Download DOUBLE TRAIT INHERITANCE Dihybrid cross: a type of cross that

DOUBLE TRAIT INHERITANCE
Dihybrid cross: a type of cross that involves _______________________________
each consisting of non-identical alleles.
Ex. Mendel crossed a pure-breeding yellow-round seed (
green-wrinkled seed (
).
), with a pure-breeding
Each gamete produced by the yellow-round seed will either be YR or YR (same)
Each gamete produced by the green-wrinkle seed will be yr or yr (same)
The F1 offspring are shown below.
possible gamete
combinations
for each parent
Therefore the F1 offspring are all:
Mendel then allowed the F1 offspring to self-fertilize to produce another generation
(F2). What genotypes and phenotypes did he find? What ratios are they present in?
1. Consider the possible combinations of the gametes (4 in total). (Use FOIL – “first”,
“inner”, “outer”, “last”)
2. Use a Punnett square to determine the F2 genotypes and phenotypes. This time there
are four possible gamete combinations from each plant.
Gametes
Remember, inheritance of the colour gene is not affected by the round or wrinkled
alleles.
Recall the law of:_______________________________________________
3. The ratios are determined by the probability of inheriting a certain trait.
Probability = number of ways that a given event could occur
Total number of possible events
 The probability of each event is independent
 The probability of independent events occurring together is equal to the product
(x) of the probability of the events occurring separately.
Example problem: In humans, free earlobes are controlled by the dominant allele E, and
attached earlobes by the recessive allele e. The widow’s peak hairline is regulated by
the dominant allele H, while the straight hairline is controlled by the recessive allele h.
a) Consider the mating of EeHh x EeHh parents, what are the probabilities of having F1
offspring with the following characteristics:
● Widow’s peak and free earlobes
● Straight hairline and free earlobes
● Widow’s peak and attached earlobes
● Straight hairline and attached earlobes
For the specific genotypes, do the full dihybrid cross. (This will also give you the
phenotypic ratios ).
b) What is the probability of having a child that is male with a widow’s peak and
attached earlobes from the same parents?
Page 140 (sample problem) and practice problem #1, handout.
You can also treat the dihybrid alleles as two monohybrids and use the rules of
probabilities to determine the PHENOTYPES of the F1 generation.
The probability of having
 Free earlobes _________
 Widow’s peak _________
attached earlobes __________
straight hairline __________
Therefore, the chances of producing the following offspring are:
 Widow’s peak and free earlobes
____________
 Straight hairline and free earlobes
____________
 Widow’s peak and attached earlobes
____________
 Straight hairline and attached earlobes ____________