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Recessive allele: an allele that only has an effect on the phenotype when present in the homozygous state. Codominant alleles: pairs of alleles that both affect the phenotype when present in a heterozygote. Locus: the particular position on homologous chromosomes of a gene.Homozygous: having two identical alleles of a gene. Heterozygous: having two different alleles of a gene. Carrier: an individual that has one copy of a recessive allele that causes a genetic disease in individuals that are homozygous for this allele.ocus: the particular position on homologous chromosomes of a gene. Homozygous: having two identical alleles of a gene. Heterozygous: having two different alleles of a gene. Carrier: an individual that has one copy of a recessive allele that causes a genetic disease in individuals that are homozygous for this allele.
Alleles can be dominant or recessive. A dominant allele is always expressed while the recessive allele is expressed when only dominant allele is absent. If two alleles present in the organism that are identical, then it is called â Homozygous â, while if two alleles are different, and then called â Heterozygous â.ecessive Allele. From the two alleles, the weaker one is known as recessive allele. This recessive allele is expressed only in the case of homozygous. In the case of heterozygous, the dominant allele masks the phenotype of the recessive allele.