Chapter 7: Sex Chromosomes

Which statement best describes human sex determination?

-Individuals with one X chromosome are male.

-Individuals with at least twice as many X chromosomes as Y chromosomes are female

-Individuals with a Y chromosome are male

-Individuals with two X chromosomes are female

-Females are heterogametic

Individuals with a Y chromosome are male; XXY individuals are male and X0 individuals are female.

True or False: The heterogametic sex determines the sex of the progeny.

True; the heterogametic sex, possessing different chromosomes (e.g. XY in males), produces gametes with varying sex chromosome compositions (X or Y), thus determining the sex of the offspring.

In humans, the male is the ________ sex.

heterogametic

homogametic

heteromorphic

homomorphic

monogametic

Heterogametic Sex; sex forming two kinds of gametes, one which determines the offspring of both types of sexes.

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

one X chromosome

high levels of estrogen

a portion of the Y chromosome

a balance between the number of X chromosome and the number of haploid

multiple alleles scattered throughout the autosomes

a portion of the Y chromosome; specifically, the sex-determining region Y gene located on the Y chromosome initiates the development of male characteristics. The SRY gene triggers the formation of testes, which produce male hormones which lead to the formation of male physical traits and characteristics.

Under what condition might a human female have the XY sex chromosome complement?

This is not possible

The X chromosome has a mutation is always able to overcome the Y chromosome.

This female would have one complete X chromosome and a Y chromosome that lacks SRY

The Y chromosome has an active Xist gene on it

She has an XX chromosome and a Y chromsome

This female would have one compete X chromosome and a Y chromosome that lacks SRY.

The SRY (Sex-determining Region Y) gene on the Y chromosome is critical for male sex determination. If a female has an XY chromosome complement but lacks a functional SRY gene, she may develop as a female despite having a Y chromosome.

• Turner Syndrome: A rare genetic condition in which a female does not have the usual pair of X chromosomes. 

How does the Y chromosome determine maleness in humans?

It carries the genes for steroid metabolism

It carries the structural genes for fetal gonad production

It carries the autosomal genes responsible for the SRY effects.

It carries a gene coding for the TDF, which directs the development of fetal gonads into testes.

It carries a gene coding for the TDF, which direct the development of fetal gonads into testes.

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 sons

all of his daughters

half of his sons

half of his daughters

all of his children

All of his daughters; A man with an X-linked gene will pass this gene to all of his daughters because daughters inherit their father's X chromosome. 

Which of the following statements is false?

An individual with Klinefelter syndrome generally has one Barr body.

An individual with Turner Syndrome has no Barr bodies.

Dosage compensation is accomplished in humans by inactivation of the Y chromosome.

A typical XX human female has one Barr body.

In a cell with X chromosomes lacking the XIC, there is still X inactivation.

Dosage compensation is accomplished in humans by inactivation of the Y chromosome. Dosage compensation happens at the X chromosome to balance the fact that females have two X linked chromosomes and males only have one X chromosome.

FRQ: What are Barr bodies, and where can they be found in the cell?

Barr bodies are highly condensed structures that are inactivated X chromosomes, found lying against the envelope of interphase cells. Human females have two X chromosomes which could lead to an excess of X-linked genes where one of the X chromosomes is randomly “turned off” in each cell. This process creates the Barr body because the inactivated X chromosomes is condensed into a compact form usually within the nucleus of cells. Males do not have Barr Bodies because they only have one X chromosome.

• Each Barr Body contains one inactive X chromosome, that is inactivated as a mechanism of dosage compensation.

FRQ: Indicate the expected number of Barr bodies found in people with Klinefelter Syndrome, Turner Syndrome, and Karyotypes  47XXX, 48XXXX.

• Klinefelter Syndrome (47, XXY): This is a male who has an extra X chromosome. When this happens one X chromosome in each cell gets inactivated. Therefore, this person will have only 1 Barr body in each cell.  

• Turner Syndrome (45, X): This is a female who is missing one X chromosome, so she only has one X chromosome. Therefore, this person has 0 Barr body in each cell. 

• Karyotype 47XXX (Triple X syndrome): This is a female who has three X chromosomes. So, two of these chromosomes will be inactivated and only one will remain active. Therefore, this person will have 2 Barr bodies in each cell. 

• Karyotype 48XXXX: This is a female who has four X chromosomes. So, three of these chromosomes will be inactivated. Therefore, this person will have 3 Barr bodies in each cell. 

X-linked genes in female mammals often demonstrate ________.

Non-recombination during meiosis

heterochromatin formation

phenotypic mosaicism

underrepresentation in the genome

incomplete dominance

phenotypic mosaicism; Because one X chromosome is inactivated in each cell, females heterozygous for X-linked genes will express one allele or the other in different cells, creating a mosaic of expression.

FRQ: Describe in detail how cat breeders can be confident that Calico cats are almost invariably females.

• Male cats only have one X chromosome so they can't contain all of the colors required for a calico coat. This is because the color of the coat is determined by the genes on the X chromosome. The genes for black and orange fur require two different X chromosomes, which only females have because they have two X chromosomes (fur color is an X-linked trait). Therefore, almost all Calico cats are females because only females have the necessary two X chromosomes to carry both the orange and black coat genes. 

• This color variation in patches could only be caused by X-linked inactivation mechanism, which only happens in females as a dosage compensation mechanism.

Which of the following statements about individuals with Klinefelter syndrome or Turner syndrome is FALSE?

Both syndromes result from nondisjunction.

Individuals with either syndrome have 44 autosomes.

Both syndromes can be identified by a karyotype.

Individuals with either syndrome are both identified as females.

Individuals with either syndrome are both identified as females; Individuals with Klinefelter syndrome (XXY) are phenotypically male, although they may have some female characteristics. Turner syndrome (X0) results in a female phenotype.

T/F: The paternal X is always inactivated for dosage compensation in placental mammals.

False; In placental mammals, X-inactivation is a random process, meaning either the maternal or paternal X chromosome can be inactivated in different cells.