DLA 10D Hardy Weinberg Exceptions Flashcards
What are the assumptions of the Hardy Weinberg equation?
Large population
No migration
No selection
No new mutation
Alleles not gained or lost
What might occur in reality that might affect the Hardey Weinberg equation?
- Always migration; but populations are typically large enough to not upset the equilibrium
- Some alleles are lost if they are deleterious
- lost alleles are replaced by new mutation
- So equilibrium is maintained
Explain The genetics behind sick cell trait
Sickle cell disease (AR) is a point base mutation to cause a structural variation of hemoglobin
When heterozygous, person is a carrier, and is said to have sickle cell trait (relatively harmless condition)
When homozygous, person is affected with AR SCD, and the person can have severe disease
What does sickle cell cause?
Normally red blood cells are biconcave discs
In SCD, a structural hemoglobin variant causes red blood cells to take a distorted shape(a long rod)
Can lead to sickle cell crisis, red blood cells stick to sides of blood vessels and causes blockage, it’s very painful
How can sickle cell affect the Harvey Weinberg equation ?
Affects selection
People who are carriers of the allele causative of SCD are less likely to die from malaria infection
- people who are homozygous for the wild type allele are more likely to die from malaria infection
- people who are homozygous for the sickle cell disease allele often have severe disease, and are less likely to reproduce
- So there is an advantage to being a heterzygote for sickle cell
In areas that are under the selective pressure of malaria
The frequency of carriers of SCD may be higher than would be predicted by HW equilibrium
The frequency of many other genetic disorders that affect blood and hemoglobin are influenced by malaria … give another example of this…
Glucose 6-phosphate dehydrogenase deficiency (X-linked ) and many other examples
What is genetic drift?
The random fluctuations of allele frequency caused by independent assortment during meiosis
This causes random distribution changes
In small populations, alleles can be lost (extinct) in relatively few generations. When extinction occurs, the other allele is said to become “fixed”
In large populations, allele frequency is maintained close to 50% over many generations
What is the founder effect?
One example is albinism in Navajo American Indians
A single variant formed in a single individual can amplify in an isolated population so that the frequency of the trait is higher than in the general population
How exactly does the founder effect the high albinism frequency in the Navajo population?
Because of geographical isolation in the past
Founder effect can occur by an individual carrying an allele enters into a new (small) population (so this would be founder effect caused by immigration)
What a genetic bottleneck?
When a large population becomes a small population
Example- the current Ashkenazi Jewish populations possibly descended from just a few hundred individuals between 600-1000 years ago. Today there are 10 million ashkenazi Jewish people
Due to this, there are many recessive alleles that are in relatively high frequency in this population, because of the bottleneck
What is consanguinity?
Con- with
Sanguine-blood
Sharing the same blood or marriages within the same family
Increasing the probability that there will be that there will be homozygous areas within the genome
Causes deviation from Harvey Weinberg expectations
What can cause HW to deviate?
- selection (malaria and sickle cell disease)
- consanguinity
- genetic drift
- founder effect
- geographic isolation leading to founder effect
- genetic bottleneck
What are the assumptions of the hardy Weinberg equation?
- large population
- no migration
- no natural selection (evolution)
- no new mutations
- alleles not gained or lost (no genetic drift)
What factors that might cause a deviation from the hardy Weinberg equation?
- small population
- selection
- migration
- genetic drift.
- new mutations
