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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