Chapter 4: Extensions of Mendeling Inheritance

What is the Mendelian Inheritance based on?

A single gene with two different alleles

What type of allele relationship is seen in simple Mendelian Inheritance?

A dominant/recessive relationship

What are the two major Mendelian laws mentioned on this slide?

The Law of Segregation and the Law of Independent Assortment

What does the Law of Segregation state?

Alleles separate during gamete formation

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Why does the slide say “Mendel got lucky”?

Because the traits he studied followed simple inheritance patterns

What will this chapter focus on?

Traits that deviate from simple dominant/recessive inheritance

Do more complex inheritance patterns still follow Mendelian laws?

Yes, they still obey Mendelian laws

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What is simple Mendelian inheritance?

Inheritance that follows Mendel’s laws with a strict dominant/recessive relationship

What is the molecular explanation for simple Mendelian Inheritance?

One functional dominant allele produces enough protein for the dominant trait

What is incomplete dominance?

A heterozygote shows an intermediate phenotype between the two homozygotes

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What is an example of incomplete dominance?

Red flowers crossed with white flowers produce pink flowers

Why does incomplete dominance occur molecularly?

One dominant allele does not produce enough protein to fully show the dominant phenotype

What is incomplete penetrance?

A dominant allele is present, but the dominant phenotype is not always expressed

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What is an example of incomplete penetrance?

A person carrying the polydactyly allele but having a normal number of fingers and toes

Why can incomplete penetrance occur?

Environmental factors or other genes may suppress the phenotype

What is overdominance?

The heterozygote has a more beneficial trait than either homozygote

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What are some molecular reasons for overdominance?

• Increased resistance to an infection

• Better protein dimer function

• Proteins function under a wider range of conditions

What is codominance?

Both alleles are fully expressed in the heterozygote

What is an example of codominance?

AB blood type

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Why does codominance occur molecularly?

Each allele produces a functional protein with its own effect

What is X-linked inheritance?

Inheritance of genes located on the X chromosome

Why are males more affected by X-linked traits?

Because males have only one X chromosome

What does hemizygous mean?

Having only one copy of a gene instead of two

What is sex-influenced inheritance?

An allele behaves differently depending on the sex of the individual

What is an example of sex-influences inheritance?

Pattern baldness in humans

What causes sex-influences inheritance molecularly?

Sex hormones affect gene expression

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What is sex-limited inheritance?

Traits expressed in only one sex

What is an example of sex-limited inheritance?

Breast development in mammals

Why does sex-limited inheritance occur?

Certain sex hormones are only present in one sex

What is a lethal allele

An allele that can cause death of an organism

What are prevalent alleles in a population called?

Wild-type alleles

What does wild-type mean?

The most common form of an allele found in a population

How do proteins produced by wild-type alleles usually function?

They function normally and are made in the proper amounts

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What is genetic polymorphism?

When two or more wild-type phenotypes are common in a large population

What does polymorphism mean?

The existence of multiple common forms within a population

Can different appearances still be considered wild-type?

Yes, if they are common in the population

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What do the snake pictures demonstrate in this slide?

Different common phenotypes within the same population

Why is genetic polymorphism important?

It increases variation within a population

Does genetic polymorphism always indicate harmful mutations?

No

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What are the most common alleles in a population called?

Wild-type alleles

How do wild-type alleles usually function?

They produce normal functional proteins in the proper amounts

What is genetic polymorphism?

When two or more wild-type phenotypes are common in a large population

What are alleles altered by a mutation called?

Mutant alleles

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How do mutant alleles often affect proteins?

They may reduce or prevent the production of a functional protein

Are mutant alleles usually common or rare in natural populations?

Rare

How are many mutant alleles inherited?

In a recessive fashion

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Why are many harmful mutant alleles recessive?

A normal dominant allele can often still produce enough functional protein

What are some recessive human diseases?

• Phenylketonuria (PKU)

• Albinism

• Tay-Sachs disease

• Sandoff disease

• Cystic fibrosis

• Lesch-Nyhan syndrome

What are some examples of diseases cause by recessive mutant alleles?

• Phenylketonuria (PKU)

• Albinism

• Tay-Sachs disease

• Sandoff disease

• Cystic fibrosis

• Lesch-Nyhan syndrome

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What causes phenylketonuria (PKU)?

Inability to metabolize phenylalanine properly

What can happen is PKU is untreated?

Severe mental impairment and physical degeneration

What disease is associated with defective tyrosinase?

Albinism

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What causes albinism?

Lack of pigmentation in the skin, eyes, and hair

What disease is caused by defective Hexosaminidase A?

Tay-Sachs disease

What happens in Tay-Sachs disease?

Defects in lipid metabolism leading to paralysis, blindness, and early death

What recessive disease is caused by defective Hexosaminidase B

Sandhoff disease

What enzyme is defective in Sandhoff disease?

Hexosaminidase B.

What happens in Sandhoff disease?

Progressive mental and motor deterioration caused by lipid metabolism defects

What recessive disease is caused by defective chloride transport across epithelial cells?

Cystic fibrosis

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What protein is defective in cystic fibrosis?

A chloride transporter

What causes cystic fibrosis symptoms?

Defective chloride transport across epithelial cell

What are common symptoms or cystic fibrosis?

Thick lung mucus and chronic lung infections

What recessive disease is caused by defective hypoxanthine-guanine phosphoribosyl transferase

Lesch-Nyhan Syndrome

What enzyme is defective in Lesch-Nyhan syndrome?

Hypoxanthine-guanine phosphoribosyl transferase

What causes Lesch-Nyhan syndrome?

inability to properly metabolize purines

What are symptoms of Lesch-Nyhan syndrome?

• Self-mutilation behavior

• Poor motor skills

• mental impairment

• kidney failure

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Why are enzyme defects commonly discussed in recessive diseases?

Because many recessive diseases result from missing or nonfunctional enzymes

What is the main difference between wild-type and mutant alleles?

Wild-type alleles are common and functional, while mutant alleles are altered and often defective

Why is cystic fibrosis considered recessive?

Two defective alleles are usually required for the disease phenotype to appear

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Why can recessive diseases remain hidden in populations?

Heterozygous carriers often appear normal

In a simple dominant/recessive relationship, does the recessive allele affect the phenotype of the heterozygote?

No, the recessive allele does not affect the heterozygote phenotype

Why can a heterozygote still show the wild-type phenotype?

Because enough functional. protein is produced for normal function

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What is haplosufficiency?

When 50% of the normal protein is enough to accomplish the protein’s cellular function

What percentage of functional protein does a heterozygote (Pp) produce in a simple dominant/recessive relationship?

About 50%

What percentage of functional protein does PP produce?

100%

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What percentage of functional protein does pp produce?

0%

What phenotype does the genotype PP produce in the flower example?

Purple flowers

What phenotype does the genotype Pp produce in the flower example?

Purple flowers

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What phenotype does the genotype pp produce in the flower example?

White flowers

Why does Pp still produce purple flowers even though only 50% of the protein is made?

Because 50% of the protein is sufficient for normal function