Ch 12: The Chromosomal Basis of Inheritance

Question 1

Summarize the chromosome theory of inheritance.

Answer 1

Mendelian genes have specific loci (positions) along chromosomes, and it is the chromosomes that undergo segregation and independent assortment.

Question 2

A cell goes through S phase, meiosis I, and cytokinesis. As the daughter cells are beginning meiosis II, which of the following is an appropriate description of the contents of each cell?

a. ) Each has half the chromosomes and the same amount of DNA as the original cell.
b. ) Each has half the chromosomes and one-fourth the amount of DNA as the original cell.Press enter after select an option to check the answer
c. ) Each has half the chromosomes and half the amount of DNA as the original cell.Press enter after select an option to check the answer
d. ) Each has half the chromosomes and twice the amount of DNA as the original cell.

Answer 2

a.) Each has half the chromosomes and the same amount of DNA as the original cell.

Homologous pairs separate during meiosis I, reducing the number of chromosome sets from two (diploid) to one (haploid). As each daughter cell begins meiosis II, it has one set of chromosomes—half the chromosomes found in the original cell. However, since each of these chromosomes was duplicated during S phase, the daughter cell has the same amount of DNA as the original cell.

Question 3

During a single crossover event, how many strands of DNA must break? (Recall that DNA is double-stranded.)

a. ) eight strands
b. ) four strands
c. ) two strands
d. ) one strand

Answer 3

b.) four strands

After interphase, the chromosomes have been duplicated and the two members of the homologous pair associate along their length. The double-stranded DNA molecules of two nonsister chromatids—one maternal and one paternal—are broken at precisely corresponding points. During synapsis, each of the four double-stranded DNA breaks is closed up so that each broken (double-stranded) end is joined to the corresponding broken (double-stranded) end of the nonsister chromatid, producing crossovers.

Question 4

The phenotype for a character most commonly observed in natural populations is called the ____ ____.

Answer 4

wild type

Question 5

Traits that are alternatives to the wild type are known as ______ __________ because they are due to alleles assumed to have originated as changes, or mutations, in the wild-type allele.

Answer 5

mutant phenotypes

Question 6

Morgan and his students invented a notation for symbolizing alleles in Drosophila. For a given character in flies, the gene takes its symbol from the first mutant (non-wild type) discovered. Thus, the allele for white eyes in Drosophila is symbolized by . The superscript + identifies the allele for the wild-type trait: __ for the allele for red eyes, for example.

Answer 6

• w —-→ indicates the allele for white eyes*
• w⁺* —-→ indicates red eyes (the wild type)

Question 7

What did Morgan learn from his experiments with flies?

Answer 7

That the gene involved in his white-eyed mutant fly was located exclusively on the X chromosome.

Question 8

In Morgan’s experiment, he found that a female fly could have white eyes only if…..

Answer 8

….both her X chromosomes carried the recessive mutant allele (w).

Question 9

Morgan’s work indicated that genes located on a sex chromosome exhibit ______ __________ patterns.

Answer 9

unique inheritance

Question 10

The physical basis for the law of segregation is the _________ of ________ in ________ I.

Answer 10

separation of homologs in anaphase I

Question 11

The physical basis for the law of independent assortment is the ___________ ___________ of all the different homologous chromosome pairs in _________ I.

Answer 11

alternative arrangements

metaphase I

Question 12

The law of segregation states that the _______ for a particular character segregate during gamete formation.

Answer 12

alleles

Question 13

A ________ trait is expressed in an organism with one or two copies of the allele.

A _________ trait is expressed only if an organism has two copies of the same allele.

Answer 13

dominant

recessive

Question 14

An individual organism with two of the ____ ______ for a particular character is homozygous.

An individual organism with two different alleles for a particular character is ___________.

Answer 14

same allele

heterozygous

Question 15

An individual’s genotype describes which alleles are present at a _______ ____ _____.

An individual’s _________ describes which trait is expressed.

Answer 15

specific gene locus

phenotype

Question 16

A botanist has acquired a group of sweet pea plants. All of the plants have yellow pea pods (the recessive trait), except for one, which has green pea pods (the dominant trait). Pea pod color is a trait caused by a single gene.

How could the botanist best determine whether the genotype of the green-pod plant is homozygous or heterozygous?

