Final answer:
A genotype with two identical dominant alleles, such as YY, is homozygous dominant. Genotypes like yy, with two identical recessive alleles, are homozygous recessive, while a genotype with two different alleles, like Yy, is heterozygous.
Step-by-step explanation:
When an individual has a genotype with two same alleles at a locus (such as YY), it is known as a homozygous condition. Specifically, a genotype like YY, where both alleles are dominant, is described as homozygous dominant. On the other hand, a genotype such as yy, where both alleles are recessive, is termed homozygous recessive. A heterozygous genotype has two different alleles (e.g., Yy).
In Mendelian genetics, the traits of a dominant allele would manifest in both homozygous dominant (YY) and heterozygous (Yy) organisms. These organisms would have the same phenotype despite having different genotypes. However, recessive traits are only expressed phenotypically in homozygous recessive (yy) individuals.
Contrarily, in the scenario where we have codominance, such as with the MN blood groups in humans, both alleles are equally expressed in a heterozygote. For example, individuals with the LMLN genotype would exhibit both M and N antigens on their red blood cells, showcasing a display of both alleles without complete dominance of one over the other.