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Mendel and MeiosisMendel and Meiosis

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Mendel: An Austrian MonkMendel: An Austrian Monk

Why offspring resemble their parents?Why offspring resemble their parents? Pea plants to study inheritance of Pea plants to study inheritance of

characteristics.characteristics. Traits transferred.Traits transferred.

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Genetic TermsGenetic Terms HeredityHeredity: The passing of characteristics from : The passing of characteristics from

parents to offspring.parents to offspring.

GeneticsGenetics: Branch of biology that studies heredity.: Branch of biology that studies heredity.

TraitsTraits: Characteristics that are inherited.: Characteristics that are inherited.

GametesGametes: Sex Cells (two distinct cells –male and : Sex Cells (two distinct cells –male and female)female)

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Plant FertilizationPlant Fertilization

Male gamete is pollen (produced by anther)Male gamete is pollen (produced by anther)

Female gamete is ovule (produced in the pistil)Female gamete is ovule (produced in the pistil)

PollinationPollination: The transfer of male pollen grains to the pistil of : The transfer of male pollen grains to the pistil of a flower.a flower.

FertilizationFertilization: The uniting of male and female gametes. : The uniting of male and female gametes. Occurs when male gamete in the pollen grain meets and Occurs when male gamete in the pollen grain meets and fuses with the female gamete in the ovule.fuses with the female gamete in the ovule.

After the ovule gamete is fertilized, it matures into a seed.After the ovule gamete is fertilized, it matures into a seed.

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Genetics TermsGenetics Terms

Hybrid- Offspring of crosses between Hybrid- Offspring of crosses between parents with different traitsparents with different traits

Purebred- Organisms that carry only one Purebred- Organisms that carry only one

variation of a characteristic.variation of a characteristic.

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Mendel’s Experiment: Mendel’s Experiment: Why Pea Plants?Why Pea Plants?

Reproduce sexually (sex cells)Reproduce sexually (sex cells) Male and female gametes are in same flowerMale and female gametes are in same flower Reproductive parts are enclosed tightly together.Reproductive parts are enclosed tightly together. Reproduce by self-pollination (Male and Female Reproduce by self-pollination (Male and Female

coming from same plant)coming from same plant) Mendel could manipulate Mendel could manipulate

Cross-PollinationCross-Pollination: Breed-or-cross, one plant with : Breed-or-cross, one plant with anotheranother– Mendel opened petals and removed anthersMendel opened petals and removed anthers– Dusted the pistil with pollen from plant he wanted to cross Dusted the pistil with pollen from plant he wanted to cross

withwith– Covered with bagCovered with bag

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Mendel a careful researcherMendel a careful researcher

Controlled experiments and peasControlled experiments and peas

One trait at a timeOne trait at a time

Analyzed data mathematicallyAnalyzed data mathematically

Used plants that were tall for many Used plants that were tall for many generations (true breeding for tallness). Used generations (true breeding for tallness). Used plants that were short for many generations plants that were short for many generations (true breeding for shortness.)(true breeding for shortness.)

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Mendel’s Monohybrid Cross: Mendel’s Monohybrid Cross: Mendel crossed tall and short Mendel crossed tall and short plants to produce new plants.plants to produce new plants.

HybridsHybrids: Offspring of parents that have : Offspring of parents that have different forms of a trait. (short and tall).different forms of a trait. (short and tall).

MonoMono (one): (one): 11stst experiments monohybrids experiments monohybrids Each parent plants differed by one single trait Each parent plants differed by one single trait

(height).(height).

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Mendel’s Monohybrid CrossMendel’s Monohybrid Cross

(P(P11=“parents”) =“parents”) – Short pea plant x Tall pea plant Short pea plant x Tall pea plant

first generation (Ffirst generation (F11=“filial”)=“filial”)Result: All tall pea plants Result: All tall pea plants

FF11(first generation) Self-pollinate (first generation) Self-pollinate – Tall pea plant x Tall pea plantTall pea plant x Tall pea plant– 3/43/4thth’s Tall’s Tall– 1/41/4thth Short Short

FF22(second generation)(second generation)

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CrossesCrosses

PP11 generation—original parents generation—original parents““P” parentP” parent

FF11 generation—Offspring of parents generation—Offspring of parents““F” filial—son or daughterF” filial—son or daughter

FF22 generation—Cross 2 from F1 generation. generation—Cross 2 from F1 generation.

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Trait patterns Mendel ObservedTrait patterns Mendel Observed

In every case, he found that one trait of a pair seemed to disappear in the F1 generation, only to reappear unchanged in ¼ of the F2 plants.

