Punnett Squares

Punnett Squares

Middle School

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Learning Objectives

• Define the key terms gene, allele, genotype, and phenotype.

• Explain how dominant and recessive alleles determine an organism's specific traits.

• Construct a Punnett square for a monohybrid cross to show possible offspring genotypes.

• Use a Punnett square to predict probabilities of offspring genotypes and phenotypes.

Key Vocabulary

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Allele

An allele is one of the different forms or variants of a specific gene.

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Genotype

The genotype is the specific genetic makeup or the set of inherited alleles of an organism.

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Phenotype

The phenotype is the observable physical appearance or expressed characteristics of an inherited genotype.

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Punnett Square

A Punnett square is a chart used to determine the probability of offspring inheriting different genotypes.

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Dominant Allele

This is the allele that is expressed as long as at least one copy is present.

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Recessive Allele

This is the allele that is only expressed when two copies are present without a dominant allele.

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Genes, Alleles, and Traits

• A trait is a specific characteristic determined by your genes, like eye color.

• Alleles are the different forms of a gene; you get one from each parent.

• A dominant allele (B) is always expressed if it is present in the pair.

• A recessive allele (b) is only expressed when two copies are present.

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Multiple Choice

Which term describes the different forms of a single gene that determine specific traits?

• Gene

• Allele

• Trait

• DNA

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Genotype vs. Phenotype

Genotype

• An organism's genotype is the specific combination of two alleles inherited for a gene.

• A homozygous genotype has two identical alleles for a trait, like BB or bb.

• A heterozygous genotype has two different alleles for a trait, such as Bb.

Phenotype

• The phenotype is the observable physical expression of a genotype, like eye or hair color.

• The dominant phenotype is expressed with genotypes BB or Bb as a dominant allele is present.

• Only the homozygous recessive genotype, bb, results in the recessive phenotype being expressed.

Multiple Choice

An organism has the genotype 'Bb'. What is the correct classification and phenotype if 'B' is the dominant allele?

• Heterozygous, dominant phenotype

• Homozygous, dominant phenotype

• Heterozygous, recessive phenotype

• Homozygous, recessive phenotype

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Using a Punnett Square

• A Punnett square is a chart to predict an offspring's traits.

• It shows the probability of inheriting a genotype and phenotype.

• One parent's genotype goes on top, the other on the side.

• Combine alleles in each box to show possible offspring genotypes.

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Multiple Choice

What is the primary purpose of a Punnett square?

• To predict the probability of offspring's genotypes and phenotypes

• To determine the exact genetic makeup of a parent

• To show how many offspring a parent can have

• To list all the genes an organism possesses

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Example of a Monohybrid Cross

• A monohybrid cross tracks a single characteristic, like crossing two heterozygous parents (Bb x Bb).

• The possible genotypes of the offspring are BB, Bb, Bb, and bb.

• This gives a genotype probability of 25% BB, 50% Bb, and 25% bb.

• This results in a 3:1 phenotype ratio: 75% dominant and 25% recessive.

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Multiple Choice

In a monohybrid cross between two heterozygous parents (Bb x Bb), what is the probability of the offspring having the recessive phenotype?

• 25%

• 50%

• 75%

• 100%

Complex Inheritance Patterns

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Dihybrid Cross

• A dihybrid cross is a tool to track two different traits.

• It shows possible offspring outcomes from parents with different traits.

• A 9:3:3:1 ratio is typical for two heterozygous parents.

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Non-Mendelian Traits

• Codominance occurs when both alleles for a gene are expressed equally.

• Incomplete dominance results in a blended or intermediate physical trait.

• These patterns do not follow Mendel's simple rules of dominance.

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Polygenic Inheritance

• Polygenic characteristics are controlled by more than one single gene.

• This leads to a wide range of possible physical traits.

• Human height and skin color are examples of polygenic inheritance.

Multiple Choice

What type of inheritance results in a blended or intermediate phenotype, such as a red and white flower producing pink offspring?

• Incomplete Dominance

• Codominance

• Polygenic Inheritance

• Dihybrid Cross

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Common Misconceptions

Misconception	Correction
Dominant traits are always the most common.	A trait's dominance does not determine how common it is.
Recessive alleles are weaker than dominant ones.	Recessive alleles are not weaker, just masked by dominant alleles.
Genotype and phenotype are the same.	Genotype is the genetic code, while phenotype is the expressed physical trait.
Siblings inherit the same combination of traits.	Genes sort separately, creating unique gene combinations in each offspring.

Multiple Choice

A plant with green pods (Gg) is crossed with a plant with yellow pods (gg). 'G' is dominant for green. What is the best prediction for the phenotypes of their offspring?

• 50% green pods, 50% yellow pods

• 75% green pods, 25% yellow pods

• 100% green pods

• 25% green pods, 75% yellow pods

Multiple Choice

If a flower shows both red and white petals at the same time, what does this indicate about the alleles for flower color?

• The alleles for red and white are codominant.

• The allele for red is incompletely dominant over white.

• The allele for red is dominant and the allele for white is recessive.

• Flower color is a polygenic trait.

Multiple Choice

A rabbit with the genotype BB (dominant brown fur) is crossed with a rabbit with genotype bb (recessive white fur). What is the only possible genotype for their offspring, and how does this explain why all the offspring will have brown fur?

• The genotype will be Bb; since 'B' is dominant, the brown fur phenotype is expressed.

• The genotype will be bb; the brown fur is a result of environmental factors.

• The genotype will be BB; the offspring inherit only the dominant parent's genes.

• The genotype could be BB or Bb; both result in brown fur.

Multiple Choice

The given image is representing the concept of:

• Polygenic traits

• Co-dominance

• Incomplete dominance

• Plasticity

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Summary

• A Punnett square predicts the probable genotypes and phenotypes of offspring.

• Genotype is an organism's genetic makeup, while phenotype is its physical appearance.

• A heterozygous monohybrid cross typically results in a 3:1 phenotype ratio.

• Complex inheritance and independent assortment increase genetic variation.

Poll

On a scale of 1-4, how confident are you about using Punnett squares to predict offspring traits?

• 1

• 2

• 3

• 4