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Heterozygotic cells are diploid or polyploid and have different alleles at a locus (position) on homologous chromosomes. When an organism is referred to as a heterozygote or as being heterozygous for a specific gene, it means that the organism carries a different version of that gene on each of the two corresponding chromosomes. Heterozygosity refers to the state of being a heterozygote, but in population genetics, it commonly refers to the fraction of individuals in a population that are heterozygous for that locus. Heterozygosity can also refer to the fraction of loci within an individual that are heterozygous.
In a heterozygous individual, each allele produces its own unique protein or enzyme. The result is a mixture of the two rather than a combination of the two into one. If the presence of the abnormal enzyme produces an abnormal effect in the heterozygote, the abnormal allele is termed a dominant gene. In other cases, especially when the abnormal allele produces a nonfunctional enzyme, the abnormal enzyme molecules have no obvious effect on the heterozygote. The individual is an asymptomatic carrier of the abnormal allele, which is referred to as a recessive gene.
To symbolize how a gene is inherited (Main article: Mendelian inheritance), the dominant allele is indicated with an upper case character and the recessive with a lower case character. The colour of Mendel's peas are often indicated as PP for the dominant homozygote, which produces a pink flower, and pp for the recessive homozygote, which produces a white flower. When these are crossed, the F1 or first filial generation receives a chromosome with the P gene from the pink-flowered parent and a corresponding chromosome with the p gene from the white-flowered parent. All of the F1 generation are heterozygous, and this is indicated with Pp. All of the F1 plants produce pink flowers.
Some genes are neither dominant nor recessive to another allele. In such cases, both genes affect the heterozygote. Sometimes the result is intermediary: when a red carnation is crossed with a white carnation, they produce heterozygous carnations with pink flowers. If the test is sensitive enough, the heterozygote has the phenotype of both parents: when a person who is homozygous for the A blood type gene marries a person who is homozygous for the B blood type gene, they produce heterozygous children who express both A and B antibodies (see, co-dominance).
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