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NSWBiologyQuick questions
Module 5: Heredity
Quick questions on Punnett squares and Mendelian inheritance: HSC Biology Module 5
8short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is setting up a Punnett square?Show answer
A Punnett square predicts the possible genotypes and phenotypes of offspring from a given parental cross.
What is standard monohybrid cross?Show answer
<!-- Diagram: Punnett square Aa x Aa | reviewed 2026-05-21 --> <svg class="fig" viewBox="0 0 320 280" role="img" aria-labelledby="punaa-t punaa-d"> <title id="punaa-t">Punnett square for Aa crossed with Aa</title> <desc id="punaa-d">A two by two Punnett square. Parent one alleles A and a across the top. Parent two alleles A and a down the left side. Offspring genotypes in the four cells are A A, A a, A a, and a a, giving a phenotypic ratio of three dominant to one recessive.</desc> <g font-size="16" font-weight="700" text-anchor="middle" class="var"> <text x="140" y="50">A</text> <text x="220" y="50">a</text> <text x="60" y="120">A</text> <text x="60" y="200">a</text> </g> <g fill="var(--paper)" stroke="var(--ink)" stroke-width="1.4"> <rect x="100" y="70" width="80" height="60"/> <rect x="180" y="70" width="80" height="60"/> <rect x="100" y="130" width="80" height="60"/> <rect x="180" y="130" width="80" height="60"/> </g> <g font-size="15" font-weight="700" text-anchor="middle" class="var"> <text x="140" y="108">AA</text> <text x="220" y="108">Aa</text> <text x="140" y="168">Aa</text> <text x="220" y="168">aa</text> </g> <text x="160" y="226" text-anchor="middle" font-size="12">Genotypic ratio 1 : 2 : 1 (AA : Aa : aa)</text> <text x="160" y="246" text-anchor="middle" font-size="12">Phenotypic ratio 3 : 1 (dominant : recessive)</text> </svg>
What are wrong gametes?Show answer
Each parent contributes only ONE allele per gene to each gamete. A heterozygous parent (Aa) produces TWO types of gametes (A and a), not four (AA, Aa, Aa, aa).
What is independent probability for separate children?Show answer
Each child is an independent event. The "probability the next child is affected" does not depend on the previous children. 1/4 each time.
What are capitalisation matters?Show answer
A means dominant, a means recessive. Be consistent. Markers often use unfamiliar letters (e.g.
What is q1?Show answer
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.
What is q2?Show answer
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]
What is q3?Show answer
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.