Patterns of Inheritance

Patterns of Inheritance

Middle School

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Learning Objectives

• Define genes, alleles, and chromosomes, and their roles in heredity.

• Explain Mendel's Laws and use Punnett squares to predict genetic outcomes.

• Differentiate between incomplete dominance, codominance, and sex-linked traits.

• Analyze a pedigree to determine the most likely pattern of inheritance.

Key Vocabulary

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Gene

A DNA segment providing instructions for a protein that influences a specific trait.

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Allele

An allele is a specific version or variant of a gene, such as 'A' or 'a'.

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Genotype

The combination of alleles an individual possesses for a particular gene, like AA.

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Phenotype

The observable physical or biochemical characteristics of an individual, such as flower color.

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Homozygous

This describes having two identical alleles for a particular gene, for instance, AA or aa.

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Heterozygous

This describes having two different alleles for a particular gene, for example, the genotype Aa.

Key Vocabulary

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Gametes

Gametes are reproductive cells, like sperm and eggs, that carry only one allele per gene.

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Pedigree

A pedigree is a chart that shows the presence or absence of a trait within a family.

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The Core Building Blocks of Heredity

• Genes are DNA segments on chromosomes that code for traits; alleles are different versions.

• An individual's genotype is their specific combination of alleles (e.g., AA, Aa, or aa).

• The phenotype is the observable trait, like purple flowers, resulting from the genotype.

• Homozygous individuals have identical alleles (AA, aa); heterozygous individuals have different alleles (Aa).

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Multiple Choice

What is the primary function of a gene?

• To code for a specific trait

• To create different versions of an allele

• To determine the observable chromosome

• To form an entire DNA segment

Multiple Choice

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

• The genotype is the observable trait, while the phenotype is the genetic code.

• The genotype is the combination of alleles that results in the observable phenotype.

• The genotype and phenotype are both determined by the environment.

• The phenotype is a different version of a gene, and the genotype is the trait.

Multiple Choice

A plant has a genotype of 'AA' for flower color. What can be concluded about this plant?

• It is heterozygous and its phenotype cannot be determined.

• It is homozygous and its phenotype is determined by the 'AA' genotype.

• It has two different alleles for the flower color trait.

• Its phenotype will be the same as a plant with an 'aa' genotype.

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Why Mendel's Ideas Work: The Two Laws

Law of Segregation

• The two alleles for a single gene separate from each other during the formation of gametes.

• As a result, each gamete receives only one allele for that gene from the parent organism.

• This important separation happens when homologous chromosomes are pulled apart from each other during meiosis I.

Independent Assortment

• The alleles for different genes get sorted into gametes independently of one another during meiosis.

• This occurs because different pairs of homologous chromosomes line up randomly at the cell's center.

• This random alignment creates many possible combinations of alleles in the final gametes, increasing genetic diversity.

Multiple Choice

According to the Law of Segregation, what happens to the two alleles for a single gene during the formation of gametes?

• They separate, so that each gamete receives only one allele.

• They combine, so that each gamete gets a new, blended allele.

• They are both passed on, so that each gamete has two alleles.

• They are copied, so that the parent keeps the original alleles.

Multiple Choice

What is the primary effect of the Law of Independent Assortment?

• It increases genetic diversity by creating many possible combinations of alleles.

• It ensures that all gametes from a parent are genetically identical.

• It prevents different genes from being passed on to the next generation.

• It causes the two alleles for a single gene to stick together.

Multiple Choice

Which statement best explains the underlying events in meiosis that are responsible for both of Mendel's laws?

• The separation of homologous chromosomes causes Segregation, while their random alignment causes Independent Assortment.

• The random alignment of chromosomes causes Segregation, while their separation causes Independent Assortment.

• Both laws are caused by gametes receiving two alleles for each gene during meiosis.

• Both laws are caused by the alleles for different genes being physically linked together.

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Monohybrid Crosses and Test Crosses

Monohybrid Cross

• A monohybrid cross is used to study the inheritance of one specific trait.

• The genotype ratio for a heterozygous cross (Pp x Pp) is 1 PP : 2 Pp : 1 pp.

