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

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

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DNA

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

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Chromosome

A chromosome is a structure found in the cell's nucleus and is made of coiled DNA.

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Gene

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

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Allele

An allele is one of the different versions or forms of a particular gene found in DNA.

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Dominant Allele

A dominant allele is a version of a gene that shows its effect over another allele.

Key Vocabulary

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Recessive Allele

An allele that is expressed as a trait only when its paired allele is identical.

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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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Mutation

A permanent alteration or change in the nucleotide sequence of an organism's DNA or gene.

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Protein

A large, functional molecule made of amino acids that helps to determine an organism's traits.

From Grouping to 'Tree Thinking'

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Early Classification

• Carolus Linnaeus grouped living things based on their physical features in the 1700s.

• He organized organisms by studying their similar and different physical characteristics.

• This system grouped organisms into sets based on their observable traits.

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Darwin's Branching Tree

• Charles Darwin later proposed organizing life into a large branching tree pattern.

• His idea was that all living things are related to each other.

• Each branch point on the tree shows a shared common ancestor.

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Modern Cladograms

• Today, scientists use diagrams called cladograms to show relationships between species.

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

• A trait must come from the most recent common ancestor to be useful.

Multiple Choice

According to the early system developed by Carolus Linnaeus, how were living things primarily grouped?

• By their similar and different physical characteristics

• By their relationship to a common ancestor

• By the environment in which they lived

• By their genetic makeup and DNA

Multiple Choice

How did Charles Darwin's idea of a 'branching tree' represent a major change from Linnaeus's way of grouping organisms?

• It focused only on the physical features of animals, not plants.

• It introduced the idea that all living things are related through common ancestors.

• It organized all life based on diet and habitat.

• It proved that physical characteristics were not important for classification.

Multiple Choice

A scientist is creating a cladogram for a bat, a bird, and a fish. All three have a backbone, but only the bat and the bird have wings. To determine the closest relationship between the bat and the bird, which trait is more useful and why?

• The presence of a backbone, because it is a feature all three animals share.

• The presence of wings, because it comes from a more recent common ancestor for just those species.

• Both traits are equally useful for building the cladogram.

• Neither trait is useful because cladograms only use genetic information.

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Interpreting Cladograms

• A cladogram shows the order that new traits appeared over time.

• Organisms at branching points share a common ancestor, they are not descendants.

• Cladograms are hypotheses about relationships, based on physical traits or DNA evidence.

• This evidence can show surprising links, like elephants, hyraxes, and manatees.

Multiple Choice

What is the primary purpose of a cladogram?

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

• To prove that one organism evolved directly from another

• To show which organisms live in the same environment

• To map out the exact locations of ancient animals

Multiple Choice

When a cladogram shows two organisms are closely related, what does this relationship mean?

• One organism is the direct parent of the other

• They share a common ancestor

• They are guaranteed to look identical

• One organism evolved from the other

Multiple Choice

Since a cladogram is a hypothesis based on evidence, what would likely happen if new DNA evidence was discovered about an organism?

• The cladogram must be incorrect and is thrown out

• The cladogram may be updated to reflect the new evidence

• The new evidence is ignored because the cladogram is a fact

• It proves that elephants are not related to manatees

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From Traits to DNA

• Heredity is the passing of traits from parents to their offspring.

• Mendel’s ‘factors’ for traits are located on structures called chromosomes.

• Chromosomes are made of a molecule called DNA, shaped like a double helix.

• DNA’s rungs have base pairs: Adenine (A) with Thymine (T), and Cytosine (C) with Guanine (G).

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Multiple Choice

What is heredity?

• The passing of traits from parents to their offspring.

• A molecule shaped like a double helix.

• The structures that hold Mendel's 'factors'.

• A pair of bases such as Adenine and Thymine.

Multiple Choice

What is the 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.

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

• DNA is a larger structure that holds many chromosomes.

Multiple Choice

If one side of a DNA rung has the base Adenine (A), what base must be on the opposite side to form a complete pair?

• Guanine (G)

• Cytosine (C)

• Another Adenine (A)

• Thymine (T)

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Genes, Proteins, and Traits

• Each gene has the instructions for making a specific protein.

