Chapter 12Chapter 12

Inheritance, Genes, and Inheritance, Genes, and ChromosomesChromosomes

MendelMendel

Prior to Mendel scientists new that two parents equally Prior to Mendel scientists new that two parents equally contributed to offspring but that the resulting contributed to offspring but that the resulting phenotype was a blend of the two parents (red and phenotype was a blend of the two parents (red and blue makes purple)blue makes purple)

Austrian monkAustrian monk

His studies in physics and mathematics were a strong His studies in physics and mathematics were a strong influence on his use of quantitative experimental influence on his use of quantitative experimental methods.methods.

Over seven years, he made crosses with Over seven years, he made crosses with 24,034 plants24,034 plants..

MendelMendel

Mendel’s new theory of inheritance was Mendel’s new theory of inheritance was published in 1866, but was largely published in 1866, but was largely ignored.ignored.

Most biologists at the time were not used Most biologists at the time were not used to thinking in mathematical terms.to thinking in mathematical terms.

Even Darwin missed the significance of Even Darwin missed the significance of Mendel’s work.Mendel’s work.

By 1900, meiosis had been observed. By 1900, meiosis had been observed.

Garden PeaGarden PeaModel systemModel system

- Easily grown- Easily grown

- Short Life Cycle- Short Life Cycle

- Pollination easily controlled- Pollination easily controlled

- Manually pollinate- Manually pollinate

- Or allow to naturally self pollinate- Or allow to naturally self pollinate

- Easily observable traits - Easily observable traits

(seed color, shape, height)(seed color, shape, height)

Observe the segregation of traits in large enough numbers to Observe the segregation of traits in large enough numbers to be able to determine relative proportions of types of progeny be able to determine relative proportions of types of progeny (probability)(probability)

Mendel’s Traits of InterestMendel’s Traits of Interest

Genetics TermsGenetics TermsCharacter:Character: observable physical feature observable physical feature

Trait:Trait: particular form of a character (purple flower) particular form of a character (purple flower)

Heritable trait:Heritable trait: trait passed from parent to offspring trait passed from parent to offspring

True breeding True breeding – same phenotype no matter how many – same phenotype no matter how many times it is crossed with itself (or own kind) (means times it is crossed with itself (or own kind) (means homozygous)homozygous)

Parental Generation (P) Parental Generation (P) – two true-breeding plants to – two true-breeding plants to be crossed be crossed

First Filial Generation (F1) First Filial Generation (F1) – progeny of the cross – progeny of the cross between parentsbetween parents

Second Filial Generation (F2) Second Filial Generation (F2) – progeny of across – progeny of across between two F1 progeny (or a selfed F1)between two F1 progeny (or a selfed F1)

Mendel’s 1Mendel’s 1stst Experiment Experiment

Crossed plants differing in just Crossed plants differing in just oneone trait ( trait (PP).).

FF1 1 generation are generation are monohybridsmonohybrids..

The monohybrids were then allowed to self The monohybrids were then allowed to self pollinate to form the Fpollinate to form the F22 generation: A generation: A

monohybrid crossmonohybrid cross..

Mendel repeated this for all seven traits.Mendel repeated this for all seven traits.

How to Cross Pea PlantsHow to Cross Pea Plants

• Cross Pollination Cross Pollination

• Self PollinationSelf Pollination

Results of Initial Mendel ExperimentsResults of Initial Mendel ExperimentsOne trait of each pair disappeared in the FOne trait of each pair disappeared in the F11 generation and generation and

reappeared in the Freappeared in the F22—these traits are —these traits are recessive (lower case letters)recessive (lower case letters)..

The trait that appears in the FThe trait that appears in the F11 is the is the dominantdominant trait (upper case trait (upper case

letters)letters)..

The ratio of dominant to recessive in the FThe ratio of dominant to recessive in the F22 was about 3:1. was about 3:1.

Reciprocal crosses yielded the same results: it made no difference Reciprocal crosses yielded the same results: it made no difference which parent contributed pollen.which parent contributed pollen.

The idea that each parent contributes equally was supported. The idea that each parent contributes equally was supported.

Particulate TheoryParticulate Theory

Blending theory was not supported by Blending theory was not supported by Mendel’s crosses.Mendel’s crosses.

Mendel proposed that the Mendel proposed that the heritable units heritable units were discrete particles.were discrete particles.

Each plant has two particles for each Each plant has two particles for each character, one from each parent.character, one from each parent.

More Genetic TermsMore Genetic TermsDiploid: Diploid: The two copies of heritable unit in an The two copies of heritable unit in an

organism.organism.

During gamete production, only one copy is given During gamete production, only one copy is given to the gameteto the gamete——this single set is called this single set is called haploidhaploid..

Genes (Mendel’s ParticlesGenes (Mendel’s Particles): is a sequence on a ): is a sequence on a DNA molecule that resides at a particular site on DNA molecule that resides at a particular site on a chromosome—the locus—and encodes a a chromosome—the locus—and encodes a particular character..particular character..

The totality of all genes in an organism is the The totality of all genes in an organism is the genomegenome..

