No MCQ genetic pedigree series would be complete without a question about ABO blood groups!

Before we get to the pedigree, a quick definition question. The ABO system, where a person can either be Type A, Type B, Type AB or Type O blood depending on their genotype is an example of which type of inheritance?

Co-dominance

Mitochondrial

Autosomal Recessive

X-linked

Autosomal Dominant

From the information you have, what are the potential phenotypes of person #1 in the green box?

A- or B- or AB- or 0-

AB- only

AB- or B-

AB+ or AB-

A- or B- or O-

A- or AB-

Answer explanation

Click here for explanation to the question above

To work these out, you have to consider what you know. Here we know that the parents of the father of the green child were A+ and O- therefore there father has to have the genotype AO and must be –. We know less about the mother, only that she is B which could be BB or BO and is rhesus – so must be –. If both parents are Rh- then the offspring will be Rh-, that bit is easy. For the other part AO x BB or BO gives AB, AO, BO or OO as options

Note that convention is to write blood group genotypes as I^{A}I^{O} Rh^{+}Rh^{–} etc. I have simplified this to AO +- etc here!

What is the genotype of female #3 (orange circle).

BO– or BO+-

BB–

BB+-

BO+-

BAO–

Answer explanation

Click here for explanation to the question above

From the phenotype, we know that person 3 has at least one B and does not have rhesus + and Rh+ is dominant over -. Therefore the only options are BB– or BO–. Then we look at her offspring and we see a O-. We know that O- must be OO– Therefore there must be an O in the mother and her genotype must be BO– for this to be possible

What is the probability that female #2 in the yellow circle will have the same blood group (phenotype) as her brother?

1/8

1/16

1/2

1/4

1/32

Answer explanation

Click here for explanation to the question above

First let’s look at the genotype of the parents. Father is A+ but his parents were A+/O- therefore he must be AO+-. The mother is AB+ but her brother is O- therefore she must be AB+-. From that combination we can work out the answer using a Punnet square (below after the break). Or you can work it out in your head – what I did was consider the possibility of getting an A phenotype from AO x AB which is 1/2 (AA or OA, vs AB or OB) and multiply by the possibility of getting Rh-, which is 1/4 (++, +-, -+, –). Therefore 1/2 x 1/4 = 1/8.

Punnet square after the page break + another question on this pedigree