Here Comes the Sun King The tale of King Henry VIII and his six
wives Wives kept experiencing miscarriages AND COULD NOT GIVE BIRTH
TO A HEALTHY MALE HEIR.
Slide 3
The wives of King Henry VIII Catherine of Aragon - divorced
Anne Boleyn - beheaded Jane Seymour - died Anne of Cleaves -
divorced Catherine Howard - beheaded Catherine Parr - survived WERE
THE wives TO BLAME? Who is to blame for gender determination? What
could explain such misfortune?
http://www.nbcnews.com/id/42041766/#. UoqWb-JLovs
Slide 4
Rationale GENDER DETERMINATION = MALE If the sperm gamete that
fertilizes is X female If the sperm gamete that fertilizes is Y-
male POSITIVE MEETS NEGATIVE? Henry may have belonged to a rare
blood group, called Kell positive. Causes autoimmune reaction
against baby if wife is not. X-linked McLeod syndrome too?
researchers propose that he also had a rare genetic disorder called
McLeod syndrome. Carried on the X-chromosome, the disease generally
affects only men and usually sets in around age 40 with symptoms
including heart disease, movement disorders and major psychological
symptoms, including paranoia and mental decline.
Slide 5
Female and Male Sex Chromosomes The X chromosome is much larger
than the Y chromosome it carries between 100-200 genes. Because
both males and females have at least 1 X chromosome, important
genes and information are found on the X chromosome. You can
survive without a Y chromosome, but you cant survive without an X
chromosome!
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1 st Law: Law of Segregation Mendels law of segregation states
that every individual possesses a pair of alleles and passes a
randomly selected copy (one or the other) to its offspring.
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The same happens for sex chromosomes the SEX chromosomes also
carry genes. We use the letters X and Y to identify these
chromosomes.
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Sex Chromosomes and X-Linked Traits SEX LINKED TRAITS genes
located on the sex chromosomes, usually the X chromosome. NOTE: Its
the sperm/male parent that determines the sex of a child. The sex
chromosomes do more than code for gender.
Slide 9
Thomas Hunt Morgans Flies Geneticist Studied fruit flies
(Drosophila melanogaster) Context: 1908 before anyone knew the link
between chromosomes and heredity Drosophlia melanogaster are a
model organism for genetics -reproduces rapidly -can reproduce when
they are only 10 to 15 days old -therefore can study many
generations in a short time -small (many of them can be contained
in a small vial) -males and females are easily distinguishable Won
the Nobel Prize in 1933 for discovering that chromosomes carried
heritable material
Slide 10
One day in the lab Examined eye colour in Drosophila Noticed
the appearance of white eyed flies among many red eyed offspring
Concluded the white eyes must be a mutation Wanted to do a breeding
analysis to understand about white eyes Did a test cross
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The test cross Mated white eyed male with purebred red eyed
female (RR) and found all F1 generation had red eyes Mendelian
According to normal Mendelian genetics, the red eyed allele was
dominant F1F2 He then decided to mate two of the F1 generation
flies and got a 3:1 ratio of red to white in the F2 Also seemed to
support Mendelian genetics EXCEPT no females has white EYES!!
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So he crossed them Maybe white eyes are lethal in females?
Slide 13
So he crossed them I guess not.but what does this have to do
with sex?
Slide 14
Morgans Results The appearance of white eyes in females shows
that this trait is not lethal in females. All possible combinations
of white eyes and sex are possible. The white-eye trait can be
carried over to females when F 1 females are crossed with white-
eyed males.
Slide 15
Did it have to do with chromosomes? The male and females seemed
to have slightly different chromosomes Morgan found that the gene
for white eyes seemed to follow the inheritance of sex From these
and other crosses, he was able to figure out that genes were
carried on chromosomes!
Slide 16
Recall: Chromosome Structure Each human cell has 46 chromosomes
23 pairs in total 22 are autosomes (not sex chromosomes) 1 pair of
sex chromosomes Each pair is homologous (similar but not the same)
Females have two X chromosomes, (XX) Males have one X and one Y
Chromosome (XY)
Slide 17
Sex Linked Inheritance Autosomal inheritance: inheritance of
alleles located on autosomal (non-sex) chromosomes (This is all the
inheritance we have dealt with up until now) Sex-linked: describes
an allele that is found on one of the sex chromosomes (X or Y) Aa,
C D C d X H X h X H Y
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Hemizygous Just like before, females can be homozygous or
heterozygous for a trait X H X h or X H X H Males are called
hemizygous because they are neither heterozygous nor homozygous.
