Question

The ABO blood groups in humans are expressed as the \(I^{4}, I^{B},\) and \(i\) alleles. The \(I^{A}\) allele encodes the A blood group antigen, \(I^{B}\) encodes \(B\) and \(i\) encodes O. Both \(A\) and \(B\) are dominant to \(0 .\) If a heterozygous blood type A parent \(\left(I^{A} i\right)\) and a heterozygous blood type B parent ( \(I^{B} i\) ) mate, one quarter of their offspring are expected to have the AB blood type \(\left(r^{A} I^{B}\right)\) in which both antigens are expressed equally. Therefore, ABO blood groups are an example of: a. multiple alleles and incomplete dominance b. codominance and incomplete dominance c. incomplete dominance only d. multiple alleles and codominance

Short answer

The ABO blood groups are an example of multiple alleles and codominance, so the correct answer is d. multiple alleles and codominance.

Step-by-step solution

Definition of each term

First, let's understand what each term given in the options means: 1. Multiple Alleles: The presence of more than two alleles at a single gene locus. 2. Incomplete Dominance: When the heterozygous phenotype is intermediate between the two homozygous phenotypes. 3. Codominance: When both alleles of a gene in the heterozygous condition are expressed equally.

Identify the type of relationship

Now, let's analyze the relationship between the ABO blood groups based on the definitions above: 1. There are three alleles (\(I^A\), \(I^B\), and \(i\)) for ABO blood groups present, so we have multiple alleles. 2. We don't have incomplete dominance in this case, because the AB blood type (\(I^A I^B\)) doesn't show an intermediate phenotype; both antigens A and B are expressed equally. 3. The AB blood type (\(I^A I^B\)) shows an equal expression of both alleles, which is an example of codominance. Hence, the ABO blood groups are an example of multiple alleles and codominance. So, the correct answer is: d. multiple alleles and codominance

Key concepts

Multiple Alleles

In genetics, the term multiple alleles refers to a situation where, for a single gene, there are more than two possible alleles, or versions of that gene, that exist within a population. This complexity offers a more diverse pattern of inheritance than the simpler case where a gene has only two alleles.
For instance, in human ABO blood group genetics, there is not just one 'type A allele' and one 'type B allele' – there are in fact three alleles at the ABO locus: the I^A allele, the I^B allele, and the i allele. The I^A allele can produce the A antigen on red blood cells, the I^B allele produces the B antigen, while the i allele does not produce any antigen, resulting in the O blood type.

• The presence of the I^A or I^B allele results in the expression of their respective antigens.
• When an individual inherits two different alleles, such as I^A and i, they have the A blood type because I^A is dominant over i, which is recessive.

However, with multiple alleles, the inheritance patterns become more complex when interbreeding between heterozygous individuals occurs, as seen with the human blood types, which significantly increases the diversity of possible genotypes and phenotypes within a population.

Codominance

Codominance is an intriguing genetic phenomenon where different alleles of a gene are neither dominant nor recessive. Codominance occurs when both alleles in a heterozygote are fully expressed, resulting in offspring with a phenotype that is neither dominant nor recessive but a mixture of both parental traits.
In the context of ABO blood group genetics, individuals with one I^A allele and one I^B allele (genotype I^A I^B) have a type AB blood phenotype. Here, both A and B antigens are equally expressed on the surface of red blood cells. This simultaneous expression of both alleles is a textbook example of codominance because neither antigen is masked; instead, they are both present and visible.
Through codominance, the resulting phenotype is a unique display of both inherited alleles:

• In type AB blood, I^A and I^B alleles are codominant, resulting in the presence of both A and B antigens.
• This differs from complete dominance, where one allele would mask the expression of the other.

Genetic diversity is enriched by codominance, which allows for a more varied expression of alleles in the phenotype, vividly seen in the ABO blood group.

Mendelian Inheritance

Mendelian inheritance patterns emerge from principles first outlined by Gregor Mendel in the 19th century. These principles form the foundation of classical genetics and describe how characteristics are transmitted from one generation to the next. The basic units of heredity, genes, follow the rules of Mendelian inheritance, which include the concepts of dominant and recessive alleles.
Mendel's work focused on genes that exhibited simple domination patterns, where one allele is dominant and the other is recessive. However, traits governed by multiple alleles and codominance do not strictly follow this simple pattern:

• Inheritance of the ABO blood group is affected by three alleles (I^A, I^B, and i), which creates a more complex inheritance pattern than Mendel’s single gene with two alleles.
• Codominance, as seen with the AB blood type, was not accounted for in Mendel's original experiments with peas, since both alleles contribute to the phenotype.

However, Mendelian principles still underlie the inheritance of ABO blood types. When a parental combination of heterozygous A (I^A i) and heterozygous B (I^B i) occurs, the offspring exemplify Mendelian concepts of genetic probability: one quarter are expected to be type AB (I^A I^B), displaying both the A and B antigens, due to the codominance of the I^A and I^B alleles. Thus, while specific to the ABO blood group, Mendelian inheritance helps explain the probabilistic outcomes of genetic crosses, even with multiple alleles and codominance.