Lecture 3 - Discontinuous variation 2, Quantative variation 1 Flashcards Preview

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Flashcards in Lecture 3 - Discontinuous variation 2, Quantative variation 1 Deck (20)
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1

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

2

How do you estimate allele frequencies with HW when all genotypes are not known?

Assume Hardy Weinberg equilibrium, Genotype frequencies are p2:2pq:q2

3

How can Hardy Weinberg be useful when genotypes are not known?

-for estimating the liklihood of recessive disorders arising

4

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

5

What is the cloak of heterozygosity?

High frequencies of recessive carriers in a population

6

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

7

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)

8

What other words are used for Quantitative traits?

-complex traits
-multifactoral traits

9

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

10

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

11

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?

12

What are the zygote frequencies for disease alleles on X linked genes?

Females
p2 = XAXA
2pq = XAXa
q2 = XaXa

Males
p = XAY
q = XaY

13

What is the ratio of affected males/affected females for x linked recessive traits?

q/q2 = 1/q

14

Why are x-linked recessive disorders almost exclusively expressed in males?

The ratio of affected males/affected females
q/q2 = 1/q

15

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")

16

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

17

Does the Hardy Weinberg principle still apply when a genetic locus has more than two alleles?

Yes, but more complex

18

What are the allele frequencies for HW when a genetic locus has more than two alleles?

p, q, r

19

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

20

What are the features of the Female and Male offspring when calculating HW if a gene is on a sex chromosome?

-Female offspring: genotypes follow first mendelian law and Hardy Weinberg expectations
-Male offspring: genotype frequncies are equal to allele frequencies in females in the previous generation