Chapter 4

Sex

sexual phenotype

Defining difference between males and females of all species

gamete size: males have small gametes, females have large gametes

Sex Determination

the mechanism by which biological sex is determined

Hermaphroditism

both sexes in the same organism

Monoecious

both male and female reproductive structures in the same organism

Dioecious

either male or female reproductive structures in one organsim

XX-XO System

XX - Female
XO - Male

What is the homogametic sex in the XX-XO system?

Females

What is the heterogametic sex in the XX-XO system?

Males

XX-XY System

XX - females
XY - males

What is the homogametic sex in the XX-XY system?

females

What is the heterogametic sex in the XX-XY system?

male

Are the X and Y chromosomes homologous?

no, but they can pair in meiosis because they are homologous at the par regions

ZZ-ZW System

ZZ - Male
ZW - Female

What is the homogametic sex in the ZZ-ZW system?

Males

What is the heterogametic sex in the ZZ-ZW system?

Females

Haplodiploidy system

Haploid set - males
Diploid set - females

Genic-Sex Determining system

no sex chromosomes
genotypes at one or more loci determine sex

Sequential Hermaphroditism

each individual animal can be both male and female, although not at the same time

Example of sequential hermaphroditism

limpets
female releases chemicals that attract another female, the female that settles on top becomes male, they mate, the male becomes female again, the cycle repeates

Genic Balance System

Sex-determining system in which sexual phenotype is controlled by a balance between X chromosome number and autosomal chromosome number

Turner Syndrome

XO
1/3000 female births
sterile female with some male phenotypes

Klinefelter Syndrome

XXY, XXXY, XXXXY, XXYY
1/1000 male births
sterile male with some female phenotypes

Poly X Females

1/1000 female births
potential fertility struggles

XYY Males

1/1000 male births
not many issues

Is the X chromosome essential for all humans?

yes, both sexes have to have at least one X chromosome to develop past fertilization

How does the presence of a Y chromosome affect the sex of a human?

results in a male phenotype, even in the presence of multiple X chromosomes

How does the absence of a Y chromosome affect the sex of a human?

results in a female phenotype

How many copies of a X chromosome do females need to be fertile?

2

What does extra copies of a X chromosome result in?

issues in both men and women

SRY Gene

Sex-determining Region Y Gene
determines maleness
encodes a protein called a transcription factor which binds to DNA and stimulates the transcription of genes that promote the differentiation of the testes

Where is the SRY gene located?

on the Y chromosome

Androgen-insensitivity Syndrome

XY individuals who have a defective androgen receptor
cells don't respond to testosterone results in phenotypical female with internal testes

Sex-linked characteristics

determined by genes located on the sex chromosomes

Linked genes

when one trait is present, the other one will be too

Who exhibits higher rates of recessive X-linked Characteristics?

males

Hemizygous

males have only one X chromosome, can't be homo or heterozygous, are hemizygous

Thomas Hunt Morgan

big researcher in sex linked traits in fruit flies

Nondisjunction

when the 2 X chromosomes fail to separate in anaphase 1

Y-linked characteristics

only present in males
all male offspring will exhibit the trait

What do mutations in the SRY gene result in?

a sterile male

Has the Y chromosome always been small?

no, it lost DNA over time

What can be used to trace female lineage?

mitochondrial DNA

What can be used to trace male lineage?

Y chromosome mutation sequences

Dosage Compensation

mechanisms to equalize the amount of protein produced between males and females for X-linked genes

Lyon Hypothesis

within a cell, both X chromosomes aren't being used, results in a Barr Body
as the number of X chromosomes increases, so does the number of Barr Bodies

Barr Body

highly condensed state that prevents the expression of any genes on that X chromosome

Random X Inactivation

each cell can inactivate a different X chromosome, independent of the other cells

What is the result of random X inactivation?

females are functionally hemizygous at the cellular level for X-linked genes females are mosaics for the expression of X-linked genes