Part 1: Terminology

Beginning students of biology always learn about Mendelian genetics. Inevitably, the study of inheritance always leads to additional questions. In fact, Mendelian inheritance patterns are exceedingly rare, especially in humans. We now know that inheritance is much more complex, usually involving many genes that interact in varied ways. Nonetheless, a clear understanding of basic inheritance patterns that follow Mendel’s original observations will provide a springboard for understanding current scientific exploration.

Inheritance patterns that follow Mendelian rules are as follows:

• Traits are governed by single genes
• There are two alternate forms of a gene, known as alleles
• Alleles are expressed as dominant and recessive

It just so happened that the traits Gregor Mendel observed in his pea plants did indeed conform to these rules. After collecting and analyzing his data, Gregor Mendel developed 2 laws of inheritance: The Law of Segregation and the Law of Independent Assortment.

1. Describe these laws:
   1. The Law of Segregation
   2. The Law of Independent Assortment
2. Before you can work with problems involving Mendelian inheritance, you need to be comfortable with the following terms:
   1. Diploid
   2. Haploid
   3. Allele
   4. Dominant
   5. Recessive
   6. Genotype
   7. Homozygous
   8. Heterozygous
   9. Phenotype

Part 2: Mendel’s First Law: Law of Segregation

The Law of Segregation states that alternative alleles of a trait segregate independently during meiosis.

Using a technique known as Punnett Square analysis, we will see how Mendel analyzed his monohybrid crosses to come up with the Law of Segregation.

Procedure

Carefully follow each step to create a Punnett square analysis. You can use these same general procedures to analyze every Punnett Square you do!

Problem: In pea plants, height is coded for by the “T” gene. The dominant allele (T) codes for the tall phenotype while the recessive allele (t) codes for the short phenotype. Make a cross between a true  breeding tall pea plant and a true breeding short pea plant.

1. What are the phenotypes of the parent plants? The parents are considered the P generation.
2. Determine the genotypes of each parent plant.
3. Imagine each parent goes through meiosis to produce gametes. List the genotype(s) of the possible gametes that each parent would produce.
4. Create a Punnett square that displays the genotypes of the possible offspring. Also label the phenotypes of the possible offspring. These offspring are considered the F₁ (first filial) generation.
5. Now allow the F₁ generation to self-pollinate. What are the possible gametes that each F₁ parent can produce?
6. Create a Punnett square that displays the genotypes of the possible offspring. Also label the phenotypes of the possible offspring. These offspring are considered the F₂ (second filial) generation.

Note: Always reduce the phenotypic and genotypic ratios to their lowest terms.

1. What is the phenotypic ratio of the F₁ generation?
2. What is the genotypic ratio of the F₁ generation?
3. What is the phenotypic ratio of the F₂ generation?
4. What is the genotypic ratio of the F₂ generation?

Part 3: Probability

Do the expected and observed phenotypic and genotypic ratios always match up in real life? In the case of flipping coins, we would expect to see heads 50% of the time and tails 50% of the time. But, does this always occur? Let’s explore!

Materials

2 coins

Procedure

1. Working with a partner, take two coins and assume that heads represent the dominant allele (A) and tails represents the recessive allele (a). The genotype for each coin is heterozygous (Aa).
2. Assume that each coin represents one parent. When a single coin is flipped, one gamete is formed (through the process of meiosis). If the flipped coin is on heads, then the gamete has the dominant allele (A). When both coins are flipped simultaneously, there will be two possible gametes that can combine through fertilization to form a zygote. Each time you flip both coins, you will record the “genotype” of the offspring.
3. Flip the coins 100 times and record your results in the chart below. (Download the chart here.)
   

   	Expected results (after 100 flips)		Your results (# of flips with each outcome)		Class results
   Genotype	Expected count	Ratio (4 × count total flips)	Observed Count	Ratio	Observed count	Ratio
   AA
   Aa
   aa
   Total flips	100		100

   Calculate ratios using this formula:
   [latex]\displaystyle\text{Genotypic Ratio}=\frac{\text{number of possible combinations (4)}\times\text{number of flips of a given genotype (observed from tally) }}{\text{total number of flips counted (100)}}[/latex]
   Note: If calculating class totals, the denominator in this equation is equal to the total of all flips counted by all students in the class.

4. Report your results to your instructor so they can be included in the “Class Totals” column.
5. What is the expected genotypic ratio for a cross between two Aa coins?

Part 4: The Law of Independent Assortment

The Law of Independent Assortment states that genes located on different chromosomes assort independently from one another. To see the effects of this law, you must examine two different genes that are carried on two different chromosomes. We can investigate this phenomenon by looking at baby.

For this experiment, each group will examine a special ear of corn. These ears were created when a mama and papa corn plant, both heterozygous for seed color (P = purple, p= yellow) and seed shape (S = smooth, s = wrinkled), made baby corns. Corn is cool, because an ear of corn is just a whole bunch of babies held in one place! By counting the corn babies (each kernel is a baby), you can investigate the principle of Independent Assortment.

Materials

1 ear of corn/group

Procedure

1. What were the phenotypes of the mama and papa corn plants that gave rise to your cob of babes? (Read the previous paragraph to answer this question!)
2. What were the genotypes of the mama and papa corn plants that gave rise to your cob of babes?
3. What were all the possible gametes each parent corn could produce?
4. Make a dihybrid cross illustrating all the possible baby corns produced by these parents. Then calculate the expected phenotypic ratios of the babies.
5. Count and record the phenotypes of 100 kernels on your cob. Record your results below. (Download the chart here.)