Consequences of the Fact That Genes Come in Pairs

Genes exist on long DNA strands that are organized into chromosomes, similar to beads strung along a wire. Each chromosome typically comes in pairs. Within these chromosome pairs, genes can occupy specific locations known as loci (singular: locus). The genes at a given locus may be identical or different. When the genes are identical, the individual is described as homozygous at that locus, pronounced as [home-oh-zai-gus]. Conversely, when the genes are not identical, the individual is termed heterozygous, pronounced as [het-er-oh-zai-gus].

Alleles

Different genes that can occupy the same locus, and hence can potentially pair with one another, are referred to as alleles.

Example of Alleles

For instance, consider the traits for hairlines: the gene for a straight hairline and the gene for a widow's peak in humans are alleles that share the same locus because they can be paired. If an individual is homozygous for the widow's peak allele, they possess two copies of the gene responsible for the enzyme that creates a dip in the hairline at the forehead.

In contrast, if an individual is heterozygous regarding hairline traits, with one copy of the widow's peak allele and one copy of the straight hairline allele, they will exhibit the widow's peak phenotype. The reason for this is that the widow's peak allele is dominant, while the straight hairline allele is recessive.

Dominant vs. Recessive Alleles

A dominant gene (or allele) will manifest its observable effects in either homozygous or heterozygous forms. However, a recessive gene (or allele) will only produce effects in the homozygous condition.

Non-Mendelian Patterns of Inheritance

It is crucial to note that not all allele pairs exhibit simple dominant-recessive relationships. Some alleles may blend their effects, a phenomenon exemplified by certain flower colors. For example, crossing red snapdragons with white snapdragons produces offspring with pink flowers. In this case, neither allele is strictly dominant over the other, leading to a blending of phenotypes rather than a clear dominance.

Mendelian Pattern of Heredity

The foundation of modern genetics can be traced back to the nineteenth century with the work of Gregor Mendel. Mendel's hypothesis centered around the idea that the units of heredity (genes) come in pairs. Furthermore, it was established that one member of each pair could influence the characteristics of an organism, setting the groundwork for the understanding of heredity and genetic inheritance that still informs the study of genetics today.