4 Sex Determination and Sex Linked Characteristics

Organisms that bear both male and female reproductive structures

Monoecious

Species in which an individual organism has either male or female reproductive structures such as humans

Dioecious

True or False: A female will always produce a gamete with an X chromosome

True

True or False: A male will always produce a gamete with a Y chromosome

False. Males have a 50/50 chance of passing down an X or Y chromosome to a gamete. They are responsible form making a male

The non sex chromosomes (1-22)

Autosomes

True or False: The presence of sex chromosomes is what determines sex

False. The individual genes ON those sex chromosomes and on the autosomes (1-22) are what determines sexual phenotype. So just because you have an XY doesn’t always mean you are a male and vice versa

• Females have two XX chromosomes

• Males posses only one X chromosome (XO)

• Males segregate half of their X chromosomes to their gametes while the other half gets none (O)

• A sperm that lacks an X will fuse with an X from the eggs making a male

XX-XO System

Two different types of gametes with respect to the sex chromosomes (ex: XY, XO)

Heterogametic Sex

Gametes that are all the same with respect to the sex chromosomes (XX)

Homogametic Sex

What allows the X and Y to pair up to create a male even though they’re not fully homologous pairs?

Their pseudoautosomal regions (primary and secondary) that contain the same genes. They pair during Prophase I

• female is heterogametic (ZW)

• Male is homogametic (ZZ)

• recall that in the XX-XO and XX-XY system the males were heterogametic

• common in birds and butterflies

ZZ-ZW Sex determination

How does the heterogametic sex differ from the homogametic sex?

a. The heterogametic sex is male; homogametic sex is female

b. gametes of the heterogametic sex have different sex chromosomes; gametes of the homogametic sex all have the same sex chromosome

c. gametes of the heterogametic sex all contain a Y chromosome; gametes of the homogametic sex all contain an X chromosome

b.

NOT A because heterogametic can be female too (exL ZW)

NOT C because ZZ-ZW is not X

• genotypes at one or more loci determine the sex of an individual

• no obvious differences in the chromosomes of females and males

• sex is determined by genes on autosomes (1-22) ONLY. This indicates that there are no distinct sex chromosomes

  • in chromosomal sex determination, the genes on the sex chromosomes work together with the autosomes to create the sexual phenotypes.

• undistinguishable sex chromosomes

• common in fungi and plants

Genic Sex Determination

• the genes on the sex chromosomes work with the autosomes (1-22) to generate sexual phenotype

• sex chromosomes look different in males and females

• distinguishable sex chromosomes

Chromosomal Sex Determination (XY XX)

Each individual animal can be both male and female, although not at the same time (ex: slipper limpet)

Sequential Hermaphroditism

Temperature for example can alter the sex chromosomes of eggs (ex: warmth can produce more female turtles)

Environmental Sex Determination

What gene and on what chromosome is responsible for the male phenotype and is only found in males?

The SRY gene (sex determining region) on the Y chromosome

• 45, XO

• can have some cells with full XX while others are XO only (mosaicism)

• 1/3000 female births

• underdeveloped, shield like chest, webbed neck

• normal intelligence

• often LETHAL

• sex chromosome aneuploidy

Turner Syndrome

• 47, XXY male (or XXXY, XXXXY, etc)

• 1/1000 male births

• male genitalia but testes underdeveloped

• normal intelligence

• often taller

• production of less testosterone

• sex chromosome aneuploidy

Klinefelter Syndrome

• 47, XXX female

• 1/1000 female births

• normal appearance and fertility

• often taller

Rare XXXX or XXXXX females

• normal female anatomy

• intellectual disability

sex chromosome aneuploidy

Triple X- Syndrome

• 47, XYY

• 1/1000 male births

• fertile, above average height, risk of personality disorders or subnormal intelligence

• sex chromosome aneuploidy

XYY Males

True or False: X chromosome contains genetic information essential for both sexes; at least one copy of an X chromosome is required for human development.

True

True or False: Genes affecting fertility are located on the X and Y chromosomes. A female usually needs at least ONE copy of the X chromosome to be fertile.

False. A female needs at LEAST TWO copies of the X to be fertile (ex: Turner syndrome)

True or False: Additional copies of the X chromosome may affect development in both males and females, influencing physical features and intellectual ability

True (ex:poly X females)

True or False: The primary male-determining gene is located on the Y chromosome. A single copy of a Y chromosome, even in the presence of several X chromosomes, usually produces a male phenotype

True (ex: Klinefelter Syndrome)

True or False: You do not need an X chromosome for human development

False. You need at least one copy of the X

Do human embryos have differentiated gonads until 6 weeks of development?

Yes, after six weeks a gene on the Y chromosome (if male) is activated and causes testes formation and inhibits ovary formation by the TDF protein. If a female, ovary formation will occur because the SRY gene is not on XX chromosomes and thus cannot be activated.

Differentiated gonads produce ANDROGEN or ESTROGEN to direct male or female secondary characteristics

• individuals have female external sexual characteristics, but the uterus, oviducts, and ovaries are absent.

• Inside the abdominal cavity, a pair of testes produce levels of testosterone typically seen in men.

• genetically male XY but looks female

• Receptor is defective and thus cannot respond to testosterone to make male characteristics and instead female characteristics do.

