Monohybrid Cross

Monohybrid Cross

High School

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Learning Objectives

• Understand how a single trait is inherited from parents to offspring.

• Learn to complete Punnett Squares for monohybrid crosses.

• Predict phenotypic and genotypic ratios for monohybrid crosses based on the Punnett Square.

Key Vocabulary

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Dominant Allele

An allele that masks the effect of others and is represented by a capital letter.

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Recessive Allele

An allele whose effect is only seen when two copies are present, written as a lowercase letter.

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Homozygous

Having two identical alleles for a particular gene, for example, the TT or tt genotype.

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Heterozygous

Describes having two different alleles for a particular trait, such as the Tt genotype.

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Fundamentals of Heredity

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

• A gene is a section of DNA that carries information for a trait.

• Alleles are different versions of a gene, such as for flower color.

• Genotype is the genetic makeup (Tt); phenotype is the observable trait.

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Solved Example 1
In pea plants, purple flowers (P) are dominant to white (p). If two heterozygous (Pp) plants are crossed, what are the predicted genotypic and phenotypic ratios of their offspring?

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Step 1: Analyze and Sketch the Problem

• Goal: Find the genotypic and phenotypic ratios of the offspring.

• Knowns: Both parents are heterozygous (Pp). Purple (P) is dominant, and white (p) is recessive.

• Unknowns: The genotypic ratio (PP:Pp:pp) and the phenotypic ratio (purple:white).

• Method: Use a Punnett square to show the possible gene combinations.

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Solved Example 1
In pea plants, purple flowers (P) are dominant to white (p). If two heterozygous (Pp) plants are crossed, what are the predicted genotypic and phenotypic ratios of their offspring?

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Step 2: Solve for the Unknown

• Draw a 2x2 Punnett square. The alleles from one parent (P, p) go on top, and the alleles from the other parent (P, p) go on the side.

• Fill in the squares: The top-left is PP, top-right is Pp, bottom-left is Pp, and bottom-right is pp.

• Count the genotypes: 1 PP (homozygous dominant), 2 Pp (heterozygous), 1 pp (homozygous recessive). The genotypic ratio is 1:2:1.

• Determine the phenotypes: PP and Pp result in purple flowers; pp results in white flowers. This gives 3 purple and 1 white. The phenotypic ratio is 3:1.

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Solved Example 1

In pea plants, purple flowers (P) are dominant to white (p). If two heterozygous (Pp) plants are crossed, what are the predicted genotypic and phenotypic ratios of their offspring?

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Step 3: Evaluate the Answer

Multiple Choice

An organism's physical appearance is its __________, while its genetic makeup is its __________.

• gene; allele

• allele; gene

• phenotype; genotype

• genotype; phenotype

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Monohybrid Crosses and Punnett Squares

• A monohybrid cross tracks a single trait between two crossed individuals.

• ​A Punnett square is a tool used to predict genetic outcomes.

• The Law of Segregation states that allele pairs separate during gamete formation.

• Use it to determine potential genotypes and phenotypes of the offspring.

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Solved Example 2
In pea plants, purple flowers (P) are dominant to white flowers (p). If a heterozygous plant is crossed with a white-flowered plant, what are the predicted genotypic and phenotypic ratios of the offspring?

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Step 1: Analyze and Sketch the Problem

• Goal: Determine the genotypic and phenotypic ratios of the offspring.

• Knowns: Parent 1 genotype is heterozygous (Pp). Parent 2 genotype is white-flowered, which is homozygous recessive (pp).

• Unknown: The genotypic and phenotypic ratios of the offspring.

• Formula: A Punnett Square is used to predict the outcome of a genetic cross.

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Solved Example 2
In pea plants, purple flowers (P) are dominant to white flowers (p). If a heterozygous plant is crossed with a white-flowered plant, what are the predicted genotypic and phenotypic ratios of the offspring?

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Step 2: Solve for the Unknown

• The heterozygous parent (Pp) produces two types of gametes: P and p. The homozygous recessive parent (pp) produces only one type of gamete: p.

• Construct a Punnett Square. The possible genotypes of the offspring are Pp and pp.

• Genotypic Ratio: By counting the squares, we find 2 Pp and 2 pp. The ratio is 2:2, which simplifies to 1 Pp : 1 pp (or 50% Pp, 50% pp).

• Phenotypic Ratio: Since P is dominant, Pp plants are purple. The pp plants are white. This gives 2 purple flowers and 2 white flowers. The ratio is 2:2, which simplifies to 1 Purple : 1 White (or 50% Purple, 50% White).

