IB BiologyName: ______Date: ______

Investigating Monohybrid and Dihybrid crosses in Maize Cobs

The corns on a maize cob are all created from individual ovules which have been fertilized by individual pollen grains. They are each an individual offspring from two parent plants.

This means we can count the individual corns and work out F1 phenotype ratios of maize. This is a bit like the work of Gregor Mendel on pea plants in the 1800s.

There are four grain phenotypes in the above ear of genetic corn: Purple & Smooth (A), Purple & Shrunken (B), Yellow & Smooth (C) and Yellow & Shrunken (D).

These four grain phenotypes are produced by two genes:

  • The Colour Gene (Alleles P=Purple coloured & p=plain yellow)
  • The Shape Gene (Alleles S=smooth & s= shrunken)

These genes are located on two separate pairs of homologous chromosomes so the genes are not linked.


Shrunken and Smooth Phenotypes /
Conscientious students counting the phenotypes at IST in 2003. /
Coloured and Colourless phenotypes

Theory of Ratios in Monohybrid Crosses

There are just three possible ratios of offspring phenotypes in monohybrid crosses:

F1 Phenotypes / Parent Genotypes possible
1 smooth : 1shrunken / Ss x ss
3 smooth : 1 shrunken / Ss x Ss
All offspring identical / SS x ss OR ss x ss OR SS x SS

We can count the number of offspring with each phenotype and predict the genotypes of the parents. The same ratios are true for purple coloured and yellow corns.

Prediction

Look at your Maize cob, and choose which gene you are going to study.
The Colour Gene or The Shape Gene.

Make a prediction about the ratio of corns, choose from the table above.

My Prediction of the phenotype ratio is: ______

I predict that the parent phenotypes were: ______and ______

Method

  1. Count the number of corns of each of your two chosen phenotypes in five rows of corn.
  2. Record your results in a table – which is your design.
  3. Calculate the phenotype ratio from the observed data.
  4. Calculate expected values using the prediction you made.
  5. Test the ‘goodness of fit’ using a suitable statistical test. Remember to show all your workings.
  6. Write a conclusion
  7. And an Evaluation.

OLD NOTES - Method

  1. Count the number of corns of each phenotype in four rows of corn.
  2. Record your results in the table below:

Row number / 1 / 2 / 3 / 4 / 5 / 6 / 7 / 8 / 9 / TOTAL
Number of Smooth
Number of Shrunken
Total number of corns counted
Ratio of Shrunken to Smooth.

What do you predict are the genotypes of the parents? Show the theory using a Punnet Square.

Expected theoretical ratio / Smooth / Shrunken
Expected values for the experiment
Observed values in the experiment
(O-E)2 divided by E
Sum of the two values / this is the value of Chi-Squared

What would the exact ratio be in theory?

We can test the results using a Chi-squared Test.

Expected theoretical ratio / Smooth / Shrunken
Expected values for the experiment
Observed values in the experiment
(O-E)2 divided by E
Sum of the two values / this is the value of Chi-Squared

Now compare you value with this table.

Probability of there being no difference between observed and expected values / 99% / 10% / 5% / 1%
Chi-squared value (with 1d.f.) / 0.001 / 2.71 / 3.84 / 6.35

If your value is greater that 3.85 then in less than 5% of cases you have no difference between your observed and expected values. You have to say that there is a "SIGNIFICANT DIFFERENCE" between your observed values and your theoretical expected values. There could be some co-dominance, multiple alleles or linkage affecting this gene.

If your value of Chi-squared is more than 3.85 you can assume that you are correct in your theory. There is no significant difference between observed and expected values.

Dihybrid Phenotype Calculations HL only !!

The alleles can be represented:

  • S=smooth (dominant), s=shrunken (recessive)
  • C=coloured (dominant), c=colourless (recessive)

How many different possible crosses are there. SSCC*sscc , SSCc*sscc, SsCc*sscc, Sscc*sscc, sscc*sscc,

How many ratios are possible (assuming no linkage), what are they.

Special Difficult Chromosome Mapping problem for Poly