• gene defect is inherited from the mother since the gene is location on the X chromosome

Androgen Insensitivity Syndrome

Characteristics determined by genes located on the sex chromosomes

Sex Linked Characteristics

What sex can be homozygous or heterozygous and why

Females because they have two X chromosomes that can carry different or the same alleles

What sex can only be be hemizygous and why?

Males because they only have one X chromosome and thus can only carry one allele. They must always have a single allele inherited from the mother

Failure of homologous chromosomes or sister chromatids to separate in meiosis or mitosis

• ex:X^wX^wY from a white eyed female usually would separate in the gametes as (X^w leaving it to pair up with a male with another X^w or Y)) and the other (X^wY making a male). What can happen though is, the X^wX^wY can fail to separate and both X^wX^w go into one gamete and the Y into another gamete

  • the progeny resulting from this process includes X^wX^wX^w(dies), X^wX^wY, X⁺Y, and YY (dies)

Nondisjunction

Sex-linked genes are located on the X chromosome which was confirmed through Bridge’s experiment

Chromosome Theory of Inheritance

What caused the degeneration of the Y chromosome over time?

Natural selection prevented crossing over of the X and Y chromosomes except on the pseudoautosomal regions. The lack of crossing over resulted in the chromosome to shrink or degenerate so that genes that encode for male traits did not end on the X chromosome and vice versa.

• Is said to help compensate full degeneration of the Y chromosome by allowing internal recombination between the two arms of the Y

• Though, can have harmful effects that can lead to rearrangements on the Y-chromosome causing anomalies of sexual development, deletion of the SRY gene, deletion of a gene on the Y chromosome that is responsible for sperm production

Palindromic Sequences on the Y chromosome

How can you tell if males are related to each other just from their Y chromosome?

Typically, the Y chromosome accumulates mutations that are inactive and show up just once down their ancestral line. They are inactive because much of the Y chromosome is nonfunctional. Thus, males that have the same set of mutations are assumed to be related.

How can female lineages be traced?

Through the sequences of their mitochondrial DNA since it’s passed down from their mother.

• A mechanism to equalize the amount of protein produced by X-linked genes and autosomal genes.

• In placental mammals, is accomplished by the random inactivation of one X chromosome in the cells of females

• fruit flies double the the activity of the X chromosome in males to compensate for the absence in another X

Dosage Compensation

An inactive chromosome

Barr Body

Why do female placental mammals function as though each cell only has a single X chromosome?

Even though female mammals are XX, the Lyon hypothesis states that one of the 2 Xs become inactivated randomly

How come in people with Turner Syndrome (XO) differ from XX females, and those with Klinefelter (XXY) differ from XY males?

The inactivation of a an X chromosome isn’t 100%. In fact 15% escape inactivation resulting in an excess of gene protein resulting in anomalies

• Lives on the X chromosome and is responsible for coating the X chromosome that is meant to be inactivated by recruiting protein complexes that alter chromatin structure

• however, some genes repress this specific gene and never inactivates the X chromosome

Xist Gene

Presence of 2 different cell populations in one individual (ex: calico cats

Mosaic

• Trisomy 21 (extra chromosome) due to nondisjunction'

• also an autosomal aneuploidy

Down Syndrome

True or False: autosomal aneuploidy is better tolerated than aneuploidy with sex chromosomes

False. autosomal aneuploidy is often lethal

Carrying only one type of an allele (ex: X^cY or XO)

Hemizygous

Is X^c X^c hemizygous?

No because it has two copies of the same allele

What is X^c X^c?

Homozygous

What is X^c X⁺

Heterozygous

Both sexes on the same organism

Hermaphroditism

The addition or loss of less than a full set of chromosomes or chromosome pairs

Aneuploidy

In the fruit fly experiment, what did the P generation consist of in both trials?

• Red eyed female (X⁺X⁺) with White eyed male (X^wY)

• White eyed female (X^wX^w) with red eyed male (X⁺Y)

In the fruit fly experiment, what did the F₁ generation consist of in both trials?

• Red eyed female (X⁺X^w) and red eyed male (X⁺Y)

  • since X⁺ is a wild type a female’s X^w from the dad can still be masked. Though, since males can only get their Y chromosome from their dad, that means their X chromosome has to be from their mom. In this case, the mom only had X⁺X⁺ creating red eyed males

• Red eyed female (X⁺X^w) and red eyed male (X^wY)

  • Females can only obtain the X^w from their mom and the X⁺ from their dad but still, the X⁺ masks the white eyed allele. However, since the mother has two X^wX^w this means the male can only get an X^w creating a white eyed male.

In the fruit fly experiment, what was the recessive and what was the wild type?

• X^w was the recessive allele for white eyes

• X⁺was the wild type (in this case dominant) allele for red eyes

The fruit fly experiment by Thomas Hunt Morgan proved what?

White eyed trait is an X-linked characteristic

What is only present in males and only inherited from the father?

Y-linked trait

Sex chromosomes are believed to have evolved from?

A pair of autosomes

X:A is 1.5 (XXX:AA or XXXY:AA )

Metafemale

X:A is 0.67 (XX:AAA)

Intersex

X:A is 0.33 (XO:AAA) or 1.3 (XXXX:AAA)

Metamale