Concepts of Genetics

William S. Klug, Michael R. Cummings, Charlotte A. Spencer

Chapter 3 Mendelian Genetics - all with Video Answers

Educators

Chapter Questions

03:36

Problem 1

In this chapter, we focused on the Mendelian postulates, probability, and pedigree analysis. We also considered some of the methods and reasoning by which these ideas, concepts, and techniques were developed. On the basis of these discussions, what answers would you propose to the following questions:
(a) How was Mendel able to derive postulates concerning the behavior of "unit factors" during gamete formation, when he could not directly observe them?
(b) How do we know whether an organism expressing a dominant trait is homozygous or heterozygous?
(c) In analyzing genetic data, how do we know whether deviation from the expected ratio is due to chance rather than to another, independent factor?
(d) since experimental crosses are not performed in humans, how do we know how traits are inherited?

Celine Ibrahim

Numerade Educator

00:45

Problem 2

Review the Chapter Concepts list on $\mathrm{p}$. 74 . The first five concepts provide a modern interpretation of Mendelian postulates. Based on these concepts, write a short essay that correlates Mendel's four postulates with what is now known about genes, alleles, and homologous chromosomes.

Jennifer Stoner

Numerade Educator

02:20

Problem 3

Albinism in humans is inherited as a simple recessive trait. For the following families, determine the genotypes of the parents and offspring. (When two alternative genotypes are possible, list both.)
(a) Two normal parents have five children, four normal and one albino.
(b) A normal male and an albino female have six children, all normal.
(c) A normal male and an albino female have six children, three normal and three albino.
(d) Construct a pedigree of the families in (b) and (c). Assume that one of the normal children in (b) and one of the albino children in (c) become the parents of eight children. Add these children to the pedigree, predicting their phenotypes (normal or albino).

Jackson Miner

Numerade Educator

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Problem 4

Which of Mendel's postulates are illustrated by the pedigree that you constructed in Problem 3 ? List and define these postulates.

Isobel Haskamp

Numerade Educator

02:57

Problem 5

Discuss how Mendel's monohybrid results served as the basis for all but one of his postulates. Which postulate was not based on these results? Why?

April Townson

Numerade Educator

01:52

Problem 6

What advantages were provided by Mendel's choice of the garden pea in his experiments?

Asma Venkitta

Numerade Educator

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Problem 7

Mendel crossed peas having round seeds and yellow cotyledons (seed leaves) with peas having wrinkled seeds and green cotyledons. All the $F_{1}$ plants had round seeds with yellow cotyledons. Diagram this cross through the $\mathrm{F}_{2}$ generation, using both the Punnett square and forked-line, or branch diagram, methods.

Kaela Piechowicz

Numerade Educator

02:47

Problem 8

Based on the preceding cross, what is the probability that an organism in the $\mathrm{F}_{2}$ generation will have round seeds and green cotyledons and be true breeding?

Christina Sorrentino

Numerade Educator

01:19

Problem 9

Which of Mendel's postulates can only be demonstrated in crosses involving at least two pairs of traits? State the postulate.

Christina Sorrentino

Numerade Educator

10:38

Problem 10

Assume that you have a garden and some pea plants have solid leaves and others have striped leaves. You conduct a series of crosses $[(a) \text { through }(e)]$ and obtain the results given in the table.
Define gene symbols and give the possible genotypes of the parents of each cross.

Heather Thornton

Numerade Educator

01:35

Problem 11

What is the basis for homology among chromosomes?

Evey Z

Numerade Educator

02:34

Problem 12

Two organisms, $A A B B C C D D E E$ and aabbccddee, are mated to produce an $\mathrm{F}_{1}$ that is self-fertilized. If the capital letters represent dominant, independently assorting alleles:
(a) How many different genotypes will occur in the $\mathrm{F}_{2}$ ?
(b) What proportion of the $\mathrm{F}_{2}$ genotypes will be recessive for
all five loci?
(c) Would you change your answers to (a) and/or (b) if the initial cross occurred between $A A b b C C$ddee$\times$aaBBccDDEE parents?
(d) Would you change your answers to (a) and/or (b) if the initial cross occurred between $A A B B C C D D E E \times$ aabbccddEE
parents?

