Biology Urgent Assignment 21

Genetics: Pre-Lab

LEARNING GOALS

By the end of this unit, you should be able to do the following:

1. Explain the purpose of each step of isolating DNA from an onion.

2. Explain the difference between genotype and phenotype.

3. Explain Mendel's Law of Segregation including the concepts of having a pair of alleles, dominance, and passing on only one of the pair.

4. Explain Law of Independent Assortment focusing on the presence two different genes,

each with a pair of alleles.

5. Use a Punnett square to predict genotypes and phenotypes of offspring given parental genotype.

DNA

Deoxyribonucleic acid (DNA) was discovered long before we knew it was the genetic material. Since DNA is made of only four bases (A, T, G, and C), it did not make a good candidate for the "alphabet of life." Many thought that protein, which has 20 different amino acids, was more likely to be the genetic material. It took many experiments to determine that DNA really did carry the code for all life. The demonstration you will do this week is a throwback to the days when DNA was first isolated using simple soap and alcohol.

DNA isolated from fruit.

Experiment Note - You will be blending an onion and mixing it with detergents and alcohol to isolate the DNA. Pay careful attention to how each chemical helps you isolate the DNA.

Genetics - Law of Segregation

The Law of Segregation encompasses several rules.

Rule 1 - Each individual has two pieces of genetic information for any trait. One piece came from the mother and one from the father.

Rule 2 - If the two pieces of information in an individual are different, one will be dominant to the other and disguise the recessive copy.

Rule 3 - When making sperm or egg cells, these two pieces of information are separated (segregated) and only one is placed in each sperm or egg cell.

The basics of genetic inheritance follow these three rules. There are many exceptions to the Law of Segregation, such as incomplete dominant or codominant traits. These will be covered in lecture.

Consider a pea plant where tall (T) is dominant to short (t). We use the same letter to track both tall and short, but use upper and lowercase to tell them apart. Consider the mating between two pea plants, one with only tall (TT) and one with only short (tt). The Law of Segregation tells us that each parent will only pass on one of the two pieces of information; this means that the first parent will give the child "T" and the other parent will give the child "t." The child will be Tt, which results in a tall plant. (Remember rule 2 from above.)

In a more complex mating, Tt with Tt, we need to use a tool called the Punnett square. The two pieces of genetic information, called alleles, from one parent are written across the top. The two alleles from the other parent are written across the side, as below:

The offspring are then filled in by connecting rows and columns.

Here we can see that 3/4 of the offspring are tall and 1/4 are short. This ratio appears whenever two heterozygotes (individuals with two different alleles) mate.

Experiment Note - You will be using a computer simulation to mate fictional creatures called Flugals. You will mate a Flugal with black eyes to a Flugal with white eyes to see which is dominant. You will do the same for presence and absence of a Tail.

Genetics - Law of Independent Assortment

The Law of Independent Assortment simply states that, when considering two different genes, the alleles for gene 1 are segregated without any regard for how the alleles for gene 2 are segregated. For example, consider both height (T is tall and t is short) and flower color (P is purple and p is white). Mate a mother that is TTPP with a father that is ttpp. The offspring are all TtPp (tall and purple). When these offspring make sperm or egg cells, they can make them containing TP (both from mother), Tp (height from mother and color from father), tP

(height from father and color from mother) or tp (both from father). All occur with equal frequency; there is no bias for the T and the P to go together and there is no bias for the t and p to go together. The color allele inherited from the mother is just as often matched with the height allele inherited from the father.

Since inheritance is independent for these two genes, you can also deal with them as two independent Punnett squares:

Approximately 3/4 will be tall and 1/4 will be short, just as before. Additionally, 3/4 will be purple and 1/4 will be white. But what fraction will be both purple and tall? Purple and short? What about white and short?

To do this, you can either make a Punnett square with four rows and four columns or we can do some simple math. First, convert 3/4 and 1/4 to 0.75 and 0.25. (These are proportions and can be plugged into a calculator much more easily than fractions.) Then choose the two conditions you wish to combine and multiply the proportions:

Tall and Purple = 0.75 * 0.75 = 0.56

Tall and white = 0.75 * 0.25 = 0.19

short and Purple = 0.25 * 0.75 = 0.19

short and white = 0.25 * 0.25 = 0.06

There are many exceptions to the Law of Independent Assortment, such as linked traits and sex-linked traits. These will be covered in lecture.

Experiment Note - You will be using a computer simulation to mate Flugals with black eyes and a tail to white eyed, tailless Flugas to see if these two traits assort independently.