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Role of Unit factorsRole of Unit factors Two factors control each traitsTwo factors control each traits Genes located on chromosomes.Genes located on chromosomes. Different forms of genes are called Different forms of genes are called

ALLELESALLELES..Example: Alleles for heightExample: Alleles for height

Two alleles for tallnessTwo alleles for tallness Two alleles for shortnessTwo alleles for shortness One allele for tallness and one for One allele for tallness and one for

shortnessshortness Alleles are located on different copies of Alleles are located on different copies of

the chromosomesthe chromosomes

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The Rule of DominanceThe Rule of Dominance

Dominant traitDominant trait: Observed trait –: Observed trait –Masks recessiveMasks recessive

(Example: Mendel’s F(Example: Mendel’s F11—All tall plants/tall allele is —All tall plants/tall allele is dominant trait)dominant trait)

Recessive traitRecessive trait: Trait hidden by : Trait hidden by dominant trait dominant trait

(Example: Mendel’s F(Example: Mendel’s F11—All tall plants/short allele that reappears in F—All tall plants/short allele that reappears in F22 is is recessive trait)recessive trait)

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Allele ShorthandAllele Shorthand

– Same letter for different allelesSame letter for different alleles– Upper case for dominant allele Upper case for dominant allele – Lower case for recessive allele Lower case for recessive allele – Dominant written firstDominant written first

Example: Allele for tallness = TExample: Allele for tallness = T Allele for shortness = tAllele for shortness = t

TtTt

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Law of Segregation (Mendelian)Law of Segregation (Mendelian)

Each organism has two different alleles, it can Each organism has two different alleles, it can produce two different types of gametes. produce two different types of gametes.

During fertilization, male and female gametes During fertilization, male and female gametes randomly pair to produce four combinations of randomly pair to produce four combinations of

alleles.alleles.

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PhenotypePhenotype

Greek words Greek words phaineinphainein, meaning “to show,” , meaning “to show,” and and typostypos, meaning “model.” , meaning “model.”

The visible characteristics (appearance and The visible characteristics (appearance and behavior) of an organism makes up it behavior) of an organism makes up it phenotype.phenotype.

Example:Example:Round, WrinkledRound, WrinkledYellow, GreenYellow, GreenBrunette, BlondeBrunette, BlondeBlue Eyes, Brown Eyes Blue Eyes, Brown Eyes

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GenotypeGenotype

From the Greek words From the Greek words gen gen or or genogeno, meaning , meaning “race,” and “race,” and typostypos, meaning “model.” , meaning “model.”

The genetic characteristics of an organism The genetic characteristics of an organism make up its genotype.make up its genotype.

Example:Example:GenotypeGenotype of a tall plant that has two alleles for tallness is of a tall plant that has two alleles for tallness is TTTT..GenotypeGenotype of a tall plant that has one allele for tallness and of a tall plant that has one allele for tallness and one allele for shortness is one allele for shortness is TtTt..

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HomozygousHomozygousTrait for two alleles is the Trait for two alleles is the

samesame

Example: Example: Two alleles for tallness (TT) Two alleles for tallness (TT)

homozygous dominanthomozygous dominant

Two alleles for shortness (tt)—Two alleles for shortness (tt)—homozygous recessive.homozygous recessive.

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HeterozygousHeterozygous

Two different alleles for one trait.Two different alleles for one trait.TtTt

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Punnett Squares.Punnett Squares.

(1905) Reginald Punnett, an English (1905) Reginald Punnett, an English biologist, created way to expected biologist, created way to expected proportions of possible genotypes in the proportions of possible genotypes in the offspring of cross-Punnett Square.offspring of cross-Punnett Square.

Know the genotypes of the parents, you can Know the genotypes of the parents, you can use a Punnett square to predict the possible use a Punnett square to predict the possible genotypes of their offspring. genotypes of their offspring.

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Making a Punnett SquareMaking a Punnett Square

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Monohybrid CrossesMonohybrid Crosses Mendel’s cross Tt x TtMendel’s cross Tt x Tt Half the gametes of each parent would Half the gametes of each parent would

contain the T allele, and the other half contain the T allele, and the other half would contain the t allele.would contain the t allele.

Gametes that each parent forms

Tt x Tt

One parent

Other Parent

It doesn’t matter which set of gametes are on top and which are on the side.

