Dihybrid Cross
Genetic Analysis
Dihybrid cross, TtRr x TtRr
• 9/16 with both
dominant traits,
• 3/16 with one dominant
and one recessive,
• 3/16 with one recessive
and one dominant,
• 1/16 with both
recessive traits.
• The genotype ratio
would be…
TR
Tr
tR
tr
TR
TTRR TTRr
TtRR
TtRr
Tr
TTRr
TTrr
TtRr
Ttrr
tR
TtRR
TtRr
ttRR
ttRr
tr
TtRr
Ttrr
ttRr
ttrr
1. How many different types of gametes would each of the
following parent plants be capable of producing? List the
possible combinations.
A) TT
b) Tt
c TtRr
d) TTrr
e) ttRr
f) TtRrSs
• 2. a)1 ‑ T; b) 2 ‑ T, t ; c) 4 ‑ TR, Tr, tR, tr; d) 1 ‑ Tr; e) 2 ‑ tR, tr; f) 8 ‑
TRS, TRs, TrS, Trs, tRS, tRs, trS, trs
• 2 In mice, grey coat colour, G, is dominant to
white coat, g, and long tail, T, is dominant to
short, t. The genes for the two traits are
located on different chromosomes. What
fraction of F1 mice resulting from each of the
following crosses would you expect to have
grey coats and short tails? (Gg, GG, tt)
•
•
•
•
a) Ggtt x Ggtt
b) ggtt x GGtt
c) GgTt x GgTt
d) GgTt x ggTt
3/4
all of them
3/16
2/16 or 1/8
3. Two black‑furred animals can be represented by B_ x B_
(The blanks indicate that we know that each of the parents
possesses the dominant allele but we don't know if they are
homozygous or heterozygous.)
‑ clue #1: 147 black‑furred offspring are either BB or Bb
‑ clue #2: 51 white‑furred offspring are bb
‑ clue #3: note the 3:1 ratio (147:51)
‑ clue #4: the white‑furred offspring can only result if both
parents passed on a "b"
The probable genotypes of the parents are Bb x Bb. You. would expect.
2/3 of the black‑furred offspring to be heterozygous.
4.In your own opinion, should all family practitioners
have a general background in genetics? Explain.
• One could easily argue that family practitioners
should have a background in genetics especially as
we become more knowledgeable about the
importance of genetics.
• Others may argue that genetics today is sufficiently
complicated and should be dealt with by specialists
only.