Mastering Biology Chapter 15

Chapter 15 Pre-Test Question 9

How are human mitochondria inherited?

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Chapter 15 Pre-Test Question 2

What name is given to the most common phenotype in a natural population?
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Chapter 15 Pre-Test Question 3

A white-eyed female Drosophila is crossed with a red-eyed male Drosophila. Which statement below correctly describes the results?
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Chapter 15 Question 1

When Thomas Hunt Morgan crossed his red-eyed F1 generation flies to each other, the F2 generation included both red- and white-eyed flies. Remarkably, all the white-eyed flies were male. What was the explanation for this result?

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Chapter 15 Pre-Test Question 4

 In humans, what determines the sex of offspring and why?

Chapter 15 Pre-Test Question 10

Which of the following is true of an X-linked gene, but not of a Y-linked gene?

Chapter 15 Question 3

Males are more often affected by sex-linked traits than females because _____.

Chapter 15 Question 25

What does a frequency of recombination of 50% indicate?

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Chapter 15 Question 26

What is the definition of one map unit?

Chapter 15 Pre-Test Question 5

In general, the frequency with which crossing over occurs between two linked genes depends on what?

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Chapter 15 Pre-Test Question 7

What results if a fragment of a chromosome breaks off and then reattaches to the original chromosome at the same place but in the reverse direction?
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Chapter 15 Question 40

If cell X enters meiosis, and nondisjunction of one chromosome occurs in one of its daughter cells during meiosis II, what will be the result at the completion of meiosis?

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Chapter 15 Pre-Test Question 8

What phenomenon occurs when a particular allele will either be expressed or silenced, depending on whether it is inherited from a male or a female?

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AP Exam Prep Question 29

 Inheritance patterns cannot always be explained by Mendel’s models of inheritance. If a pair of homologous chromosomes fails to separate during meiosis I, select the choice that shows the chromosome number of the four resulting gametes with respect to the normal haploid number (n)?Screen Shot 2017-02-01 at 1.39.58 PM.png

Misconception Question 72

Which of these descriptions of the behavior of chromosomes during meiosis explains Mendel’s law of independent assortment?

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Chapter 15 Question 43

Of the following human aneuploidies, which is the one that generally has the most severe impact on the health of the individual?

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Chapter 15 Question 23

Which of the following statements is true of linkage?

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Chapter 15 Pre-Test Question 6

Which of the following results in a situation in which the chromosome number is either 2n+1 or 2n-1 ?

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Chapter 15 Question 41

One possible result of chromosomal breakage is for a fragment to join a nonhomologous chromosome. What is this alteration called?

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Chapter 15 Question 42

A nonreciprocal crossover causes which of the following products?


Chapter 15 Question 27

Recombination between linked genes comes about for what reason?


Chapter 15 Question 24

How would one explain a testcross involving F1 dihybrid flies in which more parental-type offspring than recombinant-type offspring are produced?

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Chapter 15 Question 31

In a series of mapping experiments, the recombination frequencies for four different linked genes of Drosophila were determined as shown in the figure above. What is the order of these genes on a chromosome map?


Chapter 15 Question 32

Use the following information to answer the question(s) below.

A plantlike organism on the planet Pandora can have three recessive genetic traits: bluish leaves, due to an allele (a) of gene A; a feathered stem, due to an allele (b) of gene B; and hollow roots due to an allele (c) of gene C. The three genes are linked and recombine as follows:

A geneticist did a testcross with an organism that had been found to be heterozygous for the three recessive traits and she was able to identify progeny of the following phenotypic distribution (+ = wild type):

Phenotypes Leaves Stems Roots Number
1 a + + 14
2 a + c 0
3 a b + 32
4 a b c 440
5 + b + 0
6 + b c 16
7 + + c 28
8 + + + 470
Total 1000

Which of the following are the phenotypes of the parents in this cross?


Sex Linkage

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?


Part B – A sex-linked gene for eye color in Drosophila

The inheritance of eye color in Drosophila is controlled by genes on each of the fly’s four chromosome pairs. One eye-color 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.

Predict the eye colors 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.

Part C – The inheritance of both a sex-linked trait and an autosomal trait in humans

Red-green color 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 color vision marries a color-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.

