Genetics 3000 Sex Chromosomes

All living organisms reproduce asexually or …

sexually, most diploid organisms utilize sexual reproduction

Advantages to asexual reproduction

Quick (doesn’t require another partner), and requires less investment (energy)

One of the basic requirements of life is …

the ability to reproduce

Advantages of sexual reproduction

genetic diversity, easier to adapt to new environments, and increased risk of survival

Asexual reproduction does not have …

recombination

Disadvantages of sexual reproduction

increased investment of time and resources and slower (requires a partner)

Example of an increased investment of time would be …

when peacocks have to fluff up their feathers and dance just to attract a female, and then they have to carry these heavy feathers around with them which could make escaping predators more difficult

Even fruit flies have …

mating rituals, they will rustle their wings and follow females around

Biological sex is based off the …

size of the gametes produced, males = smaller gametes like sperm or pollen, females = larger gametes like eggs or ovules, this is the case for humans

The production of gametes often, but not always, is concurrent with …

different genetic, anatomical, and physiological characteristics which create the sexual phenotypes

Sex determination is …

the mechanism by which biological sex is established

What is the term used for individuals with a mix of female and male anatomical traits?

intersex

An example of an anatomical phenotype is …

when the skin around an intact male tom cat’s face puffs up, this is to protect their faces from other tom cats for when they get in fights over females

Biological sex is not the same as …

gender (gender = social construct)

In most species the sex chromosomes determine …

the biological sex

What is the term for production of gametes with unlike chromosomes?

Heterogametic sex (so their homologous pair is not identical, two different chromosomes)

What is the term for the production of gametes with like chromosomes?

Homogametic sex

In most species XX =

female and XY = male, exceptions include fish, reptiles, and moths (such as the ZZ and ZW system where ZW = female)

Sex chromosomes separate from each other during …

meiosis, just like autosomal chromosomes, each gamete gets only one sex chromosome, biological sex of the offspring will be determined by the pair the offspring receives

Differentiation begins in …

early development, male and female reproductive structures are indistinguishable until 6 weeks after fertilization

Undifferentiated embryos have …

two sets of ducts present that terminate the cloaca (one the 6 week mark is hit and the cell can tell which sex it is one duct is discarded while the other develops)

The reproductive system development is …

closely linked to the urinary system development

The Mesonephric (Wolffian) ducts develop into …

male genital ducts when testosterone is present

The Paramesonephric (Mullerian) ducts develop into …

female genital ducts when there is an absence of testosterone

The development of one duct and discarding another is …

imperfect and a complicated system

Genes on the Y chromosome direct the development of …

the male system, the Y chromosome is smaller and therefore has fewer genes than the X chromosome

Some genes in male specific region are …

homologous to those on X

Pseudoautosomal regions (PAR) allow …

smaller Y chromosomes to pair with X during meiosis (these are the places where the X and Y chromosomes are homologous)

In the sex determining region of a Y (SRY) chromosome there is a …

gene called testis-determining factor (TDF) which codes for attributes that lead to the production of testosterone

SRY is Y linked because …

it is found on the Y chromosome

Disorder of sex development (DSD) is an umbrella term for …

“congenital conditions associated with atypical development of internal and external genital structures” - these individuals can have for example the incorrect number of X and Y chromosomes

In humans at least one X chromosome is …

necessary for life

Turner Syndrome =

when only one X chromosome is present, patients have female external genitalia and internal ducts, but ovaries are diminished (consequences for not having pair

Additional affects of Turner Syndrome

short stature, congenital heart defects, and increased risk for chronic conditions

Patients with Turner Syndrome have how many chromosomes?

