CHAPTER 12 Test Your Knowledge
Driving Question 1
How do chromosomes determine sex, and how does sex influence the inheritance of certain traits?
By answering the questions below and studying Infographics 12.1–12.5, you should be able to generate an answer for the broader Driving Question above.
KNOW IT
Which of the following most influences the development of a female fetus?
a. the presence of any two sex chromosomes
b. the presence of two X chromosomes
c. the absence of a Y chromosome
d. the presence of a Y chromosome
e. either b or c
c
Why are more males than females affected by X-linked recessive genetic diseases?
Males have only one X chromosome, while females have 2. This means that if a male inherits an X-linked recessive allele (from his mother’s X chromosome), that is the only allele of that gene he has—there is no dominant allele on another X chromosome to mask the recessive. In this case, the male will develop the disease. If a female inherits an X-linked recessive allele on one of her two X chromosomes, she will not develop the disease if her other X chromosome has the dominant allele to mask the recessive allele.
If a man has an X-linked recessive disease, can his sons inherit that disease from him? Why or why not?
No. Sons cannot inherit X-linked conditions from their fathers because sons inherit their father’s Y chromosome, not the X. The son will inherit his X chromosome from his mother.
USE IT
Which of the following couples could have a boy with Duchenne muscular dystrophy (DMD)?
a. a male with Duchenne muscular dystrophy and a homozygous dominant female
b. a male without Duchenne muscular dystrophy and a homozygous dominant female
c. a male without Duchenne muscular dystrophy and a carrier female
d. a and c
e. none of the above
c
Predict the sex of a baby with each of the following pairs of sex chromosomes. Use your answer to check your answer to Question 1.
a. XX
b. XXY
c. XY
d. X
a: female; b: male; c: male; d: female
Consider your brother and your son.
a. If you are female, will your brother and your son have essentially identical Y chromosomes? Explain your answer.
b. If you are male, will your brother and your son have essentially identical Y chromosomes? Explain your answer.
a: If you are female, your brother will have inherited his Y chromosome from your father. However, your son will inherit his Y chromosome from his father (not your father). In this case, your brother and your son will not have the same Y chromosomes. b: If you are male, you and your brother both inherited your Y chromosome from your father. Your son will inherit this same Y chromosome from you. In this case, your brother and your son will both have the same (essentially identical) Y chromosome.
A wife is heterozygous for Duchenne muscular dystrophy alleles and her husband does not have DMD. Neither has any other notable medical history. What percentage of their sons, and what percentage of their daughters, will have:
a. Duchenne muscular dystrophy (which is determined by a recessive allele on the X chromosome)
b. an X-linked dominant form of rickets (a bone disease)
a: Duchenne muscular dystrophy is an X-linked recessive condition. If the mother is a carrier and the father does not have the disease, then none of the daughters will have Duchenne muscular dystrophy, and 50% of the sons will have the disease. b: As neither the mother nor the father has rickets, neither has an X-linked dominant allele of the rickets gene. Thus there are no dominant rickets alleles to transmit, and none of the children are at risk.
Driving Question 2
Some traits are not inherited as simple dominant or recessive inheritance patterns. What are some of these complex inheritance patterns?
By answering the questions below and studying Infographics 12.6–12.11, you should be able to generate an answer for the broader Driving Question above.
KNOW IT
What aspects of height make it a polygenic trait?
Many genes contribute to height. As there are multiple genes contributing to the phenotype, height has a polygenic component.
Which of the following inheritance patterns includes an environmental contribution?
a. polygenic
b. X-linked recessive
c. X-linked dominant
d. multifactorial
e. none of the above
d
What is the difference between polygenic inheritance and multifactorial inheritance?
In polygenic inheritance, multiple genes influence the phenotype. Multifactorial traits are those on which environment has an influence.
How does incomplete dominance differ from co-dominance?
In incomplete dominance, heterozygotes have a phenotype that is intermediate between the phenotypes of the homozygous dominants and the homozygous recessives. In codominance, heterozygotes display traits of both alleles present.
If you are blood type A-positive, to whom can you safely donate blood? Who can safely donate blood to you? List all possible recipients and donors and explain your answer.
If you are type A-positive, then you can donate to other A-positive people, as well as to AB-positive people. If you are A-positive, you can receive type O-negative, O-positive, A-postivie, and A-negative blood.
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USE IT
If two women have identical alleles of the suspected 20 height-associated genes, why might one of those women be 5 feet 5 inches tall and the other 5 feet 8 inches tall?
The environment can influence height. If the two women did not have the same diets and nutrition while growing up, they may not reach the same height. Similarly, if one of the women develops osteoporosis, she may lose height over time.
Look at Infographic 12.11. How do the data given support the hypothesis that both genes and the environment influence at least some cases of clinical depression?
The hypothesis that both genes and the environment influence at least some cases of clinical depression is supported by data showing that people with two copies of the short allele have substantially more cases of depression when they experience more than three stressful life experiences. If genes were the only contributor, then people with the shorter alleles would have higher rates of depression, regardless of stressful life experiences. If stressful life experiences were the only determining factor, then all people with four or more stressful life experiences would have the same rates of depression, regardless of which alleles they inherited.