AP Exam Prep Question 28

 During meiosis, a defect occurs in a cell that results in the failure of microtubules, spindle fibers, to bind at the kinetochores, a protein structure on chromatids where the spindle fibers attach during cell division to pull sister chromatids apart. Which of the following is the most likely result of such a defect?


Experimental Inquiry: What Is the Inheritance Pattern of Sex-Linked Traits?

The study of modern genetics began with the work of Thomas Hunt Morgan in the early 1900s. Morgan’s experiments with the fruit fly Drosophila melanogaster were the first to demonstrate that genes are located on chromosomes and are the basis of heredity.One of Morgan’s first challenges was to find fruit flies with mutant phenotypes. After nearly two years of tedious work, he discovered a single male fly with white eyes, instead of the usual (wild-type) red eye color. Through his genetic experiments with the white-eyed fly, Morgan deduced that a fruit fly’s eye color was somehow linked to its sex.

Part A – Experimental technique: Reciprocal crosses

When Gregor Mendel conducted his genetic experiments with pea plants, he observed that a trait’s inheritance pattern was the same regardless of whether the trait was inherited from the maternal or paternal parent. Mendel made these observations by carrying out reciprocal crosses: For example, he first crossed a female plant homozygous for yellow seeds with a male plant homozygous for green seeds, and then crossed a female plant homozygous for green seeds with a male plant homozygous for yellow seeds.Unlike Mendel, however, Morgan obtained very different results when he carried out reciprocal crosses involving eye color in his fruit flies. The diagram below shows Morgan’s reciprocal cross: He first crossed a homozygous red-eyed female with a white-eyed male, and then crossed a homozygous white-eyed female with a red-eyed male.

Drag the labels to their appropriate locations to complete the Punnett squares for Morgan’s reciprocal cross.

  • Drag blue labels onto the blue targets to indicate the genotypes of the parents and offspring.
  • Drag pink labels onto the pink targets to indicate the genetic makeup of the gametes (sperm and egg).

Labels can be used once, more than once, or not at all.


Part B – Experimental results: The F2 generation

In one of Morgan’s experiments, he crossed his newly discovered white-eyed male with a red-eyed female. (Note that all of the females at that time were homozygous for red eyes because the allele for white eyes had not yet propagated through Morgan’s flies.) All of the F1 flies produced by this cross (both males and females) had red eyes.

Next, Morgan crossed the red-eyed F1 males with the red-eyed F1 females to produce an F2 generation. The Punnett square below shows Morgan’s cross of the F1 males with the F1 females.

Drag the labels to their appropriate locations to complete the Punnett square for Morgan’s F1 x F1 cross.

  • Drag pink labels onto the pink targets to indicate the alleles carried by the gametes (sperm and egg).
  • Drag blue labels onto the blue targets to indicate the possible genotypes of the offspring.

Labels can be used once, more than once, or not at all.


Part C – Experimental prediction: Comparing autosomal and sex-linked inheritance

You now know that inheritance of eye color in fruit flies is sex-linked: The gene encoding eye color is located on the X chromosome, and there is no corresponding gene on the Y chromosome.How would the inheritance pattern differ if the gene for eye color were instead located on an autosome (a non-sex chromosome)? Recall that for autosomes, both chromosomes of a homologous pair carry the same genes in the same locations.
Suppose that a geneticist crossed a large number of white-eyed females with red-eyed males.
Consider two separate cases:

  • Case 1: Eye color exhibits sex-linked inheritance.
  • Case 2: Eye color exhibits autosomal (non-sex-linked) inheritance. (Note: In this case, assume that the red-eyed males are homozygous.)
For each case, predict how many of the male and female offspring would have red eyes and white eyes.Drag the correct numbers on the left to complete the sentences on the right. Numbers can be used once, more than once, or not at all.

Linked Genes and Linkage Mapping

If two genes are found on different chromosomes, or if they are far enough apart on the same chromosome that the chance of a crossover between them is very high, the genes are considered to be unlinked. Unlinked genes follow Mendel’s law of independent assortment.If, however, two genes tend to “travel together” because they are near one another on the same chromosome, they are said to be linked. Linked genes do not follow Mendel’s law of independent assortment.

