Question

If two genes are found on different chromosomes, or if they are far enough apart on the same chromosome that the chance of a crossover between them is very high, the genes are considered to be unlinked. Unlinked genes follow Mendel’s law of independent assortment. If, however, two genes tend to "travel together" because they are near one another on the same chromosome, they are said to be linked. Linked genes do not follow Mendel’s law of independent assortment. In this tutorial, you will compare the inheritance patterns of unlinked and linked genes.
Part A - Independent assortment of three genes
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)
Leaves Height Fruit Plant 1 solid normal Smooth (PP) (MM) (DD) Plant 2 mottled (mm) dwarf peach (Pp) (dd)
In a cross between these two plants (ADOPPx mmddpp), all offspring in the F, generasion are wild type and heterozygous for all three traits (AmDdPp)
Now suppose you perform a testcross on one of the F, plants (mOoPpx mmodpp). The F, generation can include piants with these eight possible phanotypes
a) solid normal, smooth
b) solid norma peach
c) solid dwarf smoom
d) solid dvarf, peach
a) motted nomal, amooth
b) motted, normal pesch
c) mottied, dwad smooth
d) moted dwaf peach
Assuming that the three genes undergo independent assortment, predict the phenotypio ratio of the offspring in the F, generation.

Answer 1

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:

Cross 1:

Parental) MMDDPP  x  mmddpp

F1) MmDdPp

Cross 2: Testcross

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:

Leaves Gene

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

High Gene

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

Skin Gene

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