Answer 16

A cross between a plant of unknown genotype and one that is known to be homozygous recessive is called a test cross because the recessive homozygote tests whether there are any recessive alleles in the unknown. Because the recessive homozygote will contribute an allele for the recessive characteristic to each offspring, the second allele (from the unknown genotype) will determine the offspring’s phenotype.

Question 17

Sex classification can be thought of as…

Answer 17

….classification into a group with a shared set of anatomical and physiological traits.

Question 18

How do the X and Y chromosomes compare in size?

Answer 18

The Y chromosome is much smaller than the X.

Question 19

While all eggs have an X chromosome, half the sperm a male produces….

Answer 19

….receives an X chromosome, and half receives a Y.

Question 20

A gene on the Y chromosome – called SRY for sex-determining region of Y is required for the development of ______. In its absence, the gonads develop into ovaries, even in an __ embryo.

Answer 20

testes

XY

Question 21

What is a sex-linked gene?

Answer 21

A gene located on either sex chromosome.

Question 22

Since males and females inherit a different number of X chromosomes, this leads to a pattern of inheritance different from that of genes located on _________.

Answer 22

autosomes

Question 23

If an X-linked trait is due to a recessive allele, a female will express the phenotype only if she is…..

Answer 23

….homozygous for that allele.

Question 24

Because males only have one locus for an X-linked trait, the terms homozygous and heterozygous are not appropriate to use. Instead, the term __________ is used. Any male receiving the recessive allele from his mother will _______ ____ _____.

Answer 24

hemizygous

express that trait

Question 25

Duchenne muscular dystrophy is an X-linked disorder which is due to the absence of a key muscle protein called __________.

Answer 25

dystrophin

Question 26

What is hemophilia?

Answer 26

An X-linked recessive disorder defined by the absence of one or more clotting factors.

Question 27

Even though females have two X chromosomes, almost all of one X chromosome in each cell of female mammals becomes ___________ during early embryonic development

Answer 27

inactivated

Question 28

The inactive X in each cell of a female condenses into a compact object called a ____ ____, which lies along the inside of the nuclear envelope.

Answer 28

Barr body

Question 29

Interestingly, in the ovaries, Barr-body chromosomes are reactivated in the cells that give rise to ____, resulting in every female gamete (egg) having an active X after _______.

Answer 29

eggs

meiosis

Question 30

Inactivation of an X chromosome involves modification of the DNA and proteins bound to it called histones, including attachment of ______ ______ to DNA nucleotides.

Answer 30

methyl groups

Question 31

There is a particular region of each X chromosome that contains several genes involved in the inactivation process. The two regions, one on each X chromosome, associate briefly which each other in each cell at an early stage of embryonic development. Then, one of the genes called XIST (__________ ________ __________) becomes active only on the chromosome that will become the Barr body. Many copies of the RNA product of this gene they attach to the X chromosome on which they are made, eventually almost covering it.

Answer 31

X-inactive specific transcript

Question 32

When geneticists follow linked genes in breeding experiments, the results _______ from those expected from Mendel’s law of independent assortment.

Answer 32

deviate

Question 33

What is meant by the term “linked genes”?

Answer 33

This indicates two or more genes on the same chromosome that tend to be inherited together.

Question 34

Refer to the image. A 50% frequency of recombination in such testcrosses is observed for any two genes that are located on different chromosomes (non-linked genes). What does this mean?

Answer 34

50% of offspring will have new combinations of traits not seen in the parents, while the other 50% will have phenotypes which match one of the parents.

Question 35

If you complete a full Punnett square for two characters, how many total small boxes will you need?
Imagine you have Bb and Nn

Answer 35

16 (4 x 4)

Question 36

Deviations from Mendel’s patterns include

–when alleles are not completely dominant or recessive

–when a gene has more than two alleles

Question 37

In the examples of pedigrees shown in lecture, the shading in of a shape indicates that the individual HAS the trait being examined.

The pedigree tells us (explicitly) only the phenotype of the individuals, not the genotype.

We can infer the genotype from examining the pattern.

Answer 37

When every individual with the trait has at least 1 parent with the trait, this is a hint that the trait is inherited in an autosomal dominant fashion.

Question 38

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

A. the arrangement of each pair of homologous chromosomes on the metaphase plate during metaphase I is random with respect to the arrangements of other pairs.

B. Sister chromatids separate during anaphase II.

C. The two alleles for each gene separate as homologous chromosomes move apart during anaphase I.

Answer 38

C.

