Chapter 10, Problem 7GP

Summary Introduction

To determine:

Which baby belongs to which couple by using the given information?

Concept introduction:

The ABO blood group is determined by three alleles of gene I: $I^A$, $I^B$ and i. The surface of red blood cells have specific surface antigens either ‘A’ or ‘B’ which are encoded by gene I. The recessive allele i does not produce any surface antigen which forms blood group O (genotype ii). If gene I encodes molecule A only, the person will have blood group A (genotype $I^{A}i$ or $I^A{I}^A$). If it encodes only molecule B, the person will have B blood group (genotype $I^B{I}^B$ or $I^{B}i$).

If gene I encodes for both molecule A and B, it give rise to blood group type AB (genotype $I^A{I}^B$). Here both $I^A$ and $I^B$ alleles are equally or codominantly expressed to give rise to a new phenotype. The person with genotype $I^A{I}^B$ will have both A and B molecules on red blood cell surface.

Explanation of Solution

Tabular representation:

Table 1: Cross between father with genotype $I^B{I}^B$ and mother with genotype $I^A{I}^B$.

Table 2: Cross between father with genotype $I^{B}i$ and mother with genotype $I^A{I}^B$.

Table 3: Cross between father and mother with genotype $I^A{I}^A$.

Table 4: Cross between father and mother with genotype $I^{A}i$.

Table 5: Cross between one parent with genotype $I^{A}i$ and the other have genotype $I^A{I}^A$.

Table 6: Cross between father with genotype ii and mother with genotype $I^B{I}^B$.

Table 7: Cross between father with genotype ii and mother with genotype $I^{B}i$.

Couple 1: Father with B blood type (genotype $I^B{I}^B$ or $I^{B}i$) and mother with AB blood type (genotype $I^A{I}^B$).

If the father has genotype $I^B{I}^B$ and mother has genotype $I^A{I}^B$, half of their child will be with blood type AB ( $I^A{I}^B$) and half will be with blood type B ( $I^B{I}^B$). (Refer table 1).

If the father has genotype $I^{B}i$ and mother has genotype $I^A{I}^B$, 50% of their children will have blood type B (with genotype $I^B{I}^{B}and{I}^{B}i$), 25% of A with genotype $I^{A}i$, and 25% with blood type AB ( $I^A{I}^B$). (Refer table 2).

Thus, couple 1 could produce children with blood type A, B or AB.

Couple 2: Father have blood type A and mother have blood type A. Both could have genotypes $I^{A}i$ or $I^A{I}^A$.

If both father and mother have genotype $I^A{I}^A$ , all of their children will have blood type A (genotype $I^A{I}^A$). (Refer table 3).

If both father and mother have genotype $I^{A}i$ , 75% of their children will have blood type A (50% with $I^{A}i$ genotype and 25% with $I^A{I}^A$ genotype ) and 25% will be of blood type O (genotype ii). (Refer table 4).

If one parent have genotype $I^{A}i$ and the other have genotype $I^A{I}^A$ , all of the children will have blood type A (50% with $I^{A}i$ and 50% with $I^A{I}^A$). (Refer table 5).

Thus, couple 2 could produce children with blood type A, or O.

Couple 3: Father with blood type O (genotype ii) and mother with blood type B (genotype $I^B{I}^B$ or $I^{B}i$).

If father has genotype ii and mother has genotype $I^B{I}^B$ , all of their children will have blood type B (genotype $I^{B}i$). (Refer table 6).

If father has genotype ii and mother has genotype $I^{B}i$, 50% of their children will have blood type B (genotype $I^{B}i$) and 50% will have blood type O (genotype ii). (Refer table 7).

Thus, couple 3 could produce children with blood type B, or O.

According to the given information:

Baby X has blood type B.

Baby Y has blood type AB.

Baby Z has blood type O.

As only couple 1 could produce children with blood type AB, so baby Y belongs to couple 1.

As only couple 3 could produce children with blood type B, so baby X belongs to couple 3.

As couple 2 could produce children with blood type O, so baby Z belongs to couple 2.

Conclusion

Baby X belongs to Jordan and Taylor, baby Y belongs to Logan and Leslie and baby Z belongs to Sam and Casey.