Volume 27, Number 3, February 2015
(1) Text
 (1) Text (with bullet)
Answers
Grandma’s secret: a genetics
puzzle
Question 1
What is the genetic basis of the ABO blood group system?
Answer
The key to understanding this question is an understanding of how ABO blood groups and short
tandem repeats are inherited.
ABO blood group phenotypes are determined by three common alleles (IA, IB and IO) that belong to a
single gene called the I gene. The relationship between phenotype and genotype is show in the table.
Phenotype
Genotype
Blood group A
IA IA or IA IO
Blood group B
IB IB or IB IO
Blood group O
IO IO
Blood group AB
IA IB
In this case the IA and IB alleles are codominant, and in addition are both dominant to the I O allele. The
biochemical reason for this is quite simple as the IA allele encodes the A version of the blood group
cell surface protein and the IB allele encodes the slightly different B version of the blood group cell
surface protein, while the IO allele doesn’t encode anything, or at least doesn’t encode a functional
protein that can sit on the cell surface. The three ABO alleles are inherited in a standard Mendelian
fashion, in that you receive one allele from your mother and one allele from your father.
Question 2
What are the limitations of testing relatedness using ABO blood groups?
Answer
Working on the assumption that we only know the ABO phenotypes (as would have been the case in
the 1960s), then a parent of blood group AB cannot have a child of blood group O, and vice versa. All
other combinations are possible. In the 1960s paternity testing of this type could only exclude
individuals from being the genetic father, and could not be used to absolutely confirm parentage. ABO
data were usually used in conjunction with known inherited variation that could be measured
Philip Allan Updates © 2015
1
biochemically and was known to be associated with alleles at a single gene. This strategy was used to
narrow down the number of suspects in the first forensics case using the new technique of DNA
fingerprinting developed by Sir Alec Jeffreys in the 1980s.
Question 3
What could the ABO genotypes of your grandma, Paul Cool and Aunt Brenda be, if your grandma and
Paul Cool are indeed Aunt Brenda’s parents?
Answer
As the child is blood group A and the alleged father is blood group B, then the only combination that
works is if the child receives an IO allele from the alleged father, as shown below:
Subject
Blood group
Genotype
Mother (your grandma)
AB
IA IB
Child (Aunt Brenda)
A
IA IO
Alleged father (Paul Cool)
B
IB IO
Question 4
What could the genotypes of your grandma, Paul Cool and Aunt Brenda be, if your grandma is Aunt
Brenda’s mother but Paul Cool is not Aunt Brenda’s father?
Answer
If either the child or the alleged father is homozygous (the child being IA IA or the alleged father being
IB IB) then they cannot be first-degree relatives.
Subject
Blood group
Genotype
Mother (your grandma)
AB
IA IB
Child (Aunt Brenda)
A
IA IA or IA IO
Alleged father (Paul Cool)
B
IB IB or IB IO
Of course in reality both could be heterozygous (the child being IA IO and the alleged father being IB IO)
and still not be related as so many people have those genotypes. In other words ABO phenotype data
cannot prove that someone is the parent of a specific child. It can only be used for exclusion.
Question 5
Why is Doctor Robert’s interpretation correct?
Answer
Since we know the genotype of the mother (IA IB) and the child has blood group A, the child must
inherit an IA allele from her mother and either an IO or IA allele from her real genetic father. As the
alleged father may carry an IO allele, we cannot exclude the possibility that he is the genetic father of
the child. This illustrates the limitations of using ABO blood groups in paternity disputes, as only AB
parents of O children (and vice versa) are impossible.
Philip Allan Updates © 2015
2
Question 6
What are short tandem repeats?
Answer
A short tandem repeat (STR) is a short repetitive sequence of base pairs. STRs are usually 2, 3 or 4
base pairs long. For example, if the core of the repetitive sequence is TAG then an STR with five
repeats would be TAGTAGTAGTAGTAG. Within a species every individual has the same STR at the
same position on the chromosome. However, individuals vary in the number of repeats they have. So
using the TAG repeat as an example, there may be several different TAG repeat numbers in the
human population such as five repeats (TAGTAGTAGTAGTAG), six repeats
(TAGTAGTAGTAGTAGTAG), seven repeats (TAGTAGTAGTAGTAGTAGTAG) and so on.
Question 7
Briefly describe an experiment that would enable you to determine the length of an STR.
Answer
For humans and many other organisms the chromosomal position and sequence of STRs are well
documented. Therefore, for the STR used in this example, we will know the sequence of the DNA
flanking the repeat. So we can design primers that recognise the sequence flanking the repeat and
use the polymerase chain reaction (PCR) to amplify the sequence between the two flanking sites. As a
result we will amplify two fragments of DNA (one from each chromosome) whose sizes reflect the
number of STR repeats. We can then separate these DNA fragments according to size on an
electrophoretic gel.
Question 8
Why are STRs used to determine relatedness, especially paternity?
Answer
STRs are inherited in a Mendelian fashion. In other words, each STR is present in two copies, one on
the chromosome you inherit from your mother and the other from the chromosome that you inherit
from your father. Again you pass on one of each of your STRs to each of your children. In theory any
piece of DNA can be used to study relatedness, but STRs are particularly useful as they are very
variable.
Question 9
Who is Aunt Brenda’s genetic father?
Answer
At this STR your grandma has the genotype 8/17, while Aunt Brenda’s genotype is 8/11. Therefore
Aunt Brenda receives the 8 STR allele from your grandma and the 11 STR allele from her genetic
father. On the limited data you have, this excludes Paul Cool (14/16) as the father, and suggests that
your grandfather (11/19) is in fact Aunt Brenda’s father.
Philip Allan Updates © 2015
3
Question 10
Draw a family tree to show the apparent relationship between everyone in table 2.
Answer
Question 11
What are you going to tell your mother?
Answer
It would appear that your genetic parents are your father and your Aunt Brenda (not your ‘mother’).
Presumably your ‘mother’ knows this, but she may not expect you to have realised it.
This resource is part of BIOLOGICAL SCIENCES REVIEW, a magazine written for A-level students by
subject experts. To subscribe to the full magazine go to
www.hoddereducation.co.uk/biologicalsciencesreview
Philip Allan Updates © 2015
4