CHAPTER 11 NOTES

 

Multiple Alleles: when more than two different alleles exist for the same trait.

    (Remember: each individual will only have two alleles for a trait but there are several alleles to choose from.)

 

In rabbits, coat color is controlled by multiple alleles. Full color (C), white (c), light-gray or chinchilla (cch) and white with black points or a Himalayan (ch). Full color is dominant to all the other alleles. Chinchilla is dominant to Himalayan and white.

 

 

cchc x Cch

 

 

 

 

cch

 

c

 

C

 

Ccch

 

Cc

ch

 

cchch

chc

 

Genotypes:

1 Ccch: 1Cc : 1 cchch: 1 chc

Phenotypes:

 

2 dark gray : 1 light gray: 1 himalayan

 

In humans, blood type has multiple alleles.  IA and IB are codominant and i (type O blood) is recessive.

 

 

Blood Type

Type of Antigen

Possible Genotypes

Type A

 

A antigen

IAIA, IAi

Type B

 

B antigen

IBIB, IBi

Type AB

A and B antigens

IAIB

Type O

 

No antigens

ii

 

 

 

Cross a heterozygous type A with a heterozygous type B.

IAi x IBi

 

 

 

 

IA

 

i

 

IB

 

 

IAIB

 

IBi

i

 

IAi

 

ii

 

 

 

 

 

 

 

 

Genotypes:

1 IAIB: 1 IBi : 1 IAi : 1 ii

 Phenotypes:

1 Type AB: 1 Type B: 1 Type A: 1Type O

 

 

Cross a person with type AB blood with another person with type O blood.

 

IAIB x ii

 

 

 

IA

IB

 

i

 

IAi

IBi

i

 

IAi

IBi

 

 

 

 

 

 

 

 

 

 

Genotypes:

2 IBi : 2 IAi

 

Phenotypes:

 Type B: 2 Type A

Polygenic Inheritance: the inheritance pattern is controlled by two or more genes each with two alleles.

 

In humans, eye color is the result of polygenic inheritance.  At the present, three gene pairs controlling human eye color are known. Two of the gene pairs occur on chromosome pair 15 and one occurs on chromosome pair 19. The bey 2 gene, on chromosome 15, has a brown and a blue allele. A second gene, located on chromosome 19 (the gey gene) has a blue and a green allele. A third gene, bey 1, located on chromosome 15, is a central brown eye color gene.

 

          The brown allele is always dominant over the blue allele so even if a person is heterozygous (one brown and one blue allele) for the bey 2 gene on chromosome 15 the brown allele will be expressed. The gey gene also has two alleles, one green and one blue. The green allele is dominant to the blue allele on either chromosome but is recessive to the brown allele on chromosome 15. This means that there is a dominance order among the two gene pairs. If a person has a brown allele on chromosome 15 and all other alleles are blue or green the person will have brown eyes. If there is a green allele on chromosome 19 and the rest of the alleles are blue, eye color will be green. Blue eyes will occur only if all four alleles are for blue eyes.

 

    Polygenic inheritance usually shows up as a range of variation such has height, skin color or hair color.

    The results of an average population will result in a bell-shaped curve.