Hemizygous organisms contain a single form (allele) of a gene without a complimentary or corresponding allele. Because the number of alleles present in an organism is referred to as the gene dosage for that organism, hemizygous organisms have the gene for which they are hemizygous in a single dose. The hemizygous state can result from the loss of chromosomal material (chromosomal deletion) or, in the normal condition, as a result of a sex chromosome linked genes, where only one copy of a chromosome may be present. A normal human male, for example, carrying only one X chromosome will contain only one allele of each gene normally found on the X chromosome.
In diploid organisms (organisms with a complete set of homologous autosomal chromosomal pairs), such organisms are hemizygous for a particular gene if there is only one copy of a particular gene that codes for a specific character or trait in a diploid organism. In the hemizygous state, recessive genes are expressed because there is no corresponding allele to dominant or mask the recessive allele. Accordingly, when an organism is hemizygous for a particular trait, the organism will exhibit even a recessive phenotype.
In contrast to the hemizygous state, organisms that are homozygous or heterozygous contain identical or differing pairs of alleles of a gene at a particular locus on a chromosome.
In humans, there are two copies of each autosome (non-sex chromosomes) and two sex chromosomes (X and Y chromosomes). Human males carry one X and one Y chromosome (XY genotype), and females have two X chromosomes (an XX genotype). Genes located on sex chromosomes are termed sex-linked genes.
In human females, the two X chromosomes are homologous (they are similar and contain the same types of genes). The X and Y sex chromosomes carried by males are not homologous. Because normal human females have two copies of each X linked gene they may be homozygous or heterozygous for those genes. In contrast, males carry only one copy of the genes found on their lone X chromosome.
Genes found in the hemizygous state may be hemizygous dominant or hemizygous recessive.
Eye coloration in fruit flies provides an excellent example of the results of the hemizygous state. In Drosophilae, the normal (wild type) fruit fly eye coloration is red. The alleles for eye color are denoted Xw+ for red eyes and Xw for alleles resulting in white colored eyes. Although female fruit flies can be homozygous (Xw+Xw+ or (XwXw) or heterozygous (Xw+Xw) with respect to the gene for eye coloration, the fact that the gene is sex-linked to the X chromosome and does not appear on the Y chromosome dictates that male genotypes be either Xw+Y or XwY. Accordingly, hemizygous dominant fruit fly males have red eyes and hemizygous recessive male fruit flied have white eyes.
Because a male only passes along a Y chromosome to male offspring, with respect to any X linked gene, any mating cross between hemizygous recessive male and a homozygous dominant female results in a hemizygous dominant male offspring.
Sex-linkage and hemizygosity play an important part in the determination of carrier and affected status for many sex-linked genetics diseases, including hemophilia and Duchenne muscular dystrophy.
Hemizygous-like patterns can be found in human females because one of the two X chromosomes in every cell becomes inactive to form a Barr body. Accordingly, with respect to functioning X-linked genes, there is a similar functional gene dose of X-linked genes in both human females and males.
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