Heterozygote
Heterozygotes are zygotes (a fertilized ovum or egg) that, with regard to a specific gene at a particular chromosomal location (locus), contain different forms (alleles) of that gene. Heterozygotes develop into organisms that are heterozygous with regard to the gene in question. As a result of meiosis, heterozygotes produce gametes that are dissimilar with respect to the genes for which they are heterozygous.
In contrast to heterozygotes, homozygotes contain identical alleles of a gene at a particular locus on a chromosome.
In many cases a heterozygote has a greater degree of fitness than a homozygote for a particular gene. This advantage in fitness is termed a heterozygote advantage (also termed heterozygote superiority) and leads to a relatively stable balance (equilibrium) regarding the representation of alleles in a population (i.e., the percentage of alleles in a population). This stability also helps to preserve a variance in alleles of a gene that are present in a population.
Zygotes are diploid cells resulting from the fertilization of a haploid female sex cell (gamete) by a haploid male gamete. After fertilization, the resulting zygote may be homozygous (carry identical alleles for one character or trait) or heterozygous (carrying different alleles) with respect to specific characters or traits. Accordingly, whether a zygote is heterozygous or homozygous depends on the particular gene and set of alleles in question.
With dominant and recessive genes, the dominant allele of a particular gene is designated by capital letter (For example, the capital letter "T" for a gene responsible for tallness in plants). The recessive allele for that particular gene would be assigned the corresponding lowercase letter (e.g., "t"). Organisms that are heterozygous carry different alleles on a pair of homologous chromosomes. Accordingly, a heterozygous organism's genotype would be designated as "Tt" with regard to the tallness gene.
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). 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 heterozygote during embryogenesis and in all the cells that arise by mitotic cell division in the heterozygous adult organism, carry the differing alleles. Cells that are part of the organism's own sex cell line (germ cell line), become different in their genetic character as the different alleles, because they reside on different chromosomes, become segregated during meiosis. Accordingly, the resulting haploid gametes in heterozygous organisms, because they carry only one copy of each allele differ with respect to which allele they carry. In the absence of mutation, an organism with a homozygous "Tt" genotype can contribute either a "T" or "t" allele to its offspring.
Heterozygosity, dependent upon differing alleles also implies that the base sequences in the region of the chromosomes containing those allele also differ.
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