Ch 7: Genetic Disorders and Human Health

What are genetic disorders?

Caused by deleterious mutations in single loci, usually present in very low frequencies

Why are mutant alleles generally driven to extinction by genetic drift?

They are usually arrive in extremely low frequencies, and because of this (especially in smaller populations) are driven out quickly and not given time to accumulate

How does directional selection act on disease alleles?

Acts against mutant alleles, usually keeping them in extremely low frequencies

What will result in a greater frequency of a disease allele, a dominant or recessive allele?

Recessive, because it can be present in heterozygotes without being expressed, as well as being present in homozygotes, though that is rarer.

What is a carrier of a genetic disease?

Someone who is heterozygous for a recessive genetic disease allele, who doesn’t express the symptoms of the disease but can pass down the allele nonetheless

How would the disease allele being dominant affect the frequency of the allele?

Frequency would decrease! Because EVERY individual who has the allele will express it, directional selection can act more strongly against the allele and effectively lower its frequency

True or false: different genes can have different rates of mutations

True!

What are the two main factors affecting the frequency of a disease allele?

Strength of selection against the allele and rate of mutation

What conditions are ideal for a disease allele to occur at a high frequency?

On a large gene, recessive allele, and only mildly deleterious

What conditions would significantly reduce the frequency of a disease allele?

Small gene, dominant, highly deleterious

If mutation rate causing a disease allele increases, how is the frequency of said allele affected?

Frequency increases

How does inbreeding in certain populations affect the frequency of disease alleles?

Due to the limited variation of sexual partners, the population loses genetic diversity, and individuals are more likely to become homozygous for recessive disease alleles that would otherwise be more likely masked if the population size was larger. They are subject to more serious effects of genetic drift because of the small population, AND an increased likelihood of expressive recessive disease alleles.

What is overdominant selection?

When the heterozygous genotype has a higher relative fitness than either homozygous genotype

What is an important example of over dominant selection?

In areas with a very high frequency of malaria, heterozygous individuals for sickle cell anemia have greater fitness than either homozygote, so the sickle allele is higher in frequency than expected

What does over dominant selection result in?

Balanced polymorphisms between allele frequencies

What is a balanced polymorphism

When both alleles are maintained in a population as a result of overdominant selection

Let’s say overdominant selection is at play here, and the relative fitnesses of each genotype are as follows:

• A1A1- 1.0

• A1A2- 1.2

• A2A2- 0.9

Which allele, A1 or A2, is likely to have a higher frequency at the balance point?

A1, because the A1A1 genotype has a higher relative fitness than the A2A2 genotype

Let’s say overdominant selection is at play here, and the relative fitnesses of each genotype are as follows:

• A1A1- 1.0

• A1A2- 1.2

• A2A2- 1.0

Which allele, A1 or A2, is likely to have a higher frequency at the balance point?

Neither! They will be equal in frequency, since A1A1 and A2A2 have the same relative fitness

What is underdominant selection?

When the heterozygous genotype is the lowest in fitness

does underdominant selection result in balanced polymorphisms?

No! It results in an unstable equilibrium and usually in the loss of an allele and fixation in the other, which is dependent on allele frequency

What is NOT a necessary condition for natural selection to occur?

One allele being consistently favored over others- balancing selection (over dominant) is possible, and it is still a form of selection