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Lesson 1: Foundations of Genetics

Lesson 2: Understanding Inheritance

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heredity

genetics

dominant

recessive

gene

law of segregation

law of independent assortment

4.1 Foundations of Genetics

allele

phenotype

genotype

homozygous

heterozygous

Early Ideas About Heredity

• Combined genetic material from a sperm and an egg determines the traits or features of an offspring.

4.1 Foundations of Genetics

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

Early Ideas About Heredity (cont.)

• The idea of blending inheritance is offspring are a blend of genetic material from both parents.

4.1 Foundations of Genetics

– The genetic material mixed or blended like colors of paint.

– Over many generations, populations would eventually look alike.

– Blending inheritance cannot explain why some traits skip a generation.

Gregor Mendel and His Experiments

• Gregor Mendel was the first to record evidence that traits are determined by factors passed from parents to offspring.

• Mendel established the basic laws of heredity.

• Genetics is the study of how traits of organisms are passed from parents to offspring.

4.1 Foundations of Genetics

Mendel’s Experimental Methods

• Mendel conducted breeding experiments by studying seven traits of pea plants and each traits had only two variations.

4.1 Foundations of Genetics

Controlled Experiments

• Mendel controlled fertilization in the pea plants, allowing him to see how traits pass from one generation to another.

4.1 Foundations of Genetics

• Mendel allowed some flowers to self-fertilize.

• He also performed cross-fertilization by transferring pollen from one pea flower to another.

Mendel’s Unique Methods

• Used true-breeding plants for each trait—plants that always produce offspring with that trait when they self-pollinate

4.1 Foundations of Genetics

• Recorded the inheritance of traits for several generations

• Used a mathematical approach

Mendel’s Experimental Results

• Mendel concluded that two factors control each inherited trait.

4.1 Foundations of Genetics

• When organisms reproduce, each gamete—sperm or egg—contributes one factor for each trait.

Dominant Factors

• A genetic factor that blocks another genetic factor is called dominant.

4.1 Foundations of Genetics

• A dominant trait is observed when offspring have one or two dominant factors.

Recessive Factors

• A genetic factor that is hidden by the presence of a dominant factor is recessive.

4.1 Foundations of Genetics

• A recessive trait can be observed only when two recessive genetic factors are present in offspring.

Mendel’s Laws of Heredity

• Law of segregation: the two factors for each trait segregate—separate from each other—during meiosis when gametes form

4.1 Foundations of Genetics

• Law of independent assortment: the factors for one trait separate independently of how factors for other traits separate

Modern Definitions of Mendel’s Ideas

• Mendel did not know about DNA or how cells reproduce, but his ideas about inheritance are still true today.

4.1 Foundations of Genetics

Genes and Alleles

• A gene is a section of DNA that has information about a trait in an organism.

4.1 Foundations of Genetics

• Each form of a gene with different information is called an allele.

Genes and Alleles (cont.)

4.1 Foundations of Genetics

Phenotype and Genotype

• The observable traits and all characteristics of an organism make up the organism’s phenotype.

4.1 Foundations of Genetics

• The alleles that make up an organism is the organism’s genotype.

• The alleles of a particular gene is that gene’s genotype.

Homozygous and Heterozygous Genotypes

• Because eukaryotes have pairs of chromosomes, a genotype for a gene has two alleles.

4.1 Foundations of Genetics

• If the two alleles have the same information, the genotype is homozygous.

• If the two alleles have different information, the genotype is heterozygous.

Homozygous and Heterozygous Genotypes (cont.)

4.1 Foundations of Genetics

Law of Segregation Explained• The movement of chromosomes during meiosis

explains Mendel’s law of segregation.

– Each set of chromatids separates into different gametes during meiosis II.

– Each gamete receives only one allele.

4.1 Foundations of Genetics

Law of Segregation Explained (cont.)

4.1 Foundations of Genetics

Law of Independent Assortment Explained

• The daughter cells produced by meiosis receive only one chromosome from each pair of homologous chromosomes.

– A daughter cell might receive the A or a chromosome from pair 1 and the B or b chromosome from pair 2.

– This results in four possible allele combinations for two homologous pairs of chromosomes.

4.1 Foundations of Genetics

Law of Independent Assortment Explained (cont.)

4.1 Foundations of Genetics

Importance of Mendel’s Genetic Studies

• In the 1860s, no one knew about chromosomes or meiosis so it was hard to understand Mendel’s discoveries.

• All the research of modern genetics is based on Mendel’s conclusions from his work with pea plants.

4.1 Foundations of Genetics

Lesson 1 Review

What is the passing of traits from parents to offspring called?

A inheritance

B genetics

C heredity

D allele

4.1 Foundations of Genetics

Lesson 1 Review

What are the alleles that make up an organism called?

A genes

B genotype

C phenotype

D factors

4.1 Foundations of Genetics

Lesson 1 Review

If two alleles for a gene have the same information, what kind of genotype does that gene have?

A homologous

B recessive

C heterozygous

D homozygous

4.1 Foundations of Genetics

End of Lesson 1

Punnett square

pedigree

incomplete dominance

codominance

multiple alleles

sex chromosomes

polygenic inheritance

genetic disorder

4.2 Understanding Inheritance

Modeling Inheritance

– Punnett square

– pedigree

• Two tools can be used to identify and predict traits among genetically related individuals.

4.2 Understanding Inheritance

Heredity

Punnett Squares

• If the genotypes of the parents are known, the genotypes and phenotypes of the offspring can be predicted.

• A Punnett square is a model used to predict possible genotypes and phenotypes of offspring.

