We have two copies of every gene; each copy is called an allele. If both copies of alleles are the same, it’s homozygous. On the other hand, if they are different, it’s heterozygous. The terms dominant and recessive mean two different inheritance patterns of traits.
Imagine hair colour as a trait. The dominant is black, and the recessive is red. Note this doesn’t mean black hair dominates or anything like that. It only means:
black allele + black allele = black trait
black allele + red allele = black trait
red allele + red allele = red trait
Simply put, you need both the recessive alleles to get a recessive trait, whereas one dominant allele is sufficient to get the dominant trait.
Also, the allele pairs (black, black), (black, red) and (red, red) are all genotypes. On the other hand, the traits – black hair and red hair are two phenotypes. If a genotype is what your genes are, then a phenotype is what you look like!
Before Hardy and Weinberg, people used to get puzzled by the fact that the population did not end up having only the dominant traits. Their principle (developed independently) states that frequencies of alleles and genotypes in a population will remain constant over time in the absence of other evolutionary influences.
