Epigenetics

• Epistatic interactions are often the result of two or more genes each affecting different steps of a biochemical pathway

• starting substrate → intermediate substrate → final product

• The A gene codes for a protein it catalyzes the first step. The B gene codes for a protein that catalyzes the second step. The recessive alleles produce non-functional proteins

• Most traits are not due to the expression of a single gene. Many are due the expression of many genes that each have small effects on the phenotype. Such traits are said to have polygenic inheritance

• The simplest polygenic system is a system where each gene has additive allelic effects:

• AABBCC - ++++++• AaBbCc - +++• Aabbcc - +• aabbcc -• Polygenic traits commonly show continuous variation

• Human skin colour variation is due to a polygenic system with at least 5 and as many as 8 genes that contribute to skin colour

• A single gene system would have 3 genotypes and 3 phenotypes

• AA - ++• Aa - +• aa -• A two gene system would have 8 genotypes

and 4 phenotypes• AABB - ++++• AaBB, AABb - +++• AaBb, AAbb, aaBB - ++• Aabb, aaBb - +• aabb -

• A three gene system would have 27 genotypes and 7 phenotypes

• AABBCC - ++++++• AABBCc, AABbCC, AaBBCC - +++++, etc• The human system with at least 5 genes

would have at 243 genotypes and at least 11 phenotypic grades of skin colour

• Most continuously varying traits - height, weight, shoe size, IQ, eyesight, athletic ability, etc. - are controlled by multiple genes each having small effects on the phenotype

• The environment in which an organism lives also influences its phenotype. Poor diet, regular physical exercise, exposure to sunlight, regular intellectual exercise, exposure to disease or toxins, all have phenotypic effects

• A measure of the influence of genes and the environment on the phenotype is heritability

• Heritability - the proportion of phenotypic variation in a population that is due to underlying genetic variation

• For example - skin colour differences can be due to genetic differences and/or due to difference in exposure to sunlight in

the recent past. If the differences among individuals in a population was due primarily to genetic differences then we could say the heritability of skin colour is high. If the differences in skin colour was primarily due to differences in exposure to sunlight then we could say the heritability of skin colour is low

• Heritability ranges in value from 0 to 1• H = 0 means all of the phenotypic

variation is due to environmental differences - e.g. language spoken

• H = 1 means all of the phenotypic variation is due to genetic differences - e.g. tongue rolling

• Many traits are due to a combination of genetic and environmental influences

• Normally heritability is measured as a statistic - a ratio of measures of variation

• Heritability can also be measured by raising a large number of pairs of identical twins in different environments

• If the twins, when raised apart, always exhibit the same trait, then the heritability is high (near 1)

• If the twins almost always exhibit different traits, then the heritability is low (near 0)

• Measuring heritability can be tricky because the measure always depends on the environment in which the measurement was made

• For example, if all twins were raised in Norway then we might conclude that speaking Norwegian is a highly heritable genetic trait

• If all twins are raised in different countries with different languages then we would conclude the language has a low heritability

• Many human behavioural traits have been shown to have both a genetic and environmental component: introversion/extroversion, the tendency to smoke, the tendency to curse, sexual orientation, marital fidelity

• Heritability estimates:• Height: High or Low?• Weight: High or Low?• Math aptitude: High or Low?• Science aptitude: High or Low?

Hardy–Weinberg equilibrium

Godfrey Hardy(1877-1947)

Wilhelm Weinberg(1862-1937)

• The Hardy–Weinberg principle (also known as the Hardy–Weinberg equilibrium, model, theorem, or law) states that Allelic and genotypic frequencies remain constant in the absence of evolutionary influences. These influences include non-random mating, mutation, selection, genetic drift, gene flow and meiotic dive

http://www.youtube.com/watch?v=JsuJlewaPX0

Hardy-Weinberg equilibrium equation

• p² + 2pq + q² = 1• p = AA + ½Aa• q = aa + ½Aa• p + q = 1 p = 1 – q THIS means!!!

• p² + 2pq + q² = 1• p² is the predicted frequency of homozygous

dominant (AA) • 2pq is the predicted frequency of heterozygous

(Aa)• q² is the predicted frequency of homozygous

recessive (aa)

Significance of the Hardy-Weinberg Equation

• 20th century, geneticists were able to use Punnett squares to predict the probability of offspring genotypes –Mendelian rules of dominance and

recessiveness

• Hardy-Weinberg equation essentially allowed geneticists to do the same thing for entire populations

Albinism• Albinism is a rare genetically inherited trait that is

only expressed in the phenotype of homozygous recessive individuals (aa). The most characteristic symptom is a marked deficiency in the skin and hair pigment melanin. This condition can occur among any human group as well as among other animal species. The average human frequency of albinism in Africa is only about 1 in 20,000.

The frequency of homozygous recessive individuals (aa) in a population is q². Therefore: q² = 1/20,000 = 0.00005 and q = 0.007Use p + q = 1 and p² + 2pq + q² = 1 and calculate the predicted Genotypes in the population

p² =predicted frequencyof homozygousdominant individuals

= .986 = 98.6%

2pq =predicted frequencyof heterozygousindividuals

= .014 = 1.4%

q² =predicted frequencyof homozygousrecessive individuals (the albinos)

= .00005 = .005%

Humans inherit either the M or the N antigen which is determined by two different alleles at the same gene locus. If we let the frequency of allele M=p and the frequency of the other allele N=q, then the next generation's genotypes will occur as follows:Frequency of MM genotype = p2 Frequency of MN genotype = 2pq Frequency of NN genotype = q2

We can take a sample of the population and count the number of people with each genotype. For example, a sample of 5000 from Forensic Town, USA, has

990 of type NN, that is 990/5000 or 19.8%

Which statement describes the swallow with the greatest evolutionary fitness?

a. A swallow that lives to be 3 years old and has four offspring, two of which survive to reproduce themselves

b. A swallow that lives to be 5 years old and has five offspring, one of which survives to reproduce

c. A swallow that lives to be 2 years old and has four offspring, all of which survive to reproduce themselves

d. A swallow that lives to be 7 years old and has three offspring, all of which survive to reproduce

e. All are equally fit

Questions

Genetic analysis of a large population of mink inhabiting an island in Michigan revealed a surprising large number of loci with a fixed allele—that is, loci without any genetic variation in the population. Which of the following is the most probable explanation for this genetic homogeneity?

a. The population exhibited nonrandom mating, producing homozygous genotypes.

b. The gene pool of this population never experienced mutation or gene flow.

c. A very small number of mink may have colonized this island, and this founder effect and subsequent genetic drift fixed many alleles.

d. The colonizing population may have had much more genetic diversity, but in the last year or two, genetic drift fixed these alleles by chance.