Solving Genetic Problems

What is a Genetic Problem?

A genetic problem is a type examination question that involves both a knowledge of Mendel's experiments, and an analysis of data produced during one of his genetic crosses. In the question the solver is presented with information concerning the genotypes and phenotypes of individuals involved in a genetic cross, and the genotypes and phenotypes of their offspring. Some of this critical information is missing, and the solver's task is to supply that missing data.
Example #1.... A red flowered plant is fertilized with pollen from a plant of unknown phenotype. 62 seeds are collected, planted the next year and grow into the F1 offspring. 16 of these offspring have white flowers and the rest have red flowers. What was the phenotype and genotype of the unknown parental plant?
The Four Steps

Almost all the genetic problems used in Simple Mendelian Genetics can be analyzed and solved using four basic steps, or processes.


(1) Use Your Course Work

From information given in the problev write down what you already know about this situation.
What you should know already ...
Flower color is controlled by a single gene.
The dominant form of this gene is often written R
When ever the R gene is present in the genotype of a plant, the flower color is always red.
The mutant form of this gene, which is recessive, is often written r.
Plants with the genotype RR or Rr have red flowers.
Plants with the genotype rr have white flowers.

(2) Use the Information in the Question

Every question includes a lot of valuable information (some of it not so obvious - think for a moment). Take this information and write it out in a logical sequence as it appears in a typical genetic cross. See what you have and what is missing.
Logical sequence ...
Phenotypes and Genotypes of parents
Parental Genotypes ... unknown, but the red flowered parent must have contained at least one R gene, with a genotype of RR or Rr.
Parental Phenotypes ... one red flowered, one unknown
Genotypes of all possible gametes from each of the parents
NOTE: The pollen grains used in this cross came from the unknown plant, and the egg cells came from the red flowered plant. The pollen grains contain the male gametes, and the egg cells (in the red flowered plant) are the female gametes.
Male gametes ... genotypes unknown (these came from the unknown male plant)
Female gametes ... At least half the gametes must have been R for the plant to have red flowers. (see above)
Genotypes and Phenotypes of Offspring (F1 generation)
The data given was for 62 F1 offspring.
Phenotypes: 16 white flowers, 46 red flowers.
Genotypes: all 16 white flowered plants = rr. All red flowered plants = R?

(3) Reduce all raw data to ratios

Not all genetic problems involve raw data, but in those that do, use Mendel's approach and reduce all such raw numbers to ratios. In classical one factor and two factor crosses, there are only three ratios that matter.
Heterozygous x Heterozygous
Aa x Aa
which gives a ratio of 3:1 among the offspring.
Heterozygous x Homozygous recessive
Aa x aa
which gives a ratio of 1:1 among the offspring.
Heterozygous x Heterozygous (two factors)
Aa.Bb x Aa.Bb
which gives a ratio of 9:3:3:1 among the offspring.
There are, of course, other combinations of genes, but their results can usually be reduced to one or other of these ratios.
If you learn (know or even understand) how these ratios arise during a genetic cross, you can often go straight to the final answer in many genetic problems. Even if it is not possible to deduce the correct answer directly from these ratios, they are valuable clues.



(4) Diagram and Fill in the Blanks

Once you have assembled the evidence, write it out in a typical genetic cross diagram. Use question marks (?) when you encounter something you cannot fill in yet.
Often the missing parts of the puzzle will be obvious once you have done this, but even if they aren't, a short trial and error session filling in the missing pieces will give the right answer after one or two tries.
In the example we have worked on so far ...

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