Problem 8 - Biology Concepts and Applications

Question

DNA fingerprinting is a method of identifying individuals by locating unique base sequences in their DNA molecules (Section 15.4 ). Before researchers refined the method, attorneys often relied on the ABO bloodtyping system to settle disputes over paternity. Suppose that you, as a geneticist, are asked to testify during a paternity case in which the mother has type A blood, the child has type \(\mathrm{O}\) blood, and the alleged father has type \(\mathrm{B}\) blood. How would you respond to the following statements? a. Attorney of the alleged father: "The mother's blood is type \(A,\) so the child's type \(O\) blood must have come from the father. My client has type B blood; he could not be the father." b. Mother's attorney: "Because further tests prove this man is heterozygous, he must be the father."

Step-by-Step Solution

Answer

The father could potentially be the parent if he has a BO genotype, providing the necessary O allele to the child.

Step 1: Understand Blood Type Inheritance

Blood type is determined by alleles. There are three main alleles: A, B, and O. Type A blood can be AA or AO, type B can be BB or BO, type AB is AB, and type O is OO. The child with type O blood must inherit one O allele from each parent.

Step 2: Analyze the Mother's Blood Type A

The mother has type A blood, which means her genotype could either be AA or AO. Since the child has an O allele, the mother must have an AO genotype, providing the O allele to the child.

Step 3: Address the Alleged Father's Blood Type B

The alleged father has type B blood, which means his genotype could either be BB or BO. To contribute an O allele to the child, his genotype must be BO.

Step 4: Evaluate Attorney of Alleged Father's Statement

The attorney claims the child must have gotten the type O blood from the father, implying type B cannot contribute an O allele. However, with the BO genotype, the father could provide the necessary O allele. Therefore, this statement is not necessarily valid.

Step 5: Consider Mother's Attorney Statement

The mother's attorney states the father is heterozygous. If the father is indeed BO, he can contribute an O allele. Thus, if the father is proven as heterozygous, he could potentially be the father, supporting the attorney's claim.

Key Concepts

Blood Type Inheritance

Blood type is determined by specific alleles: A, B, and O. These alleles combine in pairs to establish the blood type an individual has. Here's the basic breakdown of how these combinations work:

• Type A can include genotypes AA or AO.
• Type B can include genotypes BB or BO.
• Type AB is a combination of A and B, resulting in AB genotype.
• Type O results from two O alleles, or the OO genotype.

Understanding these combinations is crucial for determining possible blood types of children based on their parents' genotypes. In the case where the child has an O blood type and the mother has type A, the child's O type indicates it inherited an O allele from both parents. Therefore, even though the mother appears to be type A, she must carry an AO genotype to pass an O allele to her child. Similarly, understanding the father's type B as a BO genotype, rather than simply assuming it as BB, enables him to potentially pass on the O allele.

Paternity Testing

Paternity testing has evolved over the years. Initially, the simple ABO blood typing system was used, but it had its limitations. This method could sometimes help eliminate potential fathers but wasn't definitive. For example, if a child has blood type O, both parents must contribute an O allele. So if a potential father cannot produce an O allele, he can be excluded.

Paternity tests today take advantage of much more accurate DNA fingerprinting techniques. These methods examine specific sequences within the DNA that are unique to individuals. DNA from the child, mother, and alleged father is compared to see if the genetic patterns indicate a biological relationship. This process provides much more accurate results than relying solely on blood type, making it the preferred method in legal and personal cases of establishing paternity.

Genetics

Genetics is the study of heredity and the variation of inherited characteristics. Modern genetics began with Gregor Mendel, who discovered the fundamental laws of inheritance through his experiments with pea plants. He established that traits are inherited from parents to offspring through discrete units known as genes.

Every individual receives two alleles for each gene, one from each parent. These can be dominant or recessive. Dominant alleles can mask the presence of recessive ones. This is foundational in understanding how traits like blood type are inherited. For instance, a parent with type A blood doesn’t always pass an A allele; if they're carrying AO genotype, an O allele can be passed instead.

Beyond blood types and basic Mendelian genetics, more complex patterns emerge in gene interactions, known as polygenic inheritance, which can determine traits such as height, skin color, and even predispositions to certain diseases. While our understanding of genetics has grown significantly, many mysteries about how our genes influence our health and behavior remain to be explored.