Genotype, Phenotype, and Inheritance Quiz

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Which statement best describes a polygenic trait?

A A phenotype determined by the combined effect of many genes, each contributing a small amount ✓
B A phenotype determined by many genes that each have a major effect
C A phenotype determined by the additive effect of a small number of genes
D A characteristic determined by genes located close together on the same chromosome and inherited as a unit
E A phenotype determined by many genes but inherited only from the mother

The answer is A: A phenotype determined by the combined effect of many genes, each contributing a small amount

Explanation

Why A is correct

See Inheritance patterns. Option A is correct. The word polygenic means many genes: poly is the Greek for many. A polygenic trait is not controlled by one gene but by the combined, usually small, effects of many genes, so the effect of any single gene is modest. Many of these traits are also influenced by lifestyle and environment, such as diet, exercise or pollutants, and traits caused by many contributing factors are called complex or multifactorial disorders. Heart disease, type 2 diabetes and obesity are examples. Because so many genes are involved, these conditions do not follow the clear single-gene patterns (autosomal dominant, recessive and so on) seen with cystic fibrosis or Huntington's disease. They often cluster in families, but family members share environments and habits as well as genes, which makes the genetic risk hard to separate out. This is why it is hard to give a simple figure for the risk of passing on a polygenic condition. The memory aid is poly means many, as in polygon (many sides), so polygenic means many genes.

Polygenic: many genes, each with a small effect.
Environment: lifestyle and surroundings also contribute, so these are multifactorial conditions.
Inheritance pattern: does not follow clear single-gene patterns, as in heart disease and type 2 diabetes.

Why the other options are incorrect

B. A phenotype determined by many genes that each have a major effect: In polygenic inheritance each gene usually contributes a small effect; if each gene has a major effect, the trait is not polygenic in the usual meaning.
C. A phenotype determined by the additive effect of a small number of genes: A polygenic trait depends on many genes, not a small number. A small number of genes with additive effects would be an oligogenic trait, so this does not define polygenic inheritance.
D. A characteristic determined by genes located close together on the same chromosome and inherited as a unit: Genes located close together on one chromosome and inherited as a unit are described as linked. Linkage is about how genes are inherited together, not about many genes each adding a small effect to one trait.
E. A phenotype determined by many genes but inherited only from the mother: Polygenic traits are usually influenced by many nuclear genes inherited from both parents, not the mother alone. Maternal-only inheritance is characteristic of mitochondrial DNA, not polygenic inheritance.

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More questions from this set. Start the quiz to answer them and see each explanation.

1
Which statement best describes the inheritance pattern of an autosomal recessive condition when both parents are unaffected carriers?
2
A woman with a known mitochondrial DNA mutation is planning a family. Which inheritance pattern best describes how this mutation would be passed on to her children?

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