[(A) refers to the law of independent assortment, not segregation.]

Question 39

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 whether they just happen to assort together?

Answer 39

Use the X² test to analyze phenotypes of F1 testcross progeny in order to see whether two genes are linked or unlinked.

Question 40

If genes are unlinked and assorting independently, the phenotypic ratio of offspring from an F₁ testcross is expected to be _____.

However, if the two genes ARE linked, the phenotypic ratio of the offspring will ___ _____ that ratio.

Answer 40

1:1:1:1

not match

Question 41

The chi-square data 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 ______ ____________.

Answer 41

random fluctuations

Question 42

If the probability corresponding to the chi-square value is low the null hypothesis should be ________.

Answer 42

rejected

Question 43

If the probability corresponding to the chi square value is 0.05 (5%) or less, then the differences between the observed and expected values are considered ____________ ____________.

Answer 43

statistically significant

Question 44

If the probability is above 0.05, the results are not statistically significant, and the observed data are __________ with the ___________.

Answer 44

consistent with the hypothesis

Question 45

What is a genetic map?

Answer 45

An ordered list of the genetic loci along a particular chromosome.

Question 46

What is the probability that a male will inherit an X-linked recessive allele from his father?

Answer 46

0%

Question 47

In an X-linked, or sex-linked, trait in humans, it is the contribution of __________ that determines whether a son will display the trait.

Answer 47

the mother

Question 48

If a mother is heterozygous for a recessive sex-linked trait and her husband has the dominant allele, which one of the following is true about the probabilities for their children?

Answer 48

Their sons will have a 50% chance of inheriting the recessive allele.

Question 49

A woman is red-green color-blind. What can we conclude, if anything, about her father?

a. ) There is a 50% probability that he has normal vision.
b. ) We have too little information to tell.
c. ) He has two Y chromosomes.
d. ) He is red-green color-blind.
e. ) None of the listed responses is correct.

Answer 49

d.) he is red-green color-blind.

For the woman to be affected with this sex-linked recessive trait, she must be homozygous; therefore, she must have received a recessive allele from her father. The father’s single X must have the recessive allele.

Question 50

A woman is a carrier for red-green color blindness, a sex-linked trait. Her husband is normal (not color-blind) for this trait. What are the chances that their newborn daughter will be red-green color-blind?

Answer 50

0%

Since the father is not color blind, he only has a normal allele to give her. The father only has that one X to give her, and since he is normal, she will NOT be color blind.

Question 51

Duchenne muscular dystrophy is caused by a sex-linked recessive allele. Its victims are almost invariably boys, who usually die before the age of 20. Why is this disorder almost never seen in girls?

Answer 51

To express an X-linked recessive allele, a female must have two copies of the allele.

Question 52

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 _____________ gene on the Y chromosome.

Answer 52

corresponding

Question 53

Case 1: Eye color exhibits 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.

How many of the male and female offspring would have red eyes and white eyes?

Answer 53

Case 1: Eye color exhibits sex-linked inheritance

If there were 100 female offspring, 100 would have red eyes and 0 would have white eyes.

If there were 100 male offspring, 0 would have red eyes and 100 would have white eyes.

Question 54

Case 2: Eye color exhibits autosomal dominance.

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.

How many of the male and female offspring would have red eyes and white eyes?

Answer 54

Case 2: Eye color exhibits autosomal (non-sex-linked) inheritance

3. If there were 100 female offspring, 100 would have red eyes and 0 would have white eyes.
4. If there were 100 male offspring, 100 would have red eyes and 0 would have white eyes.

Question 55

Which of the following statements correctly describes gene linkage?

a. ) Linked genes are found on different chromosomes.
b. ) The observed frequency of recombination of two genes that are far apart from each other has a maximum value of 100%.
c. ) The closer two genes are on a chromosome, the lower the probability that a crossover will occur between them.
d. ) Linked genes always assort independently.

Answer 55

c.) The closer two genes are on a chromosome, the lower the probability that a crossover will occur between them.

Question 56

What does a frequency of recombination of 50% between two genes indicate?

Answer 56

The two genes are likely to be located on different chromosomes.

Question 57

What is the relationship between recombinant frequency and the proximity of two genes to one another?

Answer 57

The lower the frequency of recombination, the closer two genes likely are to one another.

Question 58

Name 4 alterations to chromosome structure that can occur.

Answer 58

deletion, duplication, inversion, translocation