Transcript Sex Determination

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
Sex Determination
and
Sex Chromosomes
Timothy G. Standish, Ph. D.
©2000 Timothy G. Standish
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
©2000 Timothy G. Standish
Many Plants and Animals are
Hermaphrodites
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
©2000 Timothy G. Standish
Sex Determination
Two ways in which sex can be determined:
Environment:
Turtles - Temperature of development
Some fish - Social structure
Chromosomes - Three methods:
XO - Haploid/diploid ie bees, haploid males
diploid females
ZW - Heterogametic (ZW) females, homogametic
(ZZ) males, ie birds
XY - Heterogametic (XY) males, homogametic
(XX) females, ie humans and Drosophila
©2000 Timothy G. Standish
X Chromosome Human and
Drosphila Genes Are Easy To Find
In humans and Drosophila, males are
XY
Thus males are haploid for the X
chromosome
Because of this, recessive genes on the
X chromosome show up far more
commonly in male than female
phenotypes
©2000 Timothy G. Standish
Human X-linked Recessive
Genes
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
©2000 Timothy G. Standish
X-linked Recessive Genes
Related 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
deficency of vitimin A
Mocrophthalmia - Eyes fail to develop
Optic atrophy - Degeneration of the optic
nerves
©2000 Timothy G. Standish
Human Y Chromosome
p arm
Centromere
q arm
©2000 Timothy G. Standish
Human Y Chromosome
Pseudoautosomal region (PAR)
p arm
Centromere
q arm
Pseudoautosomal region (PAR)
©2000 Timothy G. Standish
Human Y Chromosome
Pseudoautosomal region (PAR)
Sex-determining region Y (SRY):
Testis-determining factor (TDF) location
Centromere
Nonrecombining
region of the Y
(NRY)
Heterochromosin
(essentially blank?)
Pseudoautosomal region (PAR)
©2000 Timothy G. Standish
Variation In Chromosome
Number - 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
1n Gamete
+
Interruption
of meiosis
Pro or Metaphase I Metaphase II
2n
Gametes
3n Zygote
©2000 Timothy G. Standish
Variation In Chromosome
Number - 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
+
Zygote
Metaphase I
Anaphase I
+
Zygote
©2000 Timothy G. Standish
Aneuploidy 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
©2000 Timothy G. Standish
Gene Dosage
There 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
©2000 Timothy G. Standish
The Lyon Hypothesis
Having extra chromosomes causes problems (ie
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
©2000 Timothy G. Standish
Consequences of X Chromosome
Dosage 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
©2000 Timothy G. Standish
Consequences of X Chromosome
Dosage Compensation
XX
Zygote
XX
XX
At some point (probably later than the 4 cell
stage) half the X chromosomes are turned off
Daughter cells inherit the mother cell’s
XX XX
combination off and on X chromosomes
XX
Cell division
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
XX
Different patches of cells
inherit different act X
chromosomes
©2000 Timothy G. Standish
Why Calico Cats
Are 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.
©2000 Timothy G. Standish
©2000 Timothy G. Standish
Why Have More Than One
Chromosome?
If only one chromosome, no crossing over, thus all
bad or good genes go to offspring ...
©2000 Timothy G. Standish