Chapter 12The Chromosomal Basis of Inheritance

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?

A) The gene involved is on an autosome, but only in males.

B) The gene involved is on the X chromosome.

C) Other male-specific factors influence eye color in flies.

D) The gene involved is on the Y chromosome.

E) Other female-specific factors influence eye color in flies.

B

Thomas Hunt Morgan's choice of Drosophila melanogaster has been proven to be useful even today.

Which of the following has/have continued to make it a most useful species?

I. its four pairs of chromosomes

II. a very large number of visible as well as biochemically mutant phenotypes

III. easy and inexpensive maintenance

IV. short generation time and large number of offspring

A) I, II, and III only

B) II, III, and IV only

C) I, II, III, IV, and V

D) II and III only

E) I and IV only

C

SRY is best described in which of the following ways? 3)

A) a gene required for development, and males or females lacking the gene do not survive past

early childhood

B) a gene region present on the Y chromosome that triggers male development

C) a gene present on the X chromosome that triggers female development

D) an autosomal gene that is required for the expression of genes on the Y chromosome

E) an autosomal gene that is required for the expression of genes on the X chromosome

B

Cinnabar eyes is a sex-linked recessive characteristic in fruit flies. If a female having cinnabar eyes

is crossed with a wild-type male, what percentage of the F1 males will have cinnabar eyes?

A) 75% B) 0% C) 50% D) 25% E) 100%

E

Women (and all female mammals) have one active X chromosome per cell instead of two. What

causes this?

A) crossover between the XIST gene on one X chromosome and a related gene on an autosome

B) activation of the Barr gene on one of the two X chromosomes that then inactivates

C) inactivation of the XIST gene on the X chromosome derived from the male parent

D) modification of the XIST gene so that it is active only on one X chromosome, which then

becomes inactive

E) the removal of methyl (CH3) groups from the X chromosome that will remain active

D

Three genes at three loci are being mapped in a particular species. Each has two phenotypes, one

of which is markedly different from the wild type. The unusual allele of the first gene is inherited

with either of the others about 50% of the time. However, the unusual alleles of the other two

genes are inherited together 14.4% of the time. Which of the following describes what is

happening?

A) The first gene is linked but the other two are not.

B) The first gene is assorting independently from the other two that are linked.

C) The genes are showing independent assortment.

D) The first gene is located 14.4 units apart from the other two.

E) The three genes are linked.

B

Why does recombination between linked genes continue to occur?

A) The forces on the cell during meiosis II always result in recombination.

B) Without recombination there would be an insufficient number of gametes.

C) Recombination must occur or genes will not assort independently.

D) Recombination is a requirement for independent assortment.

E) New allele combinations are acted upon by natural selection.

E

If nondisjunction occurs in meiosis II during gametogenesis, what will be the result at the

completion of meiosis?

A) There will be three extra gametes.

B) 1/4 of the gametes will be n + 1, 1/4 will be n - 1, and 1/2 will be n.

C) All the gametes will be diploid.

D) Half of the gametes will be n + 1, and half will be n - 1.

E) Two of the four gametes will be haploid, and two will be diploid.

B

One possible result of chromosomal breakage is for a fragment to join a nonhomologous

chromosome. What is this alteration called?

A) translocation

B) deletion

C) transversion

D) duplication

E) inversion

A

In humans, male-pattern baldness is controlled by an autosomal gene that occurs in two allelic

forms. Allele Hn determines nonbaldness, and allele Hb determines pattern baldness. In males,

because of the presence of testosterone, allele Hb is dominant over Hn. If a man and woman both

with genotype HnHb have a son, what is the chance that he will eventually be bald?

A) 33% B) 25% C) 75% D) 50% E) 0%

C

A nonreciprocal crossover causes which of the following products?

A) nondisjunction

B) duplication only

C) duplication and nondisjunction

D) deletion only

E) deletion and duplication

E

Abnormal chromosomes are frequently found in malignant tumors. Errors such as translocations

may place a gene in close proximity to different control regions. Which of the following might then

occur to make the cancer worse?

A) death of the cancer cells in the tumor

B) expression of inappropriate gene products

C) a decrease in mitotic frequency

D) sensitivity of the immune system

E) an increase in nondisjunction

B

A couple has a child with Down syndrome. The mother is 39 years old at the time of delivery.

Which of the following is the most probable cause of the child's condition?

A) One member of the couple underwent nondisjunction in gamete production.