• This results in a phenotype ratio of 3 purple flowers to 1 white flower.

Test Cross

• This is used to find an unknown genotype by crossing it with a homozygous recessive (pp).

• If all offspring are purple, the unknown parent is homozygous dominant (PP).

• If half of the offspring are white, the unknown parent must be heterozygous (Pp).

Multiple Choice

What is the main purpose of a monohybrid cross?

• To study the inheritance of a single trait

• To find the unknown genotype of a parent

• To combine the best traits of two different species

• To create an organism with only recessive alleles

Multiple Choice

In a heterozygous cross (Pp x Pp), what is the reason for the 3:1 phenotype ratio of purple to white flowers?

• Because the PP genotype is the only one that produces purple flowers

• Because the Pp genotype is the only one that produces purple flowers

• Because both the PP and Pp genotypes result in a purple phenotype

• Because the pp genotype produces a purple phenotype

Multiple Choice

If a test cross is performed on a purple-flowered plant and about half of the offspring are white, what can be concluded about the parent plant?

• The parent plant's genotype is homozygous dominant (PP)

• The parent plant's genotype is heterozygous (Pp)

• The parent plant's genotype is homozygous recessive (pp)

• The experiment did not provide enough information

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Dihybrid Crosses

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Multiple Choice

What is a dihybrid cross?

• A cross that involves two different genes.

• A cross that results in only one type of offspring.

• A cross that can only be used for animals.

• A cross that tracks a single trait from one parent.

Multiple Choice

What does the 9:3:3:1 ratio found in the results of a dihybrid cross describe?

• The number of parents involved in the cross.

• The variety of possible phenotypes in the offspring.

• The number of dominant genes each parent has.

• The specific combination of gametes from one parent.

Multiple Choice

Based on the principles of a dihybrid cross, why is the chance of getting round and yellow peas specifically 9/16?

• It is the total number of offspring produced.

• It is the chance of inheriting at least one recessive trait.

• It is the combined probability of inheriting both dominant traits.

• It is the ratio of parent gametes to offspring.

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Variations: Incomplete Dominance vs. Codominance

Incomplete Dominance

• The heterozygote shows a blended phenotype that is intermediate between the two homozygous phenotypes.

• For example, a cross between a red (RR) and a white (rr) snapdragon results in offspring.

• All of the offspring from this cross will have pink (Rr) flowers, showing a blended color.

Codominance

• The heterozygote expresses both alleles completely and distinctly, without any blending in the phenotype.

• For example, roan cattle with the RW genotype have a coat with both red and white hairs.

• Human AB blood type is another great example, where both A and B antigens are expressed.

Multiple Choice

What is the main difference between incomplete dominance and codominance?

• In incomplete dominance, the heterozygote's phenotype is a blend, while in codominance, both alleles are expressed distinctly.

• In codominance, the heterozygote's phenotype is a blend, while in incomplete dominance, both alleles are expressed distinctly.

• Incomplete dominance happens only in plants, while codominance happens only in animals.

• Incomplete dominance is when both alleles are recessive, while codominance is when both are dominant.

Multiple Choice

When a red snapdragon is crossed with a white snapdragon, all the offspring have pink flowers. Why is this an example of incomplete dominance?

• The pink phenotype is an intermediate blend of the red and white parent phenotypes.

• The pink allele is dominant and masks the white allele.

• Both the red and white alleles are expressed separately on the flowers.

• The offspring inherited traits from only the red-flowered parent.

Multiple Choice

A scientist crosses a plant with pure red flowers and a plant with pure white flowers. If the offspring have flowers with separate, distinct patches of red and white, which principle is best demonstrated?

• Codominance, because both the red and white traits are expressed completely and distinctly.

• Incomplete dominance, because the offspring's phenotype is a blend of the parents.

• A dominant allele, because the red trait is masking the white trait.

• A recessive allele, because the white trait is hidden by the red trait.

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Variations: Multiple Alleles and Epistasis

Multiple Alleles

• Some genes can have more than two different forms, or alleles, in a population.

