Flashcards in Lecture 3 - Discontinuous variation 2, Quantative variation 1 Deck (20)
Can the Hardy Weinberg model still be used in cases where we do not know all the genotypes?
-heterozygotes and homozygotes cannot be distinguished with the dominant phenotype
-but can be useful to estimate allele frequences
-have to assume hardy-weinberg eqilibrium
-not possible to compared observed and expected
How do you estimate allele frequencies with HW when all genotypes are not known?
Assume Hardy Weinberg equilibrium, Genotype frequencies are p2:2pq:q2
How can Hardy Weinberg be useful when genotypes are not known?
-for estimating the liklihood of recessive disorders arising
Why might the Hardy weinberg estimate be wrong when calculated not knowing the genotypes?
Other processes acting - doesn't conform to assumptions
-e.g. selection, non random mating
What is the cloak of heterozygosity?
High frequencies of recessive carriers in a population
Given the Hardy Weinberg equilibrium, how are genotype and allele frequencies linked? Are there limits for the values of genotype frequenies?
-Rare alleles mostly found in heterozygotes
-the maximum genotype frequency for heterozygots is 0.5
What are the three types of quantitative trait?
Continuous traits (e.g. height, weight, milk yield...)
Categorical traits (e.g. Number of petals)
Dichotomous/threshold traits - present or absent (Underlying risk, muliple genetic and evironmental factors, schizophrenia, most common form of diabetes)
What other words are used for Quantitative traits?
Why is it difficult to study the genetic effects of Quantitative traits?
-usually controlled by several loci, each with small effects -often continuous phenotypes
-the environment often has a substantial influence
-the substitution of one allele for another is often undetectable
-different genotypes can produce the same phenotype
How are mendelian traits a special form of quantitative trait?
-Mendelian traits are only influenced by a single genetic locus
-whereas quantitative traits often have several Mendelian loci, and different phenotypes can produce the same phenotype
What questions arise from quantitative genetics?
-how much of the phenotypic variation is explained by the genotype and how much by the environment?
-can we predict offspring by parental phenotypes?
-how is the response to selection affected by the heritability of the trait?
What are the zygote frequencies for disease alleles on X linked genes?
p2 = XAXA
2pq = XAXa
q2 = XaXa
p = XAY
q = XaY
What is the ratio of affected males/affected females for x linked recessive traits?
q/q2 = 1/q
Why are x-linked recessive disorders almost exclusively expressed in males?
The ratio of affected males/affected females
q/q2 = 1/q
How is Hardy Weinberg useful in forensic DNA profiling?
-to predict how many people are likely to have a particular multilocus genotype (e.g. identify murderer)
-probability of multiple loci can be combined
However: DNA can be present without a person ever having been at the scene ("phantom of Heilbronn")
What was the "phantom of Heilbronn"?
-a woman who's DNA profile was found at muliple crime scenes
-the crimes ranged from murder to stealing a few bottles of coke
-commited over 16 years
-apart from the DNA profile, no other evidence that this person existed
-actually factory contaminated material
Does the Hardy Weinberg principle still apply when a genetic locus has more than two alleles?
Yes, but more complex
What are the allele frequencies for HW when a genetic locus has more than two alleles?
p, q, r
How do you calculate HW for the next generation if a gene is on a sex chromosome?
-assume the allele frequency for the sex linked gene is the same for males and females
-calulate female offspring and male offspring
-generally applies to haplo-diploid systems
-possible to apply when males and females have different allele frequencies