Chapter 7: Sex Determination and Sex-Chromosomes Flashcards
Which of the following statements about double fertilization is true?
It is the mechanism by which two sperm nuclei fertilize an oocyte and two endosperm nuclei, respectively.
Double fertilization occurs only in plants.
Monozygotic twins result from ……
Monozygotic twins result from a fertilized egg splitting into two zygotes.
True or False?
In humans, the male is the homogametic sex.
FALSE
Female humans are referred to as homogametic because they have only one type of sex chromosome, the X.
Klinefelter syndrome in humans, which leads to underdeveloped testes and sterility, is caused by which chromosomal condition?
47, XXY
In humans, the genetic basis for determining the sex “male” is accomplished by the presence of ________.
a portion of the Y chromosome
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?
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).
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 ________.
all of his daughters
Individuals have been identified who have two different karyotypes, such as 45,X/46,XY or 45,X/46,XX. Such individuals are called ________.
mosaic
Klinefelter and Turner syndromes have how many chromosomes, respectively?
Klinefelter: 27
Turner: 45
True or False?
Normally in humans, all the sons of a male showing a sex-linked phenotype will inherit the trait.
FALSE
True or False?
Normally in humans, all the sons of a female homozygous for a sex-linked recessive gene will inherit that trait.
TRUE
True or False?
Sex-influenced genes are those that cause males to be males and females to be females.
FALSE
True or False?
Sex-limited genes are those that cause males to be males and females to be females.
FALSE
How many Barr bodies does an individual with Klinefelter syndrome generally have?
One
Barr bodies in a cell equal the number of X chromosomes minus one.
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?
2:1 male to female
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.)
1/4
Contrast the Protenor and Lygaeus modes of sex determination.
The Protenor form of sex determination involves the XX/XO condition, while the Lygaeus mode involves the XX/XY condition.
In Drosophila, the sex of a fly with the karyotype XO:2A is ________.
male
isogamous
Having male and female gametes that are morphologically identical. They produce isogametes
Which one of the following statements best describes the major difference between sex determination in Drosophila and in humans?
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.
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?
In females, one of the X chromosomes is condensed and largely inactive so that each sex has one active X chromosome.
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?
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.
Indicate the expected number of Barr bodies in interphase cells of individuals with the following karyotypes.
46 XX
1
Indicate the expected number of Barr bodies in interphase cells of individuals with the following karyotypes.
47 XXY (Klinefelter syndrome)
1
Indicate the expected number of Barr bodies in interphase cells of individuals with the following karyotypes.
47 XYY
0
Indicate the expected number of Barr bodies in interphase cells of individuals with the following karyotypes.
48 XXXX
3
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.
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.
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?
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.
Examples of mechanisms of sex determination in animal systems
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)
Focus on mammals
Primary sex determination: gonads (from mesoderm)
Secondary sex determination: overall male or female phenotype
Early vertebrate development similar
Early vertebrate gonad development also similar
gonads develop from the _____
mesoderm
Secondary sex determination
Ovary: estrogens
(estradiol)
Testis: androgens
(testosterone)
Steroids
Powerful regulators, each sex has specific receptors for each
Essential difference between estradiol and testosterone is a methyl group
Y chromosome determines maleness
XX female
XO female
XXX female
XXXX female
XY male
XXXY male
XXY male
OY (lethal)
XO: Turner’s syndrome
XXY: Kleinfelters syndrome
Klinefelter’s Syndrome
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
Turner syndrome
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
What determines maleness on Y?
Observe XX males and XY females.
HOW?
SRY: Sex determining region
SRY
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)
Evidence of SRY as male determining region
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
Experimental demonstration of SRY
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
Another experiment:
Transgenic mouse experiments
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
Barr Body
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
One copy of X inactivated
Result is Barr body
Mechanism of X inactivation
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
If X inactivation “turns off” all but one X chromosome, why aren’t Klinefelters individuals XXY normal?
The inactivated X chromosome isn’t completely inactivated, just mostly inactivated.
Genetics of the Calico Cat
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
