Genes can have different forms. For simple
inherited genes, there exist
two possibilities, a dominant one and a recessive one. Let's use the
of the gene for leaf form. There are regular leafs and potato leafs
One can think of each image below as a packet of tomato seed
a variety that expresses the two different forms of leaves.
|Regular leaf population
|Potato leaf population
What happens to the eggs when we cross a dominant regular
leaf with a recessive potato leaf?
Select one plant to to be male and another female.
Such a cross is reciprocal in
that it doesn't matter which you chose for the male or female.
In this example below, the recessive is male and the dominant is female.
We get another kind of gene pair called a heterozygote (think unlike-egg)
So what would that resulting seed packet look like?
All the pairs become
This is called a F1 hybrid.
It's genes are referred to as a heterozygous population.
The entire population is completely uniform.
So what does the hybrid's leaf of such a cross look like?
It comes out a regular leaf.
[photo Paul Gantz]
This is because the regular leaf gene dominates over the
expression of the potato leaf gene (recessive).
With simple (or completely) dominant genes
- the dominant trait can
be expressed in a homozygous or heterozygous gene pairing
- the dominant trait is
expressed in the hybrid (good to known when developing
hybrid lines, ie incorporating disease resistance)
Recessive traits are only expressed when their genes
- a potato leaf
self-pollinating or crossing with another potato leaf variety will
produce a potato leaf plant.
- a recessive
self-pollinating or crossing with a similar recessive will only produce
- because only expressed
when in a homozygous pairing, easy to recover during segregation
the next page we will look at what happens to the heterozygotes of an
inbreeding population when they go through a process known as ...