Question

From a large-scale screen of many plants of Collinsia grandiflora, a plant with three cotyledons was discovered (normally, there are two cotyledons). This plant was crossed with a normal pure-breeding wild-type plant, and 600 seeds from this cross were planted. There were 298 plants with two cotyledons and 302 with three cotyledons. What can be deduced about the inheritance of three cotyledons? Invent gene symbols as part of your explanation.
A) Define allele symbol.
B) Provide the genotype for wild-type plant:

Answer 1

Answer:

A) Allele symbol: C/c

B) Wild-type: there are two options: Cc is 3 cotyledons are recessive, and cc if 3 cotyledons are dominant.

Explanation:

According to this example, the mutant plant with three cotyledons was crossed with a wild-type one. There were 298 wild-type plants and 302 mutants (three cotyledons). This means a 1:1 ratio.

So, to obtain a 1:1 ratio, one parent should be heterozygous and the other one should be homozygous recessive.  As parents are phenotypically different, this trait can be recessive or dominant,  but in both cases, one parent has to be heterozygous. If it is assumed that the trait is recessive, the mutant plant would be cc, and wild-type would be Cc. Another option is that the trait will be dominant, in this case, the mutant plant would be Cc and wild-type would be cc.  

To know is the trait is dominant or recessive is necessary to make a new cross between the mutant plants of this F1.

In the attached Punnett Square is showed the two possible crosses.