©2000 Timothy G. Standish Genesis 1:27 27 So God created man in his own image, in the image of God created he him; male and female created he them

©2000 Timothy G. Standish

Genesis 1:2727 So God created man in his

own image, in the image of God created he him; male and female created he them.


Sex DeterminationSex Determinationand and

Sex ChromosomesSex ChromosomesTimothy G. Standish, Ph. D.


What Is Male and Female?What Is Male and Female? What characteristics define being male or

female? Male:

– Produce sperm . . . – What are sperm?

Female:– Produce eggs . . .– What are eggs?

Organisms like Chlamydomonas are isogamous, producing morphologically indistinguishable gametes


Many Plants and Animals are Many Plants and Animals are HermaphroditesHermaphrodites

Plants like corn and peas have both male and female parts which produced sperm and eggs respectively

Some animals, for example Caenorhapditis elegans, are also hermaphroditic

In both cases there are not genetic differences producing dimorphic genders, instead the various parts develop as a result of developmental differences within the organism

C. elegans produces males via non-disjunction


Sex DeterminationSex Determination Two ways in which sex can be determined: Environment:

Turtles - Temperature of development Some fish - Social structure

Chromosomes - Three methods: XO - Haploid/diploid, i.e., bees, haploid males diploid females ZW - Heterogametic (ZW) females, homogametic (ZZ) males, i.e.,

birds XY - Heterogametic (XY) males, homogametic (XX) females, i.e.,

humans and Drosophila


X Chromosome Human and X Chromosome Human and DrosophilaDrosophila Genes Are Easy To Find Genes Are Easy To Find In humans and Drosophila, males are XYThus males are haploid for the X

chromosomeBecause of this, recessive genes on the X

chromosome show up far more commonly in male than female phenotypes


Human X-linked Recessive Human X-linked Recessive GenesGenes

Brown enamel - Tooth enamel appears brown rather than white

Hemophilia - Two types:– A - Classic hemophilia, deficiency of blood

clotting factor VIII

– B - Christmas disease, deficiency of blood clotting factor IX


X-linked Recessive GenesX-linked Recessive GenesRelated to sightRelated to sight

Coloboma iridis - A fissure in the eye’s iris Color Blindness - Two types:

– Deutan - Decreased sensitivity to green light– Protan - Decreased sensitivity to red light

Congenital night blindness - Not due to a deficiency of vitimin A

Mocrophthalmia - Eyes fail to develop Optic atrophy - Degeneration of the optic nerves


Human Y ChromosomeHuman Y Chromosome

Centromere

q arm

p arm



Centromere

q arm

p arm

Pseudoautosomal region (PAR)




Centromere

Nonrecombining region of the Y (NRY)



Heterochromosin(essentially blank?)

Sex-determining region Y (SRY):Testis-determining factor (TDF) location


Variation In Chromosome Variation In Chromosome Number - PolyploidyNumber - Polyploidy

Polyploid individuals have more than two sets of chromosomes

Many important commercial plants are polyploid:– Roses– Navel oranges– Seedless watermelons

Polyploid individuals usually result from some sort of interruption during meiosis

Pro or Metaphase I

Interruption

of meiosis

Metaphase II

2n Gametes

+ 1n Gamete

3n Zygote


Variation In Chromosome Variation In Chromosome Number - AneuploidyNumber - Aneuploidy

Polyploid humans are unknown, but individuals with extra individual chromosomes are known.

Having extra chromosomes or lacking some chromosomes is called aneuploidy

Aneuploid individuals result from nondisjunction during meiosis

Metaphase I Anaphase IZygote

+

Zygote

+


Aneuploidy In HumansAneuploidy In Humans Most human aneuploids spontaneously abort The most viable variations in chromosome number are those that deal with

the sex chromosomes: XO - Turner’s Syndrome - Phenotypically females XXX…- “Super” females XYY… - “Super” Males - On average tend to be larger and less intelligent XXY - Klinefelter’s Syndrome - Phenotypically male Of the non-sex chromosome aneuploidys, Down’s Syndrome, extra

chromosome 21, tends to be the most viable Down’s Syndrome is more common in children of mothers who gave birth

after age 40


Gene DosageGene DosageThere seem to be elegant mechanisms for

maintaining the correct dosage of genetic material in each cell

When aneuploidy causes a change in the relative dose of one chromosome, problems result

Another way in which dosage of genetic material can be changed is via macromutations


The Lyon HypothesisThe Lyon Hypothesis Having extra chromosomes causes problems (i.e.,

Downs Syndrome) Men have only one X chromosome and they are

normal (at least they think so) Women have two X chromosomes and they are

normal Mary Lyon proposed that the extra dosage of X

chromosome that women have is compensated for by turning off one of the X chromosomes.

This turned off chromosome can be observed as a “Barr Body” in metaphase female nuclei


Consequences of X Chromosome Consequences of X Chromosome Dosage CompensationDosage Compensation

During early development, X chromosomes are randomly turned off in female cells

All daughter cells have the same X chromosome inactivated as their parental cell.

Thus, females are a mosaic of patches of cells some patches expressing the genes on the paternal X chromosome, other patches expressing the maternal X chromome


Consequences of X Chromosome Consequences of X Chromosome Dosage CompensationDosage Compensation

XXXXXXXX

At some point (probably later than the 4 cell stage) half the X chromosomes are turned off

Daughter cells inherit the mother cell’s combination off and on X chromosomes

Because of dosage compensation, females are thought to be a mosaic of patches of cells with each patch expressing the same X chromosome, but none expressing both chromosomes

Different patches of cells inherit different act X chromosomes

XXXX

Cell division

XX

Zygote XXXXXXXX


Why Calico Cats Why Calico Cats Are Usually FemaleAre Usually Female

Orange coat color is a sex-linked trait in cats (it is on the X chromosome)

A female cat heterozygous for orange has skin patches expressing the orange X with the other X chromosome turned off. In other patches the opposite occurs.



Why Have More Than One Why Have More Than One Chromosome?Chromosome?

If only one chromosome, no crossing over, thus all bad or good genes go to offspring ...

Documents

©2000 Timothy G. Standish Genesis 1:27 27 So God created man in his own image, in the image of God created he him; male and female created he them