Look at Infographic 12.11. At approximately how many stressful experiences does the homozygous short-allele genotype begin to influence the depression phenotype?
It appears that three or more stressful experiences influence the depression phenotype.
From what you have read in this chapter, how can you account for two people with the same genotype for a predisposing disease allele having different phenotypes?
The two people could have different alleles at other genes that influence the phenotype, or they could have different environmental influences (e.g., diet, stressful experiences, sun exposure) that could influence their phenotype.
MINI CASE
A serious car crash on a freeway has resulted in multiple injuries causing substantial blood loss in three members of a family—a mother, a father and their 2-year-old daughter. The local blood bank will be challenged to supply blood, as their supplies of every blood type were drained after the roof of a shopping plaza collapsed the week before and many transfusions were required.
a. The EMTs must give blood immediately to all three members of the family. What blood type should they use (consider both ABO blood type and Rh factor)? Explain your answer.
b. Both parents have a blood donor card in their wallets. The mother is O-negative and the father is A-positive. From this information, what (if any) additional blood types (beyond your answer to part a) can be given to either parent? Explain your answer.
c. Does knowing the parents’ blood types allow you to infer enough information about the daughter’s possible blood type to use a different blood type for her transfusion? Why or why not? (Hint: Consider possible blood types for the daughter and the implications of, for example, using A-negative donor blood. Could you guarantee that this would be safe?)
a: Type O-negative is the universal donor, so this blood should be used when no information is known about the blood types of the recipients. b: As the mother is O-negative, she can receive only O-negative blood. As the father is A-positive, he can also receive O-positive blood, A-positive blood, and A-negative blood. c: We know that the daughter will have inherited one O allele from her mother and one Rh-negative allele. However, we don’t know if her father is heterozygous for the A allele (e.g., AO) or homozygous (AA). We also don’t know if he is homozygous or heterozygous for the Rh gene (+/+ or +/–). If he is a double heterozygote, then the daughter could be O-negative. If she is O-negative, then she can receive only O-negative donor blood—any other blood type would be attacked by her immune system. Because we don’t know the father’s genotype, it is not safe to use any blood type other than O-negative.
Driving Question 3
How do numerical abnormalities of chromosomes occur, and what are the consequences of these abnormalities?
By answering the questions below and studying Infographics 12.12 and 12.13, you should be able to generate an answer for the broader Driving Question above.
KNOW IT
What is the normal chromosome number for each of the following?
a. a human egg
b. a human sperm
c. a human zygote
a. 23; b. 23; c. 46
When looking at a karyotype, for example to diagnose trisomy 21 in a fetus, is it possible to use that analysis also to tell if the fetus has inherited a cystic fibrosis allele from a carrier mother?
Karyotype analysis can be used to detect trisomy 21, as an extra chromosome is easily visible at this level. However, cystic fibrosis is caused by mutations that change the nucleotide sequence of the gene- these cannot be detected by simply looking at the chromosomes.
USE IT
Which of the following can result in trisomy 21?
a. an egg with 23 chromosomes fertilized by a sperm with 23 chromosomes
b. an egg with 22 chromosomes fertilized by a sperm with 23 chromosomes
c. an egg with 24 chromosomes, two of which are chromosome 21, fertilized by a sperm with 23 chromosomes
d. an egg with 23 chromosomes fertilized by a sperm with 24 chromosomes, two of which are chromosome 21
The cases described in choice c and choice d can result in trisomy 21.
From what you have read in this chapter, which of the possibilities in Question 20 is most likely? Explain your answer.
Most cases of trisomy 21 occur because of a nondisjunction during meiosis in egg development that results in an egg with 24 chromosomes, 2 of which are chromosome 21).
BRING IT HOME
What factors would lead you to consider prenatal genetic testing? In your opinion, what is the value of having this information?
Factors to consider include age of the mother, abnormal prenatal tests, a previous child born with a genetic disorder or birth defect, and the family history of both parents. Opinions will vary about the value of the information obtained through prenatal genetic testing.
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INTERPRETING DATA
The graph at right shows the average (“Mean”) age of women who had children with trisomy 21 (“Cases”), of those who did not (“Controls”), and the average age of women giving birth in the population. The data are presented for 15 years.
a. In general, how does the age (at time of birth) of women giving birth to a baby with Down syndrome compare to the age of women giving birth to a baby without Down syndrome?
b. During which year was the average age of the cases closest to the average age of the controls? How close were the average ages in this year?
c. During which year was the average age of the cases the most different from the average age of the controls? How different were the average ages in this year?
d. Using the data points for each year over this 15-year period, calculate the overall average age of women having babies with Down syndrome, and the overall average age of women having babies who do not have Down syndrome.
a: In general, women giving birth to children with Down syndrome are older than women giving birth to babies without Down syndrome. b: In 1995, the average age of controls was just slightly less than 28, and the average age of controls was approximately 29.5 years. This is a difference of a little more than 1.5 years. c: In 2003, the average age of cases was 34, and the average age of controls was approximately 27.5. This is a difference of 6.5 years. d: From the data shown, over the 15 year period the average age of women having babies with Down syndrome was approximately 31.3 years, and the average age of women having babies without Down Syndrome was approximately 27.5 years.