Heredity

Heredity

Middle School

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Learning Objectives

• Explain the roles of DNA, chromosomes, and genes in how traits are inherited.

• Describe how gene mutations can affect an organism's proteins and traits.

• Explain the difference between genotype, phenotype, dominant alleles, and recessive alleles.

• Use Punnett squares to predict the outcomes of specific genetic crosses.

• Interpret cladograms to understand evolutionary relationships between different species.

Key Vocabulary

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Heredity

The process of passing physical and mental traits from parents to their children.

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DNA

Deoxyribonucleic acid is the molecule carrying genetic instructions for an organism's development.

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Gene

A specific section of a DNA molecule that holds instructions for a certain trait.

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Allele

One of the different versions or forms of a particular gene found in DNA.

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Genotype

The set of genes in an organism's DNA that is responsible for a particular trait.

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Phenotype

The observable physical properties or characteristics of an organism, like its appearance and development.

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From Traits to DNA: The Blueprint of Life

• Heredity is how traits pass from parents to their offspring.

• Traits are located on chromosomes, which are made of a molecule called DNA.

• DNA has a special twisted-ladder shape that is called a double helix.

• The DNA rungs are base pairs: A pairs with T, and C pairs with G.

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Multiple Choice

What is the fundamental relationship between traits, chromosomes, and DNA?

• Chromosomes are structures made of DNA that contain the 'factors' for traits.

• DNA is a type of trait passed on by chromosomes.

• DNA is a larger structure that holds many chromosomes.

• Chromosomes and DNA are separate parts of a parent's offspring.

Genes, Proteins, and Traits

• Genes are DNA sections that code for specific proteins.

• Proteins perform essential functions within cells.

• The actions of proteins determine an organism's traits.

Multiple Choice

What is the primary role of a gene?

• It contains the instructions for making a specific protein.

• It directly creates the physical traits of an organism.

• It is a molecule that performs tasks within the cell.

• It is an observable characteristic of an organism.

Mutations and Their Effects

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Beneficial

• These mutations help an organism survive in its environment.

• An example is a color change that provides better camouflage.

• This increases the organism's chances of reproduction and survival.

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Harmful

• Harmful mutations can cause diseases or other health problems.

• They decrease an organism's chances of survival and reproduction.

• A green bunny in a desert is more visible to predators.

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Neutral

• Neutral mutations have no observable effect on the organism.

• They do not help or harm the organism's survival chances.

• A slight, non-advantageous variation in fur color is an example.

Multiple Choice

What determines if a mutation is considered beneficial, harmful, or neutral?

• The mutation's effect on the organism's ability to survive.

• How quickly the mutation appears in a population.

• The size of the gene that the mutation affects.

• Whether the mutation affects a plant or an animal.

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Mendel's Pea Plant Experiments

• Mendel crossed pure-breeding tall and short pea plants (P generation).

• All the offspring in the next generation (F₁) were tall.

• The F₁ generation self-pollinated, creating the F₂ generation.

• This revealed dominant (visible) and recessive (hidden) traits in a 3:1 ratio.

Multiple Choice

Based on Mendel's experiments, what is the definition of a dominant trait?

• The trait that is visible and hides the other trait.

• The trait that is hidden and reappears later.

• A trait that only appears in pure-breeding plants.

• A trait that is a blend of both parents' traits.

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Predicting Traits

• An organism's genotype is its allele combination, determining its physical phenotype.

• A homozygous genotype has two identical alleles for a trait (TT or tt).

• A heterozygous genotype has two different alleles for a trait, like Tt.

• A Punnett square is a grid used to predict genetic cross outcomes.

Multiple Choice

What is the primary purpose of a Punnett square?

• To predict the probability of offspring genotypes and phenotypes.

• To identify the exact number of chromosomes in a cell.

• To show the physical appearance of an adult organism.

• To change the alleles of an organism during its life.

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Understanding Evolutionary Relationships

• Early scientists like Carolus Linnaeus grouped organisms by their physical features.

• Charles Darwin proposed all living things are related through common ancestors.

• Scientists use cladograms to show these evolutionary relationships.

• Cladograms are based on shared traits inherited from a common ancestor.

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Multiple Choice

What is the primary purpose of a cladogram?

• To show the order in which different traits appeared in history.

• To map out the exact locations of ancient animals.

• To prove that one organism evolved directly from another.

• To show which organisms live in the same environment.

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Common Misconceptions

Misconception	Correction
All mutations are harmful.	Mutations can be helpful, harmful, or have no effect at all.
Organisms that look alike are always the most closely related.	DNA analysis reveals true evolutionary relatives, not just physical appearance.
Traits that disappear in one generation are lost forever.	Recessive traits can be carried unseen and reappear in later generations.

Multiple Choice

A mutation in a gene causes a protein to fold into a different shape. How would this most likely affect the organism?

• It could alter the protein's function, leading to a new or modified trait.

• The organism's traits would not change, but the gene would be different.

• The gene would create multiple new proteins to fix the change.

• The protein would function correctly, but the trait would disappear.

Multiple Choice

If an organism has a heterozygous genotype (Tt) for a trait where 'T' (tall) is dominant, what will its phenotype be?

• Tall

• Short

• Medium height

• A mix of tall and short

Multiple Choice

If a homozygous dominant tall plant (TT) is crossed with a homozygous recessive short plant (tt), what is the probability that their offspring will be short?

• 0%

• 25%

• 50%

• 100%

Multiple Choice

A cladogram shows that birds and crocodiles share a more recent common ancestor with each other than either does with lizards. What does this imply?

• Birds and crocodiles share more inherited traits with each other than with lizards.

• Lizards evolved from crocodiles.

• Birds are not related to lizards.

• Crocodiles are the ancestors of birds.

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Summary

• Traits are passed down through genes on chromosomes, which are made of DNA.

• An organism's genotype determines its phenotype; Punnett squares predict trait inheritance.

• A mutation is a change in a gene that can be beneficial, harmful, or neutral.

• Cladograms show evolutionary relationships based on shared, inherited traits.

Poll

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

• 1

• 2

• 3

• 4