Quick questions on Punnett squares and Mendelian inheritance: HSC Biology Module 5

A Punnett square predicts the possible genotypes and phenotypes of offspring from a given parental cross.

Using the example of pea plant height, where tall (T) is dominant over short (t), a cross between two heterozygous tall plants (Tt) is the textbook monohybrid cross.

Each child is an independent event. The "probability the next child is affected" does not depend on the previous children. $\tfrac{1}{4}$ each time.

T means dominant, t means recessive. Be consistent. Markers often use unfamiliar letters (e.g.

A labelled Punnett square is the fastest way to earn the "suitable working" mark, and it prevents arithmetic slips when you read off the ratio.

Pick fractions OR percentages and stick to one; markers explicitly penalise mixing (e.g. writing both $\tfrac{1}{4}$ and 33%). Each child is independent - never let an earlier outcome change the probability.

In labrador retrievers, black coat (B) is dominant over chocolate coat (b). A breeder crosses two heterozygous black labradors. From a litter of eight puppies, predict the most likely number of chocolate puppies and identify the relevant probability rule.

A heterozygous individual (Aa) for a recessive disease allele has three children with another heterozygote. Calculate the probability that (a) exactly one child is affected, and (b) at least one child is affected. [2+2 marks]

In a dihybrid cross between two AaBb pea plants (round yellow), a student observes the following F2 phenotypes among 320 seeds: 175 round yellow, 65 round green, 60 wrinkled yellow, 20 wrinkled green. (a) State the expected ratio under Mendelian inheritance. (b) Calculate the expected numbers and compare them with the observed values.