B) One member of the couple underwent nondisjunction in somatic cell production.

C) The mother had a chromosomal duplication.

D) The woman inherited this tendency from her parents.

E) One member of the couple carried a translocation.

A

Genomic imprinting is generally due to the addition of methyl (-CH3) groups to C nucleotides in

order to silence a given gene. If this depends on the sex of the parent who transmits the gene,

which of the following must be true?

A) The imprints are transmitted only to gamete-producing cells.

B) Genes required for early development stages must not be imprinted.

C) Methylation of this kind must occur more in males than in females.

D) Methylation of C is permanent in a gene.

E) Methylation must be reversible in ovarian and testicular cells.

E

In a series of mapping experiments, the recombination frequencies for four different linked genes

of Drosophila were determined as shown in Figure 15.2. What is the order of these genes on a

chromosome map?

(view image on practice quiz)

A) rb-cn-vg-b

B) vg-cn-b-rb

C) cn-rb-b-vg

D) b-rb-cn-vg

E) vg-b-rb-cn

D

The pedigree in Figure 15.3 shows the transmission of a trait in a particular family. Based on this

pattern of transmission, the trait is most likely

(view pedigree on practice quiz)

A) autosomal recessive.

B) autosomal dominant.

C) mitochondrial.

D) sex-linked recessive.

E) sex-linked dominant.

C

A man who is an achondroplastic dwarf with normal vision marries a color-blind woman of normal height. The man's

father was 6 feet tall, and both the woman's parents were of average height. Achondroplastic dwarfism is autosomal

dominant, and red-green color blindness is X-linked recessive.

17) They have a daughter who is a dwarf with normal color vision. What is the probability

that she is heterozygous for both genes?

100%

18) What proportion of their sons would be color-blind and of normal height?

Half

19) How many of their daughters might be expected to be color-blind dwarfs?

None

Problem Sets: A white female Drosophila is mated with a red-eyed (wild type) male, the reciprocal cross of the one shown in fig 15.4. What phenotypes and genotypes do you predict for offspring?

Because the gene for this eye-color character is located on the X chromosome, all female offspring will be red-eye heterzygous; all male offspring will inherit a Y chromosome from the father and be white eyed.

Neither Tim nor Rhoda has Duchenne muscular dystrophy, but their firstborn son does have it. What is the probability that a second child of this couple will have the disease? Probability if second child is girl or boy?

1/4 (1/2 chance that the child will inherit a Y chromosome from the father and be male x 1/2 chance that he will inherit the X carrying the disease allele from his mother). If the child is a boy, there is a 1/2 chance that he will have the disease, a female would have 0 chance. (but 1/2 chance of being a carrier).

Consider what you learned about dominant and recessive alleles. If a disorder were caused by a dominant X-Linked allele, how would the inheritance pattern differ from what we see for recessive X-linked disorders?

With a disorder caused by a dominant allele there is no such thing as a carrier, since those with the allele have the disorder. Because ther allele is dominant, the females lose any advantage in having two X chromosomes since one disorder-associated allele is sufficient to result in the disorder. All fathers who have the dominant allele will pass it along to all their daughters, who will also have the disorder. A mother who has the alllee will pass it to half of her sons and half of her daughters.

Genes A,B and C are located on the same chromosome. Testcrosses show that the recombination frequency between A and B is 28% and between A and C is 12% . Can you determine the linear order of these genes? Explain?

No. The order could be A-C-B or C-A-B. To determine which possiblity is correct, you need to know recombination frequency between B and C.

A man with heophilia (a recessive sex linked condition) has a daughter of normal phenotype. She marries a man who is normal for the trait. What is the probability that a daughter of this mating will be hemophilliac? The son to be a phemophiliac? If the couple has 4 sons, what is the probability that all 4 will be born with hemophilia?

0, 1/2, 1/16

Pseudohypertrophic muscular dystrophy is an inherited disorder. It is seen almost exclusively in boys born to apparently normal parents. Is this disorder caused by a dominant or recessive allele? Is it sex linked or autosomal? How do you know? Explain why this is almost never seen in girls.

Recessive; if the disorder were dominant it would affect at least one parent of a child born with the disorder. The disorder inheritance is sex linked because it is seen only in boys. For a girl to have the disorder, she would have to inherit recessive alleles from both parents. This is very rare, since males with the recessive allele on their X chromosome die in their early teens.

