Q: A couple, both heterozygous for the cystic fibrosis gene (CFTR), are planning to have three children. What is the probability that exactly two of their children will have cystic fibrosis? (Assume cystic fibrosis is autosomal recessive.)

$$P = \frac{3!}{2!1!} \times \left(\frac{1}{4}\$$right)^2$$ \times \left(\frac{3}{4}\$$right)^1$$ = 3 \times \frac{1}{16} \times \frac{3}{4} = \frac{9}{64}$$

Q: A cross between two heterozygous brown-eyed individuals (Bb) produces five children. What is the probability that exactly two of the children will have blue eyes (bb)?

$$P = \frac{5!}{2!3!} \times \left(\frac{1}{4}\$$right)^2$$ \times \left(\frac{3}{4}\$$right)^3$$ = 10 \times \frac{1}{16} \times \frac{27}{64} = \frac{270}{1024} \approx 0.26$$

Q: In a Drosophila cross, the observed F2 phenotypes are: 193 straight wings/gray bodies, 69 straight wings/ebony bodies, 64 curved wings/gray bodies, and 26 curved wings/ebony bodies (total 352). If the expected Mendelian ratio is 9:3:3:1, what is the expected number of flies with curved wings and ebony bodies?

$$E = \frac{1}{16} \times 352 = 22$$

Q: Which formula correctly represents the probability of obtaining a specific combination of unordered outcomes in a genetic cross involving three categories (e.g., n events, a in category 1, b in category 2, c in category 3)?

$$P = \frac{n!}{a!b!c!} \times p^a q^b r^c$$

Question 1

A couple, both heterozygous for the cystic fibrosis gene (CFTR), are planning to have three children. What is the probability that exactly two of their children will have cystic fibrosis? (Assume cystic fibrosis is autosomal recessive.)

Accepted Answer

$$P = \frac{3!}{2!1!} 	imes \left(\frac{1}{4}\$$right)^2$$ 	imes \left(\frac{3}{4}\$$right)^1$$ = 3 	imes \frac{1}{16} 	imes \frac{3}{4} = \frac{9}{64}$$

Question 2

In a pedigree, which symbol is used to represent a female who is a carrier for an autosomal recessive trait?

Accepted Answer

A half-filled circle

Question 3

A genetic counselor is working with a couple, both heterozygous for a recessive disease allele (Pp). What is the probability that their first three children will all be unaffected by the disease?

Accepted Answer

$$P = \left(\frac{3}{4}\$$right)^3$$ = \frac{27}{64}$$

Question 4

Which of the following best describes the purpose of the chi square test in genetics?

Accepted Answer

To evaluate how well observed data fit the expected outcomes based on a hypothesis

Question 5

A cross between two heterozygous brown-eyed individuals (Bb) produces five children. What is the probability that exactly two of the children will have blue eyes (bb)?

Accepted Answer

$$P = \frac{5!}{2!3!} 	imes \left(\frac{1}{4}\$$right)^2$$ 	imes \left(\frac{3}{4}\$$right)^3$$ = 10 	imes \frac{1}{16} 	imes \frac{27}{64} = \frac{270}{1024} \approx 0.26$$

Question 6

In a chi square test with 4 phenotype categories, what is the correct formula for calculating the degrees of freedom?

Accepted Answer

$$df = n - 1 = 4 - 1 = 3$$

Question 7

A pedigree shows two unaffected parents with an affected child for a particular trait. Which inheritance pattern is most consistent with this observation?

Accepted Answer

Autosomal recessive

Question 8

Which of the following statements about random sampling error in genetic crosses is correct?

Accepted Answer

Random sampling error decreases as sample size increases

Question 9

In a Drosophila cross, the observed F2 phenotypes are: 193 straight wings/gray bodies, 69 straight wings/ebony bodies, 64 curved wings/gray bodies, and 26 curved wings/ebony bodies (total 352). If the expected Mendelian ratio is 9:3:3:1, what is the expected number of flies with curved wings and ebony bodies?

Accepted Answer

$$E = \frac{1}{16} 	imes 352 = 22$$

Question 10

A chi square value of 1.06 is calculated for an experiment with 3 degrees of freedom. According to the chi square table, what is the approximate P value, and what does this indicate?

Accepted Answer

P value is close to 0.80; deviations are likely due to random chance

Question 11

Which formula correctly represents the probability of obtaining a specific combination of unordered outcomes in a genetic cross involving three categories (e.g., n events, a in category 1, b in category 2, c in category 3)?

Accepted Answer

$$P = \frac{n!}{a!b!c!} 	imes p^a q^b r^c$$

Question 12

If a chi square test yields a P value less than 0.05, what is the appropriate conclusion regarding the null hypothesis?

Accepted Answer

Reject the null hypothesis

Question 13

Which of the following is NOT a standard symbol used in human pedigree analysis?

Accepted Answer

A triangle representing a male

Question 14

A couple, both heterozygous for a recessive allele (Pp), have four children. What is the probability that all four children will be carriers (Pp)?

Accepted Answer

$$P = \left(\frac{1}{2}\$$right)^4$$ = \frac{1}{16}$$

Question 15

Which of the following best describes a consanguineous mating in a pedigree diagram?

Accepted Answer

A double horizontal line connecting a male and a female

Question 16

Which of the following is a correct interpretation of a low chi square value in a genetic experiment?

Accepted Answer

There is a high probability that observed deviations are due to random chance

Question 17

In the context of Mendelian inheritance, what is the probability that two unaffected heterozygous parents for a recessive disease will have an affected child?

Accepted Answer

$$\frac{1}{4}$$

Question 18

Which of the following is a key difference between dominant and recessive inheritance patterns in pedigrees?

Accepted Answer

Dominant traits usually appear in every generation, while recessive traits can skip generations

Question 19

A geneticist observes that the offspring from a cross deviate from the expected Mendelian ratio. Which statistical method should be used to determine if this deviation is significant?

Accepted Answer

Chi square test

Mendelian Inheritance: Pedigrees, Probability, and Chi Square

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