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Determining Phenotypes Determining Phenotypes (characteristics)(characteristics)

If organism has at least one dominant If organism has at least one dominant allele, dominant trait will be expressed.allele, dominant trait will be expressed.(TT, BB, Tt, Bb )(TT, BB, Tt, Bb )

For the recessive trait to be For the recessive trait to be expressed, organism must lack expressed, organism must lack dominant allele and have two dominant allele and have two recessive alleles. (bb or tt)recessive alleles. (bb or tt)

Of the offspring ¼ will be homozygous Of the offspring ¼ will be homozygous dominant (TT/BB) 2/4 or ½ will be dominant (TT/BB) 2/4 or ½ will be heterozygous (Tt, tT, Bb, bB) and ¼ heterozygous (Tt, tT, Bb, bB) and ¼ will be homozygous recessive. will be homozygous recessive.

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Mendel’s Dihybrid CrossesMendel’s Dihybrid Crosses

DiDi means “two.” means “two.” Dihybrid cross: A cross involving two different traits.Dihybrid cross: A cross involving two different traits. Mendel did true-breed dihybrid cross between Mendel did true-breed dihybrid cross between

round yellow seeds (RRYY) and wrinkled green round yellow seeds (RRYY) and wrinkled green seeds (rryy)seeds (rryy)

Mendel’s results of dihybrid cross:Mendel’s results of dihybrid cross:– PP11 round yellow x wrinkled green round yellow x wrinkled green– FF11 All round yellow (round yellow dominant) All round yellow (round yellow dominant)– FF22 9 round yellow, 3 round green, 3 wrinkled 9 round yellow, 3 round green, 3 wrinkled

yellow, 1 wrinkled green. yellow, 1 wrinkled green. Mendel found 2 dominate traits for round and yellow seeds.Mendel found 2 dominate traits for round and yellow seeds.

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Mendel’s dihybrid crossMendel’s dihybrid cross

Dihybrid cross led to Mendel’s Law of Independent Assortment.

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Dihybrid CrossesDihybrid Crosses

Think of the RRYY x rryy cross (Round Think of the RRYY x rryy cross (Round Yellow x Wrinkle Green seeds)Yellow x Wrinkle Green seeds)

Mendel found that seed shape and seed Mendel found that seed shape and seed color would be inherited independently of color would be inherited independently of each other.each other.

Punnett square you will need four boxes on Punnett square you will need four boxes on each side.each side.

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Dihybrid CrossDihybrid Cross

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Law of Independent Law of Independent AssortmentAssortment

Mendelian principle stating that genes for Mendelian principle stating that genes for different traits are inherited independently different traits are inherited independently

of each other.of each other.

Example: Genotype RrYy produces gametes:Example: Genotype RrYy produces gametes:Rr will separateRr will separateYy will separate Yy will separate

Recombine in 4 different ways.Recombine in 4 different ways.

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Dihybrid CrossDihybrid Cross

Punnett squares are good for showing all the possible combinations of gametes and the likelihood that each will occur. However, you don’t get the exact ratio of results shown in the square.

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ProbabilityProbability

Genetics follows the rules of chance.Genetics follows the rules of chance. Probability or chance that an event will occur Probability or chance that an event will occur

can be determined by dividing the number of can be determined by dividing the number of desired outcomes by the total number of desired outcomes by the total number of possible outcomes.possible outcomes.

desired # of outcomes / total # of possible outcomesdesired # of outcomes / total # of possible outcomes(Example: Toss a coin the probability of getting (Example: Toss a coin the probability of getting

heads would be one in two chances, written 1:2 or ½. heads would be one in two chances, written 1:2 or ½. A Punnetts square can be used to determine A Punnetts square can be used to determine

the probability of the event.the probability of the event.

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ProbabilityProbability

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MeiosisMeiosis

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GenesGenes Tens of thousands of genesTens of thousands of genes Lined up on chromosomesLined up on chromosomes

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ChromosomesChromosomes

Occur in pairs Occur in pairs (Male, (Male, Female)Female)

DIPLOIDDIPLOID—A cell with —A cell with two of each kind of two of each kind of chromosome is said to chromosome is said to be diploid, or 2be diploid, or 2nn, , number of number of chromosomeschromosomes

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GametesGametes

Male (sperm) and Female (egg)Male (sperm) and Female (egg) Contain one of each kind of chromosomes.Contain one of each kind of chromosomes. A cell with one of each kind of chromosome A cell with one of each kind of chromosome

is called a is called a HAPLOID HAPLOID and is said to contain a and is said to contain a haploid, or haploid, or nn, number of chromosomes., number of chromosomes.

