DOMINANT
/ RECESSIVE GENES
Recall
earlier that the yellow peas dominated over the green.
The yellow trait is said to be dominant and the green
trait of the peas is said to be recessive. Recall
also that half of your genes in your chromosomes are
derived from your dad and half from your mom? This
is also true for the peas.
If
a trait is dominant, then as long as you inherit that
trait from either of your parents, you will posssess
that trait. If a trait is recessive, you will only
possess that trait if you inherit that trait from
both of your parents. To illustrate this, suppose
we cross a green pea with a yellow pea. Let us use
Y to denote the yellow trait inherited from either
parent and G to denote the green trait inherited from
either parent. Since the yellow trait is dominant,
the yellow pea must be either YY or YG. Since the
green trait is recessive, the green pea must be GG.
When
the green pea is crossed with the yellow pea, two
sets of possibilities arise as illustrated:
Possibility 1:
|
| |
|
Yellow
Pea
|
|
|
Green
Pea
|
|
Y |
Y |
| G |
GY |
GY |
| G |
GY |
GY |
|
In
this case, all resulting crosses are GY (4 possibilities).
Since yellow is the dominant trait, all resulting
crosses will be yellow.
Possibility 2:
|
| |
|
Yellow
Pea
|
|
|
Green
Pea
|
|
Y |
G |
| G |
GY |
GG |
| G |
GY |
GG |
|
In
this case, the resulting crosses are either GY or
GG. If the resulting cross is GY, then the pea will
be yellow. However, if the resulting cross is GG,
then the pea will be green
When
the offspring gets unlike traits from their parents
such as GY, it is known as heterozygous for that
trait. When the offspring gets like traits from
their parents such as GG, it is known as homozygous
for that trait.
As
can be seen above, yellow offsprings occur in six
out of eight possibilities - that is 3 times more
likely.
