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Chapter 4: Problem 6

List all possible genotypes for the \(\mathrm{A}, \mathrm{B}, \mathrm{AB},\) and \(\mathrm{O}\) phenotypes. Is the mode of inheritance of the \(\mathrm{ABO}\) blood types representative of dominance, recessiveness, or codominance?

Short Answer

Expert verified
Question: What is the mode of inheritance for the ABO blood types, and list the genotypes for each blood type. Answer: The mode of inheritance for the ABO blood types is codominance. The genotypes for each blood type are as follows: - Blood type A: IAIA or IAi - Blood type B: IBIB or IBi - Blood type AB: IAIB - Blood type O: ii

Step by step solution

01

Understand the ABO blood group system

The ABO blood group system classifies human blood into A, B, AB, and O types, based on the antigens present on the surface of red blood cells. There are three different alleles for blood types: IA, IB, and i. The IA and IB alleles are codominant, meaning that they both express themselves when paired together. The i allele is recessive to both IA and IB.
02

List the genotypes for blood type A

Blood type A can have two genotypes: IAIA or IAi. These occur when there is at least one IA allele and no IB allele present.
03

List the genotypes for blood type B

Blood type B can also have two genotypes: IBIB or IBi. These occur when there is at least one IB allele and no IA allele present.
04

List the genotypes for blood type AB

Blood type AB has only one genotype: IAIB. This occurs when both IA and IB alleles are present, and they are both expressed due to their codominant relationship.
05

List the genotypes for blood type O

Blood type O has a single genotype: ii. This occurs when both alleles are the recessive 'i,' which means neither the A nor B antigens are present on the red blood cells.
06

Determine the mode of inheritance

The mode of inheritance for the ABO blood types is codominance because both IA and IB alleles can express themselves simultaneously when paired together, resulting in the AB blood type. The A and B blood types show dominance (or partial dominance) over the O blood type due to the presence of the IA or IB allele over the recessive 'i.'

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Genotypes of Blood Types
Understanding the genotypes of blood types is fundamental in comprehending how our bodies determine blood type. The ABO blood system is based on the presence or absence of two main antigens, A and B, on the surface of red blood cells. These antigens are governed by three alleles: IA, IB, and i.

For blood type A, the possible genotypes are IAIA (homozygous) or IAi (heterozygous). This means an individual with type A blood could have received the IA allele from both parents, or the IA allele from one parent and the i allele from the other.

In the case of blood type B, we observe a similar pattern with genotypes IBIB and IBi. Individuals inherit the IB allele from one or both parents, leading to the expression of the B antigen.

As for type AB blood, it is unique because its only genotype is IAIB. The presence of both codominant alleles results in the expression of both antigens A and B.

Lastly, blood type O's genotype is ii, consisting of two recessive alleles, which means neither the A nor B antigens are present.

Through understanding these genetic combinations, we can predict blood type inheritance and potential donor-recipient compatibility in blood transfusions.
Codominance in Genetics
Codominance is a form of inheritance where two different alleles at a gene locus are both fully expressed in a heterozygous individual. In the ABO blood group system, this is illustrated by the presence of both A and B antigens on the red blood cells of individuals with type AB blood.

Unlike incomplete dominance, where a blend of traits is observed, codominance shows both traits distinctly. For example, the phenotype of someone with genotype IAIB will express attributes of both type A and type B blood due to the codominant nature of the IA and IB alleles.

This genetic phenomenon showcases that neither allele is recessive in the pair, which can lead to more diverse presentations within offspring when both parents carry different alleles. Understanding codominance is crucial when studying inheritance patterns as it can influence the probability of particular traits within a population.
Inheritance of Blood Types
The inheritance of blood types is a classic example of Mendelian genetics with a twist due to the presence of multiple alleles. Each person inherits one allele from each parent, resulting in a combination that determines their ABO blood type.

When discussing inheritance, we need to remember the dominance hierarchy. The alleles IA and IB are dominant over i, which is why the O type is expressed only when an individual is homozygous recessive (ii). However, IA and IB exhibit codominance with each other, resulting in the AB blood type.

