Heredity

Canine Similarities and Differences

This introductory slide presentation stimulates a shift in thinking about similarities among living things to noting their differences. It depicts increasingly specific levels of classification: canid species, domestic dog breeds, and individual Dalmatians (a breed of dog). The presentation concludes by introducing the idea that variations in genes underlie differences in traits.

• There are similarities and differences in traits among related species, within populations, and between individuals.

10 minutes

View as a whole group, discussing similarities and differences on each side.

Canine Similarities and Differences (presentation)

What is Mutation?

This brief video introduces mutation at the DNA level as the source of variation in genes. The next two activities will explore the mechanism and result of mutation in further detail.

You may wish to review the ideas in "Mutation Generates New Alleles," just under the video.

• Mutation is a natural process that generates variation in DNA sequences

5 minutes

What is Mutation? (video)

Mutate a DNA Sequence

Using a paper model, students make a mutation of their choice (substitution, insertion, or deletion) in a gene during DNA replication. Then they transcribe and translate the mutated sequence to reveal the resulting amino acid sequence.

After completing the activity, students learn about the example gene and protein—Human Leukocyte Antigen (HLA-B)—including known variants.

Notes:

You may wish to review the following:

• DNA replication follows base-paring rules: A-T, C-G
• Sometimes during DNA replication, a base is inserted, deleted, or substituted with a different one, changing the DNA sequence of a gene.
• Changes in the DNA sequence of a gene can lead to changes in the protein it codes for.
• Only mutations in germ cells (eggs or sperm) can be passed to offspring.

As in reality, the mutations students make are random. There will be variation in the resulting amino acid sequence.

Students may be tempted to skip using the “molecular machinery” (ribosome) in this model. Encourage them to use it as a visual reminder of where proteins are assembled.

• During DNA replication, occasional errors change DNA sequences. This process is called mutation.
• Changes in DNA sequences can lead to changes in proteins.

Using a paper model, students make a mutation and determine the effect on the resulting protein.

Students see the effect on a protein's structure caused by a change in a DNA sequence.

30-40 minutes

Scissors and tape

The Outcome of Mutation

How does a difference in a gene’s DNA sequence lead to a difference in an observable trait? This piece explores 8 real examples of changes in protein structure and/or function that lead to remarkably different traits.

• Variations in DNA sequences lead to variations in proteins, which lead to variations in traits.

This piece explores the remarkable variations in traits (effect) that are caused by small differences in DNA.

10-15 minutes

How Often Do Mutations Happen?

This video walks through the calculations for estimating how frequently dark fur mutations should occur in a population of rock pocket mice.

Students fill in a worksheet as they follow along.

• Mutation frequency varies, depending on the reproductive rate and population size. In elephants, mutations would accumulate much more slowly. In bacteria, mutations would accumulate much more quickly.
• Mutation happens naturally, all of the time.
• Given enough time, it is highly likely that even very specific genetic variations will arise in a population.
• This calculation estimates how often a new allele will arise. Propagation of the allele through reproduction is a separate thing.

• Mutation frequency can be calculated.
• Only when it happens in gametes can mutation generate new alleles that can be passed to offspring.

10 minutes

What is Inheritance?

Here we look at genetic variation in populations by examining how sexual reproduction “shuffles” existing alleles, increasing variation within a group.

This introductory video provides a very general overview of asexual and sexual reproduction and how sexual reproduction contributes to genetic variation. Subsequent activities will focus on allele shuffling in further detail.

• During reproduction, genetic information [DNA] passes from parent to offspring.
• During sexual reproduction, individuals inherit two copies of each gene, one from each parent.
• Sexual reproduction contributes to genetic variation.

5 minutes

What is Inheritance? (video)

Allele Shuffling

This video offers a more detailed look at how alleles are shuffled during sexual reproduction. It highlights how recombination, gamete formation (independent assortment), and random pairing of gametes each contribute to genetic and phenotypic variation.

• Variations in the DNA sequences of genes are called alleles.
• Alleles are shuffled during sexual reproduction (recombination, independent assortment, and fertilization).
• Allele shuffling during sexual reproduction contributes to genetic variation in a population.

Students see an overview of how recombination and independent assortment (cause) contribute to genetic variation (effect).

5 minutes

Allele Shuffling (video)

Build-A-Bird

This paper model of sexual reproduction uses real pigeon traits to demonstrate how two parents can produce highly varied offspring. Students recombine parental chromosomes, make gametes, then randomly combine two gametes. Finally, they decode the resulting allele combinations to draw the traits of a pigeon offspring.

Note: For simplicity, we’ve placed alleles on one chromosome.

After students complete their pigeons, hang them (along with the gametes they used to make them) all on a large wall space or white board. Discuss the following:

• Where the alleles came from in the first place (answer: in a previous geneation, there was a mutation in a reproductive cell)
• How allele “shuffling” during sexual reproduction contributes to genetic and phenotypic variation in offspring
• The amount of genetic and phenotypic variation you see in the offspring from just two pigeons

To find more information about pigeon traits, visit Pigeon Breeding: Genetics At Work

• Variations in the DNA sequences of genes are called alleles.
• Alleles are shuffled during sexual reproduction (recombination, independent assortment, and random fertilization).
• Allele shuffling during sexual reproduction contributes to genetic variation in a population.

Paper chromosomes are manipulated to model crossing over and independent assortment as generators of genetic variation.

Students visualize how crossing over and independent assortment (cause) increases genetic variation (effect).

45 minutes

Scissors, tape, colored pencils

Genetic Variation

This video highlights the context-dependency of harmful, helpful, and neutral genetic variations. A common misconception is that all mutations and their resulting traits are “bad.”

• DNA variations that arise through mutation can be beneficial, harmful, or neutral (to the organism).
• DNA variations that decrease viability or reproduction are not propagated, so they are eliminated from the population.
• Most DNA variations have no effect on viability or reproduction, and through random chance many are maintained in the population.

5 minutes

Identifying Reasoning

Students practice their argumentation skills while reviewing concepts from heredity, including mutation, alleles, and sexual reproduction.

• (Argumentation) In a scientific argument, reasoning is the justification for why the evidence supports the claim; it contains logic and relevant science ideas.
• (Argumentation practice) Identify reasoning that links a given claim and its supporting evidence.

Students practice choosing the reasoning that best connects a claim and evidence.

20 minutes

Formative Assessment

This quick formative assessment checks to see how well students understand the contributions of mutation and recombination to genetic variation in a population.

10 minutes

Before moving on, make sure your students understand the following:

• Mutation gives rise to variations in genes, called alleles.
• During sexual reproduction, allele shuffling generates new allele combinations.
• Mutation and allele shuffling increase variation within a population.