sex for the purposes of this class refers to 4 components gonadal sex gonads or where gametes are...

Sex for the purposes of this class refers to 4 components

Gonadal sex

Gonads or where gametes are produced by meiosis

Somatic sex

Somatic cells are cells that undergo mitosis.

They can be divided into primary and secondary characteristics

Secondary somatic sex characteristics are divided further into hair and body

Sexual identification

Sexual orientation

FOR HUMAN MALESGonadal sex

Consists of the testes where the gametes or sperm are produced

Somatic sex characteristicsPrimary and Secondary

Male primary somatic sex characteristics penis and scrotum

Male secondary somatic sex characteristicsHair

facial hair chest hair body hairBodypelvic build straight hipsmuscular build upper body

Ability to generate muscle mass at a faster rate than females following puberty

FOR HUMAN FEMALESGonadal sex

Ovaries where eggs are produced

Female primary somatic sex characteristics

Clitoris, labia, vagina, cervix, uterus, fallopian tubes, and the ability to bear children

Female secondary somatic sex characteristicsHair vellus rather than terminal hair

Body

rounded hips, breasts, ability to nurse offspring, menstrual cycle, increased body fat composition, decreased upper body strength, decreased ability to generate muscle mass at a fast rate

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Sex chromosomes

At the cellular level the

sex of an individual is

determined genetically

by the sex chromosomes.

X and Y -> male

X and X -> female

© Biophoto Associates/Photo Researchers

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Sex chromosomes determine gender

•Human males are the heterogametic sex with two different sex chromosomes, (XY).

•Human females are the homogametic sex (XX).

•In other species sex determination differs:

male birds ZZ

female birds ZW

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Sexual development

An embryo develops as a

male or female using

information from the Y

chromosome.

At the beginning of human

development either male or

female development is

possible.

Unspecialized gonads and

two sets of reproductive

ducts exist until 6th week of development.

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Genes on the Y chromosome

Y chromosome first seen 1923

Tips of Y chromosome are pseudoautosomal regions PAR1 and PAR2

They make up 5% of the chromosome

Contain 63 pseudoautosomal genes that cross over with the X chromosome

Most of the Y is male specific region or MSY

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Genes on the Y Chromosome

Y chromosome has 2 pseudoautosomal regions whose genes match genes on the X chromosome

And a large central region that does not recombine with the X chromosome

This non-recombining region makes up about 95% of the chromosome

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3 Classes of Genes on the Y

Genes shared with X chromosome define the pseudoautosomal regions (PAR)

•Genes similar to X chromosome genes are X-Y homologs

•Genes unique to the Y including SRY gene

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Genes on the Y chromosome

Y chromosome is small and gene poor and prone to deletionSince the Y can not recombine with X, its genes began to rotGenes decayed except for SRY and the tipsThe Y is degrading fast, losing genes at the rate of 5/million yearsMay have a way out of complete degredationMany of the DNA segments are palindromes

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Y chromosome is highly repetitive

During male meiosis a synaptonemal complex forms and one chiasma will always form in this region

This results in regular exchange of material between the tips of XP and YP

Any genes in this region are inherited as thought they are autosomal and that is why it is called the pseudoautosomal region PAR

Contains 10 -20 known genes including the RSP gene for making ribosomes

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This mutant hybrid chromosome is called an X (TDF) chromosome

When it fertilizes an X bearing egg it results in a 46 XX (TDF) male

When the corresponding mutant y chromosome fertilizes an egg it results in a 46 XY female

SEX REVERSAL

Karyotype is XX but this individual will develop as an XY male because of the presence of the testes determining factor gene

Remember the presence of testes, determines gonadal sex in males

Even though these individual will look male they will suffer from testicular atrophy or small testes and sterility

What causes the sterility?

SEX REVERSAL

In humans if there are 2 X chromosomes in a male germ line , it acts as a poison to the germ cells and kills them during meiosis

The gonadal sex of this individual will be male because they will have testes

But they can not make viable sperm

SEX REVERSAL

When a sperm carrying a Y (delTDF) chromosome fertilizes an X bearing egg the result is an

XY(delTDF) zygote

This individual develops as female even though the karyotype is XY

Ovaries are reduced and eggs that are produced will not survive

Have a female build but little pubic or underarm hair

SEX REVERSAL

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SRY activates a cascade of developmental events

Sexual development

External genitalia differentiation

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Mutations in genes involved in sexual development affect gender

Mutation/absence of:

SRY gene Normal female development

Anti-Mullerian hormone Mullerian ducts persist in male

gene

Testosterone gene Early development as female

Masculinization at puberty

DHT converting enzyme External structures lack signal

and develop as female,

internal structures are male.

