how is our sex determined?. pre-assessment questions what are the sex chromosomes? what does it mean...
TRANSCRIPT
How is our sex determined?
Pre-assessment Questions
What are the sex chromosomes?
What does it mean when a trait is sex-linked?
What are some examples of sex-linked traits or genes?
Punnett Squares for Sex Determination
In females both chromosomes are known as X chromosomes while in males one chromosome is X while the other is Y.
The Y chromosomeThe y chromosome is known as a
degenerate chromosome because it does not have many genes on it◦Only has genes related male
anatomy
Unlike the X chromosome which has many
Sex-linked Inheritance
Sex linked inheritance occurs when certain genes are located on the X or Y chromosome (almost always the X).
This means that some traits will come directly from one parent or another, or even from one grandparent or another.
Punnet Square for Sex-linked Inheritance
When illustrating sex linked inheritance we always use the X and Y letters. If we are talking about gene “A” on the X chromosome we will illustrated this way: XA or Xa.
Why doesn’t the Y have any allele on it?
Sex-Linked InheritanceImagine if the dominant allele “A”
means normal vision and “a” means colour blind.
Pretend the mother is heterozygous and the father has normal vision
What percentage of males will be colour blind? How about females?
Sex linked diseases
E.g. Haemophilia
Sex linked diseaseMuscular Dystrophy
Sex linked diseaseFragile-X syndrome
Sex linked traitRed-green colour blindness
Sex linked diseasesWhile there are diseases that
exist on the Y chromosome we will only look at diseases on the X chromosome
X-linked diseases can come in two forms:
1. Dominant2. Recessive
Dominant X linked diseaseSome X-linked diseases are
dominant. This means they will be expressed regardless of whether the other X chromosome contains the disease.
e.g. Disease = XDXD or XDXd or XDY (males)
Recessive X-linked diseaseOther X-linked diseases are
recessive. This means they will only be expressed if the individual has both alleles with the recessive diseased allele.
e.g. Disease = XdXd or XdY (males)
Carriers of the diseaseCarriers of the disease are those
individuals that have the diseased allele within them but do not have expression of the disease because the allele is recessive.
Example of X-linked carriers:
- XDXd (where Xd is the recessive diseased allele)
Pedigree ChartsGenetic researchers use pedigree charts to track
traits from parents to children
◦ Pedigree charts are similar to family trees
Pedigree Charts
X-Linked Recessive Disease Example
Assume d = diseased and D = normal
What is the genotypes for the F1 generation?
F1
F2
P
Practice: X-linked Dominant
Here is an example of a pedigree chart for an X-linked dominant disease. Let us pretend that the X-linked dominant disease is Fragile X syndrome
XFY (where F is fragile X syndrome dominant allele)
Solve the genotypes for the II generation
Homework#13, 15-18 (p.258)#10 – 13 (p.259)