Khalida Dawar

Numerade Educator

01:17

Problem 13

Albinism, lack of pigmentation in humans, results from an autosomal recessive gene (a). Two parents with normal pigmentation have an albino child.
(a) What is the probability that their next child will be albino?
(b) What is the probability that their next child will be an albino girl?
(c) What is the probability that their next three children will be albino?

Bryan Valdivia

Numerade Educator

00:28

Problem 14

Dentinogenesis imperfecta is a rare, autosomal, dominantly inherited disease of the teeth that occurs in about one in 8000
people (Witkop 1957 ). The teeth are somewhat brown in color, and the crowns wear down rapidly. Assume that a male with dentinogenesis imperfecta and no family history of the disease marries a woman with normal teeth. What is the probability that
(a) their first child will have dentinogenesis imperfecta?
(b) their first two children will have dentinogenesis imperfecta?
(c) their first child will be a girl with dentinogenesis imperfecta?

Dennis Howard

Numerade Educator

02:04

Problem 15

In a study of black guinea pigs and white guinea pigs, 100 black animals were crossed with 100 white animals, and each cross was carried to an $\mathrm{F}_{2}$ generation. In 94 of the crosses, all the $\mathrm{F}_{1}$ offspring were black and an $\mathrm{F}_{2}$ ratio of 3 black: 1 white was obtained. In the other 6 cases, half of the $\mathrm{F}_{1}$ animals were black and the other half were white. Why? Predict the results of crossing the black and white $\mathrm{F}_{1}$ guinea pigs from the 6 exceptional cases.

Jackson Miner

Numerade Educator

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Problem 16

Mendel crossed peas having round green seeds with peas having wrinkled yellow seeds. All $\mathrm{F}_{1}$ plants had seeds that were round and yellow. Predict the results of testcrossing these $\mathrm{F}_{1}$ plants.

Kaela Piechowicz

Numerade Educator

03:03

Problem 17

Thalassemia is an inherited anemic disorder in humans. Affected individuals exhibit either a minor anemia or a major anemia. Assuming that only a single gene pair and two alleles are involved in the inheritance of these conditions, is thalassemia a dominant or recessive disorder?

Susan Hallstrom

Numerade Educator

02:30

Problem 18

A certain type of congenital deafness in humans is caused by a rare autosomal (not X-linked) dominant gene.
(a) In a mating involving a deaf man and a deaf woman (both heterozygous), would you expect all the children to be deaf? Explain your answer.
(b) In a mating involving a deaf man and a deaf woman (both heterozygous), could all the children have normal hearing? Explain your answer.
(c) Another form of deafness is caused by a rare autosomal recessive gene. In a mating involving a deaf man and a deaf woman, could some of the children have normal hearing? Explain your answer.

John Barone

Numerade Educator

01:49

Problem 19

In assessing data that fell into two phenotypic classes, a geneticist observed values of $250: 150 .$ She decided to perform a $\chi^{2}$ analysis by using the following two different null hypotheses:
(a) the data fit a 3: 1 ratio, and (b) the data fit a 1: 1 ratio. Calculate the $\chi^{2}$ values for each hypothesis. What can be concluded about each hypothesis?

Jackson Miner

Numerade Educator

01:09

Problem 20

The basis for rejecting any null hypothesis is arbitrary. The researcher can set more or less stringent standards by deciding to raise or lower the $p$ value used to reject or not reject the hypothesis. In the case of the chi-square analysis of genetic crosses, would the use of a standard of $p=0.10$ be more or less stringent about not rejecting the null hypothesis? Explain.

Tyler Moulton

Numerade Educator

10:59

Problem 21

Among dogs, short hair is dominant to long hair and dark coat color is dominant to white (albino) coat color. Assume that these two coat traits are caused by independently segregating gene pairs. For each of the crosses given below, write the most probable genotype (or genotypes if more than one answer is possible for the parents. It is important that you select a realistic symbol set and define each symbol below. Assume that for cross (d), you were interested in determining whether fur color follows a 3: 1 ratio. Set up (but do not complete the calculations) a Chi-square test for these data [fur color in cross $(\mathrm{d})]$.