HORMONES

Travel in the blood stream to receptors on the target cell

Receptors bind the hormone and carry it across the plasma membrane of the cell to the nucleus

Once inside the nucleus the hormone binds to DNA to promote gene expression

It is the protein products of these genes that lead to sexual differentiation

HORMONES

Chemical messengers that are produced by one cell type, released into the bloodstream and received by the target cell

Hormones regulate gene expression in the target cell

We will look at:

testosterone

excreted by testes and adrenal cortex in males

estrogen

excreted by ovaries and adrenal cortex in females

HORMONE RECEPTORS AND GENE REGULATION

The TDF gene is on the Y chromosome and codes for the development of testes which produce testosterone

But the receptor for testosterone is regulated by a gene on the X chromosome

This AR gene or androgen receptor gene codes for the testosterone receptor

A mutation in this gene cause testicular feminization

Individuals are XY with a normal Y chromosome with a normal TDF gene

HORMONE RECEPTORS AND GENE REGULATION

They can not respond to testosterone’s masculinizing effects because they have no receptors to transport testosterone across the cell membrane

Affected individuals develop as phenotypic females who are sterile

This mutation is known as the AIS mutation, androgen insensitivity syndrome

ANDROGEN INSENSITIVITY SYNDROME

Produce testosterone

This is where they differ from the XY (TDF del) individuals

They have no receptors to transport testosterone across the cell membrane

Referred to as AIS females

Occur 1/20,000 births

Taller than average

External genitals are female

Vagina looks normal from the outside

Newborn is declared a girl and raised as such

ANDROGEN INSENSITIVITY SYNDROME

In utero AIS females develop testes because TDF gene present so their gonadal sex is male

Produce MIF and Mullerian ducts regress

But the cells of the embryo cannot sense testosterone

Instead they respond to the low levels of estrogen being produced by the adrenal cortex and develop as female

At birth appear female

No vagina, cervix, uterus or fallopian tube

3 WAYS TO MANIFEST TESTICULAR FEMINIZATION

XY (TDF del)no testosterone

XY point mutation of TDF geneno testosterone

XY with normal Y but with AIS mutation so no receptors for testosterone

All these individuals will be tall

SEX DETERMINATION IN HUMANS REQUIRES 4 ELEMENTS TO BECOME MALE

Y chromosome intact TDF gene (no point mutations)

Sensing mechanism in the indifferent gonad to the testes determining factor

Testosterone or estrogen produced by the gonads

Testosterone or estrogen receptors in the somatic tissues intact. No AIS (androgen insensitivity syndrome)

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SRY activates a cascade of developmental events

GUEVEDOCES

Born XY with female genitalia

At puberty the adrenal gland begins to produce testosterone

Causes masculinization

voice deepens

muscles develop

no breast development and no menses

adrenal testosterone causes clitoris to enlarge and resemble a penis

SEX CHROMOSOME ANEUPLOIDY

Aneuploidy refers to an abnormal number of chromosomes

Such anomolies occur often enough that they have been recognized and studied by medical science

We will discuss 3 different cases of aneuploidyKlinefelter males XXYXYYTurner syndrome XO

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Nondisjunction in Sex Chromosomes

X Chromosome

XXX

XXY

Klinefelter syndrome

XO

Turner Syndrome

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What determines gender?

Number of X chromosomes, or

presence of Y chromosome?

Individuals with unusual chromosome combinations provide a clue:

X0 XXY XY femalesXX males

Female with Turner syndromeMale with Klinefelter syndrome

=> Y chromosome determines gender.

SRY gene missingSmall bit of Y with SRY gene present

=> SRY gene on Y chromosome determines gender.

KLINEFELTER

Klinefelter males have an extra X chromosome so they karyotype as XXY males

Occurs 1/1000 live births

1940 Harry Klinefelter at Massachusetts General Hospital described a syndrome occurring in males not usually detected until after puberty

KLINEFELTER

The testes are atrophied and do not produce much testosterone

The level may be so low as to be about the same amount as the estrogen produced by the adrenal cortex

Because the level is so low during puberty these individuals do not undergo normal development of male secondary sex characteristics, including muscular upper body, narrow hips and chest and facial hair

KLINEFELTER

If treated during puberty with testosterone then breast development will stop and the hips will narrow and become more male like

They will develop muscular upper bodies, broad shoulders, square jaw and deep voices

Remain sterile because the undeveloped testes can not produce sperm

But outwardly they will look like normal males

XYY MALES

1965 Jacobs in Scotland looked at the chromosomes of 197 mentally subnormal males with violent tendencies

They found 7 of those males had a 47 XYY karyotype

7/197 = 1/28 a much higher ratio than among other groups of males tested

XYY occurs 1/1000 live births

Among tall males about 1/325

Among tall penal inmates 1/30

XYY MALES

XYY males are over represented in mental and penal institutions

Large scale study done by the Danish government suggested that tall males with lower intellectual function are more likely to be convicted of crimes regardless of their karyotype

XYY karyotype is over represented in tall males 1/325

More than 95% of all XYY males are not in prison

sex for the purposes of this class refers to 4 components gonadal sex gonads or where gametes are...

Documents

y chromosome y chromosome

chromosome slide

heterogametic sex

homogametic sex

y genes

different sex chromosomes

y male x

tips of y chromosome