid cells, as is the case with humans, whose eggs and sperm are haploid.

Each homologous copy of a chromosome contributes one copy of the same gene, and each copy of the same gene is called an allele. Although they are the same gene, the two alleles do not have to be the same. For example, an individual may have the brown allele on one chromosome and the blue allele on the other for the gene that determines eye color. The final trait in the eyes of the individual will determine which one is expressed, depending on patterns of genetic inheritance and the possible intervention of external factors.

This individual with two different alleles of the same gene would be heterozygous for that gene, whereas if the two alleles are the same, it would be homozygous. The same individual can be heterozygous for one gene and homozygous for another gene.

There are cases where the homologous copies of a chromosome are different (heterochromosomes), not with different alleles of the same gene, but with different genes on each chromosome. These genes with no copy on the homologous chromosome can be neither homozygous nor heterozygous. These cases are called hemizygous (The root “hemi” comes from the Greek and means half).

One of the best examples of hemizygous alleles can be found in organisms that follow a chromosomal sex determination system. Humans, for example, follow the XX/XY System, a sex determination system shared with all other mammals, echinoderms, molluscs, and some arthropods.

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Males will be hemizygous for all alleles on the X and Y chromosomes because they have only one copy of these chromosomes. There would be no hemizygous alleles in females.

More generally, it can be said that hemizygosis occurs in diploid genotypes with genes that are present in only one of the chromosome copies, as is the case described for heterochromosomes, but it can also occur in more cases.

Hemizygosity can affect the inheritance of some genetic traits and diseases. For example, the hemophilia A gene is an X-linked recessive gene. Males, having only one copy of the X chromosome, are hemizygous for that gene, and without an alternate copy will develop the disease. However, being a recessive gene, women develop the disease only if they are homozygous for the defective allele of that gene.

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