Homozygote
Homozygotes are zygotes (a fertilized egg or ovum) that, with regard to a specific gene, contain identical forms (alleles) of that gene at a particular location (locus) on a chromosome. Homozygotes develop into organisms that are homozygous with regard to a particular gene. Homozygotes can also exist as polyploids (organisms with more than two of each type of chromosomes) as long as all of the alleles with respect to a particular gene are identical.
In contrast to homozygotes, heterozygotes contain different alleles of a gene at a particular locus on a chromosome.
Zygotes are diploid cells resulting from the fertilization of a haploid female sex cell (gamete) by a haploid male gamete. In humans, the fusion of an ovum with spermatozoa, each of which contains one chromosome from each parental homologous (corresponding) chromosome pair plus one sex chromosome (i.e., the haploid state), restores the diploid state. Accordingly, in the human diploid zygote, there are two copies of each autosome (non-sex chromosomes) and two sex chromosomes (X and Y chromosomes).
Depending on the gene in question, a zygote may be considered a homozygote or heterozygote. A homozygote is homozygous only with regard to a specific gene that codes for a specific protein, or that determines a particular character or trait. Accordingly, a zygote may be homozygous (carry identical alleles for one character or trait) and heterozygous (carrying different alleles) with respect to other characters or traits.
With regard to a particular gene located on a non-sex chromosome (autosome), all of the cells that result from the subsequent divisions of a homozygote during embryogenesis and in all the cells that arise by mitotic cell division in the homozygous adult organism, except those cells that are part of the organism's own sex cell line (germ cell line), contain a pair of homologous (corresponding) autosomal chromosomes containing identical alleles of the gene.
When using traditional dominant and recessive nomenclature to designate alleles of a gene, a dominant allele of a particular gene is designated by capital letter (For example, the capital letter "A"). The recessive allele of a particular gene is assigned the corresponding lowercase letter (e.g., "a"). Because individuals who are homozygous carry identical alleles on both chromosomes of a pair of homologous chromosomes, the individual's genotype concerning to gene "A" would be designated as either "AA" or "aa," with regard to the types of alleles carried by the individual.
Homozygosity (the state of being homozygous) means that that the products of the instructions contained in the homozygous genes are similar enough to produce identical visible expression (identical phenotypic expression). At the genetic level there are, however, multiple codes that represent multiple sequences of bases in the nucleic acids to convey essentially identical instructions for the construction of proteins. Because multiple instructions can result in the formation of the same product, the genetic code is a considered a degenerate code. As a result, although an individual may be homozygous for a particular gene, this does not necessarily mean that the base sequence found in those genes is identical.
Because homozygotes contain identical alleles on their chromosome, in the absence of mutation they can only produce gametes (sex cells) that contain those same alleles. For example, an organism with a homozygous "AA" genotype can only contribute an "A" allele to its offspring. Such organisms, when mated with other homozygotes, breed true with regard to a particular trait.
Not all genes exist as dominant and recessive alleles. Often, there are varying and sometimes complex mechanisms of masking and expression that give rise to observable traits and characters. Many characters and traits are due to the interactions of many genes and are, therefore, considered mutagenic characters or traits. Regardless, with regard to dominant and recessive genes, homozygotes carrying dominant alleles usually express the dominant trait or character. In contrast, because there is no dominant allele to mask the expression of a recessive allele, homozygotes that carry recessive alleles will result in an organism that usually expresses the recessive trait or character. For example, in humans, the earlobes may be separated or attached to the scalp. The majority of people have free hanging or detached earlobes. Because the attached condition is determined by a recessive gene (designated "l"), only those zygotes that carry two copies of the recessive allele (i.e., have a "ll" genotype) will develop into individuals with ear lobes attached directly to the side of the head. In contrast, the ability to roll the tongue is caused by the presence of a dominant gene (designated "R") and therefore individuals who are homozygous (RR) and heterozygous (Rr) are able to roll their tongues.
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