Module 6 Lesson 3 - 4

Multiple Alleles

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Read pages 604 -606

From his work with peas, Mendel concluded that two kinds of factors control every trait and that each individual has a pair of these factors, one inherited from the mother and one from the father. In each case, one factor was dominant over the other. As you have just learned with the colour of four-o'clock flowers and roan horses, exceptions to dominance occur. You might not be surprised to discover that more than two alleles may affect the same trait.

Genes with more than two alleles are referred to as having multiple alleles. However, an individual can have a maximum of only two copies of the alleles - one from the mother and one from the father.

An example of multiple alleles is the coat colour in rabbits. Four alleles are involved in determining the coat colour: agouti (C), chinchilla (c^ch), Himalayan (c^h), and albino (c).

The order of dominance is C > c^ch > c^h > c. The hierarchy works to show that the allele on the left, C (agouti), is dominant to all the alleles on the right. The next allele, c^ch (chinchilla), is dominant to all the alleles on the right except for C (agouti).

All possible genotype combinations for rabbit coat colours are as follows:

C > c^ch > c^h > c

Agouti	CC   Ccch    Cch    Cc
Chinchilla	cchcch    cchch    cchc
Himalayan	chch    chc
Albino	cc

Example 1

A chinchilla rabbit ( c^chc^h ) is crossed with a Himalayan rabbit ( c^hc ). The order of dominance is C > c^ch > c^h > c. What is the phenotypic ratio of the offspring?

Solution

P1 Generation Phenotype = chinchilla x Himalayan

P1 Generation Genotype = c^chc^h  x  c^hc

	c ch	ch
ch	c chch	chch
c	c chc	chc

F1 Generation Genotypes = 1 c^chc^h : 1 c^chc : 1 c^hc^h : 1 c^hc

F1 Generation Phenotype = chinchilla : chinchilla : Himalayan : Himalayan = 2 chinchilla : 2 Himalayan

Answer: 1 chinchilla : 1 Himalayan (Reduce your ratios to the simplest form.)

Example 2

A chinchilla rabbit and Himalayan rabbit are crossed. Some of the offspring are albino. What are the genotypes of the parents?

Solution

The key is that albino rabbits resulted from the P1 cross. This means each parent had to pass the albino (c) allele to the offspring. To pass the albino allele, chinchilla and Himalayan rabbits had to be heterozygous with an albino allele. That leaves only one genotype combination.

P1 Generation Phenotype = chinchilla x Himalayan

P1 Generation Genotype = c^chc  x  c^hc

	c ch	c
ch	c chch	chc
c	c chc	cc

This is the only cross that will result in albino rabbits. Therefore, the genotypes of the parents are: c^chc  x  c^hc.

Watch and Listen

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Watch the following video on alternate patterns of inheritance.

Respond to the following questions for your own understanding:

1. What are multiple alleles?
   
   
2. How many alleles can one organism have for a gene with multiple alleles?
   
   
3. Define or explain codominance.
   
   
4. What is incomplete dominance?
   
   
5. How do the phenotypic and genotypic ratios compare for an incomplete dominant trait?