Spring 2009: Section 4 – Lecture 4

Reading: Chapter 4

Chapter 7

Variation in Mendelian ratios

- incomplete dominance

- over-dominance

- co-dominance

- multiple alleles

- environment

- epigenetics

- epistasis (gene interactions)

- lethal genes

- gene linkage

Multiple alleles - when there are more than two alleles for a given gene. Can result in combinations of complete dominance and co-dominance expression.

examples:

- eye color in Drosophila

- self-incompatibility in plants

- blood type in humans

- coat color in animals

White eye alleles in Drosophila

Self-incompatibility in plants

example: Brassica - four allele system

S1, S2, S3, and S4

If the same allele is present in the male and female gamete the pollen tube will stop growing before it reaches the ovary.

Cross S1S2 x S1S2

sterile

Cross S1S2 x S2S3

S2S3

S1S3

Cross S1S2 x S3S4

S1S3

S1S4

S2S3

S2S4

Blood type in humans

- Three alleles, A, B, and O

- A and B are co-dominant

- Both A and B are dominant to O

blood type genotype

A IAIA or IAIO

B IBIB or IBIO

AB IAIB

O IOIO

AB - universal recipient

O - universal donor

Coat color in rabbits

c+ - wild type

ch - himalayan

cch - chinchilla

c – albino

There is a gradation in dominance for coat color

c+ cch ch c

With multiple alleles the number of possible genotypes increases greatly.

You can calculate the number of different genotypes with the following formula: n(n+1)/2

Where n is the number of alleles

examples:

rabbit coat color: n = 4number of genotypes = 4(4+1)/2

= 20/2 = 10

blood type: n = 3number of genotypes = 3(3+1)/2

= 12/2 = 6

With the presence of multiple alleles it can be difficult at times to determine if the observed variation for a trait is due to two genes or allelic variation at one gene locus.

The way to determine if the variation you are observing is allelic is to do a complementation test.

example:

You have two individuals that are both white variants from the normal red flower color.

If you cross them and the progeny are red then the trait is controlled by more than one gene and the two white variants have a mutation in different genes.

But if you cross them and the progeny are all white then the two variants have a mutation in the same gene and the trait may be controlled by only one gene.

Biochemically it would work like this:

substrate intermediate product (white) A (white) B (red)

‘A’ is an enzyme that converts the substrate to an intermediate and is controlled by gene A.

‘B’ is an enzyme that converts the intermediate to the final product and is controlled by gene B.

2 gene model

plant 1 white aaBB x plant 2 white AAbb F1 AaBb all red

The flowers are red because the F1 individuals have one functional gene/allele at each gene loci. Hence the genes compliment each other.

1 gene model

plant 1 white a1a1 x plant 2 white a2a2

F1 a1a2 all white

The flowers in the F1 individuals are white because they do not have a dominant allele at the A locus. The shades of white may vary depending on the mutation in each parent.

So it is possible to have multiple alleles (a1, a2, a3, etc.) at a single gene locus that give various shades of white depending on the location of the mutation in the gene.

Environment - the environment can influence the expression and level of expression of a gene. Factors such as temperature, light, and nutrition can reduce or prevent expression of a gene or genes.

Example: temperature response in fur color.

Rabbits – Himalayan

As temperature decreases the fur on the extremities darkens.

How to measure environmental effects:

plants - grow large populations of a single genotype in several environments then compare the differences in expression of a trait. Any differences observed have to be due to the different environments.

animals - multiple matings to produce individuals with similar genotypes and study the F1 progeny in several environments.

humans - work with twins

1. Separate identical twins at birth and place them in separate environments.

2. Study difference between identical and fraternal twins for expression of various traits.

If the similarity (concordance) within the sets of twins for a trait is the same between identical and fraternal twins than the expression of that trait is under more environmental than genetic control.

If the level of concordance differs significantly between identical and fraternal twins with a higher level of concordance in the sets of identical twins then the trait is under more genetic control than environmental control.

Level of concordance among identical and fraternal twins.

percent concordance

trait identical fraternal

hair color 89 22

diabetes mellitus 65 18

measles 95 87

schizophrenia 80 13

manic-depressive 77 19

coffee drinking 94 79

You can also see the effect of environment by looking at just the concordance of identical twins.

example: diabetes mellitus

A concordance of 65% in identical twins means that out of 100 sets of identical twins 65 sets were the same for the trait while 35 sets had only one of the two showing the genetic disorder.

The difference observed is due to the environment. Differences in the environment (diet?) must be causing the differences because with identical twins there are no genetic differences.