FINAL 07 2020, questions and answers PDF

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Module_8_ Ch_15...

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6/13/2020

Module #8: Ch 15

Module #8: Ch 15 Due: 11:59pm on Thursday, July 1, 2021 You will receive no credit for items you complete after the assignment is due. Grading Policy

Make Connections: Chromosomal Inheritance and Independent Assortment of Alleles Mendel’s law of independent assortment (click on the figure on the left) tells us that the alleles for one character segregate into gametes independently of the alleles for a different character. Independent assortment occurs when the genes for two characters are found on different chromosomes. The reason behind this case is explained by the way in which, gametes inherit chromosomes during meiosis (click on the figure on the right).

Part A - Reviewing independent assortment of alleles At the time of Mendel’s pea plant experiments, no one knew how organisms formed gametes. As Mendel studied the inheritance of two different characters, h wondered how the alleles for the two characters segregated into gametes. Mendel had two hypotheses for how this might work. Under the hypothesis of dependent assortment, the alleles inherited from the parental generation should always be transmitted to the next generation in the same combinations. Under the hypothesis of independent assortment, alleles for different characters should segregate independently of each other, meaning that alleles should be packaged into gametes in all possible combinations, as long as each gamete has one allele for each gene. The figure below shows the experiment that Mendel used to distinguish between these two hypotheses. The results of the experiment confirmed that the alleles for these characters undergo independent assortment.

Drag the labels to the appropriate blanks to complete the sentences below. Not all labels will be used. ANSWER:

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Correct When Mendel studied the inheritance of two characters in pea plants (such as seed colour and seed shape), he found that alleles for different characters segregate independently. This means that genes are packaged into gametes in all possible allelic combinations, as long as each gamete has one allele for each gene. If genes are packaged into gametes in all possible combinations of alleles, then when you self-fertilize a plant heterozygous for two different traits (such as the YyRr plants in the example here), you see four different phenotypes in the F2 generation in a ratio of 9:3:3:1.

Part B - Chromosomal inheritance during meiosis In the figure below, you can see Mendel’s experiment again, this time superimposed on the events of meiosis and fertilization. How does chromosomal inheritance during meiosis explain Mendel’s law of independent assortment?

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Module #8: Ch 15

Drag the labels to their appropriate locations in the table below. The number at the top of each column corresponds to the same number in the image above. Each column describes what happens at that numbered stage. Use only white labels for white targets, blue labels for blue targets, and pink labels for pink targets. ANSWER:

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Correct When alleles for two characters are found on different chromosomes, they segregate independently during gamete formation because the chromosomes segregate independently during meiosis. At metaphase I, two different arrangements of chromosomes are possible: Y can line up with R and y with r, or Y can line up with r and y with R. When homologous chromosomes separate at anaphase I, independent assortment occurs. This accounts for Mendel’s observation of a 9:3:3:1 ratio in the F2 generation.

Part C - Do the alleles for different characters always sort independently? In biology lab, you conduct a breeding experiment to test Mendel’s law of independent assortment. You study two characters in a new plant species recently discovered on campus: Flower colour, which can be blue (BB) or purple (bb) Petal shape, which can be pointy (PP) or rounded (pp) You use the following procedure. In the parental generation, you breed a plant that you know to be homozygous for blue-pointy flowers (BBPP) with a plant that you know to be homozygous for purple-rounded flowers (bbpp). In the F1 generation, all your plants have blue-pointy flowers (BbPp). You then allow the F1 plants to self-pollinate to produce F2 offspring. In the F2 generation, you obtain 80 plants with the following phenotypes. Note that an underscore “_” in the genotype indicates that the second allele for that gene could be either dominant or recessive: Phenotype

Number of individuals

Blue flower/pointy petal (B_P_)

59

Blue flower/rounded petal (B_pp)

1

Purple flower/pointy petal (bbP_)

0

Purple flower/rounded petal (bbpp)

20

To try to explain this unusual data, you come up with two alternate hypotheses in addition to your original hypothesis of independent assortment. Hypothesis 1: The alleles for flower colour and petal shape are found on different chromosomes. (This is independent assortment as observed by Mendel wit the characters of seed colour and shape.) Hypothesis 2: The alleles for flower colour and petal shape are found on different chromosomes, but the blue-rounded (B_pp) and purple-pointy (bbP_) phenotypes typically do not survive, for a reason that has yet to be determined. Hypothesis 3: The alleles for flower colour and petal shape are found close to each other on the same chromosome.

