Genetics

Genetics

Middle School

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Learning Objectives

• Define key genetic terms including gene, allele, genotype, and phenotype.

• Differentiate between dominant and recessive alleles, and homozygous and heterozygous genotypes.

• Use a Punnett square to predict the probable genotypes and phenotypes of offspring.

• Describe patterns of non-Mendelian inheritance such as codominance and incomplete dominance.

Key Vocabulary

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Gene

A segment of DNA that codes for a protein and controls expressed characteristics.

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Alleles

Different variants or forms of a specific gene that determine inherited traits.

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Genotype

The inherited genetic makeup of an organism, represented by its combination of alleles.

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Phenotype

The observable physical appearance or expression of an organism's inherited genotype.

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Dominant Allele

The allele that is expressed as long as just one copy is present.

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Recessive Allele

The allele that is only expressed when no dominant allele is present.

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The Basics of Heredity

• Genetics is the science of heredity, passing traits from parents to their offspring.

• These characteristics, such as eye and hair color, are controlled by genes.

• Chromosomes are thread-like structures made of a chemical called DNA.

• A gene is a segment of DNA that holds instructions for a trait.

Multiple Choice

Which statement correctly describes the relationship between DNA, genes, and chromosomes?

• A gene is a section of DNA, and chromosomes are structures that carry many genes.

• DNA is a section of a gene, and genes are structures that carry chromosomes.

• A chromosome is a section of DNA, and genes are structures that carry DNA.

• Genes and chromosomes are both small sections of DNA.

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Genotype vs. Phenotype

• A genotype is the specific combination of alleles inherited from an organism's parents.

• The phenotype is the observable physical trait, like purple flowers or freckles.

• Dominant alleles are expressed if present; recessive alleles need two copies to show.

• Homozygous means two identical alleles (BB, bb); heterozygous means two different alleles (Bb).

Multiple Choice

An organism has the genotype 'Bb' for a specific trait, where 'B' is the dominant allele. What is the correct description of this organism?

• It is heterozygous and will express the dominant trait.

• It is homozygous and will express the dominant trait.

• It is heterozygous and will express the recessive trait.

• It is homozygous and will express the recessive trait.

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Predicting Inheritance with Punnett Squares

Monohybrid Cross

• A Punnett square is a chart used to predict the inheritance of a single trait from parents to offspring.

• In a cross of two heterozygous parents (Bb x Bb), the typical genotype ratio is 1 BB : 2 Bb : 1 bb.

• This results in a phenotype ratio where 3 offspring show the dominant trait and 1 shows the recessive trait.

Dihybrid Cross

• A dihybrid cross is used to study the inheritance of two different traits at the same time.

• This analysis uses a larger 16-square Punnett square to show all possible allele combinations for the offspring.

• When crossing two heterozygous parents, the typical phenotype ratio in the offspring is 9:3:3:1.

Multiple Choice

In a monohybrid cross between two heterozygous parents (Bb), what is the probability that an offspring will have the homozygous recessive genotype (bb)?

• 25%

• 50%

• 75%

• 0%

Non-Mendelian Inheritance

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Codominance

• ​Both alleles are expressed equally and distinctly in the organism's phenotype.

• ​​For example, a flower shows separate red and white petals.

• ​The alleles for red and white colors are both contributing.

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Incomplete Dominance

• ​This occurs when one allele is not completely dominant over another.

• ​​The result is a blended or intermediate physical trait or phenotype.

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

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Polygenic Characteristics

• ​These are specific traits that are controlled by more than one gene.

• ​​This allows for a wide range of continuous variation in organisms.

• ​Human skin color and height are examples of polygenic characteristics.

Multiple Choice

A flower with a red-petal allele and a white-petal allele produces pink petals. This is an example of what kind of inheritance?

• Incomplete Dominance

• Codominance

• Mendelian Dominance

• Polygenic Inheritance

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Common Misconceptions

Misconception	Correction
Dominant traits are always more common in a population.	Dominance is about how an allele is expressed, not how common it is.
All traits are determined by a single gene.	Many traits are polygenic, controlled by multiple genes working together.
An offspring is always a perfect blend of its parents' traits.	The combination of alleles an offspring inherits is random.

Multiple Choice

If brown eyes (B) are dominant to blue eyes (b), why can two brown-eyed parents have a blue-eyed child?

• Both parents must be heterozygous (Bb), each carrying the recessive allele.

• Eye color is a learned trait, not an inherited one.

• One of the parents must have been homozygous recessive (bb).

• This is only possible through codominance, with both traits appearing.

Multiple Choice

A scientist crosses a black chicken with a white chicken, and all the offspring are speckled with both black and white feathers. How does this inheritance pattern differ from simple Mendelian dominance?

• It is codominance, where both alleles are fully and separately expressed, unlike in simple dominance where one is hidden.

• It is incomplete dominance, where the traits blend to make gray feathers.

• It follows simple dominance, but the speckled trait is a new mutation.

• It is a polygenic trait that is controlled by many different genes.

Multiple Choice

In pea plants, tall (T) is dominant to short (t). If you use a Punnett square to cross a heterozygous tall plant (Tt) with a short plant (tt), what is the predicted ratio of tall to short offspring?

• 1 Tall : 1 Short (50% Tt and 50% tt)

• 3 Tall : 1 Short

• All offspring will be tall.

• All offspring will be short.

Multiple Choice

Analyze the genetic possibilities for a child whose mother has the genotype 'AA' and whose father has the genotype 'aa'. What can be concluded about the child's genotype and phenotype if 'A' is a dominant allele?

• The child must have the genotype 'Aa' and will express the dominant phenotype.

• The child has a 50% chance of having genotype 'AA' and a 50% chance of 'aa'.

• The child must have the genotype 'aa' and will express the recessive phenotype.

• The child's genotype will be 'AA', but their phenotype will be recessive.

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Summary

• Genetics is the study of how genes pass traits from parents to offspring.

• An organism's genotype determines its phenotype, or observable traits.

• Punnett squares are used to predict the probability of offspring inheriting specific traits.

• Some traits are controlled by multiple genes or show codominance or incomplete dominance.

Poll

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

• 1

• 2

• 3

• 4