Final answer:
If an individual has the same allele on homologous chromosomes, they are homozygous for that gene. This can be either homozygous dominant if both alleles express the dominant trait or homozygous recessive if they express the recessive trait.
Step-by-step explanation:
If an individual has the same allele on homologous chromosomes, this condition is called homozygous. When it comes to patterns of inheritance, the concept of homozygosity is fundamental. Diploid organisms, like humans, have homologous pairs of chromosomes, and for each gene, they can have either two identical or two different alleles.
A homozygous genotype means that the two alleles for a gene on these homologous chromosomes are exactly the same. For example, if both alleles are for blue eyes (bb), then the individual is homozygous recessive for the eye color gene. Conversely, if the alleles are for brown eyes (BB), the individual is homozygous dominant. The terms 'homozygous dominant' and 'homozygous recessive' refer specifically to whether the expressed trait is based on dominant or recessive alleles.
On the other hand, if an organism has one allele for brown eyes (B) and one for blue eyes (b), this condition is referred to as heterozygous, indicating the presence of two different alleles at a gene locus on homologous chromosomes. Finally, the term hemizygous is used to describe the presence of only one allele at a gene locus, as is often the case with genes found on the X chromosome in males, who are XY.