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)