Punnett Squares and Heredity

Punnett Squares and Heredity

Middle School

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Learning Objectives

• Define key genetic terms like gene, allele, genotype, and phenotype.

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

• Use a Punnett square to predict genotypes and phenotypes from a monohybrid cross.

• Differentiate between Mendelian and non-Mendelian inheritance patterns like codominance and incomplete dominance.

Key Vocabulary

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Gene

A segment of DNA that codes for a protein, controlling a specific expressed characteristic.

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Allele

Variants of a specific gene that are responsible for determining the expression of a trait.

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Genotype

The inherited genetic makeup of an organism, composed of a unique combination of its alleles.

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Phenotype

The observable physical appearance or expression of an organism that is determined by its genotype.

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Dominant Allele

An allele that is always expressed in the phenotype as long as it is present.

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Recessive Allele

An allele that is only expressed in the phenotype when no dominant allele is present.

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Genotype vs. Phenotype

• A genotype is the genetic makeup, with two alleles for each gene.

• Homozygous means identical alleles (BB, bb); heterozygous means different alleles (Bb).

• The phenotype is the physical expression or observable trait of the genotype.

• Dominant alleles are expressed in BB and Bb; recessive alleles only in bb.

Multiple Choice

An organism has two different alleles for a single trait, written as 'Bb'. Which of the following correctly describes this organism?

• It is heterozygous and will express the dominant trait.

• It is homozygous and will express the dominant trait.

• It is heterozygous and will express the recessive trait.

• It is homozygous and will express the recessive trait.

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Predicting Traits with Punnett Squares

• A Punnett square is a chart that predicts an offspring's potential traits.

• ​Parent genotypes are on the top and side; boxes show possible offspring.

• For two heterozygous (Bb) parents, offspring can be BB, Bb, or bb.

• This results in a 3:1 ratio of dominant to recessive phenotypes.

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Multiple Choice

If two heterozygous parents are crossed for a single trait, what is the probability that an offspring will express the dominant phenotype?

• 75%

• 25%

• 50%

• 100%

Beyond Mendel: Complex Inheritance

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Codominance

• ​Both alleles for a gene are expressed equally in the organism’s phenotype.

• ​​Neither of the alleles is recessive, so both phenotypes are expressed together.

• ​A flower with both red petals and white petals is an example.

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Incomplete Dominance

• ​The dominant allele is not fully expressed over the recessive allele.

• ​​This results in a third phenotype which is a blend of both alleles.

• ​For instance, a red and white flower creating a pink flower offspring.

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Polygenic Traits

• ​These characteristics are controlled by more than one gene.

• ​​This allows for a wide range of possible phenotypes to be expressed.

• ​Human height and skin color are examples of such polygenic characteristics.

Multiple Choice

A flower with red alleles and white alleles produces offspring with pink petals. What type of inheritance is this?

• Incomplete Dominance

• Codominance

• Mendelian Dominance

• Polygenic Characteristic

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Common Misconceptions

Misconception	Correction
Dominant traits are always more common in a population.	A trait's frequency depends on many factors, not just whether the allele is dominant.
Every trait is controlled by a single gene.	Many traits are polygenic, meaning they are controlled by multiple genes.
An organism's traits are determined solely by its genes.	The expression of genes can be modified by interactions with the environment.

Multiple Choice

In pea plants, tall (T) is dominant to short (t). If a homozygous tall plant (TT) is crossed with a homozygous short plant (tt), why are all offspring tall?

• All offspring have a 'T' allele, and the dominant trait is always expressed when present.

• All offspring have a 't' allele, and the recessive trait is hidden.

• The offspring are heterozygous, which results in a medium height.

• The tall allele is more common in nature than the short allele.

Multiple Choice

An individual's physical appearance is its __________, which is determined by its inherited __________.

• phenotype; genotype

• genotype; phenotype

• alleles; genes

• phenotype; chromosomes

Multiple Choice

A scientist crosses a plant with red flowers and a plant with white flowers. The offspring have flowers that are both red and white. What is the most likely explanation for this outcome?

• The alleles for red and white flowers are codominant, so both are expressed.

• The plant experienced a random mutation, resulting in a new color pattern.

• It is an example of incomplete dominance, where the traits blend together.

• Red is dominant, but the environment caused the white allele to be expressed.

Multiple Choice

As per the image, a 3:1 ratio of dominant to recessive phenotypes are shown. Which among the following is NOT correct?

• It shows that the alleles for the two traits assort independently.

• It proves that one trait is completely dominant over the other, preventing any new combinations.

• It indicates that the genes for the two traits are linked and always inherited together.

• It suggests that both traits are examples of incomplete dominance, leading to blended phenotypes.

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Summary

• Heredity is the passing of traits from parents to offspring through genes.

• Genotype is the genetic makeup, while phenotype is the observable physical trait.

• Dominant alleles are expressed if present; recessive alleles need two copies to be expressed.

• Punnett squares are used to predict the probability of offspring inheriting traits.

Poll

On a scale of 1-4, how confident are you about predicting traits using Punnett Squares?

• 1

• 2

• 3

• 4