A parental mystery: Mr. and Mrs. Smith have just arrived home with their new bab
ID: 255608 • Letter: A
Question
A parental mystery: Mr. and Mrs. Smith have just arrived home with their new baby. As soon as they walk in the door, they get a call from the hospital maternity ward. The nurse who discharged the Smiths is calling to say he is now worried that she sent the wrong baby home with them. They return to the hospital for a battery of blood tests to clear up the confusion. The results are found in the table below:
Individual
ABO Blood Type
MN Blood Type
Hemoglobin Type
(H—normal; h—mutant
hemoglobin, sickle cell)
Mr. Smith
Type A
MM
Hh
Mrs. Smith
Type B
MN
HH
Baby Emma
Type O
NN
Hh
Help them interpret the results of the blood test. Could baby Emma be their daughter? Why or why not?
Both the blood-type systems are examples of codominance, and sickle-cell anemia (hemoglobin type) is an example of an autosomal recessive trait that also exhibits codominance. Help them interpret the results of the blood test. Could baby Emma be their daughter? Why or why not?
Individual
ABO Blood Type
MN Blood Type
Hemoglobin Type
(H—normal; h—mutant
hemoglobin, sickle cell)
Mr. Smith
Type A
MM
Hh
Mrs. Smith
Type B
MN
HH
Baby Emma
Type O
NN
Hh
Explanation / Answer
Ans. Mr. Smith: Possible Genotypes (ABO type): IAIO , IAIA
Possible Genotypes (MN type): MM
Hb type: Hh
# Mrs. Smith: Possible Genotypes (ABO type): IBIO , IBIB
Possible Genotypes (MN type): MN
Hb type: HH
# Baby Emma: Possible Genotypes (ABO type): IOIO
Possible Genotypes (MN type): MM
Hb type: Hh
# Possibility based on ABO bloody type: Each parent contributes one allele to the offspring. If both the parents contribute IO allele, the resultant baby Emma has O blood type.
Thus, based on ABO blood type – Emma may be their daughter.
# Possibility based on Hb type: Each parent contributes one allele to the offspring. If one parent contribute H while the other contributes h, the resultant baby Emma can be Hh blood type.
Thus, based on Hb type – Emma may be their daughter.
# Possibility based on MM blood type: Each parent contributes one allele to the offspring. Mr. Smith can contribute allele M only whereas Mrs. Smith can contribute either of M or N allele. In no case, the resultant baby can have 2 copies of allele N because only one parent (Mrs. Smith) contributes the N allele.
Thus, based on MN blood type – Emma can NOT be their daughter.
# Conclusion: The allelic composition of the child must be a possible combination of the parental genotype only. Since the genotype of Emma with respect to MN blood type does NOT match with the possible combinations of parental genotypes, i.e. Emma can’t be NN blood type for the given MN blood type of parents; Emma is NOT the child of Mr. & Mrs. Smith.
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