2020–2021 BCSC Basic and Clinical Science Course™
2 Fundamentals and Principles of Ophthalmology
Part III: Genetics
Chapter 6: Clinical Genetics
Genes and Chromosomes
Genes on different (nonhomologous) chromosomes may or may not separate together during meiotic cell division. This random process, called independent assortment, states that nonallelic genes assort independently of one another. Because crossing over (exchange of chromosomal material between the members of a pair of homologous chromosomes) can occur in meiosis, 2 nonallelic genes originally on opposite members of the chromosomal pair may end up together on either of the 2 or may remain separated, depending on their original positions and on the sites of genetic interchange. Thus, the gametes of an individual with 2 nonallelic dominant traits, or syntenic traits, located on the same chromosome could produce 4 possible offspring. A child may inherit
both traits, if the separate alleles remain on the same chromosome and the child inherits this chromosome
neither trait, if the genes remain on 1 chromosome but the child inherits the opposite chromosome with neither allele
only 1 of the 2 alleles, if crossing over occurred between the loci, and the child receives the chromosome with that particular allele
This scheme for nonallelic traits depends on the independent assortment of chromosomes in the first division of meiosis. Approximately 50 crossovers (1–3 per chromosome) occur during an average meiotic division.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.