Heterozygote
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An organism is a heterozygote or heterozygous for a gene or trait if it has different alleles at the gene's locus for each homologous chromosome. Such an organism must be either diploid, have two homologous chromosomes in each cell, or polyploid, having more than two homologous chromosomes. In diploid organisms, the two different alleles were inherited from the organism's two parents. These different alleles contain different genetic codes. One allele may override the other, or they may both be expressed in a trait simultaneously.
Some alleles are neither dominant nor recessive to another allele. In such cases, both alleles affect the phenotype of the heterozygote. Sometimes the result is intermediary, such as when a red carnation is crossed with a white carnation producing heterozygous carnations with pink flowers. In other cases, the phenotype of the heterozygote shows both alleles, such as when a person who is homozygous for the A blood type gene mates with 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).
In a heterozygous individual, each allele produces its own unique protein or enzyme. If the presence of a different (abnormal) enzyme produces an noticable effect in the phenotype of the heterozygote, that 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.
[edit] Heterozygosity
Heterozygosity refers to the state of being a heterozygote. Heterozygosity can also refer to the fraction of loci within an individual that are heterozygous. In population genetics, it is commonly extended to refer to the population as a whole, i.e. the fraction of individuals in a population that are heterozygous for a particular locus.
Typically, the observed(Ho) and expected(He) heterozygosities are compared, defined as follows for diploid individuals in a population:
- Observed
where n is the number of individuals in the population, and ai1,ai2 are the alleles of individual i at the target locus.
- Expected
where m is the number of alleles at the target locus, and fi is the frequency of the ith allele at the target locus.
[edit] Inheritance
To symbolize how a gene is inherited, 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.
