Genetics and Heredity

Genetics and Heredity

Middle School

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Learning Objectives

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

• Use a Punnett square to predict the outcomes of a monohybrid cross.

• Explain how dominant and recessive alleles determine an organism's physical traits.

• Differentiate between Mendelian and non-Mendelian inheritance like codominance and incomplete dominance.

Key Vocabulary

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Gene

A segment of DNA with instructions for building a protein, which controls a specific inherited trait.

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Alleles

Different versions of a specific gene that are responsible for an organism's characteristics.

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Genotype

An organism’s specific inherited combination of alleles, representing its complete and entire genetic makeup.

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Phenotype

The observable physical appearance or the expressed traits of an organism, which result from its genotype.

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Dominant Allele

This is the allele that is expressed as long as at least one copy is present.

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Recessive Allele

This is the allele that is only expressed when two copies are present in the genotype.

Key Vocabulary

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Homozygote

An organism that has inherited two identical alleles for a particular gene, either BB or bb.

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Heterozygote

An organism that has inherited two different alleles for a particular gene, such as Bb.

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Punnett Square

A chart used to determine the probability of different genotypes in the offspring of two parents.

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Foundations of Heredity

• Heredity passes traits from parents to offspring; its study is called genetics.

• Gregor Mendel discovered that some traits are dominant over recessive ones.

• A gene is a segment of DNA for a trait; its forms are alleles.

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Multiple Choice

What is heredity?

• A segment of DNA that codes for a trait

• The passing of traits from parents to offspring

• The different forms a gene can have

• The study of how traits are passed on

Multiple Choice

What is the relationship between a gene and an allele?

• They are two different words for the exact same thing.

• An allele is a specific form or version of a gene.

• A gene is a segment of DNA, and an allele is a segment of a gene.

• A gene is made up of many different alleles.

Multiple Choice

If a plant has one allele for purple flowers (a dominant trait) and one for white flowers (a recessive trait), what is the most likely outcome?

• The plant will have flowers with no color.

• The plant will have white flowers.

• The plant will have purple flowers.

• The plant will have a mix of purple and white flowers.

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Expressing Traits: Genotype vs. Phenotype

• The genotype is an organism's inherited combination of alleles (e.g., BB, Bb, bb).

• The phenotype is the physical trait expressed due to the genotype (e.g., eye color).

• A homozygous genotype has two of the same alleles, either dominant (BB) or recessive (bb).

• A heterozygous genotype (Bb) has different alleles and shows the dominant trait.

Multiple Choice

Which statement best describes the relationship between an organism's genotype and its phenotype?

• The phenotype is the organism's genetic code, and the genotype is the physical trait.

• The genotype and phenotype are both determined only by dominant alleles.

• The phenotype determines the combination of alleles that make up the genotype.

• The genotype is the inherited combination of alleles, and the phenotype is the physical trait that is expressed.

Multiple Choice

Under what condition is a recessive phenotype expressed in an organism?

• When the genotype is heterozygous (Bb).

• Only when two recessive alleles are inherited by the organism (bb).

• Whenever a recessive allele is present in the genotype.

• When the genotype has at least one dominant allele.

Multiple Choice

A plant that can have purple flowers (dominant) or white flowers (recessive) shows the purple flower phenotype. What can be concluded about its genotype?

• Its genotype cannot be determined from its phenotype.

• Its genotype must be homozygous recessive (pp).

• Its genotype must be heterozygous (Pp).

• Its genotype could be either homozygous dominant (PP) or heterozygous (Pp).

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Predicting Outcomes: The Punnett Square

• A Punnett square is a chart that predicts an offspring's inherited traits.

• In a monohybrid cross, one parent's alleles are on top, the other's left.

• Each box shows a possible genotype by combining the parent alleles.

• A Bb x Bb cross results in a 3:1 dominant to recessive phenotype ratio.

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Multiple Choice

What is the primary function of a Punnett square?

• To predict the inherited traits of potential offspring.

• To map the location of alleles on a chromosome.

• To list all the genes an animal has.

• To show how an organism grows over time.

Multiple Choice

In a Punnett square, how are the possible genotypes of offspring determined?

• By looking at the parents' physical traits.

• By randomly selecting from a list of common genotypes.

• By combining the alleles from each parent in the chart's boxes.

• By counting the number of dominant alleles each parent has.

Multiple Choice

A cross between two 'Bb' parents results in a 3:1 ratio of dominant to recessive phenotypes. Which statement provides the best explanation for this outcome?

• Recessive alleles are more common than dominant alleles.

• Three of the four possible genotypes result in a dominant phenotype.

• The offspring have four completely different genotypes.

• One parent passes on only dominant alleles.

Beyond Mendel: Non-Mendelian Inheritance

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Incomplete Dominance

• The heterozygous phenotype is a blend of two homozygous phenotypes.

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

• Neither of the parent alleles is expressed fully in the offspring.

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Codominance

• Both alleles are expressed equally and separately in the heterozygote.

• A flower can show distinct patches of red and white petals.

• This means both red and white alleles are fully expressed.

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Polygenic Characteristics

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

• This results in a wide, continuous range of possible phenotypes.

• Human skin color, eye color, and height are all examples.

Multiple Choice

In inheritance patterns like incomplete dominance and codominance, how does the appearance of a heterozygous offspring compare to its homozygous parents?

• It can switch between the two homozygous parent phenotypes.

• It looks identical to one of the homozygous parents.

• It shows a phenotype that is different from either homozygous parent.

• It is always smaller than both homozygous parents.

Multiple Choice

What is the key difference between how traits are expressed in incomplete dominance versus codominance?

• In incomplete dominance both alleles are expressed, while in codominance they blend.

• Incomplete dominance is for flower colors, while codominance is for animal fur.

• In incomplete dominance alleles blend, while in codominance both alleles are expressed separately.

• Incomplete dominance is controlled by one gene, while codominance is controlled by many.

Multiple Choice

A scientist observes that the height of a particular plant species shows a wide, continuous range of variations, rather than just 'short' or 'tall'. Which genetic principle best explains this observation?

• Incomplete dominance, because the heights are a blend.

• Mendelian dominance, because there is a dominant height.

• Polygenic characteristics, because the trait results from more than one gene.

• Codominance, because different heights are expressed.

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Common Misconceptions

Misconception	Correction
Dominant traits are always the most common.	A trait's frequency depends on the mix of alleles in a population.
An organism's genotype and phenotype are the same.	Genotype is the genetic code; phenotype is the observable trait.
A recessive allele is 'weaker' and can disappear.	A recessive allele can be masked but is still passed on.
All traits are determined by a single gene.	Many traits are polygenic, controlled by multiple genes.

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Summary

• Heredity is the passing of traits through genes, which are different forms called alleles.

• An organism's genotype is its genetic code, which determines its observable phenotype.

• Punnett squares help predict the chances of offspring inheriting specific traits.

• Some traits show complex patterns like codominance or incomplete dominance.

Poll

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

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