A wild-type fruit fly (heterzygous for gray body color and normal wings) is mated with a black fly w/ vestigal wings. Phenotypic distribution for offspring: wild type 778, black vestigal 785, black normal 158, gray vestigal 162. What is the recombination frequency between these genes for body color and wing size?

17%

What pattern of inheritance would lead to a geneticist to suspect that an inherited disorder of cell metabolism is due to a defective mitochondrial gene?

The disorder would always be inherited from the mother.

A space probe discovers a planet inhabited by creatures that reproduce with the same hereditary patterns seen in humans. Height (T=tall t= dwarf) head appendages (A=antennae, a=no antennae) nose morphology (S=upturned snount, s=downturn snout). Since the creatures are not intelligent, earth scientists are able to do some controlled breeding experiments using heterozygotes in testecrosses. For tall heterozygotes with antennae, the offspring are tall-antennae 46, dwaft antennae 7, dwarf no antennae 42, tall no antennae, 5. For heterzygotes with antennae and upturned snout the offspring are: antennae upturned snout 47, antennae downturned snout 2, no antennae downturned snout 48, no antennae upturned snout 3. Calculate the recombination frequencies for both experiments.

Between T and A, 12% , between A and S, 5%

Using the information from problem 5, scientists do a further testcross using a heterzygote for height and nose. The offspring: tall upturned snout 40, dwarf upturned snout 9, dwarf downturned snout 42, tall downturned snout 9. Calculate recombination frequency. Use answer from problem 5 to determine correct sequence of 3 linked genes.

Between T and S, 18%, sequence of genes is T-A-S.

Red-green color blindness is sex-linked recessive allele. A color blind man marries a woman with normal vision whose father was color-blind. What is the probability that they will have a color blind daughter? What is the probability that their first son will be color-blind?

1/4 for each daughter (1/2 chance that child will be female x 1/2 chance of homozygous recessive genotype); 1/2 for first son

A wild-type fruit fly is mated with a black fruit fly with purple eyes. Offspring: wild type 721, black-purple 751, gray-purple 49, black-red 45. What is recombination frequency between genes for body and eye color? Using information from problem 3, what fruit flies would you mate to determine the sequency of body color, wing size, and eye color genes on the chromosome?

6%. wild-type heterozygous for normal wings and red eyes x recessive homozygous for vestigial wings and purple eyes

Women born with an extra X chromosome (XXX) are generally healthy and appear normal as XX women. What is likely explanation for this finding? How could you test this explanation?

The inactivation of two X chromosomes in XXX women would leave them with one genetically active X, as in woman with the normal number of chromosomes.

Determine the sequence of genes along a chromosome based on following recombination frequencies: A-B 8 map units, A-C 28 map units, A-D 25 map units, B-C 20 map units, B-D 33 map units.

D-A-B-C

Two genes of a flower, one controlling blue (B) versus white (b) petals and the other controlling round (R) vs. oval (r) stamens, are linked and are 10 map units apart. You cross a homozygous blue-oval plant with a homozygous white-round plant. The resulting F1 progeny are crossed with homozygous white oval plants and 1,000 F2 progeny are obtained. How many F2 plants of each of the 4 phenotypes do you expect?

450 each of blue-oval and white-round (parentals) and 50 each of each blue-round and white-oval (recombinants)

15.5 (1-3)

Gene dosage, the number of active copies of a gene, is important to proper development. Identify and describe two processes that establish the proper dosage of certain genes.

Inactivation of an X chromosome in females and genomic imprinting. Because of X inactivation, the effective dose of genes on the X chromosome is same in males and females. As a result of genomic imprinting, only one allele of certain genes is phenotypically expressed.

Reciprocal crosses between two primrose varieties A and B produced the following results: A female X B male----> offspring with all green leaves. B female x A male --->offspring with spotted leaves. Explain these results.

The genes for lead coloration are located in plastids within the cytoplasm. Normally, only the maternal parent transmits plastid genes to offspring. Since variegated offspring are produced only when the female parent is of B variety, we can conclude that variety B contains both the wild type and mutant alleles of pigment genes, producing variegated leaves. (Variety A contains only the wild-type allele of pigment genes).

Mitochondrial genes are critical to the energy metabolism of cells, but mitochondrial disorders caused by mutations in these genes are generally not lethal. Why not?

The situation is similar to that for chloroplasts. Each cell contains numerous mitochondria and in affected individuals, most cells contain a variable mixture of normal and mutant mitochondria. The normal mitochondria carry out enough cellular respiration for survival.

