Laboratory 11: Mendelian Genetics

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Objectives

After completing this lab, you should be able to:

• Discuss the differences between traits, genes, and alleles.

• Distinguish between homozygous and heterozygous conditions.

• Construct and interpret a Punnett square.

• Perform monohybrid and dihybrid crosses.

• Understand and perform linked, sex-linked, and multiple allelic crosses.

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Key Terms

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Introduction

Genetics is the study of how traits are inherited through genes. Each trait is determined by one or more genes, which can exist in alternative forms called alleles.

• Dominant alleles mask the expression of recessive alleles.

• Recessive alleles are only expressed when an organism is homozygous for that allele.

• The genotype represents the genetic makeup, while the phenotype is the physical expression of that genotype.

For example:

• BB = brown eyes (homozygous dominant)

• Bb = brown eyes (heterozygous)

• bb = blue eyes (homozygous recessive)

To predict genetic outcomes, biologists use a Punnett square, which models how alleles from parents combine in offspring.

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Traits and Alleles

When geneticists write about traits:

• Capital letters represent dominant alleles (e.g., F for freckles).

• Lowercase letters represent recessive alleles (e.g., f for no freckles).

Each person has two alleles for each trait—one from each parent.

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Pre-Lab Activity

Before attending lab, collect phenotypic data from yourself and two blood relatives (parents or siblings). Record this data in Table 1. You will use these traits in lab to determine genotypes and complete genetic analyses.

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Table 1. Phenotypes and Genotypes for Student and Family Members

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Determining Genotypes

Use the phenotypes from Table 1 to deduce possible genotypes.
Remember:

• Recessive phenotypes always mean homozygous recessive (e.g., rr).

• Dominant phenotypes could be homozygous dominant (RR) or heterozygous (Rr).

• If only one parent shows the recessive trait, their child’s genotype can often be inferred.

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Laboratory Activity

You will use Punnett squares to predict expected genotypic and phenotypic ratios, then compare these to observed ratios using corn kernels as a model for inheritance.

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Monohybrid Cross

P generation cross:
PP (purple) × pp (yellow)

Gametes produced:
PP → P  pp → p

F₁ Generation:
All offspring = Pp (purple)

F₁ generation cross:
Pp × Pp

Gametes produced:
P →  p →  P →  p →

Punnett Square:
Predict and record expected offspring genotypes.

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Table 2. Expected and Observed Results for F₂ Generation (Monohybrid Cross: Purple × Yellow Corn)

*The expected ratio is derived from your Punnett square.
**Expected numbers are based on total kernels counted ÷ 4, multiplied by the appropriate ratio (e.g., 3:1).

Question:
Which count was closer to the expected values—row or ear? ___________________________

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Dihybrid Cross

P generation cross:
PPSS (purple, starchy) × ppss (yellow, sweet)

Gametes produced:
PPSS → PS  ppss → ps

F₁ Generation:
All offspring = PpSs (purple, starchy)

F₁ generation cross:
PpSs × PpSs

Gametes produced:
PS Ps pS ps

Punnett Square:
Complete and use it to predict expected genotypic and phenotypic ratios.

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Table 3. Expected and Observed Results for F₂ Generation (Dihybrid Cross: Purple/Starchy × Yellow/Sweet Corn)

Question:
Which count was closer to the expected values—row or ear? ___________________________

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Genetics Practice Problems

Answer the following on your exercise sheet or in your digital submission.

1. Write the gametes formed from each genotype:
   a. AA  b. Aa  c. AAbb  d. AABb  e. AaBb

2. Complete Punnett squares and provide expected genotypic and phenotypic ratios:
   a. WW × ww
   b. Dd × Dd
   c. Hh × hh

3. Guinea Pig Fur Color:

   • Black (B) is dominant over white (b).
     Determine phenotypic and genotypic ratios for F₁ generation in:
     a. Homozygous dominant × heterozygous
     b. Homozygous recessive × heterozygous
     c. Homozygous dominant × homozygous recessive
     d. Heterozygous × heterozygous

4. White Forelock (Humans):
   The white forelock trait is dominant.

   • A woman with a white forelock (whose father did not have one) marries a man without one.

   • What is the probability that their child will have a white forelock?

5. Cheek Dimples (D) and Cleft Chin (C):

   • Cross: DDCC × ddcc

   • Determine F₂ genotypic and phenotypic ratios.

6. Complex Crosses:
   Using Table 1 traits, determine expected genotypic and phenotypic ratios for:
   a. RRtt × rrTT
   b. RrTT × rrTt
   c. RrTt × RrTt