Inheritance and Genetic Variation

Inheritance and Genetic Variation

Middle School

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Learning Objectives

• Define and tell the difference between an organism's genotype and its phenotype.

• Use Punnett squares to predict the results of genetic crosses between two organisms.

• Tell the difference between Mendelian and non-Mendelian patterns of inheritance.

• Explain how the environment can influence the traits that an organism displays.

Key Vocabulary

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Genetics

The study of how traits are passed from parents to their offspring through genes.

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Allele

A different version of a gene that is responsible for determining a specific genetic trait.

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Genotype

The specific combination of two inherited alleles that determines an organism's unique genetic traits.

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Phenotype

The physical expression or the outward appearance of an organism's specific genetic makeup or genotype.

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Dominant Allele

An allele that completely masks the effect of a recessive allele when it is present.

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Recessive Allele

An allele whose effect is masked whenever the dominant allele is present in an organism.

Key Vocabulary

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Homozygous

Having two of the same forms of an allele for a particular trait.

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Heterozygous

Having two different forms of an allele for a particular trait.

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Punnett Square

A chart used to find and analyze all possible gene combinations of offspring.

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Co-dominance

A pattern where both traits are fully expressed in a heterozygous genotype.

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Incomplete Dominance

A pattern where a heterozygous genotype results in a blended third phenotype.

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Genotypes, Phenotypes, and Alleles

• Alleles are different versions of a gene that determine a specific trait.

• An organism's genotype is its allele combination; the phenotype is its physical trait.

• A dominant allele (S) masks a recessive allele (s) in the phenotype.

• Genotypes SS and Ss show the dominant trait; ss shows the recessive trait.

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Multiple Choice

What is the difference between an organism's genotype and its phenotype?

• A genotype is the allele combination, while the phenotype is the observable physical trait.

• A genotype is the physical trait, while the phenotype is the allele combination.

• A genotype describes a dominant allele, while a phenotype describes a recessive allele.

• A genotype is a single gene, while a phenotype is the entire organism.

Multiple Choice

What is the relationship between a dominant allele and an organism's phenotype?

• It is expressed in the phenotype even if a recessive allele is also present.

• It is only expressed if two dominant alleles make up the genotype.

• It creates the recessive trait in the phenotype.

• It is hidden by the recessive allele in the phenotype.

Multiple Choice

If an organism has a genotype of 'Ss,' what conclusion can be drawn about its physical traits?

• It will show the dominant trait.

• It will show the recessive trait.

• It will show a blend of both the dominant and recessive traits.

• It is impossible to determine the trait from the genotype.

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Predicting Inheritance: Punnett Squares

Monohybrid Cross

• A monohybrid cross is a diagram used to track the inheritance of one single trait.

• It predicts the probability of an offspring inheriting a characteristic from its parents.

• For heterozygous parents, there is a 75% chance of showing the dominant phenotype.

Dihybrid Cross

• A dihybrid cross is a method used to track the inheritance of two different traits.

• This more complex square helps predict how combinations of traits are inherited together.

• For example, it can predict a plant's chances of having both yellow seeds and a tall stem.

Multiple Choice

What is the main purpose of using a Punnett square in genetics?

• To predict the probability of an offspring inheriting traits.

• To show the exact traits an offspring will have.

• To determine the age of the parents.

• To change the genetic traits of an organism.

Multiple Choice

What is the key difference between a monohybrid cross and a dihybrid cross?

• A monohybrid cross tracks one trait, while a dihybrid cross tracks two traits.

• A monohybrid cross is for plants, while a dihybrid cross is for animals.

• A monohybrid cross shows dominant traits, while a dihybrid cross shows recessive traits.

• A monohybrid cross is an exact prediction, while a dihybrid cross is a probability.

Multiple Choice

A scientist wants to predict the chances of a pea plant inheriting both tall stems and purple flowers from its parents. Which tool should the scientist use for this specific prediction?

• A dihybrid cross, because it tracks the inheritance of two different traits together.

• A monohybrid cross, because it is used for predicting characteristics in plants.

• A dihybrid cross, because it guarantees a 75% chance of dominant traits.

• A monohybrid cross, because it tracks the probability of a dominant phenotype.

Beyond Mendel: Other Forms of Inheritance

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Co-dominance

• ​A heterozygous genotype results in both traits being fully and separately expressed.

• ​​Neither of the alleles is dominant or recessive to the other allele.

• ​For example, a chicken with both black and white feathers shows this.

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Incomplete Dominance

• ​This pattern creates a blended phenotype when the genotype is heterozygous.

• ​​The resulting phenotype is a new, third phenotype that is a mixture.

• ​For instance, red and white snapdragons can create pink-flowered offspring.

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Environmental Influences

• ​An organism's environment can directly influence how its genes are expressed.

• ​​Factors like temperature or hormones can change the final phenotype.

• ​For example, temperature can change the fur color of some rabbit species.

Multiple Choice

Which pattern of inheritance results in a blended phenotype, where the heterozygous genotype creates a new, third trait?

• Co-dominance

• Incomplete Dominance

• Environmental Influences

• Recessive Inheritance

Multiple Choice

What is the key difference between co-dominance and incomplete dominance?

• In co-dominance both traits are fully expressed, while in incomplete dominance the traits are blended together.

• In co-dominance the traits are blended, while in incomplete dominance both traits are fully expressed.

• Co-dominance is caused by environmental factors, while incomplete dominance is purely genetic.

• Co-dominance results in a new, third phenotype, while incomplete dominance results in both original phenotypes showing.

Multiple Choice

A certain rabbit species has genes for black or white fur. If a rabbit raised in a cold environment grows white fur, but a genetically similar rabbit raised in a warm environment grows black fur, what is the best explanation?

• The rabbit is heterozygous, so both black and white fur traits are appearing separately.

• The rabbit's genes for black and white fur are blending to create a gray phenotype.

• Environmental factors like temperature can influence how genes for fur color are expressed.

• One allele for fur color is completely dominant over the other allele, regardless of the environment.

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Common Misconceptions

Misconception	Correction
Dominant traits are always more common in a population.	Dominance describes expression, not how common a trait is.
All traits follow simple dominant/recessive patterns.	Many traits are determined by more complex patterns.
An organism's traits are determined only by its genes.	The environment also plays a crucial role in traits.

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Summary

• An organism's genotype is its inherited alleles, which determines its observable phenotype.

• Dominant alleles mask the traits of recessive alleles in heterozygous individuals.

• Co-dominance and incomplete dominance are examples of more complex inheritance patterns.

• An organism's phenotype is a result of its genes and environmental influences.

Poll

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