Predicting Inherited Traits
Explain how alleles are inherited in sexual reproduction and use Punnett squares to predict the probability of offspring inheriting a characteristic, including dominant and recessive alleles
What a learner can do afterwards
- Defines allele, dominant, and recessive in the context of inheritance
- Constructs a Punnett square for a monohybrid cross and calculates probability ratios
- Explains the difference between genotype and phenotype
- Gives an example of a human characteristic controlled by dominant/recessive alleles
The lesson
You already know a gene is a piece of DNA that carries instructions, and that you get one set of chromosomes from each parent. For many traits, a gene comes in two different versions called alleles: one from your mother and one from your father. A dominant allele shows up in the body even with just one copy. A recessive allele only shows up when both copies are the recessive version. Scientists write the dominant allele as a capital letter, like B for brown eyes, and the recessive allele as the same lowercase letter, like b for blue eyes.
The two alleles a person carries for a trait are called their genotype, like Bb or bb. The trait you can actually see, like brown or blue eyes, is called their phenotype. Bb and BB look the same on the outside, brown eyes, even though their genotypes are different, because one B is enough to cover up the b.
Two brown-eyed parents each have the genotype Bb. Cross Bb with Bb and the Punnett square gives four boxes: BB, Bb, Bb, and bb. Three of the four boxes have at least one B, so three out of four children are expected to have brown eyes. One box is bb, so one out of four is expected to have blue eyes: a 3:1 ratio.
A Punnett square shows the chance of each outcome, not a guarantee. A 3:1 ratio means about 3 in 4 children are expected to have the dominant trait, but any single child could still be the 1 in 4.
A Punnett square lines up each parent's alleles to show every possible combination in their child, so you can predict the odds of a trait like eye color.
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