More Genetics TermsMore Genetics TermsAlleles: Alleles: Different forms of a geneDifferent forms of a gene

True-breeding individuals have two copies of the True-breeding individuals have two copies of the same allele—they are same allele—they are homozygoushomozygous for the allele for the allele (e.g., (e.g., ssss).).

HeterozygousHeterozygous individuals have two different alleles individuals have two different alleles (e.g., (e.g., SsSs).).

PhenotypePhenotype: Physical appearance of an organism : Physical appearance of an organism (e.g., spherical seeds).(e.g., spherical seeds).

GenotypeGenotype: The genetic makeup (e.g., : The genetic makeup (e.g., SsSs).).

Spherical seeds can be the result of two Spherical seeds can be the result of two different genotypes—different genotypes—SSSS or or SsSs..

The Law of Segregation The Law of Segregation Mendel’s First LawMendel’s First Law

The two copies of a The two copies of a gene separate when gene separate when an individual makes an individual makes gametes (Meiosis).gametes (Meiosis).

Allele combinations can Allele combinations can be predicted using a be predicted using a Punnett squarePunnett square..

All basedAll basedon Meiosis!on Meiosis!

Test CrossTest Cross

Determines whether an individual is Determines whether an individual is homozygous or heterozygous for a trait homozygous or heterozygous for a trait by crossing it with the homozygous by crossing it with the homozygous recessiverecessive

Mendel crossed the FMendel crossed the F11 with known with known

homozygotes (e.g., wrinkled or homozygotes (e.g., wrinkled or ssss) )

Law of Independent Law of Independent AssortmentAssortment

Mendel’s Second LawMendel’s Second Law

Dihybrid CrossesDihybrid Crosses

Alleles of different genes assort independently Alleles of different genes assort independently during gamete formation.during gamete formation.

Doesn’t always apply to genes on the same Doesn’t always apply to genes on the same chromosome chromosome (Linked)(Linked); but chromosomes do ; but chromosomes do segregate independently.segregate independently.

Dihybrid CrossesDihybrid Crosses

Phenotypic Ratio: 9:3:3:1

Two traits segregate independently of each other

PedigreesPedigreesGenetic HistoryGenetic History

Humans have few Humans have few offspring; pedigrees do offspring; pedigrees do not show the clear not show the clear proportions that the pea proportions that the pea plants showed.plants showed.

Geneticists use pedigrees Geneticists use pedigrees to determine whether a to determine whether a rare allele is dominant or rare allele is dominant or recessive.recessive.

Exceptions to Mendelian Exceptions to Mendelian InheritanceInheritance

Not always “One Gene Not always “One Gene One Trait”One Trait”

Many examples in Many examples in Genetics where Genetics where Mendel’s Laws are Mendel’s Laws are not followednot followed

• Examples:Examples:– Incomplete Incomplete

InheritanceInheritance– Co-dominanceCo-dominance– Hybrid VigorHybrid Vigor– Quantitative Trait Quantitative Trait

LociLoci– Linked GenesLinked Genes

Incomplete InheritanceIncomplete Inheritance

Some alleles are Some alleles are neither dominant neither dominant nor recessive—a nor recessive—a heterozygote has an heterozygote has an intermediate intermediate phenotypephenotype

Co-DominanceCo-Dominance

Two alleles at one Two alleles at one locus produce locus produce phenotypes that are phenotypes that are both present in the both present in the heterozygote.heterozygote.

Hybrid VigorHybrid Vigor

A cross between two A cross between two different true-different true-breeding breeding homozygotes can homozygotes can result in offspring result in offspring with stronger, larger with stronger, larger phenotypes: phenotypes: “Hybrid vigor” or “Hybrid vigor” or heterosis.heterosis.

Quantitative Trait LociQuantitative Trait Loci

Several Genes determine the expression Several Genes determine the expression of one traitof one trait

““You get a gradient of phenotypes”You get a gradient of phenotypes”

Example: HeightExample: Height

Linked GenesLinked Genes

Genes located on the Genes located on the same Chromosomesame Chromosome

Segregate together Segregate together (not independently) (not independently)

• When a gene is When a gene is linked to the sex linked to the sex Chromosome it is Chromosome it is said to be said to be sex-linkedsex-linked

Sex-Linked TraitsSex-Linked Traits

Mammals:Mammals:Female has two X chromosomes (XX).Female has two X chromosomes (XX).Male has one X and one Y (XY).Male has one X and one Y (XY).

Male mammals produce two kinds of Male mammals produce two kinds of gametes—half carry a Y and half carry gametes—half carry a Y and half carry an X.an X.

The sex of the offspring depends on The sex of the offspring depends on which chromosome fertilizes the egg.which chromosome fertilizes the egg.

Sex-Linked TraitsSex-Linked Traits

Genes on sex chromosomes don’t follow Genes on sex chromosomes don’t follow Mendelian patterns.Mendelian patterns.

The Y chromosome carries few genes; the The Y chromosome carries few genes; the X chromosome carries many.X chromosome carries many.

Thus, males have only one copy of these Thus, males have only one copy of these genes (genes (hemizygoushemizygous).).

Results in males that better display Results in males that better display recessive phenotypesrecessive phenotypes