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OrganismOrganism Body Cell (2Body Cell (2nn)) Gamete (Gamete (nn))

Fruit FlyFruit Fly 88 44

Garden PeaGarden Pea 1414 77

CornCorn 2020 1010

TomatoTomato 2424 1212

Leopard FrogLeopard Frog 2626 1313

AppleApple 3434 1717

HumanHuman 4646 2323

ChimpanzeeChimpanzee 4848 2424

DogDog 7878 3939

Adder’s tongue Adder’s tongue fernfern

12601260 630630

Chromosome Numbers of Some Chromosome Numbers of Some Common OrganismsCommon Organisms

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Homologous ChromosomesHomologous Chromosomes

Pair chromosomes are Pair chromosomes are called called homologous homologous chromosomes—chromosomes—determine phenotypedetermine phenotype..

Gene for same traitGene for same trait– same order,same order,– chromosomes in a chromosomes in a

homologous pair are not homologous pair are not always identical.always identical.

(Chromosome 4 (Chromosome 4 contains 3 traits contains 3 traits Mendel Studied)Mendel Studied)

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MeiosisMeiosis

From the Greek word From the Greek word meiounmeioun, , meaning “to diminish”. meaning “to diminish”.

Cell division that results in a Cell division that results in a gamete containing half the gamete containing half the

number of chromosomes of its number of chromosomes of its parents.parents.

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MeiosisMeiosis Divisions: Meiosis I and Meiosis IIDivisions: Meiosis I and Meiosis II Begins with Begins with

one diploid (2one diploid (2nn) cell four haploid () cell four haploid (nn) cells. ) cells. Sex cells (gametes) haploid.Sex cells (gametes) haploid. Sperm fertilizes an egg-results in zygote Sperm fertilizes an egg-results in zygote

(diploid)(diploid) Zygote develops by MITOSIS into a multi-Zygote develops by MITOSIS into a multi-

cellular organism.cellular organism. Reproduction Reproduction —Production and subsequent —Production and subsequent

fusion of haploid sex cells. fusion of haploid sex cells.

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InterphaseInterphase

Chromosomes replicate Chromosomes replicate Chromosome Chromosome

– two identical sister chromatids held together by a two identical sister chromatids held together by a centromerecentromere

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Prophase IProphase I Chromosomes coil up and a Chromosomes coil up and a

spindle forms.spindle forms. Homologous chromosomes Homologous chromosomes

comes together, matched comes together, matched gene by gene, to form a four-gene by gene, to form a four-part structure called a tetrad.part structure called a tetrad.

Chromatid pair so tight that Chromatid pair so tight that sometimes non-sister sometimes non-sister chromatids from homologous chromatids from homologous chromosomes sometimes chromosomes sometimes exchange genetic material in exchange genetic material in a process known as a process known as crossing overcrossing over..

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Crossing OverCrossing Over Exchange of genetic materialExchange of genetic material Any location Any location Several locations at onceSeveral locations at once Humans-Two to three crossovers for each pair of Humans-Two to three crossovers for each pair of

homologous chromosomes.homologous chromosomes.

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Metaphase IMetaphase I Centromere attaches to a spindle fiberCentromere attaches to a spindle fiber Spindle fibers pull the tetrads into the middle, or Spindle fibers pull the tetrads into the middle, or

equator, of the spindle.equator, of the spindle. Chromosomes are lined up side by side as Chromosomes are lined up side by side as

tetrads.tetrads.

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Anaphase IAnaphase I Chromosomes separate Chromosomes separate

and move to opposite and move to opposite ends of the cell. ends of the cell.

Centromeres holding the Centromeres holding the sister chromatids sister chromatids together do not split like together do not split like they do in anaphase of they do in anaphase of mitosis.mitosis.

Ensures that each new Ensures that each new cell have only one cell have only one chromosome from each chromosome from each homologous pair.homologous pair.

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Telophase ITelophase I

Spindle is broken downSpindle is broken down Chromosomes uncoilChromosomes uncoil Cytoplasm divides Cytoplasm divides 2 2

new cells.new cells. Half of genetic Half of genetic

information of original information of original cell (one chromosome cell (one chromosome from each homologous from each homologous pair)pair)

Another cell division Another cell division neededneeded

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Meiosis IIMeiosis II

Newly formed cells go through short interphase Newly formed cells go through short interphase (chromosomes don’t replicate)(chromosomes don’t replicate)

Prophase II—Spindle forms in each of the two new cells and Prophase II—Spindle forms in each of the two new cells and the spindle fibers attach to the chromosomes.the spindle fibers attach to the chromosomes.

