Chapter 7: Sex Determination and Sex-Chromosomes

Question 1

Which of the following statements about double fertilization is true?

Answer 1

It is the mechanism by which two sperm nuclei fertilize an oocyte and two endosperm nuclei, respectively.

Double fertilization occurs only in plants.

Question 2

Monozygotic twins result from ……

Answer 2

Monozygotic twins result from a fertilized egg splitting into two zygotes.

Question 3

True or False?

In humans, the male is the homogametic sex.

Answer 3

FALSE

Female humans are referred to as homogametic because they have only one type of sex chromosome, the X.

Question 4

Klinefelter syndrome in humans, which leads to underdeveloped testes and sterility, is caused by which chromosomal condition?

Answer 4

47, XXY

Question 5

In humans, the genetic basis for determining the sex “male” is accomplished by the presence of ________.

Answer 5

a portion of the Y chromosome

Question 6

A recessive gene for red-green color blindness is located on the X chromosome in humans. Assume that a woman with normal vision (her father is color blind) marries a color-blind male. What is the likelihood that this couple’s first son will be color blind?

Answer 6

50%

First examine the possible genotypes of the parents. Color-blindness is a recessive sex linked trait. So the genotypes of the parents should be:
mother’s genotype (normal vision but whose father was color blind) XNXn

She is a carrier because of her heterozygous condition. For the recessive gene to be expressed one must be homozygous for it.

father’s genotype (color blind) XnY

From the Punnett square it is obvious that half of the children will be color blind and the other half will be normal. More specifically there will be 1 normal boy to 1 color-blind boy to 1 normal girl to 1 color- blind girl.

XNXn one normal (carrier) girl
XNY one normal boy
XnXn one color blind girl
XnY one color blind boy

Probability of having a color blind daughter is (1/2 x 1/2) or 1/4 or 25% (0.25) This is because both sex and colorblindness are involved in the solution.
The probability that their first son is color blind is 50% (0.50).

Question 7

Assume that a man who carries an X-linked gene has children. Assuming normal meiosis and random combination of gametes, the man would pass this gene to ________.

Answer 7

all of his daughters

Question 8

Individuals have been identified who have two different karyotypes, such as 45,X/46,XY or 45,X/46,XX. Such individuals are called ________.

Answer 8

mosaic

Question 9

Klinefelter and Turner syndromes have how many chromosomes, respectively?

Answer 9

Klinefelter: 27

Turner: 45

Question 10

True or False?

Normally in humans, all the sons of a male showing a sex-linked phenotype will inherit the trait.

Answer 10

FALSE

Question 11

True or False?

Normally in humans, all the sons of a female homozygous for a sex-linked recessive gene will inherit that trait.

Answer 11

TRUE

Question 12

True or False?

Sex-influenced genes are those that cause males to be males and females to be females.

Answer 12

FALSE

Question 13

True or False?

Sex-limited genes are those that cause males to be males and females to be females.

Answer 13

FALSE

Question 14

How many Barr bodies does an individual with Klinefelter syndrome generally have?

Answer 14

One

Barr bodies in a cell equal the number of X chromosomes minus one.

Question 15

The sex of birds, some insects, and other organisms is determined by a ZW chromosomal arrangement in which the males have like sex chromosomes (ZZ) and females are ZW (similar to XY in humans). Assume that a recessive lethal allele on the Z chromosome causes death of an embryo in birds. What sex ratio would result in the offspring if a cross were made between a male heterozygous for the lethal allele and a normal female?

Answer 15

2:1 male to female

Question 16

One form of hemophilia is caused by a sex-linked recessive gene. Assume that a man with hemophilia marries a phenotypically normal woman whose father had hemophilia. What is the probability that they will have a daughter with hemophilia? (In this problem, you must include the probability of having a daughter in your computation of the final probability.)

Answer 16

1/4

Question 17

Contrast the Protenor and Lygaeus modes of sex determination.

Answer 17

The Protenor form of sex determination involves the XX/XO condition, while the Lygaeus mode involves the XX/XY condition.

Question 18

In Drosophila, the sex of a fly with the karyotype XO:2A is ________.

Answer 18

male

Question 19

isogamous

Answer 19

Having male and female gametes that are morphologically identical. They produce isogametes

Question 20

Which one of the following statements best describes the major difference between sex determination in Drosophila and in humans?

Answer 20

In Drosophila, the ratio of X chromosomes to the number of haploid sets of autosomes determines the sex while in humans the presence of the Y chromosome determines maleness.

Question 21

How do mammals, including humans, solve the “dosage problem” caused by the presence of an X and Y chromosome in one sex and two X chromosomes in the other sex?

Answer 21

In females, one of the X chromosomes is condensed and largely inactive so that each sex has one active X chromosome.

Question 22

Cat breeders are aware that kittens expressing the X-linked calico coat pattern and tortoiseshell pattern are almost invariably females.
Which one of the following statements supports this conclusion?

Answer 22

Since females normally have two X chromosomes, random X inactivation during development of a heterozygous female will create a mottled pattern.
Normal males are not mottled, because they only have one X chromosome and therefore can only express one of the two potential X-linked alleles.

Question 23

Indicate the expected number of Barr bodies in interphase cells of individuals with the following karyotypes.

46 XX

Answer 23

1

Question 24

Indicate the expected number of Barr bodies in interphase cells of individuals with the following karyotypes.