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Module #8: Ch 15

Drag the labels to their appropriate locations in the table below. Labels may be used more than once. (Hint: First, figure out the predicted F1 gametes for each hypothesis; then construct a Punnett square to help you fill in the rest of the table.) ANSWER: Reset

mostly BP and bp

3 blue-pointy: 1 purplerounded

BP, Bp, bP, bp in equal numbers

9 blue-pointy: 1 purplerounded

2 predominant phenotypes

no

4 phenotypes

yes

BP, Bp, bP, bp in equal numbers

mostly BP and bp

2 predominant phenotypes

2 predominant phenotypes

9 blue-pointy: 1 purplerounded

3 blue-pointy: 1 purplerounded

no

no

Help

yes

Correct According to Mendel’s law of independent assortment, the alleles for different characters segregate independently when they are found on different chromosomes. Under independent assortment, the F2 offspring are produced in a ratio of 9:3:3:1. As you saw in this example, when alleles are found on the same chromosome, independent assortment does not hold, and you do not see a ratio of 9:3:3:1. You may remember that Mendel studied seven different characters in pea plants -- flower colour, flower position, seed colour, seed shape, pod shape, pod colour, and stem length. Mendel found that independent assortment held true for any pair of characters he looked at. We now know that this must mean that the alleles for these characters are all found on different chromosomes. As it turns out, pea plants have exactly seven chromosomes, and the alleles for each of Mendel’s characters is on a different chromosome!

Sex Linkage A trait controlled by a gene located on either sex chromosome is called a sex-linked trait. In human genetics, however, this term has historically referred specifical to a trait controlled by a gene on the X chromosome (also called an X-linked trait). In this tutorial, you will explore the pattern of inheritance of sex-linked traits in humans and other animals.

Part A - The inheritance of a skin condition in humans Consider the following family history: Bob has a genetic condition that affects his skin. Bob’s wife, Eleanor, has normal skin. No one in Eleanor’s family has ever had the skin condition. Bob and Eleanor have a large family. Of their eleven children, all six of their sons have normal skin, but all five of their daughters have the same skin condition as Bob. Based on Bob and Eleanor’s family history, what inheritance pattern does the skin condition most likely follow?

Hint 1. What are the characteristics of an autosomal dominant inheritance pattern? Recall that autosomal traits are controlled by genes located on autosomes, or non-sex chromosomes. Which statement would be true for a condition that is inherited as an autosomal dominant trait? ANSWER:

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Module #8: Ch 15 If both parents have the condition, all of their children will definitely have the condition. If a mother has the condition, none of her sons can have the condition. If a child has the condition, one grandparent must have had the condition. If a father has the condition, all of his children will definitely have the condition.

Hint 2. What are the characteristics of an X-linked dominant inheritance pattern? Which of the following statements would be true for a condition that is inherited as an X-linked dominant trait? Select all that apply. ANSWER:

A son can only inherit the condition if his mother has it. A pedigree for a family would show affected individuals in each generation. Both males and heterozygous females with the condition will pass the trait to half of their children. If a father has the condition, all of his daughters will have the condition.

Hint 3. What are the characteristics of an autosomal recessive inheritance pattern? Recall that autosomal traits are controlled by genes located on autosomes, or non-sex chromosomes. Which statement would be true for a condition that is inherited as an autosomal recessive trait? ANSWER:

If both a mother and a father have the condition, all of the children will have the condition. If both a mother and a father have the condition, their sons can have the condition, but their daughters cannot. If one parent has the condition but the other parent does not, all of the children will have the condition. If a child has the condition, one grandparent must have had the condition.

ANSWER: X-linked dominant X-linked recessive autosomal recessive autosomal dominant Y-linked

Correct If the skin condition is caused by an X-linked dominant allele, a father would pass the allele on to all of his daughters, who would all have the skin condition. In contrast, the father would not pass the allele on to any of his sons because the sons would receive the father’s Y chromosome, not his X chromosome. As a result, none of the sons would inherit the skin condition.

Part B - A sex-linked gene for eye colour in Drosophila The inheritance of eye colour in Drosophila is controlled by genes on each of the fly’s four chromosome pairs. One eye-colour gene is on the fly’s X chromosome, so the trait is inherited in a sex-linked manner. For this sex-linked trait, the wild-type (brick red) allele is dominant over the mutant vermilion (bright red) allele. A homozygous wild-type female fly is mated with a vermilion male fly.

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Module #8: Ch 15

Predict the eye colours of F1 and F2 generations. (Assume that the F1 flies are allowed to interbreed to produce the F2 generation.) Drag the correct label to the appropriate location in the table. Labels can be used once, more than once, or not at all.

Hint 1. Can you construct a Punnett square for the F1 generation? Complete this Punnett square to determine the eye colours of the F1 females and males. (Use X + for the wild-type allele, X v for the vermilion allele, and Y for the Y chromosome.) Drag the labels to their appropriate locations on the Punnett square. Labels can be used once, more than once, or not at all. ANSWER: Reset

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Hint 2. Can you construct a Punnett square for the F2 generation? Complete this Punnett square to determine the eye colours of the F2 females and males. (Use X + for the wild-type allele, X v for the vermilion allele, and Y for the Y chromosome.) Drag the labels to their appropriate locations on the Punnett square. Labels can be used once, more than once, or not at all. ANSWER:

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ANSWER: Reset

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Correct Both F1 females and F1 males will have the wild-type eye colour, but because the trait is sex-linked, F1 females will be heterozygous for the trait. As a result, F2 males have a

chance of inheriting each allele, and thus of having that eye colour. F2 females, on the other hand, will all have the

wild-type eye colour because they inherit the dominant allele on the X chromosome from their fathers (the F1 males).

