Chapter 7: Complex Traits

Polygenic

a trait caused by more than one gene

Complex trait

A phenotype caused by one or more genes and an environmental influence.

Polygenic vs Mendelian traits

Polygenic traits do not exhibit Mendelian ratios because each gene contributes a small degree to the phenotype and influences of individual genes may not be equal.

Are the contributions of individual genes to a polygenic complex trait necessarily equal?

No.

• some have major effect, some have only minor influence.

Polygenic TRAIT

Arises from inputs of several genes

• AKA Quantitative trait

continuously varying

refers to degrees of a phenotype

how bell curve shows distribution of phenotype classes of polygenic traits

In a bell curve, individuals who are heterozygous for many genes are more common and form the peak. Homozygotes are less common and are represented in the two tails.

Explain how fingerprint patterns, height, and skin color are complex traits.

• Fingerprint patterns are the numbers of ridges.

• Height is recorded in inches or centimeters.

• Skin color is assessed by degree of darkness.

Empiric risk

Probability that a trait will recur based upon its incidence in a population, based on observation.

Incidence

The number of new cases of a disease during a certain time in a population.

Prevalence

The number of cases of a disease in a population at a particular time.

Heritability (H)

degree of variation in a trait due to genetics

polygenic risk score

test of whether someone will develop an illness based on SNP differences using single base differences

coefficient of relatedness

The proportion of genes that two people related in a certain way share.

Copy number variants (CNVs)

A DNA sequence present in different numbers of copies (repeats) in different individuals.

concordance

A measure indicating the degree to which a trait is inherited;

• percentage of twin pairs in which both members express a trait.

affected sibling pair study

A gene identification approach that looks for single nucleotide polymorphisms (SNPs) that siblings with a particular trait share, but that siblings who do not have the trait do not share.

homozygosity mapping

Correlation of stretches of homozygous DNA base sequences in the genomes of related individuals to certain traits or disorders.

how leptin, ghrelin, and other proteins affect weight

The protein hormone leptin stimulates the hypothalamus to suppress appetite and increases metabolic rate. Ghrelin, produced in the stomach, increases appetite. Genes involved in inflammation and the immune response may affect body weight.

significance of the difference in heritability for BMI and obesity

The fact that heritability of BMI is lower than that of obesity means that the genetic contribution to weight is greater in people who gain weight easily.

environmental influences on body weight

Environmental influences on weight appear suddenly in a population compared to genetic changes.