PRACTICE EXAM II ANSWER KEY

 Exam II will be made up of questions similar to the examples below and similar to questions you have received on the last 2 homeworks.

Matching

A generation of animals expected to differ from each other genetically (eg. The F₂ generation)    segregating generation

A psychiatric disorder characterized by long-term thought disorders, hallucinations and disorganized speech.                                                                                                                                                                                                                  schizophrenia

Twins who are genetically no more alike than ordinary brothers and sisters.   DZ twins

The process of deciding which type (either MZ or DZ) a pair of twins is. 

zygosity determination

A generation of animals not expected to differ from each other genetically. 

                                                                                    non-segregating generation

An estimate of the additive genetic contribution to variation in a population.

                                                                                                narrow heritability

Twins who are genetically identical.                                           MZ twins

An estimate of the total genetic contribution to variation in a population.

                                                                                                broad heritability

Possible answers

non-segregating generation            narrow heritability                        MZ twins

half-siblings                                   back cross                                  manic depression

zygosity determination                   selection differential                      schizophrenia

broad heritability                           DZ twins                                     dominance deviation

segregating generation                   correlation                                  allele-sharing method

concordance                                identical-by-descent method 

Twin studies

The following table gives the reported correlations for same-sex twins raised together for 4 traits. Assume that the sample sizes are large.

TRAIT                                MZ CORRELATION                   DZ CORRELATION

Body mass index (BMI)                     0.80                                             0.40

General cognitive ability                      0.86                                              0.60

Extraversion (EXT)                           0.50                                              0.13

Dust allergy (ALL)                            0.52                                              0.21

 

Basing your answers on these correlations and assuming the validity of twin study methods:

Which trait is the least influenced by non-shared environments?  general cognitive ability

Justify your answer.    Has the highest MZ correlation, indicating lowest variance due to individual experience (= non-shared e) 

Which trait is most influenced by the shared environment?     general cognitive ability

Justify your answer.   DZ correlation is > ½ the MZ correlation  - indicates the twins are MORE similar than we would expect on genetics only, shared experiences have made them more similar

What is the narrow heritability of BMI?     0.8      or 80%

Show how you arrived at this answer.    Since there is no evidence here for shared environment, the heritability simply = MZ correlation 

For which 2 traits might non-additive genetic influences (eg. dominance) be a factor?

                                                            extraversion and dust allergy

Justify your answer.   DZ correlation is < ½ MZ correlation, indicating DZ twins are LESS similar than we expect on additive gene action only.

 

  

Cross-breeding studies of animal behavior.

Two inbred strains of fruit flies (Drosophila melanogaster), P₁ and P₂, were crossed to give the F₁ generation. Flies from the F₁ were bred to give the F₂ generation. Male flies were placed with a standard "target" female and the amount of time spent in courtship was recorded. The sample sizes, mean courtship time observed for the 4 generations, and the variance for each generation are given below:

GENERATION         SAMPLE SIZE        MEAN COURTSHIP                VARIANCE    

                                                                        TIME (%)                          IN COURTSHIP

 TIME       

            P₁                           100                             74.0                                                344

            P₂                           100                             70.0                                                372

            F₁                                    100                             84.0                                                383

            F₂                           400                             74.9                                               621

Do the generation means give evidence of genetic influences on courtship activity?  yes

Justify your answer.   Inbred strains P1 and P2  have different mean scores reflecting  genetic differences between the strains

Do the generation variances give evidence of genetic influences on courtship activity? yes

Justify your answer.  F2 is a segregating generation and would be expected to have a bigger variance if genes influenced the trait – the F2 here DOES have a larger variance, hence there is evidence genes influence the trait

Given that genes seem to be important in determining courtship time, what do the generation means tell you about the nature of that genetic control?

Since the F1 generation mean does NOT fall midway between the 2 parental means, there appears to be NON-ADDITIVE gene effects   eg dominance for long courtship time.