Chapter 15 Pre-Test Question 9
How are human mitochondria inherited?
Chapter 15 Pre-Test Question 2
Chapter 15 Pre-Test Question 3
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?
Chapter 15 Pre-Test Question 4
Chapter 15 Pre-Test Question 10
Chapter 15 Question 3
Chapter 15 Question 25
What does a frequency of recombination of 50% indicate?
Chapter 15 Question 26
Chapter 15 Pre-Test Question 5
In general, the frequency with which crossing over occurs between two linked genes depends on what?
Chapter 15 Pre-Test Question 7
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?
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?
AP Exam Prep Question 29
Misconception Question 72
Which of these descriptions of the behavior of chromosomes during meiosis explains Mendel’s law of independent assortment?
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?
Chapter 15 Question 23
Which of the following statements is true of linkage?
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 ?
Chapter 15 Question 41
One possible result of chromosomal breakage is for a fragment to join a nonhomologous chromosome. What is this alteration called?
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?
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):
Which of the following are the phenotypes of the parents in this cross?
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
|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.)
Part C – The inheritance of both a sex-linked trait and an autosomal trait in humans
- 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.
AP Exam Prep Question 28
Experimental Inquiry: What Is the Inheritance Pattern of Sex-Linked Traits?
Part A – Experimental technique: Reciprocal crosses
- 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
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 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
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.)
Linked Genes and Linkage Mapping
In this tutorial, you will compare the inheritance patterns of unlinked and linked genes.
Part A – Independent assortment of three genes
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:
Part B – Gene linkage and phenotypic ratios
Part C – Building a linkage map
Use the data to complete the linkage map below.
Scientific Skills Exercise: Using the Chi-Square Test
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.
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.
Part C – Interpreting the data
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.
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 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.
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?
Part C – Trisomy 21
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?
Make Connections: Chromosomal Inheritance and Independent Assortment of Alleles
Part A – Reviewing independent assortment of alleles
- 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.
Part B – Chromosomal inheritance during meiosis
- 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?
- 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.
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
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?