Grades 6–8 · Organisms
How can inherited gene versions combine?
Inherited gene versions can combine because an offspring receives a contribution from each parent. A version of a gene is called an allele. In the fictional diploid moon plants, each plant carries two alleles for one gene, while each reproductive cell carries one.
In the diagramFictional moon-plant inheritanceAa × Aa: one allele from eachParent BParent A gametesAaAAAaaaAA / Aa:purple podsaa: whiteaa probability: 25%Probabilities do not guarantee family counts.
The model uses two alleles, A and a. An Aa parent can contribute A or a with equal probability. When two Aa parents contribute independently, fertilization restores a pair. The four equally likely combinations are AA, Aa, Aa and aa.
These allele pairs are genotypes. The visible pod color is the phenotype. Under the invented complete-dominance rule, AA and Aa both have purple pods, while aa has white pods. The square therefore gives a one-quarter probability of AA, one-half of Aa and one-quarter of aa, but a three-quarter probability of purple pods.
The square represents possibilities for each offspring, not four guaranteed offspring with one assigned to each cell. Dominant means expressed in the mixed Aa combination under this rule; it does not mean more common, stronger or better. Most real traits require a more complicated explanation than this single-gene model.
About this explanation
Starting knowledge: Use simple fractions and distinguish a probability from a guaranteed outcome.
Sources
- OpenStax Biology 2e — Characteristics and Traits
- OpenStax Biology 2e — Introduction to Mendelian Genetics