1. The alleles we inherit from our parents get expressed as outward traits. For almost all traits, one allele will be expressed and the other will not. If one of your alleles is dominant, its trait will be expressed. If both alleles are recessive, their trait will be expressed. Each of the traits in the table below follows a simple dominant/recessive system. For each trait, circle whether you have a dominant or recessive trait.
Trait Dominant Recessive
When a diploid organism (remember, carries two alleles for each gene!) has two of the same allele, we call it homozygous. When it has two different alleles, we call it heterozygous.
2. Would the following combinations be homozygous or heterozygous?
Aa __________________________________heterozygous
BB __________________________________homozygous
dd __________________________________homozygous
Ee __________________________________heterozygous
Because homozygous pairs of the same gene could be different (lowercase or capital), we have to qualify these as either homozygous dominant or homozygous recessive. Add the terms “recessive” and “dominant” to your homozygous pairs above. It is not necessary to do this for heterozygous pairs.
The expressed external characteristic, or the specific physical manifestation of a trait, is the phenotype. What might be a specific example of a phenotype? (HINT: see the table above)
Recall the method of writing an organism’s alleles, such as Aa or aa. This written representation is the genotype. The genotype is useful for describing a person’s complete genome, regardless of which allele gets expressed.
For example, let’s imagine a set of corn seedlings: small plants that look almost like overgrown grass. Normally, these corn seedlings are green, with green being dominant over white. The letter we use for this trait is G/g. The phenotype of green corn seedlings is __green______ and the genotype of green corn seedlings is ___GG or ___Gg______.
Imagine a scenario in which a farmer breeds two green corn plants, both of which have genotypes Gg. That is, they each carry one allele for green and one allele for white. This is called a monohybrid cross.
The corn farmer would need to know how many of her plants would be green and how many would be white. Of course, white plants would lack chlorophyll, not be able to photosynthesize, and die. She could calculate the numbers of green and white plants as percentages of expected values based on a single cross. To do this, she would set up a Punnett Square. Each corn plant would split up its alleles to make gametes in meiosis. EACH GAMETE ALWAYS GETS HALF THE GENOTYPE. Separate the gametes from each parent. Put one parent across the top of your square, and one parent across the left side, like this:
G g
G GG Gg
g Gg gg
How many of your four combinations have the following genotypes:
GG: __1__/4
Gg: __2__/4
gg: __1__/4
Convert these to percentages. These are your expected genotypic ratios.
GG: _25___/%
Gg: _50 _/%
gg: __25__/%
Look again at your completed Punnett Square.
How many of your four combinations have the following phenotypes:
Green: __3__/4
White: __1__/4
Convert these to percentages. These are your expected phenotypic ratios.
Green: __75__/%
White: __25__/%
If you cross two green plants, both of which have the genotype of Gg, and they produce 120 offspring plants, approximately how many would you expect to be green and how many would you expect to be white?
White Phenotype=25% of the total offspring
=25100×120=30 offspring plants
Green Phenotype=75% of the total offspring
=75100*120=90 offspring plants
