Question

In a disputed parentage case, the child is blood type \(0,\) 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: Blood type AB would exclude a male from being the father in this case.

Step-by-step solution

Understanding Blood Types and Genotypes

Blood types are determined by the presence of antigens on the surface of red blood cells: A antigen (blood type A), B antigen (blood type B), both A and B antigens (blood type AB), or neither A nor B antigens (blood type O). Blood types are inherited through the combination of parental alleles (A, B, or O) that determine the respective genotype. The O allele is recessive, while A and B alleles are dominant. For a child to have blood type O, they must inherit O allele from both parents.

Determining Mother's Genotype

As the mother has blood type A, her possible genotypes are AA or AO. However, as a child with blood type O requires O allele from both parents, the mother's genotype must be AO.

Determining Possible Father's Genotype

For a child to have blood type O when the mother's genotype is AO, the father must contribute an O allele. Therefore, the father's possible genotypes are OO, AO, and BO.

Blood Type that Excludes a Male from being the Father

If a male has blood type AB, his genotype is AB, and he cannot contribute an O allele to the child's genotype. Thus, a male with blood type AB would be excluded from being the father.

Other Blood Types Proving Paternity

Although blood types A, B, and O do not exclude a male from being the father, they do not prove that a male is the father either. Blood type paternity tests can only exclude potential fathers, not confirm paternity. More sophisticated DNA tests are needed to confirm paternity.

Key concepts

Blood Type Inheritance

Understanding how blood types are passed down from parents to children is a fundamental aspect of genetics. Each person inherits one allele from each parent, and these alleles determine the individual's blood type. These alleles can be for blood type A (represented as 'A'), blood type B ('B'), or blood type O ('O'). The A and B alleles are dominant, meaning that if an individual inherits either an A or a B allele from one parent, that blood type will be expressed. The O allele is recessive; therefore, for blood type O to be expressed, an individual must inherit the O allele from both parents.

For instance, if a child has blood type O, it means they received an O allele from each parent (OO genotype). If the mother has blood type A, her genotype could be either AA or AO. However, since the child has type O blood, the mother must have an AO genotype, contributing the O allele to the child. Understanding this, we can then use the child's and mother's blood types to infer possible genotypes for the father. This forms the basis for using blood types to examine paternity in disputed cases.

Antigens and Genetics

Antigens are substances on the surface of red blood cells that determine an individual's blood type and provoke an immune response. The presence or absence of these antigens (A and B) is what defines the four primary blood groups: A (presence of A antigen), B (presence of B antigen), AB (presence of both A and B antigens), and O (absence of both antigens).

These antigens are linked directly to the genetics of blood types. The genes that a person inherits from their parents include instructions for making these antigens. In the case of blood type O, neither the A nor the B antigen is produced, this is due to the O alleles not coding for either antigen. This basic genetic understanding is crucial when determining the compatibility for blood transfusions and understanding inherited traits, such as in the context of paternity tests.

Genotype Determination

Genotype refers to the combination of alleles that an individual possesses, which in the case of blood types, consists of two alleles. Determining a person's genotype goes beyond knowing their blood type (phenotype) as different genotypes can result in the same blood type. For example, a person with blood type A could have either an AA or AO genotype.

Knowing the genotype is especially important when examining paternity. For a child with an OO genotype (blood type O), the father's genotype must contain at least one O allele. He could have an OO, AO, or BO genotype, but not an AB genotype since that would not result in the necessary O allele for the child. Hence, when a male's blood type is AB, he cannot be the father of a child with blood type O. Although genotype determination can exclude certain individuals as potential fathers, it does not confirm paternity. For that, more detailed genetic testing is required.