Dominant and Recessive Traits

Dominant and Recessive Traits

Middle School

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Learning Objectives

• Define genotype, phenotype, and dominant and recessive alleles.

• Use a Punnett square to predict the outcomes of a monohybrid cross.

• Explain the difference between homozygous and heterozygous genotypes.

• Differentiate between Mendelian and non-Mendelian inheritance patterns.

Key Vocabulary

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Gene

A segment of DNA that codes for a specific protein, which in turn controls characteristics.

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Allele

Different versions or variants of a specific gene that are responsible for determining traits.

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Genotype

The complete genetic makeup of an organism, representing the specific set of alleles it inherited.

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Phenotype

The physical appearance or observable expression of an organism that results from its inherited genotype.

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Dominant Allele

An allele that is always expressed as a trait, even when only one copy is present.

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Recessive Allele

An allele that is only expressed as a trait when two copies are present in the genotype.

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Genotype and Phenotype

Genotype

• For each gene, an organism inherits two alleles, receiving one from each of its parents.

• This specific combination of inherited alleles (e.g., BB, Bb, or bb) makes up its genotype.

• A dominant allele is written with a capital letter, while a recessive allele uses a lowercase letter.

Phenotype

• A phenotype is the observable physical trait that results from the organism's specific genotype.

• A genotype that contains at least one dominant allele will always express the dominant trait.

• The recessive trait is only expressed if the genotype consists of two recessive alleles (bb).

Multiple Choice

An organism's allele combination, such as Bb or bb, is known as its what?

• Phenotype

• Genotype

• Allele

• Chromosome

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Homozygous vs. Heterozygous

• An organism is homozygous when it has two identical alleles for a trait.

• This can be homozygous dominant (BB) or homozygous recessive (bb).

• An organism is heterozygous if it has two different alleles for a trait.

• In heterozygous individuals (Bb), the dominant trait is the one that is expressed.

Multiple Choice

If an organism's genotype for a trait is 'gg', how would it be classified?

• Heterozygous

• Homozygous dominant

• Homozygous recessive

• Non-Mendelian

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Using a Punnett Square

• It is a chart that predicts possible offspring genotypes from two parents.

• ​To set one up, draw a four-square grid for a single trait cross.

• Place one parent's alleles on top and the other parent's on the left.

• Fill each square by combining alleles from its corresponding row and column.

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Multiple Choice

What is the primary function of a Punnett square?

• To show the exact traits an offspring will have.

• To determine the probability of different genotypes in an offspring.

• To identify the physical location of a gene on a chromosome.

• To list all the genes an organism possesses.

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Predicting Genetic Outcomes

• A Punnett square predicts outcomes from a cross of two heterozygous parents (Bb x Bb).

• Genotype probability is 25% homozygous dominant (BB) and 25% homozygous recessive (bb).

• There is a 50% probability of producing a heterozygous (Bb) offspring.

• This results in a 75% dominant phenotype, creating a 3:1 phenotypic ratio.

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Multiple Choice

In a monohybrid cross between two heterozygous parents (Bb), what is the typical ratio of dominant phenotypes to recessive phenotypes?

• 1:1

• 2:1

• 3:1

• 4:0

Beyond Simple Inheritance

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Codominance

• Both alleles for a trait are expressed equally and distinctly in the phenotype.

• For instance, a flower may show both red and white petals together.

• Neither allele is dominant or recessive over the other in this case.

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Incomplete Dominance

• One allele is not completely dominant over the other recessive one.

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

• For example, a red and a white flower can produce a pink flower.

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Dihybrid Crosses

• A dihybrid cross is used to study the inheritance of two different traits.

• Punnett squares can help predict the outcomes of these more complex crosses.

• The typical phenotypic ratio for this type of cross is 9:3:3:1.

Multiple Choice

A chicken with black feathers is crossed with a chicken with white feathers, and their offspring have both black and white feathers. What is this pattern of inheritance called?

• Incomplete Dominance

• Recessive Inheritance

• Codominance

• Polygenic Inheritance

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Common Misconceptions

Misconception	Correction
Dominant traits are always the most common.	Trait frequency depends on many factors, not only dominance.
All inheritance follows simple Mendelian rules.	Many traits follow complex, non-Mendelian patterns of inheritance.
Traits are determined only by genes.	Gene expression can be modified by the environment, influencing the final trait.

Multiple Choice

If you cross a plant that is homozygous dominant for purple flowers (PP) with one that is homozygous recessive for white flowers (pp), what will be the genotype of all the offspring?

• PP (homozygous dominant)

• Pp (heterozygous)

• pp (homozygous recessive)

• A mix of PP and pp

Multiple Choice

Why does a heterozygous individual (like Tt) express the dominant trait instead of a blend of both traits?

• Because the recessive allele is stronger.

• Because the dominant allele masks the effect of the recessive allele.

• Because heterozygous organisms only show recessive traits.

• Because the environment changes the expression.

Multiple Choice

A geneticist finds that crossing a red-eyed fly with a white-eyed fly results in all offspring having pink eyes. How can you use this information to predict the outcome of crossing two pink-eyed flies?

• All offspring will have pink eyes.

• The offspring will likely show a ratio of red, pink, and white eyes, as this is incomplete dominance.

• All offspring will have red eyes, as it is the dominant color.

• The offspring will have a mix of red and white eyes, showing codominance.

Multiple Choice

A student reports that a Punnett square for a monohybrid cross predicts a 1:2:1 genotypic ratio. What must be true about the parents of this cross?

• Both parents are homozygous dominant.

• Both parents are heterozygous.

• One parent is homozygous dominant, the other is homozygous recessive.

• One parent is heterozygous, the other is homozygous recessive.

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Summary

• Genotype is the genetic code (e.g., Bb); phenotype is the observable trait.

• Dominant alleles mask recessive ones. Genotypes can be homozygous (BB) or heterozygous (Bb).

• Punnett squares predict the probability of traits, like the 3:1 heterozygous cross ratio.

• New traits can come from mutations or complex patterns like codominance.

Poll

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

• 1

• 2

• 3

• 4