MS-LS3-2: Reproduction and Genetic Variation

MS-LS3-2

Reproduction and Genetic Variation

Middle School

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Learning Objectives

• Describe the difference between sexual and asexual reproduction.

• Explain why sexual reproduction leads to genetic variation in offspring.

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

• Use a Punnett square to show how genes are passed to offspring.

Key Vocabulary

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Heredity

Heredity is the process of passing physical or mental traits from parents to their children.

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Gene

A gene is a basic unit of heredity that is passed down from a parent to their offspring.

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Allele

Alleles are the different forms or variations that a single gene can have for a trait.

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Sexual Reproduction

Sexual reproduction is when two parents contribute genetic information to create a unique offspring.

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Asexual Reproduction

Asexual reproduction involves a single parent producing offspring that are genetically identical to itself.

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Genetic Variation

Genetic variation describes the natural differences in DNA found among individuals within a species.

Key Vocabulary

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Chromosome

A chromosome is a structure inside the cell's nucleus that is made up of DNA and holds genes.

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Dominant Allele

A dominant allele is a version of a gene that shows its effect even if only one copy is present.

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Recessive Allele

A recessive allele is a version of a gene that only shows its effect when two copies are present.

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Genotype

The genotype is the specific combination of alleles an organism has for a particular trait, for example, Ll.

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Phenotype

The phenotype is the observable physical characteristic of an organism, such as its height, hair, or eye color.

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Homozygous

Homozygous describes having two of the same alleles for a particular gene, such as either LL or ll.

Key Vocabulary

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Heterozygous

This term describes having two different alleles for a specific genetic trait, like 'Ll'.

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Punnett Square

This is a diagram used to predict the possible genotypes of offspring from a genetic cross.

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Reproduction and Genetic Variation

Asexual Reproduction

• A single parent passes all its genetic information to the offspring.

• The offspring are genetically identical to the parent, much like a clone.

• This process results in very little genetic variation among the offspring.

Sexual Reproduction

• Two parents each contribute half of the genetic information to an offspring.

• The offspring inherits a unique combination of genes from both of the parents.

• This process creates genetic variation, making offspring different from their parents.

Multiple Choice

Which statement accurately describes asexual reproduction?

• A single parent passes all of its genetic information to the offspring.

• Two parents each contribute half of the genetic information to an offspring.

• The offspring is genetically different from the parent.

• The offspring inherits a unique combination of genes.

Multiple Choice

What is the primary difference between the offspring from sexual and asexual reproduction?

• Sexual reproduction results in genetic variation, while asexual reproduction results in genetically identical offspring.

• Asexual reproduction requires two parents, while sexual reproduction only requires one.

• Sexual reproduction produces offspring that are clones of the parent.

• Asexual reproduction creates a unique combination of genes from two parents.

Multiple Choice

If a species' environment changes suddenly, why might a species that uses sexual reproduction have an advantage over a species that uses asexual reproduction?

• The genetic variation from sexual reproduction increases the chance that some individuals will be able to survive the change.

• Having two parents provides the offspring with more protection in a changing environment.

• Asexually produced offspring are identical and therefore stronger than genetically different offspring.

• A single parent in asexual reproduction can produce offspring more quickly to adapt to the change.

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Mendel's Pea Plant Experiments

• Mendel created pure-breeding parent plants that only produced offspring like themselves.

• He crossed pure-breeding tall and short plants, and all offspring were tall.

• Next, he crossed these new tall offspring with each other.

• The short trait reappeared in a ratio of 3 tall to 1 short.

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Multiple Choice

What is a key characteristic of a pure-breeding plant?

• It only produces offspring like itself.

• It is a mix of tall and short traits.

• It cannot be crossed with other plants.

• It always produces more short offspring than tall.

Multiple Choice

What happened when Mendel first crossed pure-breeding tall plants with pure-breeding short plants?

• All of the offspring were tall.

• All of the offspring were short.

• The offspring were a mix of tall and short.

• The offspring were of medium height.

Multiple Choice

What did the results of Mendel's second cross, where the short trait reappeared, reveal about how traits are passed on?

• A trait can be hidden in one generation and reappear in the next.

• Tall plants can only produce other tall plants.

• Crossing two tall plants always results in a 3:1 ratio.

• The short trait is the most common trait in pea plants.

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Dominant and Recessive Alleles

• Genes determine traits; you get two alleles for each gene.

• When alleles differ, the dominant one is expressed over the recessive one.

• A recessive trait only appears if an organism inherits two recessive alleles.

• This explains traits skipping generations and the 3:1 experimental ratio.

Multiple Choice

What happens when an organism has one dominant allele and one recessive allele for a specific trait?

• The dominant allele is expressed.

• The recessive allele is expressed.

• Both alleles are expressed equally.

• Neither allele is expressed.

Multiple Choice

Under what condition will a recessive trait appear in an organism?

• When the organism inherits one dominant and one recessive allele.

• When the organism inherits two recessive alleles for that trait.

• When the organism's parents both showed the dominant trait.

• When the trait is beneficial for survival.

Multiple Choice

Which statement provides the best explanation for how a trait can appear in a grandparent and a grandchild, but not in the parent?

• The parent carried a recessive allele that was masked by a dominant allele.

• The trait was learned by the grandchild from the grandparent.

• The grandparent's dominant allele became recessive in the parent.

• Genes for traits can disappear and then reappear randomly.

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Predicting Traits with Punnett Squares

• A Punnett square predicts offspring genotypes (Tt) and phenotypes (tall/dwarf) from sexual reproduction.

• A capital letter (T) is used for the dominant allele, lowercase (t) for recessive.

• An organism’s genotype can be homozygous (TT, tt), heterozygous (Tt).

• A cross of two heterozygous parents (Ttx Tt) results in a 3:1 ratio.

Multiple Choice

What is the primary purpose of a Punnett square?

• To predict the potential genotypes and phenotypes of offspring

• To determine the exact traits an organism will have

• To show which traits are dominant over others

• To count the number of alleles an organism has

Multiple Choice

How is a homozygous genotype different from a heterozygous genotype?

• A homozygous genotype has two identical alleles (LL or ll), while a heterozygous genotype has two different alleles (Ll).

• A homozygous genotype has only dominant alleles, while a heterozygous genotype has only recessive alleles.

• A homozygous genotype determines phenotype, while a heterozygous genotype determines the number of offspring.

• A homozygous genotype is represented by a single letter, while a heterozygous genotype is represented by two letters.

Multiple Choice

If two heterozygous parents (Ll) are crossed, what conclusion can be drawn about the likely appearance of their offspring?

• Three of the offspring are likely to show the dominant trait, while one is likely to show the recessive trait.

• All of the offspring will show the dominant trait.

• Half of the offspring will be homozygous and half will be heterozygous.

• All of the offspring will have the same genotype as the parents.

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Common Misconceptions About Heredity

Misconception	Correction
A dominant trait is always more common or better.	Dominant means the trait is expressed if the allele for it is present.
Offspring traits are a direct blend of their parents' traits, like paint.	Traits are passed as separate units (alleles) that do not blend together.
Offspring get all their traits from only one parent.	Offspring inherit a unique mix of genes from both parents.
Every trait is determined by just a single gene.	Most traits, like height, are influenced by many genes working together.

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Summary

• Sexual reproduction causes genetic variation; asexual reproduction creates identical offspring.

• Traits come from genes (alleles), with one allele from each parent.

• Dominant alleles can hide recessive ones, which can skip a generation.

• Punnett squares are models used to predict the traits of offspring.

Poll

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

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