Monohybrid cross
When looking at the probable phenotypic and genotypic make-up of offspring of a cross between two hybrids at a single character, we're looking at a monohybrid cross. This case is one of six possible combinations of parents at a character with two alleles.
Aa X Aa
We predict that 25% of the offspring will be AA
50% of the offspring will be Aa
25% will be aa
Phenotypically, we predict that 75% will show the dominant trait and 25% will show the recessive trait.
Try this
In a cross between two purple flowers, one of the 10 offspring shows the recessive trait. Determine the genotypes of the parents.
Dihybrid cross
When we look at a cross involving parents that are hybrid (heterozygous) at two characters, we're looking at a dihybrid cross. At this level, we look at the phenotypic ratio only of the offspring.
*coughpunnettsquaresareforbabiescough*
Let's try AaBb X AaBb
First, check that this is a dihybrid cross. These ratios will not correspond to any other combination of parents.
What we really want to see is the ratio of
domA/domB:domA/recB:recA/domB:recA/recB
A_B_:A_bb:aaB_:aabb
without doing a Punnett square which is error-prone.
Remember your probability rules:
For A_/B_
the probability of the offspring being dominant at A AND B is the product of the probability of being A_ and B_
That is, 3/4 X 3/4 = 9/16
Where is the 3/4 coming from?
For A_/bb
3/4 X 1/4 = 3/16
For aa/B_
1/4 X 3/4 = 3/16
For aabb
1/4 X 1/4 = 1/16
So the expected phenotypic ratios of the offspring of a dihybrid cross are
9:3:3:1
9 dom/dom:3dom/rec:3rec/dom:1rec/rec
We'll do a modified Punnett in class (a tree diagram, so to speak).
When asked to give the expected ratio of offspring showing dom/rec traits of a dihybrid cross, DO NOT ANSWER "3". That answer is meaningless.
The correct answer is 3/16.
Putting it all together (for your lab!)
Given: AaBBCcDdeeFfGg X aaBbccDDEeFfGg
What is the probability that an offspring of this cross will have the make-up
aaBBccDdEeffGG?