Traits and Reproduction

Traits and Reproduction

Middle School

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Learning Objectives

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

• Explain the difference between Mendelian and Non-Mendelian inheritance patterns.

• Use a Punnett square to predict the outcomes of genetic crosses.

• Describe how special traits affect the patterns of inheritance in organisms.

Key Vocabulary

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Gene

A gene is a part of DNA that holds instructions for building proteins and determining your traits.

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Alleles

Alleles are the different versions of a single gene that are responsible for our unique traits.

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Genotype

Your genotype is your unique genetic code, the specific combination of alleles you inherited from parents.

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Phenotype

A phenotype is the collection of your observable traits, like your eye color and hair color.

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Punnett Square

A Punnett Square is a special chart used to predict the possible genotypes of offspring.

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Dominant Allele

A dominant allele is a powerful version of a gene that always shows its trait.

Genotype and Phenotype

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Homozygous Dominant (BB)

• ​The organism has two copies of the dominant allele.

• ​​The genotype, or genetic makeup, is represented as BB.

• ​The phenotype is the physical expression of the dominant trait.

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Heterozygous (Bb)

• ​The organism has one dominant and one recessive allele.

• ​​The genotype for this combination is represented as Bb.

• ​The dominant allele's trait is expressed in the phenotype.

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Homozygous Recessive (bb)

• ​The organism has two copies of the recessive allele.

• ​​The genotype for this combination is represented as bb.

• ​The recessive allele's trait is expressed in the phenotype.

Multiple Choice

If brown eyes (B) are dominant over blue eyes (b), which genotype would result in a blue-eyed phenotype?

• B

• BB

• Bb

• bb

Mendelian Inheritance: The Monohybrid Cross

• A Punnett square predicts genotypes for a single characteristic.

• Parent alleles are placed on the outside and combined in the inner squares.

• This cross of two heterozygous parents results in a 3:1 phenotypic ratio.

Multiple Choice

In a monohybrid cross between two heterozygous parents (Bb), what is the probability of the offspring showing the dominant phenotype?

• 100%

• 75%

• 25%

• 50%

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Predicting Multiple Traits: The Dihybrid Cross

• Mendelian inheritance can track two independent traits, known as a dihybrid cross.

• A larger Punnett square is used to show all allele combinations.

• The 9:3:3:1 ratio is a common outcome for heterozygous parents.

• This ratio shows the probabilities of the four possible phenotype combinations.

Multiple Choice

What is the typical phenotypic ratio for a dihybrid cross between two parents who are heterozygous for both traits?

• 1:1:1:1

• 9:3:3:1

• 3:1

• 1:2:1

Beyond Simple Inheritance

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Codominance

• Both traits are fully and equally expressed in the offspring.

• Neither of the two alleles is dominant over the other one.

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

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Incomplete Dominance

• The dominant allele does not fully mask the recessive allele.

• Offspring show a blended phenotype that is between the parents.

• Red and white flowers can produce a pink flower offspring.

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Polygenic Traits

• These traits are controlled by more than just one gene.

• They show a very wide range of different continuous variations.

• Human height and our skin color are good examples here.

Multiple Choice

A flower with a red-petal allele and a white-petal allele produces pink petals. What pattern of inheritance does this describe?

• Polygenic Characteristic

• Incomplete Dominance

• Mendelian Dominance

• Codominance

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Sources of New Traits

New Gene Combinations

• New traits can arise when genes from two parents combine in new ways during reproduction.

• This process shuffles existing genes, creating a unique genetic combination in the offspring.

• The environment can also influence how these inherited traits are expressed in an individual.

Gene Mutations

• A gene mutation is a permanent change that occurs in the DNA sequence of a cell.

• For a mutation to be passed on, it must occur in a reproductive cell like sperm or egg.

• This can introduce a completely new trait that did not exist in the parents' genes.

Multiple Choice

Besides new combinations of genes, what is another source of new inheritable characteristics?

• The recessive genotype

• The dominant phenotype

• Environmental interaction

• Gene mutations in reproductive cells

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Common Misconceptions

Misconception	Correction
Dominant traits are always the most common ones.	Dominance is about expression, not how common a trait is.
All traits are inherited in a simple pattern.	Many traits are complex and controlled by multiple genes.
Traits are determined only by an organism's genes.	The environment also interacts with genes to influence traits.

Multiple Choice

Why does a heterozygous individual (like Bb) express the dominant trait instead of the recessive one?

• Because recessive traits are only expressed in non-Mendelian inheritance.

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

• Because the recessive allele changes the dominant allele.

• Because heterozygous means having two dominant alleles.

Multiple Choice

In a cross between a homozygous dominant parent (BB) and a homozygous recessive parent (bb), what is the probability of the offspring having a heterozygous genotype (Bb)?

• 100%

• 50%

• 25%

• 0%

Multiple Choice

A cattle breeder crosses a white cow with a red bull. The resulting calf has a coat with patches of red and patches of white. Analyze this outcome and contrast it with what you would expect from incomplete dominance.

• This is codominance; incomplete dominance would result in a solid pinkish or roan coat.

• This is codominance; incomplete dominance would also result in red and white patches.

• This is incomplete dominance; codominance would result in a solid red coat.

• This is simple dominance; incomplete dominance would result in a white coat.

Multiple Choice

Human skin color is a trait that shows wide continuous variation and is influenced by multiple genes. What pattern of inheritance does this suggest and how could it lead to a new, inheritable skin tone?

• Polygenic inheritance; a new inheritable tone could result from a mutation in one of the controlling genes.

• Codominance; a new tone appears when two different skin color alleles are equally expressed.

• Incomplete dominance; a new tone is created by blending the parents' skin tones in every generation.

• Monohybrid inheritance; a new tone can only appear if the environment changes.

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Summary

• An organism's genotype is its inherited alleles, and its phenotype is its observable trait.

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

• Inheritance can follow simple Mendelian patterns or more complex non-Mendelian patterns.

• New traits can arise from gene mutations or be influenced by the environment.

Poll

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

• 1

• 2

• 3

• 4