Background Information: There are two primary steps to alcohol metabolism. First, alcohol is converted to acetaldehyde via the enzyme alcohol dehydrogenase (ADH)

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Background Information:

There are two primary steps to alcohol metabolism. First, alcohol is converted to acetaldehyde

via the enzyme alcohol dehydrogenase (ADH). Second, acetaldehyde is converted to acetate via

the enzyme aldehyde dehydrogenase (ALDH). The ADH gene, located on chromosome 4, codes

for the ADH enzyme. The ALDH gene, located on chromosome 12, codes for the ALDH

enzyme. Acetaldehyde is a toxic substance whose accumulation leads to an “alcohol flush

reaction,” which includes facial flushing, nausea, and rapid heartbeat.

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The ALDH gene has two primary alleles. The dominant D allele results in production of

nonfunctional ALDH enzyme. This results in a build-up of acetaldehyde and a flush reaction

when an individual consumes alcohol. The recessive d allele results in production of a functional

ALDH enzyme and lack of a flush reaction. First, we are interested in the presence or absence of

the flush reaction phenotype.

1. Jasmine and her mother both get a flush reaction when they drink alcohol. Jasmine’s father

does not. Jasmine’s husband, Jake, does not get a flush reaction, but his mother and father

both do.

a. Draw a pedigree of Jasmine, Jake, and their parents. Label each individual with

his/her genotype.

b. What genotypic ratio is expected among Jasmine and Jake’s kids? Show your work

with a Punnett square.

c. What proportion of Jasmine and Jake’s kids are expected to exhibit the flush

reaction? Explain.

2. The ADH gene also has two primary alleles. One allele, A, codes for a form of the ADH

enzyme that catalyzes the conversion of alcohol into acetaldehyde approximately 80 times

faster than the form encoded by the a allele. AA individuals exhibit fast conversion, Aa

individuals exhibit medium conversion, and aa individuals exhibit slow conversion of alcohol

into acetaldehyde. In this question, we are interested in “conversion rate” as a trait. The

possible phenotypes are fast, medium, and slow.

a. Does the ADH gene exhibit dominance, incomplete dominance, or codominance with

respect to the alcohol conversion rate? Explain your answer.

b. Jasmine and Jake both exhibit a medium rate of conversion of alcohol into

acetaldehyde. What proportion of their kids are expected to exhibit fast conversion,

medium conversion, or slow conversion of alcohol? Show your work with a Punnett

square and explain.

3. Now consider both the ADH and the ALDH genes at the same time.

a. Write out Jasmine’s genotype, including both the ADH and the ALDH genes (e.g,

AADd, or whatever the correct genotype is). Do the same for Jake.

b. Draw a single Punnett square showing the expected genotypic ratio of Jasmine and

Jake’s kids, considering both genes together.

c. Write out the expected phenotypic ratio of Jasmine’s and Jake’s kids, considering

both traits together. For example, how many will have a flush reaction and fast rate

of conversion? How many will have no flush reaction and a medium rate of

conversion? Etc.

4. The figures below show possible arrangements of chromosomes during meiosis in the germ

cells of Becky, who is a double heterozygote for the ADH and ALDH genes (AaDd). Becky’s

father was AADD and her mother was aadd.

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a. Using Figure 1 below, draw how the alleles for these genes would be arranged on the

chromosomes, given that they segregate independently. Note that the figure contains

only 2 of the 23 kinds of chromosomes due to space constraints.

b. Using Figure 2 below, draw how the alleles for these genes would be arranged on the

chromosomes if they were tightly linked (i.e., assume there has been no crossing over

between them).

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c. Becky married Juan, who also had one AADD and one aadd parent. Draw two

Punnett squares representing the expected proportions of genotypes among Becky

and Juan’s children. Use the first square to represent what would happen with

independent assortment and the second square to represent what would happen if the

genes are tightly linked (i.e., there is no crossing over between them).

d. What are the expected proportions of offspring phenotypes for each case?

e. How does independent assortment affect the genetic and phenotypic diversity of the

offspring, compared to dependent assortment? Explain.

5. Both the ADH and ALDH genes contribute to an individual’s probability of developing

alcohol dependence, a disease that is relatively common and often devastating. Inability to

convert acetaldehyde to acetate results in a flush reaction that makes alcohol consumption

extremely unpleasant. Meanwhile, fast conversion of alcohol to acetaldehyde quickly

removes the pleasurable effects of alcohol.

People with a flush reaction essentially never develop alcohol dependence, no matter how

fast or slow they convert alcohol into acetaldehyde. Consuming alcohol is just too

unpleasant. Among those without a flush reaction, people with the AA genotype (fast

converters) are least likely to develop alcohol dependence. People with the Aa genotype

(medium converters) are roughly 5 times as likely and people with the aa genotype (slow

converters) are roughly 10 times as likely to develop alcohol dependence as people with the

AA genotype. Many other factors also influence whether or not an individual develops

alcohol dependence, including that person’s culture, education, religion, health care, family

environment, and social support network.

a. Given the information presented in this problem set so far, which extensions to

Mendelian genetics (listed below) are involved in the trait of alcohol dependence?

Give a brief explanation for every extension so that I can evaluate your

reasoning for choosing it or not.

1. Linkage

2. Incomplete dominance

3. Codominance

4. More than 2 alleles per gene

5. Polygenic Inheritance

6. Epistasis

7. Pleiotropy

8. Environmental Influence

9. Sex linkage

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b. So far, I have given you only part of the story of alcohol dependence; the whole

picture is more complex. There are actually many different ADH genes located very

close together on chromosome 4, many different alleles for both the ADH and ALDH

genes, and many additional genes involved in this trait, including at least one on the

X chromosome. Many of these genes influence other traits, such as anxiety or ability

to detoxify other substances, in addition to the tendency to develop alcohol

dependence. Researchers are actively discovering more about the genetics

underlying this trait each year. Given this information, which additional extensions

to Mendelian genetics apply to alcohol dependence? Give a brief explanation for

every extension that you did not choose in part a above so that I can evaluate

your reasoning for choosing it or not.

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6. A mother and her child both have blood type A.

a. What are the possible genotypes of the father? Remember to use the correct symbols

for the alleles (e.g., IB

b. The mother has a second child (with the same father) and that child has a blood type

of O. Now, considering both children, what are the possible genotypes of the father?

c. Considering both children, what is the genotype of the mother?