Download Foundations of Biology

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, 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
©2000 Timothy G. Standish
X Chromosome Human and
Drosophila 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
deficiency 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 (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

©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
 Orange
Why Calico Cats
Are Usually Female
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