They only have one possibility! X H Y
Slide 19
Morgans Flies Re-examined Purebred red eyed female is crossed
with a white male (NOTE: no symbol for eye colour on the Y
chromosome because it does not carry an allele for eye colour)
Slide 20
The F2 generation
Slide 21
Red Eye Females???? Can white eyed females possible occur in
nature? YES! For this to happen, the offspring would have to
inherit two recessive white eyed genes (therefore male must be
white eyed (X r Y) while female had at least one white eyed gene (X
R X r ) P: X R X r x X r Y Phenotype: 1 normal female: 1 white eyed
female; 1 normal male: 1 white eyed male Genotype: 1 X R X r : X r
X r :X R Y: X r Y
Slide 22
Sex-Linked Disorders Example of X Linked Genes: (Genes on
located on the X chromosome) Hemophilia (the inability to clot
blood) X h X h or X h Y Myopia (nearsightedness) X m X m or X m Y
Night blindness X n X n or X n Y Male Pattern Baldness X b X b or X
b Y Colour-blindness X c X c or X c Y
Slide 23
Recessive X-linked All the above mentioned traits are
recessive. Autosomal: You require 2 copies of the recessive allele
to show the recessive trait (Ie: Blue eyed individuals have the
genotype: bb) and having at least 1 copy of the dominant allele
causes the dominant trait to be expressed. (Ie: The genotype Bb
would result in a brown eyed individual) Sex linked traits: since
males have only 1 X chromosome, they only require 1 copy of the
recessive allele to show a recessive trait (____X h Y_____).
Females require 2 copies of the recessive allele to show a
recessive trait (____X h X h ____).
Slide 24
Carrier Carrier: someone who does not have the phenotype of a
condition but has the allele for the condition. This usually
applies to recessive genes For sex-linked genes, only FEMALES can
be carriers. Males only have one copy of the allele, so they cannot
be carriers. If they have the gene, they will express it.
Slide 25
Red Green Colour Blindness Inability to distinguish between red
and green A red green colour blind person does not see the number
29 on the right In humans normal vision is completely dominant to
red-green colour blindness
Slide 26
Slide 27
Muscular Dystrophy X d X d or X d Y Skeletal muscles lose their
normal structure and fibrous tissue develops in their place Caused
by a recessive allele carried on the X chromosome and is
sex-linked
Slide 28
Sex Linked Problems For example, hemophilia A is a blood
disease where it takes a long time for the blood to clot. The gene
for hemophilia is located on the X chromosome and is recessive. If
a woman carrying the hemophilia allele marries a man who does not
have hemophilia, what are the odds their children will have
hemophilia?
Slide 29
How does it work Let X H represent the normal allele Let X h
represent the allele for hemophilia (Y is the Y chromosome)
Slide 30
Results 50% of the males are affected 0% of the females are
affected, although one is a carrier
Slide 31
Sex-Linked Genes Male Pattern Baldness Located on the X
chromosome Recessive If you are male and your mothers father had
it, you may get it. It is rare in females. Why?
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Male Pattern Baldness - P Let X B represent the normal hair
allele, and X b represent the baldness allele P - Normal Female x
Bald Male XBXB xXbYXBXB xXbY Both sons are normal, both daughters
are carriers! XbXb Y XBXB X B X b Carrier Female X B Y Normal Male
XBXB X B X b Carrier Female X B Y Normal Male
Slide 33
Male Pattern Baldness = F 1 F 1 - Carrier Female x Normal Male
XBXb xXBYXBXb xXBY 100% of females are normal, of sons are normal,
of sons are affected Altogether, of children are affected XBXB Y
XBXB X B Normal Female X B Y Normal Male XbXb X B X b Carrier
Female X b Y Affected Male
Slide 34
What about a bald female? It could happen, but youd need Bald
or Carrier Femalex Bald Male X b X b or X B X b xX b Y There are
also Y-linked diseases Obviously, only males can get it. If your
dad has it, you will get it Less common because the Y chromosome is
smaller and has less genes
Slide 35
Example 1 In fruit flies, the gene for eye colour is X-linked.
Red eyes are dominant to white eyes. If a heterozygous red-eyed
female mated with a red- eyed male, what is the probability of a)
Producing a white eyed offspring? b) Producing a white eyed male
offspring? c) Producing a white eyed female offspring? d) A male
being white eyed?
Slide 36
Answer 1 P: X R X r x X R Y
Slide 37
Example 2 What is the probability of a homozygous red-eyed
female and a white eyed male having a carrier female offspring? P:
X R X R x X r Y
Slide 38
Example 3 What are the phenotypic and genotypic ratios when a
white eyed female mates with a red eyed male? P: X r X r x X R
Y