In this tutorial, you will compare the inheritance patterns of unlinked and linked genes.

Part A – Independent assortment of three genes

A wild-type tomato plant (Plant 1) is homozygous dominant for three traits: solid leaves (MM), normal height (DD), and smooth skin (PP).Another tomato plant (Plant 2) is homozygous recessive for the same three traits: mottled leaves (mm), dwarf height (dd), and peach skin (pp).

In a cross between these two plants (MMDDPP x mmddpp), all offspring in the F1 generation are wild type and heterozygous for all three traits (MmDdPp).

Now suppose you perform a testcross on one of the F1 plants (MmDdPp x mmddpp). The F2 generation can include plants with these eight possible phenotypes:

  • solid, normal, smooth
  • solid, normal, peach
  • solid, dwarf, smooth
  • solid, dwarf, peach
  • mottled, normal, smooth
  • mottled, normal, peach
  • mottled, dwarf, smooth
  • mottled, dwarf, peach
Assuming that the three genes undergo independent assortment, predict the phenotypic ratio of the offspring in the F2 generation.

Part B – Gene linkage and phenotypic ratios

Now, suppose that the three tomato genes from Part A did not assort independently, but instead were linked to one another on the same chromosome. Would you expect the phenotypic ratio in the offspring to change? If so, how?
Which statement best predicts the results of the cross MmDdPp x mmddpp assuming that all three genes are linked?
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Part C – Building a linkage map

Suppose that you perform the cross discussed in Part B: MmDdPp x mmddpp. You plant 1000 tomato seeds resulting from the cross, and get the following results:

Use the data to complete the linkage map below.

Drag the labels onto the chromosome diagram to identify the locations of and distances between the genes. Use the blue labels and blue targets for the genes; use the white labels and white targets for the distances. Gene m has already been placed on the linkage map.

Scientific Skills Exercise: Using the Chi-Square Test

Are two genes linked or unlinked?

Genes that are in close proximity on the same chromosome will result in the linked alleles being inherited together more often than not. But how can you tell if certain alleles are inherited together due to linkage or due to chance?

If genes are unlinked and therefore assort independently, the phenotypic ratio of offspring from an F1 testcross is expected to be 1:1:1:1. If the two genes are linked, however, the observed phenotypic ratio of the offspring will not match the expected ratio.

Given random fluctuations in the data, how much must the observed numbers deviate from the expected numbers for us to conclude that the genes are not assorting independently but may instead be linked? To answer this question, scientists use a statistical test called a chi-square (χ2) test. This test compares an observed data set to an expected data set predicted by a hypothesis (here, that the genes are unlinked) and measures the discrepancy between the two, thus determining the “goodness of fit.”

If the difference between the observed and expected data sets is so large that it is unlikely to have occurred by random fluctuation, we say there is statistically significant evidence against the hypothesis (or, more specifically, evidence for the genes being linked). If the difference is small, then our observations are well explained by random variation alone. In this case, we say the observed data are consistent with our hypothesis, or that the difference is statistically insignificant. Note, however, that consistency with our hypothesis is not the same as proof of our hypothesis.

Part A – Calculating the expected number of each phenotype

In cosmos plants, purple stem (A) is dominant to green stem (a), and short petals (B) is dominant to long petals (b). In a simulated cross, AABB plants were crossed with aabb plants to generate F1 dihybrids (AaBb), which were then test crossed (AaBb X aabb). 900 offspring plants were scored for stem color and flower petal length. The hypothesis that the two genes are unlinked predicts the offspring phenotypic ratio will be 1:1:1:1.

Using the ratio of 1:1:1:1, calculate the expected number of each phenotype out of the 900 total offspring. Drag the correct values onto the data table. Labels may be used once, more than once, or not at all.

Part B – Calculating the χ2 statistic

The goodness of fit is measured by χ2. This statistic measures the amounts by which the observed values differ from their respective predictions to indicate how closely the two sets of values match.

The formula for calculating this value is


where o = observed and e = expected.

The expected and observed data have been entered into the table below. Carry out the operations indicated in the top row. In the last column, enter your answers to two decimal places. Then add up the entries in the last column to find the χ2 value.