Jennifer Hudspeth

Numerade Educator

03:54

Problem 22

Draw all possible conclusions concerning the mode of inheritance of the trait portrayed in each of the following limited pedigrees. (Each of the four cases is based on a different trait.)
a.
b.
c.
d.

Bryan Valdivia

Numerade Educator

13:11

Problem 23

In a family of eight children, what is the probability that
(a) the third child is a girl?
(b) six of the children are boys?
(c) all the children are girls?
(d) there are four boys and four girls?
Assume that the probability of having a boy is equal to the probability of having a girl $(p=1 / 2)$.

April Townson

Numerade Educator

01:02

Problem 24

In a family of six children, where one grandparent on either side has red hair, what mathematical expression predicts the probability that two of the children have red hair?

Raj Bala

Numerade Educator

01:55

Problem 25

The autosomal (not X-linked) gene for brachydactyly, short fingers, is dominant to normal finger length. Assume that a female with brachydactyly in the heterozygous condition is married to a man with normal fingers. What is the probability that
(a) their first child will have brachydactyly?
(b) their first two children will have brachydactyly?
(c) their first child will be a brachydactylous girl?

Anand Jangid

Numerade Educator

02:27

Problem 26

Galactosemia is a rare recessive disorder caused by the deficiency of galactose- 1 -phosphate uridylyltransferase, leading to the accumulation of toxic levels of galactitol in the blood. It leads to a $75 \%$ mortality rate in infants as infants cannot metabolize galactose from breast milk. In many countries, newborns are given a heel prick test to measure the levels of metabolic enzymes. As a genetic counselor, how would you explain to a couple whose baby has tested positive for galactosemia where the disease has come from?

Jessica Wooten

Numerade Educator

01:31

Problem 27

Two true-breeding pea plants were crossed. One parent is round, terminal, violet, constricted, while the other expresses the respective contrasting phenotypes of wrinkled, axial, white, full. The four pairs of contrasting traits are controlled by four genes, each located on a separate chromosome. In the $\mathrm{F}_{1}$ only round, axial, violet, and full were expressed. In the $\mathrm{F}_{2},$ all possible combinations of these traits were expressed in ratios consistent with Mendelian inheritance.
(a) What conclusion about the inheritance of the traits can be drawn based on the $\mathrm{F}_{1}$ results?
(b) In the $\mathrm{F}_{2}$ results, which phenotype appeared most frequently? Write a mathematical expression that predicts the probability of occurrence of this phenotype.
(c) Which $\mathrm{F}_{2}$ phenotype is expected to occur least frequently? Write a mathematical expression that predicts this probability.
(d) In the $F_{2}$ generation, how often is either of the $P_{1}$ phenotypes likely to occur?
(e) If the $F_{1}$ plants were testcrossed, how many different phenotypes would be produced? How does this number compare with the number of different phenotypes in the $\mathrm{F}_{2}$ generation just discussed?

Anand Jangid

Numerade Educator

04:52

Problem 28

Tay-Sachs disease (TSD) is an inborn error of metabolism that results in death, often by the age of $2 .$ You are a genetic counselor interviewing a phenotypically normal couple who tell you the male had a female first cousin (on his father's side) who died from TSD and the female had a maternal uncle with TSD. There are no other known cases in either of the families, and none of the matings have been between related individuals. Assume that this trait is very rare.
(a) Draw a pedigree of the families of this couple, showing the relevant individuals.
(b) Calculate the probability that both the male and female are carriers for TSD.
(c) What is the probability that neither of them is a carrier?
(d) What is the probability that one of them is a carrier and the other is not? [Hint: The $p$ values in (b), (c), and
(d) should equal $1 .]$

John Barone

Numerade Educator

09:18

Problem 29

Datura stramonium (the Jimsonweed) expresses flower colors of purple and white and pod textures of smooth and spiny. The results of two crosses in which the parents were not necessarily true breeding are shown at the top of the next column.
(a) Based on these results, put forward a hypothesis for the inheritance of the purple/white and smooth/spiny traits.
(b) Assuming that true-breeding strains of all combinations of traits are available, what single cross could you execute and carry to an $\mathrm{F}_{2}$ generation that will prove or disprove your hypothesis? Assuming your hypothesis is correct, what results of this cross will support it??

