Mendelian Genetics

Mendelian Genetics

Middle School

​

Learning Objectives

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

• Explain the difference between dominant and recessive alleles.

• Use a Punnett square to predict the traits of offspring.

• Distinguish between Mendelian and non-Mendelian inheritance patterns.

Key Vocabulary

​

Gene

A gene is a section of DNA that holds the instructions for building specific proteins.

​

Allele

An allele is one of the different forms or variations of a particular gene.

​

Genotype

The genotype is the specific combination of alleles that an organism has for a trait.

​

Phenotype

The phenotype is the observable physical trait that results from an organism's genotype, like eye color.

​

Dominant Allele

This allele's trait is always expressed in the phenotype if at least one copy is present.

​

Recessive Allele

This allele’s trait is only expressed when two copies are present in the organism's genotype.

​

​

The Basics of Genetics

• Gregor Mendel discovered genes control traits passed from parents to offspring.

• An organism’s genotype is its genetic makeup; its phenotype is its appearance.

• A dominant allele (B) is expressed over a recessive allele (b).

• Organisms are homozygous (BB, bb) or heterozygous (Bb) for each trait.

​

Multiple Choice

An organism has the genotype 'Bb'. What is the correct terminology for this genotype and which allele will be expressed in the phenotype?

• Homozygous, and the recessive allele 'b' will be expressed.

• Heterozygous, and the dominant allele 'B' will be expressed.

• Homozygous, and the dominant allele 'B' will be expressed.

• Heterozygous, and the recessive allele 'b' will be expressed.

​

​

The Punnett Square

Monohybrid Cross

• A Punnett square predicts the genotypes and phenotypes of offspring from a genetic cross.

• A monohybrid cross tracks the inheritance of a single trait, such as flower color.

• The resulting phenotypic ratio between two heterozygous parents is typically 3 dominant to 1 recessive.

Dihybrid Cross

• A dihybrid cross tracks the inheritance of two different traits at the same time.

• This cross requires a larger 4x4 Punnett square to show all possible offspring allele combinations.

• The typical phenotypic ratio for this cross between two heterozygous parents is 9:3:3:1.

Multiple Choice

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

• 75%

• 50%

• 25%

• 0%

Non-Mendelian Inheritance

​

Codominance

• A phenotype where both alleles for a gene are expressed equally and distinctly.

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

• For example, a flower shows both red and white petals at once.

​

Incomplete Dominance

• This occurs when one allele is not completely dominant over the other allele.

• This results in a third phenotype that is a blend of the two.

• A red and a white flower can produce a new pink flower.

​

Polygenic Traits

• These are characteristics that are controlled by more than one single gene.

• Polygenic traits often show a wide range of variations in a population.

• Examples in humans include skin color, eye color, and overall height.

Multiple Choice

If a flower with red petals is crossed with a flower with white petals and produces offspring with pink petals, what type of inheritance is this?

• Simple Dominance

• Codominance

• Incomplete Dominance

• Polygenic Inheritance

​

Common Misconceptions

Misconception	Correction
Dominant traits are always the most common.	Dominance is about trait expression, not its frequency.
Offspring are a perfect blend of their parents' traits.	Traits are inherited as discrete units (genes), not blended.
All genetic traits follow simple dominant/recessive rules.	Many traits are complex, involving codominance or other patterns.

Multiple Choice

In pea plants, purple flowers (P) are dominant to white flowers (p). If a heterozygous purple-flowered plant (Pp) is crossed with a white-flowered plant (pp), what is the probability that the offspring will have white flowers?

• 0%

• 25%

• 50%

• 75%

Multiple Choice

How does the expression of alleles in codominance differ from incomplete dominance?

• In codominance both alleles are expressed equally; in incomplete dominance the phenotype is a blend of the two alleles.

• In codominance the phenotype is a blend; in incomplete dominance both alleles are expressed separately.

• In codominance only the dominant allele is expressed; in incomplete dominance both alleles are expressed.

• There is no difference; they are the same type of inheritance.

Multiple Choice

A tall pea plant (dominant trait) is crossed with a short pea plant (recessive trait). Half of the offspring are tall and half are short. Based on this outcome, what must the genotype of the tall parent be?

• It must be homozygous dominant (TT).

• It must be heterozygous (Tt).

• It must be homozygous recessive (tt).

• It is impossible to determine from the information given.

Multiple Choice

Two parents both have brown eyes (a dominant trait). They have a child with blue eyes (a recessive trait). How is this possible?

• This is not possible; one parent must have had blue eyes.

• One parent was homozygous dominant (BB) and the other was heterozygous (Bb).

• Both parents were homozygous dominant (BB).

• Both parents were heterozygous (Bb), carrying the recessive allele for blue eyes.

​

Summary

• Gregor Mendel established that genes, in different forms called alleles, determine traits.

• An organism's specific combination of alleles is its genotype, which determines the phenotype.

• Dominant alleles mask recessive alleles, and Punnett squares help predict offspring traits.

• Codominance and incomplete dominance are more complex non-Mendelian inheritance patterns.

Poll

On a scale of 1-4, how confident are you about the concepts covered in today's review?

• 1

• 2

• 3

• 4