BackWeek 5 Multiple Alleles, Types of Dominance, ABO Blood Groups, and Probability Rules
Study Guide - Smart Notes
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Multiple Alleles and Genetic Variation
Definition and Examples
In genetics, a gene may exist in more than two allelic forms within a population, known as multiple alleles. Although an individual can carry only two alleles for a given gene (one from each parent), the population may have many different alleles.
Wild type allele (A): The most common allele in a population.
Mutant alleles: Variants of the wild type gene (A).
Most genes in humans have only two alleles, but some genes, such as those for blood groups, have more.
Types of Dominance
Overview of Dominance Relationships
Dominance describes how different alleles interact to produce a phenotype. There are several types:
Complete Dominance: One allele is completely dominant over the other; the phenotype of the heterozygote is the same as that of the homozygous dominant.
Incomplete Dominance (Partial Dominance): The heterozygote displays a phenotype that is intermediate between the two homozygotes. For example, crossing a plant with white flowers and one with dark blue flowers may produce light blue flowers.
Codominance: Both alleles in the heterozygote are fully expressed, resulting in a phenotype that shows both parental traits. For example, a flower with both white and blue patches.
ABO Blood Groups
Genetic Basis and Inheritance
The ABO blood group system in humans is determined by three alleles: IA, IB, and i. These alleles combine to produce four possible blood types: A, B, AB, and O.
Alleles: IA and IB are codominant; both are dominant over i.
Phenotypes: The combination of alleles determines the blood group.
Mendelian Inheritance: The ABO system follows Mendelian principles of inheritance.
Phenotype (Blood Group) | Genotype |
|---|---|
O | ii |
A | IAIA or IAi |
B | IBIB or IBi |
AB | IAIB |
Probability Rules in Genetics
Product Rule (AND): Two Events Happening Together
The product rule is used when two outcomes are independent and you want the probability of both occurring together. Multiply the probabilities of each event.
Formula:
Example: The probability that a child inherits allele A from one parent and allele B from another, if each event is independent.
Sum Rule (OR): One or the Other
The sum rule is used when the outcomes are mutually exclusive and you want the probability of either event occurring. Add the probabilities of each event.
Formula:
Example: The probability that a child has a dominant phenotype from two heterozygous parents:
Example Problems
Problem 1: If the mother is ii (blood type O) and the father is either IAi or ii, what is the probability their child is blood type A?
If the father is IAi: 1/2 chance for A
If the father is ii: 0 chance for A
Problem 2: What is the probability that one child is blood type A and another is blood type O?
P(O, A) = 1/2 × 1/2 = 1/4 (assuming independent events)
P(A, O) = 1/4
Total probability = 1/2
Summary Table: ABO Blood Group Inheritance
Parent Genotypes | Possible Child Blood Types | Probability |
|---|---|---|
IAi × ii | A, O | 1/2 each |
ii × ii | O | 1 |
IAi × IBi | A, B, AB, O | 1/4 each |
Additional info: The above table summarizes possible outcomes for common parental genotype combinations in the ABO system, which is a classic example of multiple alleles and codominance in human genetics.
