Biochemical Genetics

“The study of the relationships between genes and enzymes, specifically the role of genes in controlling the steps in biochemical pathways.”

Gregor Mendel’s Pea Studies

Mendel’s Seven Traits

X

Mendel made a cross between a Tallplant and a Dwarf plant.

Progeny were all Tall >

Then, Mendel allowed the plants to self-pollinate:

Offspring were:

75% Tall

25% Dwarf

How could such a thing happen?

Alleles are often indicated with a single letter.

A CAPITAL letter denotes a dominant allele = TA lowercase letter denotes a recessive allele = t

TT x tt

Tt

Tall x Dwarf

Tall

Genotype (chemical code): Phenotype (visual):

T

t

T t

TT Tt

Tt tt

…one parent (female)

..other Parent (male)

25% TT (tall)50% Tt (tall)25% tt (dwarf)

Dr. James ReidUniversity of Tasmania

discoverer of the le mutation

Gibberellin Hormone

The Active formOf Gibberellin

Gibberellin 3 beta-hydroxylaseis non-functional in this mutant

The le allele is a point mutation (substitution) that alters the active site of the enzyme.

Which explains the differences seen between heterozygotes and homozygous-recessives !

Finally, a biochemical explanation for Dominance!!!

Gibberellin

Mutant

Dr. Reid found it using Arabidopsis mutant

Iron-binding cofactor sites

Mutation of Active Site

Pea

Arabidopsis

le mutation

Where could neutral mutations occur?

Pea

Arabidopsis

Probably NOT in these four “conserved” areas

Mendel’s Seven Traits

Another example ofusing “Biochemical Genetics” to explain

Mendel’s observations

The wrinkled seed trait in plants

“Smooth seed” is dominantover wrinkled seed

Why?

cornpea

Made by Starch Synthase

and Starch-Branching Enzyme

StarchGrains

“r” is the genedescribed by

Mendel

“r” stands for “rugosus” = “wrinkled”

Wrinkled seedsare low in starchand high in sugar

(starch is a polymerof glucose) Starch synthesis

in developing seeds

Wrinkled seedsare low in starchand high in sugar

It all has to do with

Osmosis“diffusion of water across a membrane

from high concentration tolow concentration”

Starch-BranchingEnzyme is mutated(Loss of Function)

Sugars build-up because they are not being used to

make starch.

Water movesinto cells ofseed due to

osmosis.

Tissue swells Later, as seeds mature,they dry down (so they

can survive long periods).

Dry seeds end uplooking wrinkled

(like a raisin).

So what?

Could try to overcome mutation in pea or other plants.

Could induce mutation in other species.

Learn about the function of the enzyme.

Learn about similar genes in other species:

The Pea β-Hydroxylase gene for instance is fairly similar to the ones in Tobacco and Lettuce, but much more different than the one in Cucumber.

Kiwi (Actinidia deliciosa) = interspecific hybrid from China

Hexaploid = 174 chromosomes (29 pairs)