A chromosome is a collection of genes, and these genes can take several forms known as alleles. Both parents pass on specific alleles to their children and these alleles come together to create the genetic information, or a genotype, that determines what physical characteristics offspring will display. Sometimes these alleles are the same, and these are called homozygous alleles. When the alleles are different, they're called heterozygous alleles.
Dominant and Recessive Alleles
The alleles an offspring inherits can be either dominant or recessive. When an allele is dominant, it means the genetic characteristic it is defining is more likely to occur. For example, having an immunity to poison ivy is a dominant trait in humans. When a gene is recessive, it means it is less likely to occur. For example, color blindness is a recessive trait.
Dimples: A Homozygous Dominant Trait
Let's talk about homozygous dominant genotypes and Luke. Luke's parents are Sally and John. Both Sally and John have dimples, which is a dominant trait. Furthermore, let's assume that both Sally and John have two alleles for this dominant gene, represented as "DD." In this case both Sally and John have a homozygous dominant genotype. Because Luke gets one allele from Sally and one from John, Luke has a 100 percent chance of inheriting a homozygous dominant genotype of "DD," and Luke will be born with dimples.
Curly Hair: A Homozygous Trait
Curly hair is another dominant trait. In this example, assume that both Sally and John have curly hair, but that Sally has a heterozygous genotype of "Cc." Since Luke gets one allele from Sally and one from John, Luke has a 50 percent chance of having a homozygous dominant genotype of "FF" and a 50 percent change of having a heterozygous dominant genotype of "Ff." In this example, Luke will physically display curly, but he will carry a recessive allele for straight hair.
Freckles : A Homozygous Trait
Having freckles is a third example of a dominant trait. In this example, assume that both Sally and John have freckles . Let's also assume that both Sally and John have one dominant allele for freckles and one recessive allele. Sally and John would both have a heterozygous genotype for freckles, which would be represented as "Ff." Does that mean Luke will too? Not necessarily. Because Luke gets one allele from Sally and one from John, Luke has a 25 percent change of have a homozygous dominant genotype of "FF," a heterozygous dominant genotype of "Ff" and a homozygous recessive genotype of "ff."