Part C – Interpreting the data

The χ2 value means nothing on its own–it is used to find the probability that, assuming the hypothesis is true, the observed data set could have resulted from random fluctuations. A low probability suggests the observed data is not consistent with the hypothesis, and thus the hypothesis should be rejected.
What is the hypothesis that you are testing?

Part D

A standard cut-off point biologists use is a probability of 0.05 (5%). If the probability corresponding to the χ2 value is 0.05 or less, the differences between observed and expected values are considered statistically significant and the hypothesis should be rejected. If the probability is above 0.05, the results are not statistically significant; the observed data is consistent with the hypothesis.

To find the probability, locate your χ2 value (2.14) in the χ2 distribution table below. The “degrees of freedom” (df) of your data set is the number of categories (here, 4 phenotypes) minus 1, so df = 3.

Chi-square distribution table

Between which values on the df = 3 line does your calculated χ2 value lie?

Part E

The column headings show the probability range for your χ2 number.

Chi-square distribution table

What is the probability range that your data fit the expected 1:1:1:1 ratio of phenotypes?


Part F

Based on whether there are non-significant differences (p > 0.05) or significant differences (p ≤ 0.05) between the observed and expected values, you can determine if the data are consistent with the hypothesis that the two genes are unlinked and assorting independently.
Do your results support the hypothesis that the stem color and petal length genes are unlinked and assorting independently, or do the observed values differ from the expected values enough to reject this hypothesis?

Chromosomal Mutations

Chromosomal mutations are changes in the normal structure or number of chromosomes.

  • Changes in chromosome structure can result from errors in meiosis or from exposure to radiation or other damaging agents.
  • Certain changes in chromosome number can result from nondisjunction during either meiosis or mitosis.

Both structural mutations and nondisjunction can play a role in trisomy 21, commonly known as Down syndrome.

Part A – Changes in chromosome structure

The diagram below shows two normal chromosomes in a cell. Letters represent major segments of the chromosomes.

The following table illustrates some structural mutations that involve one or both of these chromosomes. Identify the type of mutation that has led to each result shown.

Drag one label into the space to the right of each chromosome or pair of chromosomes. You can use a label once, more than once, or not at all.

Part B – Nondisjunction

Suppose a diploid cell with three pairs of homologous chromosomes (2n = 6) enters meiosis.

How many chromosomes will the resulting gametes have in each of the following cases?

Drag one label into each space at the right of the table. Labels can be used once, more than once, or not at all.

Part C – Trisomy 21

Down syndrome is caused by trisomy 21, the presence of three copies of chromosome 21. The extra copy usually results from nondisjunction during meiosis.In some cases, however, the extra copy results from a translocation of most of chromosome 21 onto chromosome 14. A person who has had such a translocation in his or her gamete-producing cells is a carrier of familial Down syndrome. The carrier is normal because he or she still has two copies of all the essential genes on chromosome 21, despite the translocation. However, the same may not be true for the carrier’s offspring.The diagram shows the six possible gametes that a carrier of familial Down syndrome could produce.

Suppose that a carrier of familial Down syndrome mated with a person with a normal karyotype. Which gamete from the carrier parent could fuse with a gamete from the normal parent to produce a trisomy-21 zygote?

Drag one of the white cells (representing gametes) to the white target in the diagram. Drag one of the pink cells (representing zygotes) to the pink target.

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. Why this is the case is explained by the way 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, he 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 terms to the appropriate blanks to complete the sentences below. Not all terms will be used.

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?

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.


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 color, 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 color and petal shape are found on different chromosomes. (This is independent assortment as observed by Mendel with the characters of seed color and shape.)

Hypothesis 2: The alleles for flower color 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 color and petal shape are found close to each other on the same chromosome.

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.)

Activity: Mistakes in Meiosis


Activity: Polyploid Plants



Misconception Question 71

Which of these descriptions of the behavior of chromosomes during meiosis explains Mendel’s law of segregation?


Misconception Question 73

Select the correct statement(s) about sex determination in animals.
Select all that apply.

Misconception Question 74

Imagine a human disorder that is inherited as a dominant, X-linked trait. How would the frequency of this disorder vary between males and females?



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