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In diploid organisms, each chromosome carries paired loci that can exist in different forms called alleles. When an organism carries identical alleles at a locus, it displays a pure or homozygous trait. Conversely, when the two alleles differ, the organism exhibits a hybrid or heterozygous trait.
Alleles are categorized as dominant or recessive. A dominant allele will manifest in the organism’s observable traits (phenotype) regardless of whether it is paired with another dominant or a recessive allele. A recessive allele only shows its effect when paired with another recessive allele. For example, the allele for white eye color is recessive; a person will have white eyes only if both alleles are recessive.
A pure trait, or homozygous condition, involves two identical alleles—either two dominant or two recessive. A hybrid trait, or heterozygous condition, consists of one dominant and one recessive allele. Because the dominant allele dictates the phenotype, a heterozygous organism will display the same observable trait as a homozygous dominant one.
The genotype refers to the genetic makeup (the specific combination of alleles) while the phenotype is the outward expression of that genotype. Understanding the distinction helps clarify why some traits appear in a population even if they are recessive.
During sexual reproduction, each parent contributes one allele from each pair to their offspring. A pure-trait parent will always pass the same allele, whereas a hybrid-trait parent may pass either the dominant or recessive allele. This assortment can result in offspring whose phenotypes differ from those of their parents, such as two heterozygous parents producing a homozygous recessive child.
To visualize the probabilities of offspring genotypes, geneticists use a Punnett square. Arrange one parent’s alleles along the top and the other’s along the side. Use a capital letter for a dominant allele and a lowercase letter for a recessive allele. For instance, crossing two heterozygous individuals (Pp × Pp) yields the following 2×2 grid:
P p
P PP Pp
p Pp pp
This cross produces one homozygous dominant (PP), two heterozygous (Pp), and one homozygous recessive (pp) genotype, illustrating the classic 1:2:1 ratio.