Explain the chromosomal basis of Mendel's laws

According to this theory, Mendelian genes have specific loci along chromosomes, and it is the chromosomes that undergo segregation and independent assortment.

Why do fruit flies make ideal model organisms for

genetic research?

- a single mating will produce hundreds of offspring and a new generation can be bred every two weeks

-only has 4 pairs of chromosomes which are easily seen within a light microscope

Compare and contrast wild type traits with mutant

phenotypes

The phenotype for a character most commonly observed in natural populations, such as red-eyes in Drosophila is called the wild type.

Traits that are alternatives to the wild-type such as white eyes in Drosophila are called mutant phenotypes because they are due to alleles assumed to have originated as changes, or mutations, in the wild-type allele.

Explain the chromosomal basis of sex

-humans and other mammals have two varities of sex chromosomes

-X and Y chromosome

-Y is much smaller than the X

-XX is female

-Xy is male

-two sex chromosomes segregate during meiosis and each gamete receives one

-each egg contains one X, in contrast, half sperm cells a male produces contain and X and half contain a Y

What is a sex-linked gene?

A gene located on either sex chromosome; those located on the Y chromosome are y-linked genes.

Human x-chromosome contains 1,100 genes which are called X-linked genes.

Explain X inactivation in females

Female mammals inherit 2 X chromosomes

-most of one X chromosome in each cell in female mammals become inactived during early embryonic development

-as a result, cells of females and males have same effective dose

-inactive X in each cell of a female condenses into a compact object called a Barr body

-lies along inside of nuclear envelope

-in the ovaries, Barr-body chromosomes are reactivated in the cells that give rise to eggs, so every female gamete has an active X

What is linkage?

Genes located near eachother on the same chromosome tend to be inherited together in genetic crosses; such genes are said to be genetically linked and are called linked genes.

-two or more genes on the same chromosome that tend to be inherited together

Explain how the frequency of parental types and

recombinants can be used to create linkage

maps

-farther apart the two genes are, the higher probability that a crossover will occur between them and a higher recombination

-distance between genes expressed in map units (1 map unit is 1% recombination frequency)

-maximum value of 50%

What is nondisjunction? Explain how it leads to

abnormal chromosome number using a diagram.

-there is an ocassional mishap called a nondisjunction, where members of a pair of homologous chromosomes do not move apart properly during meiosis I or sister chromatids fail to separate in meiosis II

-see fig 15.13 on page 297

Define polyploidy and explain how it may arise

-some organisms have more than two complete chromosome sets in all somatic cells

-general term for this chromosomal alteration

-triploidy (3n) and tetraploidy (4n)

-triploidy may arise is by fertilzation of an abnormal diploid egg produced by nondisjunction of all its chromosomes

-tetraploidy could result from failure of a 2n zygote to divide after replicating its chromosomes

-fairly common in plant kingdom

Explain deletions, duplications, inversions, and

translocation

deletions= occurs when a chromosomal fragment is lost

The affected chromosome is then missing certain genes

-the deleted fragment may become attached as an extra segment to a sister chromatid producing a duplication

-a chromosomal fragment may also reattach to the original chromosome but in reverse orientation, producing an inversion

-a 4th possibly result of chromosomal breakage is for fragment to join a nonhomologous chromosome, a rearrangement called a translocation

What is genomic imprinting and why is it an exception to the standard chromosome theory

-geneticists have identified two to three dozen traits in mammals that depend on which parent passed along the alleles for those traits

-such variation in phenotype depending on whether an allele is iniherited from the male or female parent is called genomic imprinting

-occurs during gamete formation and results in the silencing of a particular allele of certain genes

-genes are imprinted differently in sperm and eggs, a zygote expressess only one allele of an imprinted gene

-imprints are then transmitted to all body cells during development

-in each generation, old imprints are erased in gamete-producing cells

-many cases consists of -Ch3 groups (methyl)

Explain how the inheritance of organelles is an exception to the standard chromosome theory

-not all of eukaryotic cell's genes are located on nuclear chromosomes, or even in the nucleus

-some genes are located in organelles in the cytoplasm..extranuclear genes or cytoplasmic genes

-mitochondria as well as chloroplasts contain small circular DNA molecules that carry a number of genes

-source is maternal parent

-almost all mitochondria passed onto a zygote come from the cytoplasm of the egg