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Unit 4: Heredity and continuity of life
Quick questions on Codominance, incomplete dominance, multiple alleles, sex linkage and polygenic inheritance (QCE Biology Unit 4)
7short 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 definition?Show answer
Both alleles in a heterozygote are fully expressed in the phenotype. Neither masks the other.
What is cross ratio?Show answer
Heterozygote x heterozygote gives a 1:2:1 phenotype ratio (because all three genotypes are phenotypically distinguishable), not 3:1.
What is notation?Show answer
Show the allele on the chromosome, for example X^H (normal) or X^h (haemophilia). Males are X^H Y or X^h Y. Females are X^H X^H, X^H X^h or X^h X^h.
What is pattern?Show answer
Polygenic traits typically form a bell-shaped (normal) distribution in the population, with most individuals near the mean and fewer at the extremes. Adding the effect of environmental factors (diet for height, sun exposure for skin colour) further smooths the distribution.
What is q1?Show answer
Distinguish between codominance and incomplete dominance, giving one example of each. [3 marks]
What is q2?Show answer
A cross between blood group A father (homozygous) and blood group B mother (homozygous) produces a child. Predict the offspring phenotype and genotype. [3 marks]
What is q3?Show answer
Refer to X-linked recessive colourblindness in humans. (a) Explain why colourblindness affects males more often. (b) Construct a Punnett square for a carrier mother and unaffected father.
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