Genetics and Punnett Squares

Genetics and Punnett Squares

Middle School

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Learning Objectives

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

• Explain dominant vs. recessive alleles and homozygous vs. heterozygous genotypes.

• Use a Punnett square and probability to predict genetic traits in offspring.

• Distinguish between Mendelian and non-Mendelian inheritance patterns.

Key Vocabulary

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Gene

A gene is a segment of DNA that provides instructions for a specific trait or characteristic.

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Allele

An allele is a different form of a gene. We inherit two alleles for each trait.

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Genotype

The genotype is the specific combination of alleles an organism inherits for a particular trait.

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Phenotype

The phenotype refers to the observable physical traits of an organism, determined by its genotype.

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Punnett Square

A Punnett square is a chart used to predict the likely genotypes and phenotypes of offspring.

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Heredity

Heredity is the process of passing genetic traits and information from parents to their offspring.

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Fundamental Genetic Concepts

• Genes are DNA segments that act as instructions for an organism's traits.

• Alleles are different versions of a gene, with one inherited from each parent.

• Your genotype is the specific combination of alleles you have inherited.

• The phenotype is the observable physical trait that your genotype produces.

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Multiple Choice

What is the term for the physical expression of an organism's genes, such as its eye color or height?

• Genotype

• Allele

• Phenotype

• Gene

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Dominant and Recessive Alleles

Dominant Allele

• A dominant allele is expressed as a trait as long as one copy is present in the genotype.

• This type of allele is represented by a capital letter, such as B.

• An organism that is heterozygous (Bb) is called a ‘hybrid’ and shows the dominant trait.

Recessive Allele

• A recessive allele is only expressed when two copies are present without a dominant allele.

• This type of allele is represented by a lowercase letter, such as b.

• An organism that is homozygous recessive (bb) is ‘purebred’ and shows the recessive trait.

Multiple Choice

An organism has one dominant allele and one recessive allele for a trait (Bb). What is its genotype called?

• Homozygous dominant

• Homozygous recessive

• Heterozygous

• Phenotype

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Mendel's Legacy & Punnett Squares

Gregor Mendel

• ​The basic principles of inheritance were first discovered by an Austrian monk named Gregor Mendel.

• ​​His work explains how specific traits are passed down from one generation to the next one.

• ​Mendel's discoveries allow us to predict the likely characteristics of offspring based on their parents.

Punnett Squares

• ​A Punnett square is a simple chart that helps us visualize and predict genetic outcomes.

• ​​One parent's genotype is written along the top, and the other parent's is on the side.

• ​Each box shows a possible combination of alleles that an offspring could potentially inherit from them.

Multiple Choice

What is the primary purpose of using a Punnett square?

• To show the exact genetic makeup of parents

• To determine the probability of genotypes and phenotypes in offspring

• To map the entire genome of an organism

• To study environmental impacts on genes

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Using a Monohybrid Cross

• A monohybrid cross focuses on a single characteristic, like flower color.

• Each square in the Punnett square represents a 25% probability for a genotype.

• For a Bb x Bb cross, genotypes are 25% BB, 50% Bb, and 25% bb.

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

Multiple Choice

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

• 75%

• 50%

• 25%

• 0%

Beyond Simple Dominance

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Incomplete Dominance

• ​Occurs when one allele is not completely dominant over the other.

• ​​This results in a blended or intermediate physical trait.

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

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Codominance

• ​Both alleles in a gene pair are expressed fully and equally.

• ​​The traits do not blend together in the offspring.

• ​An example is a flower with both red and white petals.

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Polygenic Traits

• ​These are traits that are controlled by more than one gene.

• ​​This results in a wide and continuous range of variations.

• ​Human height and skin color are classic examples of this.

Multiple Choice

When a red-flowered plant and a white-flowered plant produce an offspring with pink flowers, this is an example of what type of inheritance?

• Codominance

• Incomplete Dominance

• Simple Dominance

• Polygenic Inheritance

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Common Misconceptions

Misconception	Correction
A dominant trait is always the most common.	Dominance describes allele expression, not its frequency in a population.
Every trait is controlled by a single gene.	Many traits are polygenic, influenced by multiple genes.
Genotype and phenotype are the same.	Genotype is the genetic code; phenotype is the physical trait.
An organism's traits are determined only by its genes.	The environment can interact with genes to influence the phenotype.

Multiple Choice

Why can an organism with a heterozygous genotype (Hh) express the dominant trait (tall) but not the recessive one (short)?

• Because the recessive allele is stronger than the dominant allele.

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

• Because two recessive alleles are needed for the trait to be expressed.

• Because the environment changes the expression of the gene.

Multiple Choice

If brown fur (B) is dominant to white fur (b) in rats, what percentage of offspring from a Bb x Bb cross are expected to have brown fur?

• 25%

• 50%

• 75%

• 100%

Multiple Choice

A flower breeder wants to produce only blue-flowered plants, which is a recessive trait (bb). What genotypes must the parent plants have to guarantee that all offspring have blue flowers?

• BB and bb

• Bb and Bb

• bb and bb

• BB and Bb

Multiple Choice

In snapdragon flowers, petal color shows incomplete dominance. A red-flowered plant (RR) is crossed with a white-flowered plant (WW). What phenotype is expected in their offspring (RW)?

• Red flowers

• White flowers

• Red and white spotted flowers

• Pink flowers

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Summary

• Heredity is how traits are passed down through genes and alleles.

• An organism's genotype determines its phenotype; dominant alleles can mask recessive ones.

• Punnett squares are a tool to calculate the probability of inheriting specific traits.

• Inheritance can be complex, involving codominance, incomplete dominance, and polygenic traits.

Poll

On a scale of 1-4, how confident are you about using a Punnett square to predict genetic outcomes?

• 1

• 2

• 3

• 4