Metaphase II—The chromosomes, still made up of sister Metaphase II—The chromosomes, still made up of sister chromatids, are pulled to the center of the cell and line up chromatids, are pulled to the center of the cell and line up randomly at the equator.randomly at the equator.

Anaphase II—Centromere of each chromosome splits, Anaphase II—Centromere of each chromosome splits, allowing sister chromatids to separate and move to opposite allowing sister chromatids to separate and move to opposite poles.poles.

Telophase II—Nuclei reform, spindles break down, and Telophase II—Nuclei reform, spindles break down, and cytoplasm divides.cytoplasm divides.

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Meiosis ResultsMeiosis Results

Four haploid sex cells have been formed Four haploid sex cells have been formed from one original diploid cell.from one original diploid cell.

Each haploid cell contains one chromosome Each haploid cell contains one chromosome from each homologous pair.from each homologous pair.

Haploid cells become gametes, transmitting Haploid cells become gametes, transmitting the genes they contain to offspring.the genes they contain to offspring.

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Genetic RecombinationGenetic Recombination Gene combinations vary based on how chromosomes Gene combinations vary based on how chromosomes

lines up during metaphase I (random).lines up during metaphase I (random). As number of chromosome increase the number of gene As number of chromosome increase the number of gene

combinations increase.combinations increase.Example:Example: Pea Plants (7 Pea Plants (7 nn) 2) 277= 128 x 128 =16,384 different offspring = 128 x 128 =16,384 different offspring

Sperm Egg Sperm Egg Human (23Human (23nn) 2) 22323= 8 million x 8 million =70 trillion = 8 million x 8 million =70 trillion

possible zygotes. possible zygotes. Reassortment of chromosomes and the genetic Reassortment of chromosomes and the genetic

information they carry, either by crossing over or by information they carry, either by crossing over or by independent segregation of homologous chromosomes is independent segregation of homologous chromosomes is called called GENETIC RECOMBINATIONGENETIC RECOMBINATION..

Provides genetic variation Provides genetic variation

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Mistakes in MeiosisMistakes in Meiosis

Nondisjunction (failure of homologous Nondisjunction (failure of homologous chromosomes to separate) results in chromosomes to separate) results in gametes with either an extra chromosome gametes with either an extra chromosome or a missing chromosome.or a missing chromosome.

Extra chromosomes often survive; those Extra chromosomes often survive; those lacking one or more usually do not.lacking one or more usually do not.

TRISOMY—Extra ChromosomeTRISOMY—Extra Chromosomee.g.—Extra chromosome 21—Down’s e.g.—Extra chromosome 21—Down’s SyndromeSyndrome

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Mistakes in Meiosis Continued…Mistakes in Meiosis Continued…

MONOSOMYMONOSOMY—When a gamete with a —When a gamete with a missing chromosomes fuses with a normal missing chromosomes fuses with a normal gamete during fertilization, resulting zygote gamete during fertilization, resulting zygote lacking chromosome.lacking chromosome.

Most zygotes don’t survive; if do organisms Most zygotes don’t survive; if do organisms generally does not.generally does not.

Turner syndrome—human females with only Turner syndrome—human females with only one X chromosome.one X chromosome.

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Mistakes in MeiosisMistakes in Meiosis

Lack of separation chromosomes- Gamete inherits Lack of separation chromosomes- Gamete inherits a complete diploid set of chromosomes.a complete diploid set of chromosomes.

TriploidTriploid—gamete with extra set of chromosome is —gamete with extra set of chromosome is fertilized by a normal haploid gamete, resulting fertilized by a normal haploid gamete, resulting offspring has a set of three chromosomes.offspring has a set of three chromosomes.

TetraploidTetraploid—Fusion of two gametes, each with an —Fusion of two gametes, each with an extra set of chromosomes, produces offspring with extra set of chromosomes, produces offspring with four sets of chromosomes.four sets of chromosomes.

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PolyploidyPolyploidy

Organisms with more than the usual number Organisms with more than the usual number of chromosome sets are called of chromosome sets are called POLYPLOIDYPOLYPLOIDY..

Animals almost always cause death in Animals almost always cause death in zygoteszygotes

Plants happens often; Larger and healthier. Plants happens often; Larger and healthier. Great commercial value.Great commercial value.

Plant breeders artificially produce polyploidy Plant breeders artificially produce polyploidy plants using chemicals that cause non-plants using chemicals that cause non-disjunction.disjunction.