Complete Dominance- Mendelian Genetics Worksheet - Free Printable

Problem Overview:

The worksheet focuses on Mendelian genetics, specifically complete dominance. We need to solve two genetic crosses using Punnett squares and determine the genotypic and phenotypic ratios for each cross.

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Step 1: Define Key Terms

Before solving the problems, let's define the key terms provided in the table:

| Term | Definition | In Your Own Words | How Will You Remember? |
|---------------|-------------------------------------------------------------------------------|-----------------------------------------------------------|-----------------------------|
| Homozygous | Having identical alleles for a particular gene (e.g., AA or aa). | Both alleles are the same. | Think "homo" means "same." |
| Heterozygous | Having different alleles for a particular gene (e.g., Aa). | The alleles are different. | Think "hetero" means "different." |
| Phenotype | The observable physical traits of an organism (e.g., black fur, tall plant). | What you see. | Think "phenotype" sounds like "physical." |
| Genotype | The genetic makeup of an organism (e.g., AA, Aa, aa). | The actual genes. | Think "genotype" sounds like "genes." |
| Allele | One of two or more alternative forms of a gene. | Different versions of a gene. | Think "allele" is part of "gene." |
| Dominant | An allele that is expressed when present in either homozygous or heterozygous form. | Stronger trait; overrides recessive. | Think "dominant" means "in charge." |
| Recessive | An allele that is only expressed when present in the homozygous state. | Weaker trait; hidden by dominant. | Think "recessive" means "hidden." |

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Problem 1: Rabbits (Black Fur vs. White Fur)

#### Given:
- Black fur is dominant over white fur.
- Cross: Heterozygous black male (Aa) × Homozygous white female (aa).

#### Solution:
1. Parental Genotypes (P):
- Male: Aa (heterozygous black)
- Female: aa (homozygous white)

2. Punnett Square:
- Place the alleles from the male (A, a) on the top row.
- Place the alleles from the female (a, a) on the left column.
- Fill in the Punnett square by combining alleles.

```
| | a | a |
|---|----|----|
| A | Aa | Aa |
| a | aa | aa |
```

3. Genotypic Ratio:
- From the Punnett square, the possible genotypes are: Aa, Aa, aa, aa.
- Count the occurrences: 2 Aa : 2 aa.
- Simplify the ratio: 1 Aa : 1 aa.

4. Phenotypic Ratio:
- Dominant allele (A) expresses black fur, and recessive allele (a) expresses white fur.
- Phenotypes: 2 black (Aa), 2 white (aa).
- Simplify the ratio: 1 black : 1 white.

#### Final Answer for Problem 1:
- P (parent genotypes): Aa × aa
- Genotypic ratio: 1 Aa : 1 aa
- Phenotypic ratio: 1 black : 1 white

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Problem 2: Pea Plants (Tall vs. Short)

#### Given:
- Tall is dominant over short.
- Cross: Homozygous short plant (tt) × Homozygous tall plant (TT).

#### Solution:
1. Parental Genotypes (P):
- Short plant: tt (homozygous recessive)
- Tall plant: TT (homozygous dominant)

2. Punnett Square:
- Place the alleles from the tall plant (T, T) on the top row.
- Place the alleles from the short plant (t, t) on the left column.
- Fill in the Punnett square by combining alleles.

```
| | T | T |
|---|----|----|
| t | Tt | Tt |
| t | Tt | Tt |
```

3. Genotypic Ratio:
- From the Punnett square, the possible genotypes are: Tt, Tt, Tt, Tt.
- All offspring have the genotype Tt.
- Simplify the ratio: 1 Tt.

4. Phenotypic Ratio:
- Dominant allele (T) expresses tall plants, and recessive allele (t) is hidden.
- All offspring will be tall (Tt).
- Simplify the ratio: 1 tall.

#### Final Answer for Problem 2:
- P (parent genotypes): tt × TT
- Genotypic ratio: 1 Tt
- Phenotypic ratio: 1 tall

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Final Answers:

1. Rabbits:
- P: Aa × aa
- Genotypic ratio: 1 Aa : 1 aa
- Phenotypic ratio: 1 black : 1 white

2. Pea Plants:
- P: tt × TT
- Genotypic ratio: 1 Tt
- Phenotypic ratio: 1 tall

Boxed Final Answer:
\[
\boxed{
\begin{array}{l}
\text{1. Rabbits:} \\
\text{P: Aa × aa} \\
\text{Genotypic ratio: 1 Aa : 1 aa} \\
\text{Phenotypic ratio: 1 black : 1 white} \\
\\
\text{2. Pea Plants:} \\
\text{P: tt × TT} \\
\text{Genotypic ratio: 1 Tt} \\
\text{Phenotypic ratio: 1 tall}
\end{array}
}
\]