Page 168 - Big Ideas Year 10 sciPAD 2025

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168 SCIPAD - BIG IDEAS - BOOK 2
THE BASIS OF HEREDITY
Hitchhiker’s Guide to Genetics
Some alleles are ‘dominant’ over others. If a copy of the allele is present it will dominate the other
allele and ‘show up’ in the organism. Hitchhiker’s thumb is a condition where the thumb can bend
to an unusual angle. For simplicity, assume it is controlled by a gene given the dominant allele ‘T’.
A person with the alleles ‘TT’ or ‘Tt’ will not have Hitchhiker’s thumb. Only a person with the alleles
‘tt’ will have Hitchhiker’s thumb.
1. State the possible alleles of:
(a) A person with Hitchhiker’s thumb. 
(b) A person without Hitchhiker’s thumb. 
2. Is Hitchhiker’s thumb the dominant condition? 
3. A person with Hitchhiker’s thumb mates with a person who is purebred for non-Hitchhiker’s
thumb.
(a) State the alleles of the parent who:
(i) has Hitchhiker’s thumb. 
(ii) is purebred for non-Hitchhiker’s thumb. 
(b) Complete the Punnett square to show the possible offspring when they reproduce.
(c) What percentage of their offspring would you expect to have Hitchhiker’s thumb?

(d) What percentage of their offspring would you expect to not have Hitchhiker’s thumb?

Hitchhikers thumb Non-Hitchhikers thumb
Please note that this is an over-simplistic representation of the genetics involved.
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