Inheritance and variation
Use a simple allele model without assuming every characteristic follows it.
Before you begin
- Cells and nuclei
- Probability as a proportion
Separate genes, alleles and traits
A gene is a DNA sequence contributing to a biological function; alleles are versions of a gene. In a simple diploid model, an individual has two alleles for a gene, one inherited from each parent. Genotype describes the alleles, while phenotype describes an observable characteristic. Many characteristics depend on multiple genes and environmental conditions.
Use a one-gene dominant model carefully
In complete dominance, one copy of a dominant allele is sufficient for its associated phenotype. A recessive phenotype requires two recessive alleles. A Punnett square lists possible allele combinations and their probabilities. It predicts chances for each offspring, not a guaranteed distribution in a small family. Dominant does not mean more common or better.
Worked example
For a hypothetical trait with complete dominance, cross Aa with Aa. Find the probability of aa.
Show the worked solution
- Each parent produces gametes with A or a with probability one-half.
- The equally likely combinations are AA, Aa, aA and aa.
- One of the four combinations is aa.
1/4, or 25%.
Try it yourself
Which genotype is heterozygous?
Common mistakes
- Do not apply a single-gene model to every human characteristic.
What can you explain now?
Cross Aa with aa under the same simple model. What proportion of offspring is expected to be aa?
Compare with the explanation
1/2.
The first parent contributes A or a equally; the second always contributes a. Aa and aa each have probability one-half.
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