• The ABO blood group system in humans is a classic example of this inheritance pattern.

• This system is controlled by three alleles: I^A, I^B, and i.

Epistasis

• Epistasis is when one gene can mask or modify the physical expression of another gene.

• For example, in some animals, one gene determines if coat pigment is produced at all.

• If no pigment is made, the animal has a white coat, hiding its color gene.

Multiple Choice

What is it called when a single gene has more than two different alleles in a population?

• Multiple alleles

• Epistasis

• A dominant gene

• A recessive gene

Multiple Choice

What is the primary difference between multiple alleles and epistasis?

• Multiple alleles involve different versions of one gene, while epistasis is when one gene affects the expression of another gene.

• Multiple alleles determine coat color in animals, while epistasis determines human blood type.

• Multiple alleles involve one gene masking another, while epistasis involves having more than two forms of a gene.

• Multiple alleles is a form of dominance, while epistasis is a form of recessiveness.

Multiple Choice

An animal has genes for brown fur, but it also inherits a separate gene that prevents any pigment from being produced at all. What would be the expected coat color of this animal?

• White, because the gene for pigment production can mask the gene for color.

• Brown, because the gene for brown fur is the primary color gene.

• A mix of brown and white, as both genes would be expressed.

• Black, because the absence of pigment would cause the fur to darken.

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Variations: Sex-Linked and Linked Genes

Sex-Linked Traits

• Traits determined by genes on sex chromosomes, most often the X chromosome.

• Males (XY) express recessive traits on their single X chromosome more often.

• Carrier females (X^NXⁿ) can pass recessive traits to their sons.

Linked Genes

• Genes located close together on the same chromosome are usually inherited together.

• These genes do not follow the law of independent assortment unless separated.

• The process of crossing over during meiosis can separate these linked genes.

Multiple Choice

What determines if a trait is considered sex-linked?

• The genes located on the sex chromosomes.

• The genes that are usually inherited together.

• The process of crossing over during meiosis.

• The law of independent assortment.

Multiple Choice

Why are recessive sex-linked traits more commonly seen in males?

• Males have only one X chromosome, so any recessive trait on it is expressed.

• Females do not have X chromosomes to carry the traits.

• Carrier females can only pass traits to their daughters.

• Genes on the Y chromosome are always dominant over genes on the X.

Multiple Choice

If two genes are found very close to each other on the same chromosome, what is the most likely outcome for their inheritance?

• They will likely be inherited together unless separated by crossing over.

• They will separate according to the law of independent assortment.

• They will only be passed from a carrier mother to her son.

• They will be located on the X and Y chromosomes.

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Variations: Polygenic Traits and Pleiotropy

Polygenic Traits

• These are traits that are influenced by the combined effect of multiple genes.

• This inheritance pattern results in a wide range of phenotypes, called continuous variation.

• Human height and skin color are examples that show a bell-shaped distribution curve.

Pleiotropy

• This occurs when a single gene influences multiple, seemingly unrelated phenotypic traits.

• This may happen if the gene’s protein is used in different cell types.

• The protein could also be part of many biochemical pathways in the body.

Multiple Choice

What is the fundamental difference between polygenic traits and pleiotropy?

• Polygenic traits are controlled by multiple genes, while pleiotropy involves one gene affecting multiple traits.

• Polygenic traits involve one gene affecting multiple traits, while pleiotropy is controlled by multiple genes.

• Polygenic traits only occur in humans, while pleiotropy only occurs in plants.

• Polygenic traits result in two phenotypes, while pleiotropy results in a wide range of phenotypes.

Multiple Choice

What is the primary consequence of a trait being controlled by many genes?

• It leads to a wide range of phenotypes within the population.

• It causes a single gene to affect unrelated traits.

• It ensures that only two possible traits can appear.

• It means the trait is influenced by the environment.

Multiple Choice

If scientists find that a single gene in a cat determines both its coat color and its susceptibility to a certain disease, what is the most likely scientific explanation?

• The gene's protein is likely used in different cell types or biochemical pathways.

• The trait is actually influenced by the combined effect of many different genes.