47 XXY (Klinefelter syndrome)

Answer 24

1

Question 25

Indicate the expected number of Barr bodies in interphase cells of individuals with the following karyotypes.

47 XYY

Answer 25

0

Question 26

Indicate the expected number of Barr bodies in interphase cells of individuals with the following karyotypes.

48 XXXX

Answer 26

3

Question 27

Which of the following are properties of a Barr body?

It is an inactivated X chromosome.
It is formed as part of a mechanism for dose compensation.
The number of Barr bodies per cell is always equal to half the number of X chromosomes present.
It is a darkly stained region near the nuclear envelope of interphase cells.
It is the same X chromosome in every cell.

Answer 27

It is an inactivated X chromosome.

It is formed as part of a mechanism for dose compensation.

It is a darkly stained region near the nuclear envelope of interphase cells.

~~~~~~~~~~~~~~~~~~~~~~~
It is NOT always the same X chromosome in every cell.

The number of Barr bodies per cell is NOT always equal to half the number of X chromosomes present.

Question 28

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 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?

Answer 28

X-linked dominant

If the skin condition is caused by an X-linked dominant allele, a father would pass the allele on to all of his daughters, who would all have the skin condition. In contrast, the father would not pass the allele on to any of his sons because the sons would receive the father’s Y chromosome, not his X chromosome. As a result, none of the sons would inherit the skin condition.

Question 29

Examples of mechanisms of sex determination in animal systems

Answer 29

Ratio of X chromosomes to Sets of Autosomes determines sex in Drosophila

Temperature-Dependent sex determination in reptiles

haploid vs diploid in honey bees:
(male bees are haploid, while female bees are diploid)

Question 30

Focus on mammals

Answer 30

Primary sex determination: gonads (from mesoderm)

Secondary sex determination: overall male or female phenotype

Early vertebrate development similar

Early vertebrate gonad development also similar

Question 31

gonads develop from the _____

Answer 31

mesoderm

Question 32

Secondary sex determination

Answer 32

Ovary: estrogens
(estradiol)

Testis: androgens
(testosterone)

Question 33

Steroids

Answer 33

Powerful regulators, each sex has specific receptors for each

Essential difference between estradiol and testosterone is a methyl group

Question 34

Y chromosome determines maleness

Answer 34

XX female
XO female
XXX female
XXXX female

XY male
XXXY male
XXY male
OY (lethal)

XO: Turner’s syndrome
XXY: Kleinfelters syndrome

Question 35

Klinefelter’s Syndrome

Answer 35

XXY

Genitalia usually male, but with rudimentary testes, unable to produce sperm —> sterile

Tall, with long arms and legs

Some feminine development occurs

Ambiguous sexual development (intersexual)

Intelligence often below normal

~2 of 1000 male births

Question 36

Turner syndrome

Answer 36

XO

Female genitalia, rudimentary ovaries

Short stature (usually under 5 feet)

Skin flaps on neck

Shieldlike chest

Intelligence often normal

~1 in 2000 female births

Question 37

What determines maleness on Y?

Observe XX males and XY females.

HOW?

Answer 37

SRY: Sex determining region

Question 38

SRY

Answer 38

SRY: Sex determining region

Encodes gene that triggers gonadal tissue to develop testes

This product is called testis-determining factor (TDF)

SRY is present in all mammals examined

Found in mouse (Sry)

Question 39

Evidence of SRY as male determining region

Answer 39

Human males with 2 X chromomsomes
Often, these individuals have SRY region attached to one of the X chromosomes

Human females, 1 X, 1 Y
Their Y is generally missing SRY region

Question 40

Experimental demonstration of SRY

Answer 40

Mouse experiments:

• SRY analogous region found in mouse (Sry)
• Mouse Sry DNA injected into normal XX (females) mouse eggs
• Most of these offspring develop into males!
• Demonstrates power of Sry region in male development
• However, the male offspring were not fertile, they need other Y genes

Question 41

Another experiment:

Transgenic mouse experiments

Answer 41

XX mice injected with human SRY

• SRY expressed correctly
• Do not get sex transformation
• Therefore, we conclude that human SRY is different enough to not function in mouse

Question 42

Barr Body

Answer 42

Dosage Compensation Mechanism

Is an inactivated X

Prevents excess of X-linked gene products

Happens mid-gestation

All subsequent daughter cells inactivate same copy of X

Multiple X genotypes inactivate all but 1 X

Question 43

One copy of X inactivated

Answer 43

Result is Barr body

Question 44

Mechanism of X inactivation

Answer 44

Region called X-inactivation center (Xic)

Expression of this region leads to inactivation of this X

Correlation of constant expression of Xic & X-inactivation

RNA product of Xic hypothesized to coat that X & inactivate it: cis acting

Many aspects still unknown

Question 45

If X inactivation “turns off” all but one X chromosome, why aren’t Klinefelters individuals XXY normal?

Answer 45

The inactivated X chromosome isn’t completely inactivated, just mostly inactivated.

Question 46

Genetics of the Calico Cat

Answer 46

X linked allele
B = brown hair color
O= orange hair color

Cell divisions; gene expression,
orange patches of hair

In another cell (and all of its subsequent daughter cells) the other X is inactivated, resulting in brown hair color areas

Calico cats are always female