By Kevin Beck • Updated August 30, 2022
In genetics, each gene exists in two forms, or alleles, typically one dominant and one recessive. Every individual carries two alleles per gene, one inherited from each parent. When both parents’ genotypes are known, basic probability allows prediction of offspring genotypes and phenotypes.
However, real‑world inheritance is complicated by recombination, the exchange of chromosomal segments during meiosis that reshuffles alleles and affects the expected ratios.
Imagine a hypothetical alien species where purple hair (P) is dominant over yellow hair (p) and round heads (R) dominate flat heads (r). If both parents are heterozygous for both traits (PpRr), the possible offspring genotypes are: PPRR, PPRr, PPrr, PpRR, PpRr, Pprr, ppRR, ppRr, and pprr.
A Punnett square shows a genotype ratio of 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1, translating to a phenotypic ratio of 9 : 3 : 3 : 1. That is, out of 16 children, you would expect 9 purple‑haired, round‑headed; 3 purple‑haired, flat‑headed; 3 yellow‑haired, round‑headed; and 1 yellow‑haired, flat‑headed. These translate to percentages of 56.3 %, 18.8 %, 18.8 %, and 6.2 %, respectively.
These calculations assume independent assortment. Gene linkage—when two genes are physically close on a chromosome—violates that assumption.
When two loci are near each other, they can be inherited together as a block through a process called crossing over. The probability of recombination depends on the physical distance between the loci: the closer they are, the less likely a crossover will separate them.
Think of it like friends standing close together at a party: the closer you are, the more likely you’ll interact. Gene linkage follows the same principle.
To estimate the physical distance between two loci using offspring data—a technique known as gene mapping—researchers compare the observed phenotypic ratios to the expected ratios from independent assortment.
In a typical experiment, a dihybrid parent (PpRr) is crossed with the double recessive (pprr) individual. Suppose 1,000 progeny are counted with the following phenotypes:
Because linked genes produce an excess of parental phenotypes and a deficit of recombinant phenotypes, the recombination frequency is calculated as:
(100 + 98) ÷ (404 + 100 + 98 + 398) = 0.20
Multiplying by 100 gives a recombination frequency of 20 %, which is expressed in centimorgans (cM). A lower recombination frequency indicates tighter linkage; a higher frequency suggests the genes are farther apart.
In summary, genes that show a high recombination frequency are likely separated by a greater chromosomal distance, whereas a low frequency points to close physical proximity.
