Extra Study Section: Common Mistakes and More Practice

Common mistakes to avoid

1. Confusing the genotype with the phenotype. "Brown eyes" is a phenotype; "Bb" is a genotype. Always say which one you are describing.
2. Thinking a dominant allele is "stronger" or more common. Dominance only describes which allele shows in a heterozygote. A dominant allele can be rare in a population.
3. Writing gametes with two alleles. A gamete carries only one allele of each gene. For Yy the gametes are Y and y, never "Yy". For RrYy each gamete carries one allele of each gene: RY, Ry, rY or ry.
4. Forgetting that ratios are probabilities. A 3 : 1 ratio does not mean that a family of four children must have exactly three dominant and one recessive child. Each child has a 75% chance of showing the dominant trait, independent of the previous child.
5. Mixing up carriers and patients. A carrier (Nn) is healthy but can pass on the disease allele; a patient (nn) shows the disease.

Probability of independent events

Each birth is an independent event. If two carrier parents have a 25% chance of an affected child, then after a first affected child the chance of the second child being affected is still 25%. To find the chance of two particular outcomes in two separate children, multiply: chance of two affected children in a row = 1/4 × 1/4 = 1/16.

Worked example 9: a Punnett square from a ratio

• A pea plant with round seeds is crossed with a plant with wrinkled seeds.
• In the offspring, 48 plants have round seeds and 52 have wrinkled seeds (close to 1 : 1).
• What are the genotypes of the parents?
• A 1 : 1 ratio appears in a cross between a heterozygote and a homozygous recessive: Rr × rr.
• The wrinkled offspring (rr) prove that the round parent had an r allele, so it was Rr.

Worked example 10: expected numbers

If two hybrid tall pea plants (Tt × Tt) produce 400 seeds that all grow, about how many plants are expected to be short? Short = tt = 1/4 of 400 = 100 plants; tall = 300 (of which about 100 are TT and 200 are Tt). Real counts usually differ a little from the expected numbers because fertilisation is a matter of chance, but with large numbers the results come close to the predicted ratio. That is why Mendel's use of hundreds and thousands of plants gave clear results.

Linking the ideas

• Genes (DNA) are the cause; the inherited traits are the effect.
• Variation within a population (different alleles) is the raw material on which natural selection works. Without variation there is nothing to select.
• Artificial selection uses the same variation, but the human breeder decides which individuals reproduce.
• Genetic diseases show what can happen when a harmful recessive allele becomes homozygous; communities reduce this through awareness, screening and counselling, which also respects human differences and protects the health of future generations.