Brief - sex linkage & pedigrees

Dr Pusey

www.puseyscience.com

Crash Course - Heredity

Sex Linkage• Autosomal Genes – Appear in Pairs.

Dominant allele wins.

• Sex Linked traits occur on either the X or the Y chromosome.

• X and Y chromosomes are unique.• Women have two X chromosomes – act

like autosomal alleles.• Men only have one X and one Y –

Therefore MEN have 100% chance of exhibiting the trait if it appears.

• If a trait is recessive, it will almost always only be expressed in males.

• E.g. Colour blindness, Haemophilia

More about Dr Pusey’s Family Colour Blindness is X-linked.

Dr Pusey has two brothers. Both of his brothers are coloured blind (true story). Dr Pusey is not (AHA!).

Since Dr Pusey’s dad is not colour blind, his mother must be a“carrier”.

Draw the punnet square to determine how this occurred.

More about Dr Pusey’s Family

X Y

X

Xcb

Dr Pusey’s Dad

Dr

Pu

sey

’s M

um

More about Dr Pusey’s Family

X Y

X XX XY

XcbXcbX XcbY

Dr Pusey’s Dad

Dr

Pu

sey

’s M

um

Can you write the Phenotypes? Boy/Girl? Affected/Not Affected/Carrier?

More about Dr Pusey’s Family

X Y

X XXUnaffected Female

XYUnaffected Male

XcbXcbX

Carrier FemaleXcbY

Affected Male

Dr Pusey’s Dad

Dr

Pu

sey

’s M

um

Boys: 50% chance of being colour blindGirls: 50% chance of being a carrier, no chance of being colour blind

If Dr Pusey’s mum is a carrier, what does this say about Dr Pusey’s Granddad?Can Dr Pusey pass on the colour-blindness gene?

Pedigrees

Draw a Pedigree

Dr Pusey has 2 brothers

Brother 1 (Chris) has 1 boy and 1 girl

Brother 2 (Leigh) has 1 boy and 1 girl

Dr Pusey will have a boy or a girl (pending birth!)

Use all of the knowledge about Dr Pusey’s family to build a Pedigree starting from his Parents.

Draw a Pedigree

or

Not Affected

Carrier

Affected

50% Chance of being a carrier 50% Chance of being a carrier

Chris Dr Pusey Leigh

Mum Dad