Ch 12: The Chromosomal Basis of Inheritance Flashcards

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1
Q

Summarize the chromosome theory of inheritance.

A

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

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2
Q

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.

A

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.

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3
Q

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

A

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.

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4
Q

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

A

wild type

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5
Q

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.

A

mutant phenotypes

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6
Q

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.

A
  • w —-→ indicates the allele for white eyes*
  • w+* —-→ indicates red eyes (the wild type)
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7
Q

What did Morgan learn from his experiments with flies?

A

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

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8
Q

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

A

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

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9
Q

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

A

unique inheritance

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10
Q

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

A

separation of homologs in anaphase I

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11
Q

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

A

alternative arrangements

metaphase I

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12
Q

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

A

alleles

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13
Q

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.

A

dominant

recessive

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14
Q

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 ___________.

A

same allele

heterozygous

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15
Q

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

An individual’s _________ describes which trait is expressed.

A

specific gene locus

phenotype

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16
Q

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?

A

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.

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17
Q

Sex classification can be thought of as…

A

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

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18
Q

How do the X and Y chromosomes compare in size?

A

The Y chromosome is much smaller than the X.

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19
Q

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

A

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

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20
Q

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.

A

testes

XY

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21
Q

What is a sex-linked gene?

A

A gene located on either sex chromosome.

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22
Q

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 _________.

A

autosomes

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23
Q

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

A

….homozygous for that allele.

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24
Q

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 _______ ____ _____.

A

hemizygous

express that trait

25
Q

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

A

dystrophin

26
Q

What is hemophilia?

A

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

27
Q

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

A

inactivated

28
Q

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.

A

Barr body

29
Q

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 _______.

A

eggs

meiosis

30
Q

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

A

methyl groups

31
Q

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.

A

X-inactive specific transcript

32
Q

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

A

deviate

33
Q

What is meant by the term “linked genes”?

A

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

34
Q

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?

A

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.

35
Q

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

A

16 (4 x 4)

36
Q

Deviations from Mendel’s patterns include

–when alleles are not completely dominant or recessive

–when a gene has more than two alleles

A
37
Q

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.

A

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.

38
Q

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.

A

C.

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

39
Q

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?

A

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

40
Q

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.

A

1:1:1:1

not match

41
Q

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 ______ ____________.

A

random fluctuations

42
Q

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

A

rejected

43
Q

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 ____________ ____________.

A

statistically significant

44
Q

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

A

consistent with the hypothesis

45
Q

What is a genetic map?

A

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

46
Q

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

A

0%

47
Q

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

A

the mother

48
Q

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?

A

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

49
Q

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.

A

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.

50
Q

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?

A

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.

51
Q

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?

A

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

52
Q

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.

A

corresponding

53
Q

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?

A

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.

54
Q

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?

A

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

  1. If there were 100 female offspring, 100 would have red eyes and 0 would have white eyes.
  2. If there were 100 male offspring, 100 would have red eyes and 0 would have white eyes.
55
Q

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.

A

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

56
Q

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

A

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

57
Q

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

A

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

58
Q

Name 4 alterations to chromosome structure that can occur.

A

deletion, duplication, inversion, translocation