12th Zoology Genetics Problems: Blood Groups and Sex-Linked Inheritance Solved
The genetics problems in Principles of Inheritance and Variation look different on the surface but use the same three steps every time: write each parent's genotype, list the gametes each parent can make, and combine them. Once that is a habit, the book back problems take under a minute each.
The ABO blood group alleles
The ABO blood group is controlled by three alleles of one gene, IA, IB and IO, which is why it is the textbook example of multiple alleles. They all occupy the same locus, and each person carries two of them.
| Blood group | Possible genotypes |
|---|---|
| A | IAIA or IAIO |
| B | IBIB or IBIO |
| AB | IAIB (co-dominance: both antigens are expressed) |
| O | IOIO only |
The key shortcut: a group O child must have received an IO allele from each parent.
Problem 1: Parents of A, AB and B children
Three children in a family have blood groups A, AB and B. What could the parents' genotypes be?
The AB child needs IA from one parent and IB from the other. Try IAIO × IBIO:
| IB | IO | |
|---|---|---|
| IA | IAIB (AB) | IAIO (A) |
| IO | IBIO (B) | IOIO (O) |
A, AB and B all appear, so the answer is IAIO and IBIO. The same square answers a second question: parents with groups A and B can have children of all four groups, but only when both are heterozygous.
Problem 2: A group O child of A and B parents
A child is group O, the father is group A and the mother is group B. What are the parents' genotypes?
The child is IOIO, so each parent must carry IO. The father is IAIO and the mother is IBIO.
Problem 3: Which group is impossible?
Which phenotype cannot appear in the children of IAIO × IAIB?
| IA | IB | |
|---|---|---|
| IA | IAIA (A) | IAIB (AB) |
| IO | IAIO (A) | IBIO (B) |
The second parent has no IO allele to give, so group O is impossible.
Problem 4: Two AB parents
IAIB × IAIB gives IAIA, IAIB, IAIB and IBIB. The children can be A, B or AB, in the ratio 1 : 1 : 2 for A : B : AB. They cannot be O.
Problem 5: Rh factor
Both parents are Rh positive with genotype Dd. What about their children?
Dd × Dd gives DD, Dd, Dd and dd. Only dd is Rh negative, so about one fourth of the children will be Rh negative. This is the ordinary 3 : 1 monohybrid ratio.
For emergencies, when there is no time to test blood, O Rh negative blood is the safest to transfuse because its red cells carry no A, B or Rh antigens.
Sex-linked inheritance: colour blindness and haemophilia
Both are recessive traits carried on the X chromosome. A male has only one X, so a single recessive allele shows its effect. A female needs two, which is why these conditions are more common in males. We write XC for the normal allele and Xc for colour blindness.
Problem 6: Colourblind man and normal woman
XcY × XCXC:
| Xc | Y | |
|---|---|---|
| XC | XCXc (carrier daughter) | XCY (normal son) |
Result: all daughters are carriers and all sons are normal. A father never passes his X chromosome to his sons.
Problem 7: Colourblind father and carrier mother
XcY × XCXc:
| Xc | Y | |
|---|---|---|
| XC | XCXc (carrier daughter) | XCY (normal son) |
| Xc | XcXc (colourblind daughter) | XcY (colourblind son) |
Two of the four possible children are colourblind, so the probability is 50%.
Related facts the problems lean on
- Sex determination. XY and XO systems show male heterogamety. In the ZW-ZZ system of birds, some reptiles and some fishes, females are heterogametic.
- Chromosomal disorders. Down's syndrome is trisomy 21, Patau's syndrome is trisomy 13, Klinefelter's syndrome is XXY and Turner's syndrome is XO.
- Universal donor and recipient. O is the universal donor and AB the universal recipient.
Practise
Draw the cross for every problem, even the easy ones, until the method is automatic. All the book back problems, with worked explanations, are on the Principles of Inheritance and Variation page, and the additional MCQs give you more crosses to try.