Genetics of an Ear of Corn

Introduction

Single Trait Inheritance

Based on the principles set forth by Mendel, we can predict what genotypes and phenotypes offspring will have based on the genotypes and phenotypes of their parents. One efficient way to do this involves using a Punnett square. A Punnett square is a grid where all the alleles of one parent are provided as the column headers while all of the alleles from the second parent are provided as the row headers (Figure 14.1). When the alleles from the two parents are combined in the grid, the internal squares predict the genotypes of their offspring. In addition, multiple Punnett squares can predict offspring genotypes across several generations. The first filial generation (F1) is the offspring that results from crossing the original, parental generation. The second filial generation (F2) is the offspring that results from crossing F1 individuals.

The phenotypes of the individuals will reflect the dominant allele, unless both parents contribute the recessive allele- in which case the recessive allele will be expressed.

Double trait inheritance

Blue-winged and Golden-winged Warblers

Two warblers that occur in Louisiana, the Blue-winged and Golden-winged Warblers, hybridize on a regular basis.  Their hybrids are known as Brewster’s and Lawrence’s Warblers.   Examine their photographs on screen (provided by the instructor), and catalog their dominant underpart color and throat color (the latter in terms of whether it matches the underparts, or is a different color instead).

As it turns out, the underpart color and throat color are each simply determined by single loci, which have two alleles, one dominant and one recessive.

The creation of a Brewster’s or a Lawrence’s Warbler requires receiving particular combinations of genes from the parents, with respect to throat color and underpart color.   Underpart color has two alleles: white or yellow.  White is dominant and yellow recessive, which means that if a bird gets a white underpart gene from one parent and a yellow gene from the other, the white allele will be what shows.  The throat can be either plain (i.e., not black- so the underlying white or yellow shows; this is dominant) or black (recessive).

We will abbreviate the above alleles as follows.  Upper case letters will designate the dominant allele.

W        white underparts

w         yellow underparts

P          plain throat (underpart color shows throough)

p          black throat

Pure-breeding Blue-winged Warblers have the genome wwPP, giving them yellow underparts and plain throats.   Pure Golden-winged Warblers have the genome WWpp, giving them white underparts and black throats.

When a pure Blue-winged mates with a pure Golden-winged, the offspring (which we call an F1 hybrid) always gets a w and P from the Blue-winged parent, and a W and p from the Golden-winged.  What would you therefore expect to be the F1 hybrid’s four alleles?

___ ___ ___ ___

Based on these alleles, what is an F1 hybrid’s underpart color? _____________Throat color? _______________

Based on the photos provided, what common name do we give to this hybrid form? ______________________

Now, consider the F2 generation, created by crossing F1 hybrids.  Each parent has the genome WwPp, so is equally likely to pass on a W as a w.  Likewise, it is equally like to pass on P as a p.  A Punnett’s Square allows geneticists to diagram out the different possible allele (gene) combinations that could result when two organisms mate.  Below you will see all the possible pairs of alleles that each parent could give, with the father’s alleles listed as the column heads and the mother’s as the row heads.  Combine these in each cell to show the four genes that the offspring would receive in each scenario.  After listing the four alleles, also write in the name of its phenotype (that is, its appearance) based on the photos.  I have done three for you.

A Punnett square allows visualization of the genotypes of the parents and possible genotypes of the offspring.

How many of 16 possible offspring have appearance of Blue-winged Warblers? _______________________

How many have the appearance of Golden-winged Warblers? ________________________

How many have the appearance of Brewster’s Warblers? __________________________

How many have the appearance of Lawrence’s Warblers? ___________________________

How many of those appearing like Blue-winged have pure Blue-winged genes (i.e., are wwPP)? ___________

How many of those appearing like Golden-winged are pure (WWpp)? _____________________

Kernels of Corn

For the variety of corn used in this lab, the kernels each have their own genetic makeup (they are the corn’s “offspring”), and can be either purple or yellow in color, and smooth or shrunken in shape.

Safety Precautions

• None

Materials

• Corn cobs

For this activity, you will work in pairs.

Methods

The corn has two parental versions of the same gene, one inherited from the female plant, the other from the female plant. For example, a corn could have Pp as a genotype for the color of the corn kernel. P from the female plant, p from the male plant. In regard for corn kernel shape, it could be SS. S from the female plant, S from the male plant.

Interactive question 1: What could be the genotype of a corn kernel when is has a purple phenotype?

1. PP only
2. Pp only
3. Pp or PP
4. pp or PP
5. pp or Pp

Interactive question 2: What could be the genotype of a corn when is has a shrunken phenotype?

1. SS only
2. Ss only
3. Ss or SS
4. ss or SS
5. ss only

Homozygous means that an organism has the same allele (or version) for a gene. Heterozygous means, that an organism has different versions for an allele of the same gene.

Interactive question 3: What would be a possible genotype for the color of a corn kernel if it would be heterozygous for color?

1. PP
2. pp
3. Pp or pP
4. All of the choices are correct

Interactive question 4: What would be a possible genotype for the shape of a corn kernel if it would be homozygous for shape?

1. Ss only
2. SS or ss
3. SS only
4. ss only
5. None of the choices are correct

A corn kernel heterozygous for color and shape of the corn kernel would be PpSs.

Interactive question 5: If both parents are heterozygous and they are crossed for offspring, what could be the alleles of the female and male reproductive cells (gametes)

1. PS
2. Ps
3. pS
4. ps
5. all choices are possible genotypes that female and male reproductive cells (gametes) donate.

A Punnett square allows visualization of the genotypes of the parents and possible genotypes of the offspring.

Does your count of the four different types of kernels, suggest that each trait was controlled by one gene that had both dominant and recessive versions, as in the warblers?  Why or why not?  (Answer in lab notebook)

If so, was purple or yellow dominant?  Shrunk or smooth?