Mendelian Inheritance

Essential Question

How do dominant and recessive alleles determine traits?

Children can look like their parents in some ways and different in others. In the 1860s, Gregor Mendel studied pea plants and found patterns that explain how traits pass from parents to offspring. In this lesson, you’ll learn the vocabulary of genetics, how dominant and recessive alleles work, and how Mendel’s pea plant crosses showed a 3 : 1 pattern.

Genes, Alleles, and Traits

• A gene is a section of DNA that codes for a trait
• Alleles are different versions of the same gene
• You have two alleles for each gene, one from each parent
• Genotype is the alleles; phenotype is the observable trait

A gene is a section of DNA that codes for a trait, such as flower color. Different versions of a gene are called alleles. In pea plants, the gene for flower color has a purple allele (P) and a white allele (p). A body cell has two alleles for each gene, one on each of a pair of homologous chromosomes (see Meiosis). The combination of alleles is the genotype, such as Pp. The trait you can observe is the phenotype, such as purple flowers. Two identical alleles make an organism homozygous (PP or pp). Two different alleles make it heterozygous (Pp). Alleles and how they change in populations are covered in Unit 4.

Dominant and Recessive Alleles

• A dominant allele shows when at least one copy is present
• A recessive allele shows only when two copies are present
• Dominant alleles get a capital letter and recessive alleles a lowercase letter
• Dominant does not mean most common or best

Genotype	Type	Phenotype (P = purple is dominant)
PP	Homozygous dominant	Purple
Pp	Heterozygous	Purple
pp	Homozygous recessive	White

When the two alleles of a gene are different, one may dominate the other. Pp plants have purple flowers because the dominant allele (P) is expressed and the recessive allele (p) is hidden. White flowers appear only in pp plants. A Pp plant carries the white allele but does not show it. Dominant describes how an allele is expressed, not how common it is. Some dominant alleles are rare, and some recessive alleles are very common.

Mendel’s Pea Plant Crosses

• Mendel crossed true-breeding plants with different traits
• All F1 offspring showed only the dominant trait
• The recessive trait returned in about 1 of 4 F2 offspring
• The ratio of about 3 dominant : 1 recessive appeared for every trait he tested

Trait in the F2 generation	Dominant	Recessive	Ratio (dominant / recessive)
Flower color	705	224	3.15 : 1
Pod shape	882	299	2.95 : 1
Seed shape	5,474	1,850	2.96 : 1

Mendel crossed true-breeding pea plants, which always produce offspring like themselves. A purple (PP) plant crossed with a white (pp) plant gave an F1 generation in which every plant was purple (Pp). When he crossed the F1 plants with each other, the F2 generation had both colors, about 3 purple to 1 white. Mendel concluded that traits are carried by separate units (what we now call genes), that each parent passes on one copy, and that the recessive version can be hidden for a generation and then reappear. The two alleles of a gene separate when gametes form, as in meiosis. The table shows some of his counts. Real counts are close to 3 : 1 but not exact, because which alleles meet at fertilization is a matter of chance.

Genes and the Environment

• Genes set the possible range for a trait
• The environment can change how much a trait is expressed
• The same genotype can give different phenotypes in different conditions
• Most traits depend on both

A phenotype is not set by genes alone. The graph shows made-up practice data for pea plant height. Plants with the same genotype are shorter in shade than in full sun, so light affects how tall a plant grows. Genotype still matters: in both settings, TT and Tt plants are taller than tt plants. Human traits such as height and skin color depend on both genes and environment, such as nutrition and sunlight.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Mendel’s Ratio

In one cross, Mendel counted 705 F2 plants with purple flowers and 224 with white flowers. (a) Find the ratio of purple to white. (b) How many of each would exactly a 3 : 1 ratio predict? (c) Is the result consistent with a 3 : 1 ratio? Compare it with the seed shape data, 5,474 round seeds and 1,850 wrinkled seeds.

Show the solution

1. (a) Ratio: 705 / 224 = 3.15, so the ratio is 3.15 : 1.
2. (b) Predicted counts: Total = 705 + 224 = 929 plants. A 3 : 1 ratio means 3/4 are purple and 1/4 are white. Purple: 3/4 x 929 = 696.75, about 697. White: 1/4 x 929 = 232.25, about 232.
3. (c) Compare: The data (705 and 224) are close to the predicted 697 and 232. The seed shape ratio, 5,474 / 1,850 = 2.96 : 1, is also close to 3 : 1. Fertilization is random, so small differences from the prediction are expected.
4. Answer: (a) 3.15 : 1. (b) About 697 purple and 232 white. (c) Yes, the data are close to 3 : 1.

Example 2: Use the Cross Model

The diagram shows a cross of true-breeding purple (PP) and white (pp) pea plants. (a) What is the genotype of every F1 plant, and why are they all purple? (b) What fraction of the purple F2 plants are heterozygous? (c) Out of 1,000 F2 plants, about how many are expected to have white flowers?

Show the solution

1. (a) F1 genotype: Each F1 plant gets P from the purple parent and p from the white parent, so all are Pp. They are purple because P is dominant.
2. (b) Heterozygous purple plants: Of the F2 plants PP, Pp, Pp, and pp, three are purple. Two of those three are Pp, so the fraction is 2/3.
3. (c) White plants: Only pp is white, which is 1/4 of the F2. 1/4 x 1,000 = 250 plants.
4. Answer: (a) Pp, and P is dominant. (b) 2/3. (c) About 250 plants.

Example 3: Genes and Environment

The graph shows made-up practice data for pea plant height. Write a claim, evidence, reasoning statement about this question: does genotype alone decide how tall the plants grow?

Show the solution

1. Claim: No. Height depends on both genotype and environment.
2. Evidence: TT plants are 96 cm in full sun but 71 cm in shade, a drop of 25 cm. Also, TT plants are taller than tt plants in both settings (96 cm vs 48 cm in sun, 71 cm vs 35 cm in shade).
3. Reasoning: The same genotype gave two different heights, so something other than genotype changed the phenotype: the amount of light. But genotype still matters, since the tt plants stay shorter than TT plants under the same conditions.
4. Answer: Height depends on genotype and environment: TT drops 25 cm in shade, and TT is still taller than tt in both settings.

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Practice

Practice with genotype and phenotype vocabulary, checking data against a 3 : 1 ratio, reading a cross model, and explaining how genes and environment work together. Try it on your own first, then check your work with the answer key.

• Mendelian Inheritance Practice Worksheet (PDF)
• Mendelian Inheritance Practice Answer Key (PDF)

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Lesson Slides

• 10.4 Mendelian Inheritance Slides (PDF)

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Big Ideas

• A gene has different versions called alleles
• Genotype is the alleles; phenotype is the trait you see
• A dominant allele shows when one copy is present
• A recessive trait shows only with two copies
• Mendel’s crosses gave about 3 dominant : 1 recessive in the F2
• Genes and environment both affect traits

Back to the essential question: Dominant alleles show in the phenotype when at least one copy is present, and recessive alleles show only when two copies are present. A recessive allele can be hidden for a generation and then reappear, which gives the 3 : 1 pattern. Next, you’ll use Punnett squares to predict the results of crosses.

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