Back to the full dot-point answer
QLDBiologyQuick questions
Unit 4: Heredity and continuity of life
Quick questions on Codominance, incomplete dominance, multiple alleles, sex linkage and polygenic inheritance (QCE Biology Unit 4)
14short 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 codominance?Show answer
Definition. Both alleles in a heterozygote are fully expressed in the phenotype. Neither masks the other.
What is incomplete dominance?Show answer
Definition. Neither allele is fully dominant. The heterozygote shows an intermediate (blended) phenotype.
What is multiple alleles?Show answer
Definition. A gene has more than two possible alleles circulating in the population, although any one individual still carries only two.
What is sex linkage?Show answer
Definition. A gene located on a sex chromosome. In humans, X-linked genes are inherited differently from autosomal genes because males (XY) have only one X.
What is polygenic inheritance?Show answer
Definition. A trait controlled by two or more genes. Each gene contributes a small additive effect to the phenotype. The result is continuous variation along a spectrum, not discrete categories.
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 codominance versus incomplete dominance?Show answer
Codominance shows both alleles at once (patches of red and white). Incomplete dominance shows an intermediate blend (pink).
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 confusing codominance and incomplete dominance?Show answer
Codominance shows both phenotypes simultaneously (AB blood, roan coat). Incomplete dominance shows a blend (pink flower).
What is writing sex-linked alleles without the X?Show answer
The notation X^h Y, not just h or Y, is required.
What is treating multiple alleles as multiple genes?Show answer
Multiple alleles is many forms of one gene. Polygenic is many genes.
What is saying polygenic traits show no Mendelian inheritance?Show answer
They still obey segregation and independent assortment; their phenotype is just the sum of many genes plus environment.