Bryan Valdivia

Numerade Educator

05:02

Problem 30

The wild-type (normal) fruit fly, Drosophila melanogaster, has straight wings and long bristles. Mutant strains have been isolated that have either curled wings or short bristles. The genes representing these two mutant traits are located on separate chromosomes. Carefully examine the data from the five crosses shown on the top of the following page (running across both columns).
(a) Identify each mutation as either dominant or recessive. In each case, indicate which crosses support your answer.
(b) Assign gene symbols and, for each cross, determine the genotypes of the parents.

Syed Vasi

Numerade Educator

02:08

Problem 31

An alternative to using the expanded binomial equation and Pascal's triangle in determining probabilities of phenotypes in a subsequent generation when the parents' genotypes are known is to use the following equation: $\frac{n !}{s ! t !} a^{s} b^{t}$ where $n$ is the total number of offspring, $s$ is the number of offspring in one phenotypic category, $t$ is the number of offspring in the other phenotypic category, $a$ is the probability of occurrence of the first phenotype, and $b$ is the probability of the second phenotype. Using this equation, determine the probability of a family of 5 offspring having exactly 2 children afflicted with sickle-cell anemia (an autosomal recessive disease $)$ when both parents are heterozygous for the sickle-cell allele.

Kari Hasz

Numerade Educator

01:29

Problem 32

To assess Mendel's law of segregation using tomatoes, a truebreeding tall variety (SS) is crossed with a true-breeding short variety $(s s) .$ The heterozygous $F_{1}$ tall plants $(S s)$ were crossed to produce two sets of $\mathrm{F}_{2}$ data, as follows.
$\begin{array}{cc}\text { Set I } & \text { Set II } \\ 30 \text { tall } & 300 \text { tall } \\ 5 \text { short } & 50 \text { short }\end{array}$
(a) Using the $\chi^{2}$ test, analyze the results for both datasets. Calculate $\chi^{2}$ values and estimate the $p$ values in both cases.
(b) From the above analysis, what can you conclude about the importance of generating large datasets in experimental conditions?

Anand Jangid

Numerade Educator

01:29

Problem 33

Albinism, caused by a mutational disruption in melanin (skin pigment production, has been observed in many species, including humans. In 1991 , the only documented observation of an albino humpback whale (named "Migaloo") was observed near New South Wales. Recently, Polanowski and coworkers (Polanowski, A., S. Robinson-Laverick, and D. Paton. 2012. Journal of Heredity $103: 130-133$ ) studied the genetics of humpback whales from the east coast of Australia, including Migaloo.
(a) Do you think that Migaloo's albinism is more likely caused by a dominant or recessive mutation? Explain your reasoning.
(b) What data would be helpful in determining the answer to part (a)?

Patina Herring

Numerade Educator

02:34

Problem 34

(a) Assuming that Migaloo's albinism is caused by a rare recessive gene, what would be the likelihood of the establishment of a natural robust subpopulation of albino white humpback whales in this population?
(b) Assuming that Migaloo's albinism is caused by a rare dominant gene, what would be the likelihood of the establishment of a natural robust subpopulation of albino white humpback whales in this population?

Cody Delk

Numerade Educator

02:34

Problem 35

Assume that Migaloo's albinism is caused by a rare recessive gene.
(a) In a mating of two heterozygous, normally pigmented whales, what is the probability that the first three offspring will all have normal pigmentation?
(b) What is the probability that the first female offspring is normally pigmented?
(c) What is the probability that the first offspring is a normally pigmented female?

Cody Delk

Numerade Educator

03:18

Problem 36

Dentinogenesis imperfecta is a tooth disorder involving the production of dentin sialophosphoprotein, a bone-like component of the protective middle layer of teeth. The trait is inherited as an autosomal dominant allele located on chromosome 4 in humans and occurs in about 1 in 6000 to 8000 people. Assume that a man with dentinogenesis imperfecta, whose father had the disease but whose mother had normal teeth, married a woman with normal teeth. They have six children. What is the probability that their first child will be a male with dentinogenesis imperfecta? What is the probability that three of their six chil-
dren will have the disease?

Jennifer Stoner

Numerade Educator