16.2: Exercise

Introduction to Mendelian Genetics - Part 1

Genes occur in pairs due to the diploid nature of human somatic cells, with one gene on each homologous chromosome. The DNA sequence of each gene in this pair may be identical or different in sequence. These possible forms are called alleles. When a person has two identical alleles, they are homozygous (i.e., AA or aa). When a person has two different alleles, they are heterozygous (i.e., Aa). Our genetic makeup, or genotype, is what determines our physical characteristics, the phenotype.

In some cases, one allele masks the expression of the other allele. This allele is called the dominant allele, and is expressed an observed in the phenotype. The allele that is masked and not observed is known as the recessive allele. Dominant traits are represented by capital letters, such as ‘A’, whereas recessive traits are represented by lowercase letters, such as ‘a’.

1. Use the letter ‘A’ to represent the following:
   • Homozygous dominant organism: _______________ Color: ____________________
   • Homozygous recessive organism: ________________ Color: ____________________
   • Heterozygous organism: _______________________ Color: ____________________

2. If the letter ‘A’ represents flower color, where ‘A’ represents red flowers and ‘a’ represents white flowers, what color would each flower be? Fill-in the blanks above.

In somatic cells, both alleles are present in each diploid cell. However, during the formation of haploid gametes, the alleles for each trait separate so that each gamete only contains one allele for each particular trait. This is known as Mendel’s law of segregation, which contributes to the haploid nature of our gametes, the sperm and egg. Additionally, Mendel’s law of independent assortment tells us that each trait is inherited independently of every other trait.

If we know the genotypes of both parents, we can calculate the expected genotypes and phenotypes of their offspring. Punnett squares allow us to calculate expected results.

3. Use Punnett Squares to calculate the expected genotypes and phenotypes of the offspring from the following crosses:

AA x aa AA x Aa

Aa x Aa (monohybrid cross) Aa x aa

Traits are not always dominant or recessive, however. Some traits exhibit incomplete dominance, where the heterozygous genotype has an intermediate phenotype. For example, in certain flowers like the snapdragon flower, a heterozygous condition results in pink flowers.

4. Use Punnett Squares to calculate the expected genotypes and phenotypes of the offspring from the following crosses, if trait ‘A’ exhibits incomplete dominance:

AA x aa AA x Aa

Aa x Aa Aa x aa

A condition known as codominance also occurs, where both alleles in a heterozygous individual are expressed equally. The most common example of codominance are the ABO blood groups, where a person with type AB blood expresses both A and B alleles equally, and O is recessive to both the A and B alleles. (Note: The alleles are simplified here. We will see them written as I^A, I^B, and i in lecture.)

5. Use Punnett Squares to calculate the expected genotypes and phenotypes of the offspring from the following monohybrid crosses, if traits ‘A’ and ‘B’ exhibit codominance:

AO x BO AO x OO

AA x BO BB x OO

Working with your lab partners, determine your phenotypes for the traits discussed in the next section. These traits are based upon two alleles and follow the normal dominance rules of Mendelian inheritance. Record your phenotype and possible genotype(s) in the table below. Once each group has completed their data collection, we will combine our class results and place it on a chart on the board.

Traits:

1. Bent little finger – bent is dominant “C”, straight is recessive “c” (clinodactyly)
2. Tongue rolling – tongue rolling is “R”, non-tongue rolling is “r”
3. Widow’s peak – V-shaped forehead hairline is “W”, straight forehead hairline is “w”
4. Free earlobe – free earlobes are “E”, attached earlobes are “e”
5. Finger hair – middle segment hair is “H”, no hair is “h”
6. Dimpled cheeks – dimples on one or both cheeks is “D”, dimple absence is “d”
7. Eyebrow raising – ability to raise eyebrows is “Y”, inability is “y”
8. Ear wiggling – ability to wiggle ears is “G”, inability is “g”
9. Long toe – if your second toe is longer than your big toe = “L”, a shorter second toe is “l”
10. Curly hair – curly hair is “A”, straight hair is “a”
11. Freckles – presence of freckles is “Z”, no freckles is “z”
12. PTC tasting – ability to taste is “P”, inability is “p” (phenylthiocarbamide)

Data Table for Student Genotypes and Phenotypes

Trait	Your Phenotype	Your Possible Genotype(s)	# Students Dominant Phenotype	# Students Recessive Phenotype	Class % Dominant Phenotype	Class % Recessive Phenotype
Bent little finger
Tongue rolling
Widow’s peak
Free earlobe
Finger hair
Dimpled cheeks
Eyebrow raising
Ear wiggling
Long toe
Curly hair
Freckles
PTC tasting

Number of students in our class: _________________

Introduction to Mendelian Genetics Part 2 – Dihybrid Cross

Dihybrid crosses involve analyzing possible combinations of two traits, ‘A’ and ‘B’.

In the problem below, the letter ‘A’ represents flower color, where ‘A’ represents red flowers and ‘a’ represents white flowers. The letter ‘B’ represents stem height, where ‘B’ represents long stems and ‘b’ represents short stems.

1. What are the possible combinations of alleles that may be found in the gametes of an organism that is heterozygous for both traits, ‘AaBb’? Remember that the gametes must have one allele for each trait: __________________________________________________________________
2. Use a Punnett square to calculate the expected genotypes and phenotypes of the offspring from the following dihybrid cross: AaBb x AaBb

Data Table for Dihybrid Cross

Gametes:

Expected genotypes and ratio: ____________________________________________________________

Expected phenotype and ratio: ___________________________________________________________