4.2 Understanding Inheritance

One-Trait Model• The Punnett square shows the possible offspring of a cross between two true-breeding pea plants—one

with yellow seeds and one with green.

4.2 Understanding Inheritance

One-Trait Model (cont.)

– The phenotype will be yellow seeds because Y is dominant to y.

– The only possible genotype for hybrid offspring is heterozygous—Yy.

4.2 Understanding Inheritance

One-Trait Model (cont.)

4.2 Understanding Inheritance

Two-Trait Model• The possible offspring of two heterozygous genotypes—Yy and Yy—would have

three different genotypes and two phenotypes.

4.2 Understanding Inheritance

Pedigrees• All the genetically related members of a family are part

of a family tree.

4.2 Understanding Inheritance

• A pedigree shows genetic traits that were inherited by members of a family tree.

• Pedigrees are important tools for tracking complex pattern of inheritance and genetic disorders in families.

Pedigrees (cont.)

4.2 Understanding Inheritance

A pedigree chart that shows three generations of a family.

Types of Dominance• Alleles show incomplete dominance when they produce

a phenotype that is a blend of the parents’ phenotypes.

• When both alleles can be observed in the phenotype, the interaction is called codominance.

4.2 Understanding Inheritance

– The human blood type AB is an example of codominance.

Multiple Alleles• Some genes have more than two alleles, or multiple

alleles.

• The human ABO blood group is determined by multiple alleles as well as codominance.

• There are three different alleles for the ABO blood type—IA, IB, and i.

4.2 Understanding Inheritance

Multiple Alleles (cont.)

4.2 Understanding Inheritance

Sex-Linked Inheritance• Chromosomes X and Y are the sex chromosomes—

they contain the genes that determine gender or sex.

4.2 Understanding Inheritance

• Except for sperm and eggs, each cell in a male has an X and a Y chromosome, and each cell in a female has two X chromosomes.

• A recessive phenotype is observed in a male when a one-allele gene on his X chromosome has a recessive allele.

Sex-Linked Inheritance (cont.)

4.2 Understanding Inheritance

In this family, the grandmother’s genome included the color blindness allele.

Polygenic Inheritance• Polygenic inheritance is when multiple genes

determine the phenotype of a trait.

4.2 Understanding Inheritance

• Many phenotypes are possible when possible when polygenic inheritance determines a trait.

Maternal Inheritance• Humans inherit mitochondrial genes only from their

mothers.

4.2 Understanding Inheritance

• Inheritance of traits related to the mitochondria can be traced from grandmother to grandchildren.

How are the traits of parents inherited and expressed in offspring?

Human Genetic Disorders• If a change occurs in a gene, the organism with the

mutation may not be able to function as it should.

4.2 Understanding Inheritance

• An inherited mutation can result in a phenotype called a genetic disorder.

Human Genetic Disorders (cont.)

4.2 Understanding Inheritance

Genes and the Environment

• An organism’s environment can affect its phenotype.

– Genes affect heart disease, but so do diet and exercise.

– Genes affect skin color, but so does exposure to sunlight.

4.2 Understanding Inheritance

Lesson 2 Review

Punnett squares model the ____ of offspring.

A genotypes

B phenotypes

C genotypes and phenotypes

D genes

4.2 Understanding Inheritance

Lesson 2 Review

What is the term for when alleles produce a phenotype that is a blend of the parents’ phenotypes?

A incomplete dominance

B codominance

C multiple alleles

D polygenic inheritance

4.2 Understanding Inheritance

Lesson 2 Review

How many Y chromosomes do females have?

A 0

B 1

C 2

D 4

4.2 Understanding Inheritance

End of Lesson 2

Chapter Assessment

California Standards Practice

Concepts in Motion

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What is the term for the idea that offspring are a blend of genetic material from both parents?

A polygenic inheritance

B sex-linked inheritance

C maternal inheritance

D blending inheritance

Chapter Assessment 1

What type of alleles can only be observed in the phenotype when they are present as a homozygous genotype?

A dominant

B recessive

C inherited

D heterozygous

Chapter Assessment 2

What is the term for the idea that inheritance of one trait is not influenced by inheritance of another trait?

A law of independent assortment

B law of heredity

C law of segregation

D maternal inheritance

Chapter Assessment 3

What is a good example of a trait that is determined by multiple alleles?

A color of camellia flowers

B human AB blood type

C color blindness

D human ABO blood group

Chapter Assessment 4

Why are male humans more likely to be color-blind than females?

A maternal inheritance

B sex-linked inheritance

C polygenic inheritance

D incomplete dominance

Chapter Assessment 5

Which pea trait did Mendel not study?

A seed color

B pod color

C flower position

D flower shape

CA Standards Practice 1

SCI 2.c

If two plants with genotypes Mm are crossed, what percent of the offspring will have phenotype M?

A 0%

B 25%

C 75%

D 100%

CA Standards Practice 2

SCI 2.d

What is the term for when more than one gene determine a trait?

A incomplete dominance

B multiple alleles

C polygenic inheritance

D sex-linked inheritance

CA Standards Practice 3

SCI 2.c, 2.d

Which does NOT describe Mendel’s experiments?

A Mendel observed several generations of plants.

B Mendel chose pea plants because they reproduce quickly.

C Mendel counted small numbers of offspring.

D Mendel used true-breeding plants.

CA Standards Practice 4

SCI 2.d

What type of genetic disorder is hemophilia?

A dominant

B X-linked recessive

C codominant

D recessive

CA Standards Practice 5

SCI 2.d

Concepts in Motion 1

Concepts in Motion 2

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