6.1 Sickle Cell Alleles

Q1

SICKLE-CELL DISEASE AND MALARIA

What is a unit of heredity info that codes for a gene product

eg. protein

gene

what are alternate versions of a gene called?

alleles

Ex of a diploid organism?

Human, have 2 copies of each gene, (1 from each parent)

What is an organism who’s 2 copies are the same allele

homozygous for that gene

An individual with 2 diff. alleles is?

heterozygous

What are certain genotypes associated with?

higher or lower fitness, depending on its environment

Define fitness

refers to how successfully an individual passes their genes to future generations

Heterozygous advantage occurs when…

the heterozygous genotype for a particular gene confers fitness than do homozygous genotypes

What 3 conditions have to to place for evolution by natural selection to occur?

1. There must be variation within the population

2. The mechanism that creates the variation must be heritable

3. the variable must lead to differences in fitness

What is an inherited disease in which people produce malformed red blood cells that are sickle-shaped instead of disc-shaped (despite high mortality will continue to persist in areas)

sickle cell disease, occurs when a person is homozygous for the sickle cell allele (HbS) of the hemoglobin gene

A deadly mosquito-borne disease, in which a human’s red blood cells are invaded by a protozoan (microorganism) parasite

malaria

what is the HbA/HbS heterotype advantage?

increased fitness where malaria is common, protection, reduces illness

Q2

THE INTERPLAY OF EVOLUTIONARY FORCES

through what are new alleles created?

through random mutations of DNA

the persistence of a new allele in a population depends on what?

the relationship with its fitness advantage upon the environment

What is a mechanism of evolution in which random events affect the frequency of alleles in a population?

genetic drift

What will happen if relative fitness of a genotype is high?

natural selection affects the allele frequency rather than the genetic drift (will only dominate if the selection is strong)

What effect will small populations have on a allele?

its frequency becomes “fixed”-disappearing/ only one in population

eg. cheetahs

What effect will large populations have on a allele?

the allele frequencies remain stable over time

Q3

MAKING PREDICTIONS: HARDY AND WEINBERG

What is happening in the allele frequencies in Hardy-Weinberg equilibrium?

they are remaining constant-not evolving

What 5 conditions have to be met in order for this equilibrium to occur?

1. No natural selection, all genotypes are equally adaptive

2. No genetic drift, randomness is not a factor

3. No mutation

4. No migration

5. Random mating, offspring genotypes are random combinations of parental alleles

What is the H-W equation used for?

to predict allele and genotype frequencies for a gene with 2 alleles in a population.

What do the symbols p and q represent?

the frequency of the 2 diff. alleles, so p+q=1

What is the H-W equation?

p²+ 2pq+q²=1

p²

the frequency of individuals that are homozygous for the first allele

2pq

the frequency for the heterzygous individuals

q²

the frequency of individuals that are heterozygous for the other allele

What are simplified systems describing expectations w/o influencing factors?

null models, allow one to generate + test alternative hypothesis

How can scientists test if evolution is occuring in a particular gene?

They can measure the genotype frequencies in a population and compare the frequencies to those predicted in H-W equation

When can a null hypothesis be rejected?

if the diff. between observed and expected frequencies is significant!

What does the H-W null model suggestif malaria were to disappear,…?

then the heterozygote advantage associated witht he sickle-cell allele, HbS would disappear. Also, if sickle-cell disease would disappear, were wouldn’t be a strong election against HbS.