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….
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
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.
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 can’t survive without an X
chromosome!
1st Law: Law of Segregation
• Mendel’s 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.
The same happens for sex
chromosomes
the SEX chromosomes also carry genes. We
use the letters X and Y to identify these
chromosomes.
Sex Chromosomes and X-Linked Traits
• SEX LINKED TRAITS – genes located on the sex
chromosomes, usually the X chromosome.
• NOTE: It’s the sperm/male parent that determines the
sex of a child.
• The sex chromosomes do more than code for gender.
Thomas Hunt Morgan’s 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
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
The test cross…
• Mated white eyed male with purebred red eyed female (RR) and
found all F1 generation had red eyes
• According to normal Mendelian genetics, the red eyed allele was
dominant
F1
F2
• 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!!
So he crossed them
Maybe white
eyes are
lethal in
females?
So he crossed them
I guess not….but
what does this have
to do with sex?
Morgan’s 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 F1
females are crossed with whiteeyed males.
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!
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)
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, CDCd
XHXh XHY
Hemizygous
• Just like before, females can be homozygous or
heterozygous for a trait
• XHXh or XHXH
• Males are called hemizygous because they are neither
heterozygous nor homozygous. They only have one
possibility!
• XHY
Morgan’s 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)
The F2 generation…
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 (XrY) while female had at least one white eyed
gene (XR Xr)
P: XRXr x XrY
Phenotype: 1 normal female: 1 white eyed female; 1 normal male: 1 white eyed male
Genotype: 1XR Xr : Xr Xr :XRY: XrY
Sex-Linked Disorders
• Example of X Linked Genes: (Genes on located on the X
chromosome)
• Hemophilia (the inability to clot blood)
• XhXh or XhY
• Myopia (nearsightedness)
• XmXm or XmY
• Night blindness
• XnXn or XnY
• Male Pattern Baldness
• XbXb or XbY
• Colour-blindness
• XcXc or XcY
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 (____XhY_____). Females require 2 copies of
the recessive allele to show a recessive trait (____XhXh____).
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.
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
Muscular Dystrophy XdXd or XdY
• 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
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?
How does it work
• Let XH represent the normal allele
• Let Xh represent the allele for hemophilia
• (Y is the Y chromosome)
Results
• 50% of the males are affected
• 0% of the females are affected, although one is a carrier
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?
Male Pattern Baldness - P
• Let XB represent the normal hair allele, and Xb represent
the baldness allele
• P-
XB
XB
Normal Female
x
XBXB
XbY
x
Bald Male
Xb
XBXb
Carrier Female
Y
XBY
Normal Male
XBXb
Carrier Female
XBY
Normal Male
Both sons are normal, both daughters are carriers!
Male Pattern Baldness = F1
• F1 - Carrier Female
XBXb
XB
Xb
x
Normal Male
x
XBY
XB
XBXB
Normal Female
XBXb
Carrier Female
Y
XBY
Normal Male
Xb Y
Affected Male
100% of females are normal, ½ of sons are normal, ½ of
sons are affected
Altogether, ¼ of children are affected
What about a bald female?
It could happen, but you’d need
Bald or Carrier Female x
XbXb or XBXb
Bald Male
x
X bY
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
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?
Answer 1
• P: XRXr x XRY
Example 2
• What is the probability of a homozygous red-eyed
female and a white eyed male having a carrier female
offspring?
P: XRXR x XrY
Example 3
• What are the phenotypic and genotypic ratios when a
white eyed female mates with a red eyed male?
P: XrXr
x
XRY