Parents with type A and type B could have children with type A, type B, type AB, or type O blood, depending on their genotype. The allele combination passed down to their offspring is subject to chance, with each parent contributing one allele.

It's important to note that a child's blood type can sometimes help infer, but not definitively determine, parentage. Determining the potential blood types of offspring can be made using a Punnett square, a tool that maps out the likelihood of various allele combinations from the parents.

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Most popular questions from this chapter

Students taking a genetics exam were expected to answer the following question by converting data to a "meaningful ratio" and then solving the problem. The instructor assumed that the final ratio would reflect two gene pairs, and most correct answers did. Here is the exam question: "Flowers may be white, orange, or brown. When plants with white flowers are crossed with plants with brown flowers, all the \(F_{1}\) flowers are white. For \(F_{2}\) flowers, the following data were obtained: Convert the \(F_{2}\) data to a meaningful ratio that allows you to explain the inheritance of color. Determine the number of genes involved and the genotypes that yield each phenotype." (a) Solve the problem for two gene pairs. What is the final \(\mathrm{F}_{2}\) ratio? (b) A number of students failed to reduce the ratio for two gene pairs as described above and solved the problem using three gene pairs. When examined carefully, their solution was deemed a valid response by the instructor. Solve the problem using three gene pairs. (c) We now have a dilemma. The data are consistent with two alternative mechanisms of inheritance. Propose an experiment that executes crosses involving the original parents that would distinguish between the two solutions proposed by the students. Explain how this experiment would resolve the dilemma.

Horses can be cremello (a light cream color), chestnut (a brownish color), or palomino (a golden color with white in the horse's tail and mane). Of these phenotypes, only palominos never breed true. \(\begin{array}{ll}\text { cremello } \times \text { palomino } & \longrightarrow \begin{array}{l}1 / 2 \text { cremello } \\ 1 / 2 \text { palomino }\end{array} \\ \text { chestnut } \times \text { palomino } \longrightarrow & \begin{array}{l}1 / 2 \text { chestnut } \\ 1 / 2 \text { palomino }\end{array} \\ \text { palomino } \times \text { palomino } \longrightarrow & \begin{array}{l}1 / 4 \text { chestnut } \\ 1 / 2 \text { palomino }\end{array} \\ & 1 / 4 \text { cremello }\end{array}\) (a) From the results given above, determine the mode of inheritance by assigning gene symbols and indicating which genotypes yield which phenotypes. (b) Predict the \(\mathrm{F}_{1}\) and \(\mathrm{F}_{2}\) results of many initial matings between cremello and chestnut horses.

In four o'clock plants, many flower colors are observed. In a cross involving two true-breeding strains, one crimson and the other white, all of the \(\mathrm{F}_{1}\) generation were rose color. In the \(\mathrm{F}_{2}\), four new phenotypes appeared along with the \(P_{1}\) and \(F_{1}\) parental colors. The following ratio was obtained: Propose an explanation for the inheritance of these flower colors.

In this chapter, we focused on extensions and modifications of Mendelian principles and ratios. In the process, we encountered many opportunities to consider how this information was acquired. On the basis of these discussions, what answers would you propose to the following fundamental questions? (a) How were early geneticists able to ascertain inheritance patterns that did not fit typical Mendelian ratios? (b) How did geneticists determine that inheritance of some phenotypic characteristics involves the interactions of two or more gene pairs? How were they able to determine how many gene pairs were involved? (c) How do we know that specific genes are located on the sexdetermining chromosomes rather than on autosomes? (d) For genes whose expression seems to be tied to the sex of individuals, how do we know whether a gene is X-linked in contrast to exhibiting sex-limited or sex-influenced inheritance?

In mice, a short-tailed mutant was discovered. When it was crossed to a normal long-tailed mouse, 4 offspring were short-tailed and 3 were long-tailed. Two short-tailed mice from the \(\mathrm{F}_{1}\) generation were selected and crossed. They produced 6 short-tailed and 3 long-tailed mice. These genetic experiments were repeated three times with approximately the same results. What genetic ratios are illustrated? Hypothesize the mode of inheritance and diagram the crosses.
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