Question

In a disputed parentage case, the child is blood type $\mathrm{O},$ while the mother is blood type $A.$ What blood type would exclude a male from being the father? Would the other blood types prove that a particular male was the father?

Short answer

Answer: A male with blood type B (BB or BO) or blood type AB (AB) would be excluded from being the father of the child, as these genotypes do not carry the O gene needed for a child with blood type O. However, it is important to note that having blood type A (AO) or type O (OO) does not necessarily prove that a particular male is the father, and only a paternity test could confirm the true biological relationship between a particular man and the child.

Step-by-step solution

Understand the ABO blood group system

Human blood types are determined by the presence, absence, or combination of two antigens, A and B. These antigens are inherited from our parents, with one gene coming from each parent. Blood type O is characterized by the absence of both A and B antigens, while blood type A has the A antigen. In total, there are 4 possible blood types: A, B, AB, and O which are determined by the combinations of A and B genes: AA, AO, BB, BO, AB, and OO.

Determine the possible blood type combinations for the mother and child

From the given information, the mother has blood type A. This means that she has either AA or AO genotypes. The child has blood type O, which means the genotype is OO. To obtain the OO genotype in the child, the mother must have given an O gene. Therefore, the mother must have AO genotype.

Determine the father's possible blood type combinations

The child's blood type is O which means he has an OO genotype. Since we know the mother gave one O gene, the father must also give an O gene for the child to have this genotype. Thus, the father's blood type must be either type O (OO) or type A (AO), since these are the only two genotypes that carry the O gene.

Find the blood type that would exclude a male from being the father

Based on the possible blood type combinations identified in step 3, a father with blood type B (BB or BO) or blood type AB (AB) would be excluded from being the father. These genotypes do not carry the O gene, and it would be impossible for a father with one of these blood types to have a child with blood type O when the mother is blood type A.

Discuss whether the remaining blood types prove that a particular male is the father

The remaining blood types that could be the father are type A (AO) or type O (OO). However, having one of these blood types does not necessarily prove that a particular male is the father of the child. This is because many other males could also have these blood types, and only a paternity test could confirm the true biological relationship between a particular man and the child.

Key concepts

ABO blood group system

The ABO blood group system is a method of classifying human blood into different types based on the presence of antigens on the surface of red blood cells. These antigens are molecules that can trigger an immune response and are the result of genetic inheritance from our parents. In the ABO system, there are two main antigens: A and B. These can combine in different ways:

• Blood Type A: This occurs if the person has at least one A antigen. Possible genotypes are AA or AO.
• Blood Type B: This occurs with at least one B antigen present. The genotypes are BB or BO.
• Blood Type AB: This type has both A and B antigens. The genotype is AB.
• Blood Type O: This type has neither A nor B antigens. The genotype is OO.

Understanding these blood groupings is crucial for medical purposes, like blood transfusions, and also for understanding genetics, particularly when determining parentage.
The ABO system is inherited in a simple Mendelian way, where each parent contributes one of their two ABO genes to their offspring. This is why knowing the genotypes involved can help solve genetic puzzles, like determining possible parentage.

genotype

In genetics, a genotype is the specific combination of alleles (forms of a gene) that an individual possesses. For the ABO blood group, alleles are represented by the genes A, B, and O. Each individual inherits one allele from each parent, leading to a pair of alleles which form their genotype.

• Individuals with genotype AA or AO, for instance, will express blood type A.
• Those with BB or BO will express blood type B.
• The AB genotype leads to blood type AB, where both types of antigens are present.
• Finally, the OO genotype results in blood type O, which is characterized by the lack of A or B antigens.

This genetic makeup not only determines blood type but can also be used in solving puzzles of genetic inheritance, such as determining the possibility of certain parent-child relationships. In legal or medical settings, understanding genotypes can provide more insight into the family lineage.

inheritance

Inheritance in genetics refers to the transmission of genetic information from parents to their offspring. Each parent contributes one half of the genetic material needed to form a new individual. For simple Mendelian traits like those in the ABO blood group system, this transmission can be easily tracked.
In the context of the exercise, the child's blood type (O) results when both parents pass on an O allele. The lacking A and B antigens are a direct result of this genetic inheritance, making the genetic makeup (or genotype) OO.

• The mother, with blood type A, has a genotype of either AA or AO, meaning she must have contributed the O allele for her child to have type O blood. Thus, her genotype is AO.
• For the father to also contribute an O to result in a child with OO genotype, his blood type must be either type O or type A with an AO genotype.

This inheritance explains why certain blood types can exclude a male from being the father, while others might support the possibility. Blood types that do not carry an O allele, like BB or AB genotypes, cannot result in a child with blood type O, thus excluding that individual as a potential father.