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Red-green colour blindness is due to an X-linked recessive allele in humans. A widow’s peak (a hairline that comes to a peak in the middle of the forehead) is due to an autosomal dominant allele. Consider the following family history: A man with a widow’s peak and normal colour vision marries a colour-blind woman with a straight hairline. The man’s father had a straight hairline, as did both of the woman’s parents. Use the family history to make predictions about the couple’s children. Drag the correct label to the appropriate location in the table. Not all labels will be used.

Hint 1. How to approach the problem To answer any of the questions in this problem, you first need to determine the following: 1. What are the man’s and woman’s genotypes for each trait? (See Hint 2.) 2. Given the genotypes of the parents for the hairline trait, what is the expected frequency of offspring with a widow’s peak and with a straight hairline? Is there a different inheritance pattern for males and females? 3. Given the genotypes of the parents for the colour vision trait, what is the expected frequency of offspring with normal vision and with colourblindness? Is there a different inheritance pattern for males and females? 4. How can you calculate the combined probability of inheriting two specific traits when the traits are inherited independently of each other? (See Hint 3.)

Hint 2. What are the genotypes of the parents? Which of the following shows the correct genotypes of the man and the woman for the two traits in the problem? (Use W for widow’s peak and w for straight hairline; X N for normal colour vision and X n for colourblindness.) ANSWER:

Man: Ww and X n Y Woman: ww and X N X n

Man: WW and X N Y Woman: ww and X n X n Man: WW and X N Y Woman: Ww and X N X n

Man: Ww and X N Y Woman: ww and X n X n

Hint 3. Can you calculate the combined probability of inheriting two traits that assort independently? Suppose that a couple has a

chance of having a son with a widow’s peak and a

chance of having a colour-blind son.

The couple’s chances of having a son with both a widow’s peak and colourblindness would be _____. ANSWER:

ANSWER:

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Correct For the hairline gene, the man’s genotype is Ww and the woman’s is ww. For the colour vision gene, the man’s genotype is X N Y and the woman’s is X n X n. Because the genes are on different chromosomes, they assort independently. Also, because one gene is sex-linked, it exhibits a different inheritance pattern in males and females. You should use the multiplication rule to calculate the chances of two events (e.g., widow’s peak and colourblindness) occurring together in a specific combination, paying attention to whether the offspring is male or female.

Activity: Mistakes in Meiosis

Click here to view this animation. Then answer the questions.

Part A What is a nondisjunction?

Hint 1. Think about the literal meaning of the word nondisjunction.

ANSWER:

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Module #8: Ch 15 An error in which a diploid cell or organism lacks a chromosome of one type, producing a chromosome number of 2n - 1 An error in which a diploid cell or organism has an extra chromosome of one type, producing a chromosome number of 2n + 1 An error in cell division that causes homologous chromosomes or sister chromatids to move to the same side of the dividing cell None of the above

Correct Nondisjunction refers to the failure of pairs of chromosomes or sister chromatids to separate during meiosis or mitosis.

Part B When can nondisjunction occur? Choose the best answer.

Hint 1. Nondisjunction can occur in cells of the germ line (cells that produce gametes) or in other body cells.

ANSWER:

In mitosis, when sister chromatids fail to separate In meiosis, when homologous chromosomes fail to separate All three answers are correct. In meiosis, when sister chromatids fail to separate

Correct Nondisjunction errors can occur in meiosis I, when homologous chromosomes fail to separate, or in either mitosis or meiosis II, when sister chromatids fail to separate.

Part C Which syndrome is characterized by the XO chromosome abnormality? ANSWER:

Klinefelter syndrome Trisomy X Turner syndrome Down syndrome

Correct Turner syndrome is characterized by a chromosome pattern of XO, or one X chromosome and no X or Y chromosome.

Part D What kind of cell results when a diploid and a haploid gamete fuse during fertilization?

Hint 1. Consider whether the terms refer to entire sets of chromosomes or to a single chromosome.

ANSWER:

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Module #8: Ch 15 A triploid cell A monosomic cell A trisomic cell A monoploid cell

Correct A triploid cell has three sets of chromosomes: the two from the diploid gamete and the one from the haploid gamete.

Part E Of the following chromosomal abnormalities, which type is most likely to be viable in humans?

Hint 1. There are only a small number of aneuploid conditions that humans can tolerate; most changes in chromosome number are lethal.

ANSWER:

Haploidy Triploidy Trisomy Monosomy

Correct Some trisomies are viable in humans, but they result in developmental abnormalities, such as Down syndrome (trisomy 21). Only humans with trisomies of the small chromosomes (13, 18, or 21) or the sex chromosomes survive past birth.

Part F If a diploid cell undergoes meiosis and produces two gametes that are normal, and one with n − 1 chromosomes, and one wit...