6.3 Alleles: Hetero-, Homo-, Hemizygosity

Mendel’s First Law (segregation of alleles) is especially remarkable because he made his observations and conclusions (1865) without knowing about the relationships between genes, chromosomes, and DNA. We now know the reason why more than one allele of a gene can be present in an individual; most eukaryotic organisms are diploid and have at least two sets of homologous chromosomes. For organisms that are predominantly diploid, such as humans or Mendel’s peas, chromosomes exist as pairs, with one copy inherited from each parent. Diploid cells, therefore, can contain two different alleles of each gene, with one allele part of each member of a pair of homologous chromosomes. If both alleles of a particular gene are the same (indistinguishable), the individual is said to be homozygous at that gene or locus. On the other hand, if the alleles are different (can be distinguished) from each other, the genotype is heterozygous. In cases where there is only one copy of a gene present, for example if there is a deletion of the locus on the homologous chromosome, we use the term hemizygous. Another example is the single X-chromosome in X/Y males, where almost all the loci on that chromosome are hemizygous.

Although a single diploid individual can have at most two different alleles of a particular gene, many more alleles can exist in a population of individuals. In a natural population the most common allelic form is usually called the wildtype allele. However, in many populations there can be multiple variants at the DNA sequence level that are visibly indistinguishable as all exhibit a normal, wild type appearance. There can also be various mutant alleles (in wild populations and in lab strains) that vary from wild type in their appearance, each with a different change at the DNA sequence level. The many different mutations (alleles) at the same locus are called an allelic series for a locus.

Take a look at the video below, Homozygous, Heterozygous, Hemizygous, Haploid, by Nikolay’s Genetics Lessons (2015) on YouTube, which discusses the terms homozygous, heterozygous, hemizygous and haploid.

Reference

Nikolay’s Genetics Lessons. (2015, September 9). Homozygous, heterozygous, hemizygous, haploid (video file). YouTube. https://www.youtube.com/watch?v=tUV90q6pzOU