Gen. Biology Notes C-9 ( Day 2)

Gen. Biology Notes C-9 ( day 2)

Objectives:

1.Differentiate Complete Dominance, Incomplete Dominance and Co- dominant problem types .

2.Differentiate Monohybrid and Dihybrid problem types.

Notes:

In the previous example yesterday, we concluded that that the brown –eyed allele was COMPLETELY dominant to the blue –eyed allele. In nature, there are all kinds of situations where one allele is NOT completely dominant to the other and a “mixing” of alleles shows up in the phenotype of the individual.

Incomplete Dominance:

In a certain kinds of flowers, Japanese 4 o’clocks for example, the flower color of the plant can be one of three colors. Red , white or pink are the possibilities. The third color is given by the recessive allele getting to contribute a little bit to the phenotype of the flower.

It works out that red flowers are INCOMPLETELY dominant to white flowers. This fact is noted in the key signature for the problem . If we use R as red we would NOT use r as white because this is not a completely dominant situation. So we use a lower case letter that is different from R. For instance the key would now look like this:

R= red flower

w= white flower

Rw= pink flower.

If the two different alleles show up in the individual, the w allele gets to add a little influence to the phenotype of the flower. Think of a can of red paint. It doesn’t take a lot of white drops to turn the color pink. The color pink is MOSTLY red, but a little bit of it’s color is white. Same thing here with incomplete dominance. Try the following example problem:

Cross a heterozygous male plant with a white flowering plant and indicate the genotypic and phenotypic ratio of the potential offspring.

Key

R=red flower

w= white flower

Rw= pink flower

The genotypes of the parents would be:

RwXww

Rw

w

w

The genotypic ratio of offspring would be 2Rw:2ww: 0RR

The phenotypic ratio would be 2 pink flowing plants : 2 white flowering plants

In reality most allele influence on genotypes is probably due to the influence of multiple genes with multiple alleles. To simplify the problem so it can be more easily understood, we assume that the flower color is due to one gene with only two alleles and they are INCOMPLETY Dominant or recessive.

Co- Dominance

There is another situation in nature in which the alleles in question are neither Completely dominant or Incompletely dominant . We usually see co-dominance in the phenotype of an individual in the form of spots or stripes, but not always. Co-dominant alleles are just that, co-dominant to each other. Each allele gets to contribute equally to the phenotype of the individual. We utilize yet a different key signature to indicate a Co-dominant situation. Different capitalized letters are employed to signify co-dominance. For instance:

B= Black

W= white

BW= black and white stripes or spots.

Consider the following problem about roan colored horses detailed for you in your text.

The color of the horse is pinkish and you might be tempted to consider the problem as incomplete dominance, but it is NOT! If you examine a roan colored horse carefully, you will see that fifty percent of its hair is red and fifty percent of its hair is white. There is no mixing like in incomplete dominance. Both alleles in this case get to contribute EQUALLY to the phenotype. The pinkish color is only an opitical illusion. At any rate, try the following example dealing with salamander color.

B=black

Y=yellow

BY= black and yellow stripes.

Cross a heterozygous male with a yellow female

BY X YYB Y

Y

Y

As you can see we have a 50 percent chance of having baby ,yellow salamanders and a 50 percent chance of having baby striped salamanders.

You need to be able to identify the problem type to set the problem up correctly. There are tips and tricks that I will identify in class but the good part is, all of these problem types will be of the monohybrid kind and I will never ask you to solve an incomplete or co-dominant problem on a di-hybrid basis.