Lecture 3 - Discontinuous variation 2, Quantative variation 1

Question 1

Can the Hardy Weinberg model still be used in cases where we do not know all the genotypes?

Answer 1

• 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

Question 2

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

Answer 2

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

Question 3

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

Answer 3

-for estimating the liklihood of recessive disorders arising

Question 4

Why might the Hardy weinberg estimate be wrong when calculated not knowing the genotypes?

Answer 4

Other processes acting - doesn’t conform to assumptions

-e.g. selection, non random mating

Question 5

What is the cloak of heterozygosity?

Answer 5

High frequencies of recessive carriers in a population

Question 6

Given the Hardy Weinberg equilibrium, how are genotype and allele frequencies linked? Are there limits for the values of genotype frequenies?

Answer 6

• Rare alleles mostly found in heterozygotes

- the maximum genotype frequency for heterozygots is 0.5

Question 7

What are the three types of quantitative trait?

Answer 7

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)

Question 8

What other words are used for Quantitative traits?

Answer 8

• complex traits

- multifactoral traits

Question 9

Why is it difficult to study the genetic effects of Quantitative traits?

Answer 9

• 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

Question 10

How are mendelian traits a special form of quantitative trait?

Answer 10

• 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

Question 11

What questions arise from quantitative genetics?

Answer 11

• 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?

Question 12

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

Answer 12

Females
p2 = XAXA
2pq = XAXa
q2 = XaXa

Males
p = XAY
q = XaY

Question 13

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

Answer 13

q/q2 = 1/q

Question 14

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

Answer 14

The ratio of affected males/affected females

q/q2 = 1/q

Question 15

How is Hardy Weinberg useful in forensic DNA profiling?

Answer 15

-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”)

Question 16

What was the “phantom of Heilbronn”?

Answer 16

• 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

Question 17

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

Answer 17

Yes, but more complex

Question 18

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

Answer 18

p, q, r

Question 19

How do you calculate HW for the next generation if a gene is on a sex chromosome?

Answer 19

• 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

Question 20

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

Answer 20

• 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