Lecure 35 Flashcards
Give examples where heterozygote is the fittest?
- Protection in malarial region of
- Thalassaemia carrier (Tt)
- Cystic Fibrosis - Cc protects against diarrhoea (Cc)
- PKU - protects against a fungal toxin causing miscarriage (Pp)
- Tay-Sachs - Tt protects against Tuberculosis
-heterozygote the fittest in a number of examples= that is why the mutations are kept in the population
thalassemia= have some irregular blodd cells but offers protection against malaria
cystic fibrosis= prootects against cholera and typhoid= don’t have as many functioning Cl channels so don’t lose as much water from their bowl
How does fitness change allele frequencies?
-can a lot
How can H-W equilibrium restored if autosomal or X linked?
-When a population is not in HW equil and the trait is autosomal it can be restored in one generation of random mating NOT so for an X linked trait
What are the three types of selection?
- stabilizing
- disruptive
- directional
What is stabilizing selection?
- selection against the extremes in the phenotype
- eg. birth weight of babies= selection against the very heavy and the very light=higher risk of death
What is disruptive selection?
-selection against the intermediate phenotypes
-the opposite of stabilizing, keep the extremes and get rid of the middle
-eg:Bird beaks
birds with intermediate sized beaks are at a disadvantage
intermediate= not good at getting small or big seeds, small beak= good for small seeds and vice versa
eg.2- Batesian mimicry= bad tasting butterfly=have to be an exact match to the bad tasting ones, so the extremes have an advantage but the ones in the middle don’t if only half-way
What is directional selection?
-selection for one phenotype from the end of the phenotypic (and presumably genotypic) range Shifts the frequency of one or more phenotypes in a particular direction
-eg moths
pre industrial revolution post industrial revolution 1952 >1850s to 1952
trees with lichen trees with soot etc., light form camouflaged dark form camouflaged allele b selected for allele B selected for f(0.95)
What happens when selection is acting but the heterozygote is the fittest genotype?
-stable or balanced polymorphism polymorphism
How does selection affect allele frequencies?
-• Selection means there are differences in relative fitnesses of the genotypes
• Relative fitness changes according to the environment • Selection may change allele frequencies quickly
What are two types of non random mating?
- Inbreeding-consaguinity-self-fertilisation
- - mating between related individuals (genetic relationship) - Assortative mating=- select individuals like themselves (positive AM)
- select individuals unlike themselves (negativeAM) -( not genetically related)
-Also at population level (often in plant and animal breeding ) - nonrandom mating if one individual contributes disproportionally to the next generation
-non random mating= between genetically related
=assertive mating: positive= seek out the one with similar phenotype
negative: seek one that is the opposite to you
-also when one bull chosen= sperm frozen= too much offspring-contributing disproportionally to the next generation
What does consanguinity and self fertilisation do?
-Consanguinity/inbreeding alter genotypic ratios by increasing homozygosity but doesn’t allele frequencies on its own
-However consanguinity provides the phenotypes (eg., ) on which
can act eg ., in humans CF, PKU, Thalassaemia etc.,
Human consanguinity…..
What is citrullineamia?
-caused by the fact that one bull carrying it had loads of offspring
-• Autosomal recessive • Mutation in gene which affects production arginosuccinate
synthetase
• calves die due to the inability to
• process ammonia
• 3000 deaths per year (incidence of disease in humans is 1/100,000)
• Artificial insemination generates 80% of dairy cattle
-can be detected in sperm now
How does mutation influence allele frequencies?
-• On its own minor change to allele frequencies
• Source of all new alleles
• With selection can be significant
How is migration calculated? and allele frequencies
-may be significant depending on m and difference between x and p
Looking at the formula, delta p = m(x-p) What makes migration significant?
When the difference in allele frequencies in locals (p) and migrants (x) is great.
When migration rate (m) is high.
-Therefore p = f(A) after one generation of migration = 0.4 + 0.02
f(A) =0.42