45 (46-1Y or 1X)

Klinefelter Syndrome

Two X chromosomes and a Y chromosome, patients have male external genitalia and internal ducts, but testes are diminished

Other features of Klinefelter Syndrome

infertility(less or non-viable sperm), gynecomastia (non-cancerous enlargement of breasts), tall stature

Cells have evolved to utilize the amount of …

protein produced from genes to autosomes, proteins are produced from both maternal and paternal chromosomes (equivalent in males and females)

If genes were expressed from both X chromosomes in females, then …

XX individuals would have twice as much protein as XY individuals

Dosage compensation is when …

the body balances X chromosome gene expression in XX individuals

Life could have evolved to solve the dosage problem by either …

providing XY individuals with twice as much information or it could repress one of the X chromosomes in XX individuals halving the amount of information (evolution followed the X repressing route)

Barr bodies are observed in the …

nuclei of XX individuals but not XY, a Barr body is a highly condensed chromosome

Condensing an X chromosome into a Barr body …

prevents the DNA from being accessed and then expressed

The process of limiting the X chromosomes to 1 is called the …

Lyon Hypothesis after Mary Lyon

Individuals with DSDs show corresponding …

numbers of Barr bodies in cells (an individual with three copies of an X chromosome have 2 Barr bodies found in their nuclei)

Random X inactivation

the X chromosome chosen for condensation is a random event, all progeny from one cell will have the same inactivated X chromosome

The coat of cats like tortoise shell come from randomly X inactivated chromosomes so …

one cell in the embryo multiplies and once the cell knows what sex it is then each cell inactivates an X chromosome, so the patches of color each come from the same original cell

What inactivates the X chromosome?

non-coding RNA

The X-chromosome inactivation center (XIC) is …

a small region on the X chromosome

The XIC contains the X-inactivation specific transcript (Xist) which produces …

the RNA (highly unstable that will coat the entire X-chromosome condensing it and preventing gene expression

The RNA produced by Xist is …

highly unstable until its time to inactivate the chromosome, this way when the cell hasn’t determined what sex it is the RNA made by Xist won’t coat the chromosome too early

Genes linked to the X chromosome show …

unique inheritance patterns, this is because the X and Y chromosomes are different sizes (making them difficult to pair during meiosis and because) so the X chromosome has way more genes resulting in unique inheritance

For x-linked genes males are called …

hemizygous (have only one X chromosome so they have only one copy of any x-linked gene/they are stuck with whatever allele they get from their mother)

The allele for white eyes in fruit flies is …

recessive but we have to consider the biological sex since this gene is carried on the sex chromosomes

Male parents have no …

choice in which chromosome goes to progeny

XX individuals can follow mendelian principles because …

they have two copies of each gene meaning they have a “choice” which allele they pass to their offspring, males do not have a choice which is what makes them hemizygous

Inactivation only happens in …

mammals, in non-mammals the expression of X chromosome in males is pushed up, while other species do not compensate for the difference in X chromosome expression at all

X-linked recessive traits are carried …

on the X chromosome, only females can be carriers since males have only 1 X chromosome (males either express the gene or they don’t)

For X-linked recessive traits, an affected father cannot pass …

the trait onto their sons because the sons have to get their Y chromosome from their father

For X-linked recessive traits, not all sons of carrier mothers …

have the trait, some sons could inherit the unaffected allele

X-linked dominant traits are more prevalent in …

females, because if an affected father has any daughters they will all express the trait, this also means fathers cannot pass it to their sons and that an affected mother will pass it to some sons if she is heterozygous

X-linked dominant genes are seen less in males because …

they are more lethal when the affected allele is the only one expressed like in males, because of random inactivation of X chromosomes in females, about half of the females body expresses the trait while the other half affects the wild-type allele

Y-linked traits do not show …

dominant or recessive traits, since there is only one allele present, traits do not skip generation, if an affected male has a son, the son is also affected

Temperature dependent sex determination (TSD) is when the …

environment of a developing embryo can determine biological sex (common in reptilians like species of turtles

A very narrow temperature range can result …

in a mix of male and female offspring (usually 2-3 degree window)

This chart represents …

a system where very low temperatures result in embryos becoming female

This chart represents …

a system where very high temperatures result in embryos becoming female

This chart represents …

a system where extreme temperatures (hot or cold) result in embryos becoming female, so a midrange temperature results in males

Hormone production during development with TSD is …

tied to temperature

Aromatase is the enzyme that …

converts testosterone into estrogen

TSD is only active for …

a specific window of development, so aromatase will not continue to turn testosterone into estrogen

In embryos who become female at high temperatures this means that …

aromatase becomes a functional enzyme at high temps, if the enzyme in a specific organism functions at low high temps but its located in a cold environment then the embryo will develop into a male