Post on 23-Feb-2016
description
Sex Linkage
Here Comes the Sun KingThe tale of King Henry VIII and Anne Boleyn
T.H. Morgan’s FliesContext: 1908 – before anyone knew
the link between chromosomes and heredity
Working with the fruit fly, Drosophlia melanogaster
A model organism for geneticsWon the Nobel Prize in 1933 for
discovering that chromosomes carried heritable material
One day in the lab…Was looking at flies, who normally have wild
type red eyes, saw a fly with white eyesWanted to do a breeding analysis to understand
about white eyesDid a test cross
Test CrossA test cross is used to determine the genotype
of an individual exhibiting a dominant trait Determine if this individual is homozygous or
heterozygous (HH or Hh)How?
Cross a dominant phenotypeindividual to one with the recessive phenotype.
By observing the resulting phenotypes, you can figure out the genotype
So he crossed themMaybe
white eyes are lethal in
females?
So he crossed themI guess not….but what does this have to do with
sex?
Morgan’s ResultsThe 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 white-eyed 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 chromosomes23 pairs in total 22 are autosomes
(not sex chromosomes)
1 pair of sex chromosomes
Each pair ishomologous (similar but not the same)
10
Females have two X chromosomes, (XX)Males have one X and one Y Chromosome (XY)
X and Y on a Karyotype A Karyotype is an arrangement of the chromosomes of
an individual that has been sorted according to size and type
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
Sex Linked InheritanceAutosomal 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
HemizygousJust 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
Sex Linked ProblemsFor 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 workLet XH represent the normal alleleLet Xh represent the allele for hemophilia(Y is the Y chromosome)
Results
50% of the males are affected0% of the females are affected, although one is
a carrier
CarrierCarrier: someone who does not have the
phenotype of a condition but has the allele for the condition.
This usually applies to recessive genesFor sex-linked genes, only FEMALES can be
carriers.
Sex-Linked GenesMale Pattern Baldness
Located on the X chromosomeRecessive If you are male and your mothers father had it,
you will get it. It is rare in females.Why?
Male Pattern Baldness - PLet XB represent the normal hair allele, and Xb
represent the baldness alleleP - Normal Female x Bald Male
XBXB x XbY
Both sons are normal, both daughters are carriers!
Xb YXB XBXb
Carrier FemaleXBYNormal Male
XB XBXb
Carrier FemaleXBYNormal Male
Male Pattern Baldness = F1
F1 - Carrier Female x Normal MaleXBXb x XBY
100% of females are normal, ½ of sons are normal, ½ of sons are affectedAltogether, ¼ of children are affected
XB YXB XBXB
Normal FemaleXBYNormal Male
Xb XBXb
Carrier FemaleXbYAffected Male
What about a bald female?
It could happen, but you’d needBald or Carrier Female x Bald Male
XbXb or XBXb x XbY
There are also Y-linked diseasesObviously, only males can get it. If your dad has it, you will get itLess common because the Y chromosome is smaller and
has less genes