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Solved Example 2
In pea plants, purple flowers (P) are dominant to white flowers (p). If a heterozygous plant is crossed with a white-flowered plant, what are the predicted genotypic and phenotypic ratios of the offspring?

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Step 3: Evaluate the Answer

• The Punnett square correctly combines the parental gametes, and the calculated ratios (1:1 for both genotype and phenotype) account for all four possible outcomes in the square.

• The results align with the principles of Mendelian genetics for a monohybrid cross between a heterozygote and a homozygous recessive.

Multiple Choice

According to the Law of Segregation, what happens to the alleles for a trait during gamete formation?

• They double in number.

• They separate from each other.

• They disappear.

• They combine to form a new allele.

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Example Crosses: Disease and Height

Cystic Fibrosis

• A cross between two heterozygous carriers (Aa x Aa), with 'a' being the recessive allele.

• The genotypic ratio is 1 homozygous dominant (AA), 2 heterozygous (Aa), and 1 homozygous recessive (aa).

• This results in a phenotypic ratio of 3 normal individuals to 1 with cystic fibrosis.

Plant Height

• A cross between a heterozygous tall plant (Tt) and a homozygous short plant (tt).

• The genotypic ratio is 1 heterozygous (Tt) to 1 homozygous recessive (tt), or simply 1:1.

• This results in a phenotypic ratio of 1 tall plant to 1 short plant.

Multiple Choice

Based on the cross between a heterozygous tall plant (Tt) and a homozygous short plant (tt), what percentage of the offspring is expected to be short?

• 0%

• 25%

• 100%

• 50%

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Example Crosses: Animal Traits

Guinea Pig Hair

• Short hair (H) is the dominant trait in guinea pigs, while long hair (h) is recessive.

• A purebreeding short-haired guinea pig (HH) is crossed with a long-haired guinea pig (hh).

• All offspring will have the genotype Hh and will have the short hair phenotype.

Mouse Fur

• In mice, grey fur (G) is dominant over the recessive brown fur (g) trait.

• A heterozygous mouse (Gg) is crossed with a mouse that has brown fur (gg).

• The genotypic ratio is 0 GG:2 Gg:2 gg, and the phenotypic ratio is 2 Grey:2 Brown.

Multiple Choice

If a purebreeding short-haired guinea pig (HH) is crossed with a long-haired guinea pig (hh), what will be the phenotype of all the offspring?

• 75% short hair, 25% long hair

• 50% short hair, 50% long hair

• Short hair

• Long hair

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Common Misconceptions in Genetics

Misconception	Correction
Dominant traits are always more common in a population.	Dominance only describes how an allele is expressed, not how frequent it is.
Genotype and phenotype are the same.	Genotype is the set of genes (e.g., Tt), while phenotype is the observable trait (e.g., tall).
An organism showing a dominant trait must be homozygous dominant.	A dominant phenotype can result from either a homozygous dominant (TT) or heterozygous (Tt) genotype.

Multiple Choice

In guinea pigs, short hair (H) is dominant over long hair (h). If a heterozygous short-haired guinea pig (Hh) is crossed with another heterozygous guinea pig (Hh), what is the expected phenotypic ratio for their offspring?

• 1 short : 3 long

• All short hair

• 3 short : 1 long

• 1 short : 1 long

Multiple Choice

A cross between two plants results in offspring with a 50% Tt and 50% tt genotypic ratio. Based on this outcome, what were the genotypes of the parent plants?

• TT and Tt

• Tt and tt

• Tt and Tt

• TT and tt

Multiple Choice

In a species of mouse, grey fur (G) is dominant to brown fur (g). If a grey mouse is crossed with a brown mouse and some of their offspring are brown, what must be the genotype of the grey parent?

• GG

• gg

• It cannot be determined from the information given.

• Gg

Multiple Choice

Imagine you are a genetic counselor. A couple, both heterozygous carriers for cystic fibrosis (Aa), are planning to have children. How would you explain the probability of them having a child with the disease (aa)?

• You would explain that there is a 75% chance with each pregnancy of having a child with the disease.

• You would tell them it's impossible for them to have a child with the disease.

• You would explain that with each pregnancy, there is a 25% chance of having a child with the disease, as predicted by a Punnett square.

• You would tell them it is guaranteed they will have a child with the disease.

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Summary

• Heredity passes traits via genes, and alleles determine genotype and phenotype.

• A dominant allele can mask a recessive allele in the phenotype.

• A monohybrid cross predicts inheritance for one trait using a Punnett square.

• The Law of Segregation and ratios predict the probability of inheriting traits.

Poll

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

• 1

• 2

• 3

• 4