Transcript Dihybrid Cross Notes

Dihybrid Notes
• Monohybrid = crossing parents while
looking at one trait; 2 letters total
• Dihybrid = crossing parents while looking
at two traits; 4 letters total (2/trait!)
• Steps still the same!
Big Issue: Gametes
How to determine the possible gamete/allele
combo that go on the sides of the Punnett
square?
Use the
FOIL
method!
FOIL
• F = First; the first letters for each allele
pair; should be 2 different letters
• O = Outer; the outer letters for each allele
pair; should be 2 different letters
• I = Inner; the inner letters for each allele
pair; should be 2 different letters
• L = Last; the last letters for each allele
pair; should be 2 different letters
Practice
• Determine all the gamete/allele combos for
each “parent.”
1.AABb First = AB; Outer = Ab; Inner = AB; Last = Ab
2.AaBb AB; Ab; aB; ab
3.aaBB aB; aB; aB; aB
Alternately: 1) AB and Ab only
2) all 4
3) aB only!
Steps
1. Assign letters (one type of letter for one
dominant/recessive trait; 2nd letter for
another)
2. Determine parental genotypes
3. Determine gamete combos of 2 traits (Foil)
4. Put alleles on sides of a punnett square (as
big as 4x4!)
5. - 7. Fill in, determine genos and phenos!
In rabbits, black fur is dominant to white fur and
presence of fur is dominant to being hairless. If a
heterozygous black, hairless male is crossed with a
white, heterozygous furry female, determine the
ratios of all genotypes and phenotypes.
1. Determine
dominant and
recessive traits
and assign letters
Carefully read the
problem!
Dominant black = B
Recessive white = b
Dominant Hairy = H
Recessive hairless = h
2. Determine
genotype for
parents
*Parents must have 4
alleles (aka 4 letters)
Heterozygous black,
hairless male
Bbhh
White heterozygous
hairy female
bbHh
3. Determine alleles/gametes
Remember FOIL!
Bbhh gametes:
Bh, Bh, bh, bh
(or Bh, bh)
bbHh gametes:
bH, bh, bH, bh
(or bH, bh)
4. Draw a 4 x 4
Punnett square:
16 boxes!
OR
2 x 2 =8 boxes
Bh
Bh
bh
bh
bH
BbHh
BbHh
bbHh
bbHh
bh
Bbhh
Bbhh
bbhh
bbhh
bH
BbHh
BbHh
bbHh
bbHh
Bbhh
Bbhh
bbhh
bbhh
bh
Genotype:
5. Determine
genotypes
w/ ratios
4 BbHh: 4 bbHh: 4 Bbhh: 4 bbhh
4:4:4:4 = 1:1:1:1
Phenotypes:
4 Black and Hairy
6. Determine
phenotypes
w/ ratios
4 White and Hairy
4 Black and Hairless
4 White and Hairless
1:1:1:1
Sample Problem
• In peas, the gene for round is dominant to
wrinkled and yellow is dominant to green.
If two peas plants that were heterozygous
for both traits were crossed, what would
the resultant generation ratios be like?
Genotype Ratio: ___ RRYY : ___ RRYy : ___ RrYY : ___ RRyy :
___ RrYy : ___ rrYY : ___ Rryy : ___ rrYy : ___ rryy
Phenotype Ratio : ___ Round and Yellow : ___ Round and Green :
___ Wrinkled and Yellow : ___ Wrinkled and Green
Genotypes-
1 RRYY : 2 RRYy : 2 RrYY : 1 RRyy
: 4 RrYy : 1 rrYY : 2 Rryy : 2 rrYy : 1 rryy
Phenotypes- 9 Round/Yellow : 3 Round/Green
: 3 Wrinkled/Yellow : 1 Wrinkled/Green
Extra tips
When determining the probability of
events
– If the problem states “and” then multiply
the chances for each
• Chance of heads and heads = ½ x ½ = ¼ or
25%
– If the problem states “or” then add the
chances for each
• Chance of heads or tails = ½ + ½ = 1 or 100%