The Principle of Independent Assortment would suggest a 9:3:3:1 phenotype ratio

Question

The Principle of Independent Assortment would suggest a 9:3:3:1 phenotype ratio (dom/dom; rec/dom; dom/rec; rec/rec) in a cross, involving parents heterozygous at each of two genes. When the same experiment is done using different traits including flower color and pollen shape, crimson flowers are dominant (red is recessive) and long pollen is dominant (round is recessive). A cross involving heterozygous parents shows the following results:

4831 (= 69.5%) with crimson flowers and long pollen,

390 (= 5.6%) with crimson flowers and round pollen,

393 (= 5.6%) with red flowers and long pollen,

1338 (= 19.3%) with red flowers and round pollen.

a. Since there is no 9:3:3:1 ratio, would you conclude that these genes are on the same chromosome or on different chromosomes?

b.  Explain how the crimson/round and red/long flowers might have arisen at a low frequency?

Explanation / Answer

Mendelian inheritance pattern does not consider,

1. Recombination

2. Gene linkage

3. Epistasis

Mendel laws are applicable only to unlinked genes.

Mendel dihybrid ratio = 9:3:3:1

In the given case,

Crimson = Dominant = C

Red = Recessive = c

Long pollen = Dominant = L

Round pollen = Recessive = l

Parental cross: CcLl X CcLl

Progeny:

C_L_ = Crimson and long

Expected = 9/16 = 0.5625 = 56.25%

Observed = 69.5%

C_ll = Crimson and round

Expected = 9/16 = 0.1875 = 18.75%

Observed = 5.6%

ccL_ = Red and long

Expected = 3/16 = 0.1875 = 18.75%

Observed = 5.6%

ccll = Red and round

Expected = 1/16 = 0.0625 = 6.25%

Observed = 19.3%

From the given results, it is clear that the parent phenotypes are over-represented than the expected values and recombinant phenotypes are under represented than expected. This clearly demonstrates that both the genes are linked. So, both the genes are located at close proximity on the same chromosome. They do not assort independently of each other during gamete formation. Hence observed recombinants are at low frequency.

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