• The organism must have experienced a random mutation.

• The trait shows a bell-shaped distribution curve in the population.

Variations: Penetrance, Expressivity, and Environment

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Penetrance

• This is the percentage of people with a gene who show the trait.

• It determines if a genetic trait will appear in an individual or not.

• For example, a gene with 80% penetrance affects 8 out of 10 people.

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Expressivity

• This describes the range of intensity for a specific genetic trait.

• Individuals with the same genotype can show the trait to different degrees.

• A condition could cause mild symptoms in one person but severe in another.

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Genes vs. Environment

• The environment plays a key role in how a phenotype is expressed.

• Genes can set a potential range for a particular trait's development.

• Factors like nutrition and sunlight can determine the final physical outcome.

Multiple Choice

What does the penetrance of a gene determine?

• Whether the genetic trait will appear in an individual or not.

• How intensely a genetic trait is expressed.

• The final physical outcome due to environmental factors.

• The potential range for a trait's development.

Multiple Choice

What is the primary difference between penetrance and expressivity?

• Penetrance is about whether a trait appears, while expressivity is about the intensity of the trait.

• Penetrance describes the intensity of a trait, while expressivity determines if it appears.

• Penetrance is caused by the environment, while expressivity is caused by genes.

• Penetrance applies to groups of people, while expressivity applies to individuals.

Multiple Choice

A brother and sister both have the same gene for a particular condition. However, the brother has very severe symptoms, while the sister's symptoms are mild. What is the best explanation for this difference?

• The gene has variable expressivity, and environmental factors may also have an influence.

• The gene has 100% penetrance in one person but low penetrance in the other.

• Only one person's phenotype was affected by their nutrition and sunlight.

• The gene sets a potential range, but the environment is the only cause of the difference.

Reading Pedigrees

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Autosomal Dominant

• A square represents a male and a circle represents a female.

• Shaded symbols mean the person has the trait being studied.

• This trait will appear in every single generation without any skips.

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Autosomal Recessive

• This trait can easily skip one or more generations.

• Two unaffected parents can have an affected child.

• This happens if both of the parents are carriers.

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X-linked Recessive

• The trait is found much more often in males than females.

• It frequently skips generations, passing through carrier females.

• An affected grandfather cannot pass it directly to his grandson.

Multiple Choice

What is the basic rule for identifying an individual who has the trait being studied in a pedigree?

• The person's symbol is shaded.

• The person's symbol is a square.

• The person's symbol is in the first generation.

• The person's symbol is unshaded.

Multiple Choice

Which statement accurately compares the inheritance patterns of autosomal dominant and autosomal recessive traits?

• Dominant traits appear in every generation, whereas recessive traits can appear in children of unaffected parents.

• Recessive traits appear in every generation, whereas dominant traits can skip generations.

• Dominant traits are only found in males, while recessive traits are only found in females.

• Recessive traits are passed from a single carrier parent, while dominant traits require two.

Multiple Choice

A pedigree shows a trait that mostly affects males and cannot be passed from a father to his son. What is the best explanation for this pattern?

• It is an X-linked recessive trait.

• It is an autosomal dominant trait.

• It is an autosomal recessive trait.

• It is a trait that only mothers can carry.

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Common Misconceptions

Misconception	Correction
Dominant alleles are better or more common.	Dominant alleles are expressed even if only one copy is inherited.
Genotype (genes) and phenotype (traits) are the same.	Genotype is the genetic makeup. Phenotype is the physical result.
Incomplete dominance and codominance are identical.	Incomplete dominance blends traits. Codominance expresses both traits separately.
Genes for different traits are always inherited separately.	Linked genes are close on a chromosome and inherited together.
Males need two copies of a recessive X-linked allele to show the trait.	Males only have one X chromosome, so one allele shows the trait.

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Summary

• Traits are passed down through genes on chromosomes, with versions called alleles.

• Mendel's laws and Punnett squares help explain and predict basic inheritance patterns.

• Inheritance is complex, with patterns like codominance, sex-linkage, and polygenic traits.

• The final phenotype is a result of genotype interacting with the environment.

Poll

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