Blood type is determined by the I gene. IAIA and IAi determines A blood type. IBIB and IBi determines B blood type. ii determines 0 blood type. Michelle's mother must at least carry one recessive allele i.
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The blood type is determined by a gene, which is triallelic.
- IA dominant over i. The person presents A type of antigens.
- IB dominant over i. The person presents B type of antigens.
- IA and IB are codominant. The person presents both types of antigens, A and B.
- i is the recessive allele. The person does not present any antigen.
⇒ H0m0zyg0us dominant individuals IAIA, or heter0zyg0us Individuals IAi, will express the A blood type.
⇒ H0m0zyg0us dominant individuals IBIB, or heter0zyg0us Individuals IBi, will express the B blood type.
⇒ Individuals with h0m0zyg0us recessive genotype, ii will express the 0 blood type.
⇒ Individuals carrying IA and IB alleles will express the AB blood type.
To solve the problem, we will use this information.
We know that
- Brother and sister are blood type A
⇒ If Michelle is blood type 0, then her genotype is ii. She must have received one recessive allele from her father and one from her mother.
⇒ This suggests that Michelle's father, who is blood type A, is heter0zyg0us IAi. He must have at least one dominant allele IA that determines his phenotype, and one i allele.
⇒ Michelle's brother and sisters are also type A. They might be either h0m0zyg0us dominant IAIA or heter0zyg0us IAi.
⇒ Michelle's mother should have at least one recessive allele i. She could be either IAi or IBi, or ii. There are three possible option for her genotype.
Let us make the Punnet squares to understand it better. We know the genotype of the father, but there are three options for the mother.
Option 1
Cross) father x mother
Parentals) IAi x IAi
Gametes) IA i IA i
Punnett square) IA i
IA IAIA IAi
i IAi ii
F1) 25% of the progeny is expected to be h0m0zyg0us dominant IAIA
50% of the progeny is expected to be heter0zygus IAi
25% of the progeny is expected to be h0m0zyg0us recessive ii
75% of the progeny is expected to be blood type A ⇒ IAIA and IAi ⇒ Sisters and brother
25% of the progeny is expected to be blood tye 0 ⇒ ii ⇒ Michelle
Option 2
Cross) father x mother
Parentals) IAi x IBi
Gametes) IA i IB i
Punnett square) IA i
IB IBIA IBi
i IAi ii
F1) 25% of the progeny is expected to be IAIB
25% of the progeny is expected to be heter0zygus IAi
25% of the progeny is expected to be heter0zygus IBi
25% of the progeny is expected to be h0m0zyg0us recessive ii
25% of the progeny is expected to be blood type AB ⇒ IAIB
25% of the progeny is expected to be blood type A ⇒ IAi ⇒ Sisters and brother
25% of the progeny is expected to be blood type B ⇒ IBi
25% of the progeny is expected to be blood tye 0 ⇒ ii ⇒ Michelle
Option 3
Cross) father x mother
Parentals) IAi x ii
Gametes) IA i i i
Punnett square) IA i
i IAi ii
i IAi ii
F1) 50% of the progeny is expected to be heter0zygus IAi
25% of the progeny is expected to be h0m0zyg0us recessive ii
50% of the progeny is expected to be blood type A ⇒ IAi ⇒ Sisters and brother
50% of the progeny is expected to be blood tye 0 ⇒ ii ⇒ Michelle
Despite the fact we can not determine for sure the mother's genotype, we know that she carries at least one recessive allele, which is why Michelle's blood type is 0.
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