• Proteins are molecules that do most of the work in cells.

• The structure of a protein determines its specific function in the body.

• The actions of proteins result in an organism’s observable traits.

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Multiple Choice

What is the primary role of a gene?

• It contains the instructions for making a specific protein.

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

• It is an observable characteristic of an organism.

• It directly creates the physical traits of an organism.

Multiple Choice

What is the relationship between a protein's structure and its function?

• The specific function of a protein is determined by its structure.

• All proteins have the exact same structure and function.

• The number of genes determines the function of a protein.

• An organism's traits determine the structure of its proteins.

Multiple Choice

If a gene undergoes a significant change, what is the most likely outcome for the organism?

• A change in the gene could alter the protein's structure, leading to a different trait.

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

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

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

Mutations and Their Effects

• A mutation is a change in the structure of a gene.

• Beneficial mutations can help an organism survive in its environment.

• Mutations can also be harmful, neutral, or have no observable effect.

Multiple Choice

What is a mutation?

• A change in the structure of a gene

• A temporary change in an organism's appearance

• The process of an organism growing older

• A behavior an organism learns from its parents

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

• Whether the mutation affects a plant or an animal

• The size of the gene that the mutation affects

Multiple Choice

A species of moth has a mutation that changes its wing color from light to dark. If the trees in their habitat become darkened by pollution, what would this mutation be considered?

• A beneficial mutation

• A harmful mutation

• A neutral mutation

• A temporary mutation

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Mendel's Experiments and Breakthrough

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

• ​All offspring in the next generation (F₁) were tall, hiding the short trait.

• In the following generation (F₂), the short trait reappeared in a 3:1 ratio.

• He called the visible trait dominant and the hidden trait recessive.

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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 is a blend of both parents

• A trait that only appears in pure-breeding plants

Multiple Choice

Why were all the offspring in the F1 generation tall, even though one of the parent plants was short?

• The tall trait was dominant, so it masked the short trait.

• The short trait was eliminated from the plants' genetics.

• Only tall plants can produce offspring.

• Mendel crossed two tall plants by mistake.

Multiple Choice

What is the best conclusion that can be drawn from the reappearance of the short trait in the F2 generation in a 3:1 ratio?

• The short trait was recessive and was hidden in the F1 generation.

• The 3:1 ratio is a random outcome with no meaning.

• The tall trait becomes weaker in the F2 generation.

• The plants created a new short trait on their own.

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Predicting Traits: Genes and Punnett Squares

• Genes are DNA sections that code for traits; different forms are called alleles.

• An organism’s allele combination is its genotype, which determines its physical phenotype.

• A homozygous genotype has identical alleles (TT, tt); heterozygous has different alleles (Tt).

• Punnett squares predict the probability of offspring genotypes and phenotypes from a cross.

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Multiple Choice

What is the main purpose of using 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.

Multiple Choice

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

• An organism's genotype is its combination of alleles, which determines its physical phenotype.

• An organism's phenotype is its combination of alleles, which determines its genetic genotype.

• An organism's genotype is its physical appearance, while its phenotype is its genetic code.

• An organism's phenotype and genotype are unrelated to its alleles.

Multiple Choice

If an organism's genotype is represented as 'Tt', what can be concluded?

• The organism has a heterozygous genotype.

• The organism has a homozygous genotype.

• The organism's phenotype is recessive.

• The organism's alleles are identical.

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Common Misconceptions

Misconception	Correction
All mutations are harmful.	Mutations can be helpful, harmful, or have no effect.
Organisms that look alike are always the most closely related.	Physical appearance can be misleading; DNA analysis reveals true relatives.
Traits that disappear in one generation are lost forever.	Recessive traits can be carried without being seen and can reappear later.

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Summary

• Cladograms show evolutionary relationships based on heredity, the passing of traits.

• Genes, sections of DNA on chromosomes, code for proteins that determine traits.

• A mutation is a gene change that can have beneficial, harmful, or neutral effects.

• Punnett squares predict an offspring's genotype and phenotype.

Poll

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

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