Complete question:
A wild-type tomato plant (Plant 1) is homozygous dominant for three traits: solid leaves (MM), normal height (DD), and smooth skin (PP).
Another tomato plant (Plant 2) is homozygous recessive for the same three traits: mottled leaves (mm), dwarf height(dd), and peach skin (pp).
In a cross between these two plants (MMDDPP x mmddpp), all offspring in the F1 generation are wild type and heterozygous for all three traits(MmDdPp).
Now suppose you perform a testcross on one of the F1plants (MmDdPp x mmddpp). The F2 generation can include plants with these eight possible phenotypes:
solid, normal, smooth
solid, normal, peach
solid, dwarf, smooth
solid, dwarf, peach
mottled, normal, smooth
mottled, normal, peach
mottled, dwarf, smooth
mottled, dwarf, peach
Assuming that the three genes undergo independent assortment, predict the phenotypic ratio of the offspring in the F2 generation.
Answer and Explanation:
<u>Cross 1:</u>
Parental) MMDDPP x mmddpp
F1) MmDdPp
<u>Cross 2</u>: <em>Testcross</em>
Parental) MmDdPp x mmddpp
Gametes) MDP MdP MDp Mdp mDP mdP mDp mdp
mdp mdp mdp mdp mdp mdp mdp mdp
Punnet Square)
mdp
MDP MmDdPp
MdP MmddPp
MDp MmDdpp
Mdp Mmddpp
mDP mmDdPp
mdP mmddPp
mDp mmDdpp
mdp mmddpp
F2) 8/64 = 1/8 MmDdPp, solid leaves, normal height, smooth skin
8/64 = 1/8 MmddPp, solid leaves, dwarf height, smooth skin
8/64 = 1/8 MmDdpp, solid leaves, normal height, peach skin
8/64 = 1/8 Mmddpp, solid leaves, dwarf height, peach skin
8/64 = 1/8 mmDdPp, mottled leaves, normal height, smooth skin
8/64 = 1/8 mmddPp, mottled leaves, dwarf height, smooth skin
8/64 = 1/8 mmDdpp, mottled leaves, normal height, peach skin
8/64 = 1/8 mmddpp, mottled leaves, dwarf height, peach skin
Another way of predicting the phenotypic ratio of the offspring in the F2 generation is by performing a testcross for each gene by separately and then multiplying their genotypic proportions. This is:
<u><em>Leaves Gene</em></u>
Parental) Mm x mm
Gamtes) M m m m
Punnet Square) M m
m Mm mm
m Mm mm
F2) 1/2 Mm
1/2 mm
<u><em>High Gene</em></u>
Parental) Dd x dd
Gamtes) D d d d
Punnet Square) D d
d Dd dd
d Dd dd
F2) 1/2 Dd
1/2 dd
<u><em>Skin Gene </em></u>
Parental) Pp x pp
Gamtes) P p p p
Punnet Square) P p
p Pp pp
p Pp pp
F2) 1/2 Pp
1/2 pp
To get the phenotypic ratio of the offspring we multiply the genotypic proportion for each trait by the genotypic proportions of the other two traits, like this:
- For plants with solid leaves, normal height, smooth skin
solid leaves ratio -> 1/2
normal height ratio -> 1/2
smooth skin ratio -> 1/2
MmDdPp ratio: 1/2 x 1/2 x 1/2 = 1/8
- For plants with solid leaves, dwarf height, smooth skin
MmddPp ratio: 1/2 x 1/2 x 1/2 x 1/2 = 1/8
- For plants with solid leaves, normal height, peach skin
MmDdpp ratio: 1/2 x 1/2 x 1/2 x 1/2 = 1/8
- For plants with solid leaves, dwarf height, peach skin
Mmddpp ratio: 1/2 x 1/2 x 1/2 x 1/2 = 1/8
- For plants with mottled leaves, normal height, smooth skin
mmDdPp ratio: 1/2 x 1/2 x 1/2 x 1/2 = 1/8
- For plants with mottled leaves, dwarf height, smooth skin
mmddPp ratio: 1/2 x 1/2 x 1/2 x 1/2 = 1/8
- For plants with mottled leaves, normal height, peach skin
mmDdpp ratio: 1/2 x 1/2 x 1/2 x 1/2 = 1/8
- For plants with mottled leaves, dwarf height, peach skin
mmddpp ratio: 1/2 x 1/2 x 1/2 x 1/2 = 1/8
