Mendel and Mendel and MeiosisMeiosis

Chapter 10Chapter 10

p. 258 - 285p. 258 - 285

Chapter OutlineChapter Outline

Mendel’s Laws of HeredityMendel’s Laws of Heredity MeiosisMeiosis

Mendel’s Laws of Mendel’s Laws of HeredityHereditySection 1 Section 1

p. 258 - 268p. 258 - 268

http://www.accessexcellence.org/AB/BC/Gregor_Mendel.html

GeneticsGenetics GeneticsGenetics is the branch of biology that is the branch of biology that

studies heredity.studies heredity.

HeredityHeredity is the passing of traits from from is the passing of traits from from parents to parents to offspring.offspring. one generation to the nextone generation to the next

TraitsTraits are characteristics that are inherited.are characteristics that are inherited.

Gregor MendelGregor Mendel is the father of genetics. is the father of genetics. He studied pea plants.He studied pea plants.

Why peas?Why peas? Mendel chose peas because they Mendel chose peas because they

reproduce sexually, and both the reproduce sexually, and both the male and female gametes are in male and female gametes are in the same flower. Therefore, the same flower. Therefore, plants could be self-pollinated or plants could be self-pollinated or cross-pollinated.cross-pollinated.

GametesGametes – – sex cells sex cells (sperm & egg = pollen & ovule). (sperm & egg = pollen & ovule). Gametes have only 1 copy of each Gametes have only 1 copy of each

chromosome,chromosome, But body cells have 2 copies of every But body cells have 2 copies of every

chromosome (1 from mom, 1 from dad).chromosome (1 from mom, 1 from dad).

Pea ReproductionPea Reproduction Pollination –Pollination – The transfer of the male pollen grains to the female ovule.The transfer of the male pollen grains to the female ovule.

FertilizationFertilization – The fusing together of male and female gametes.– The fusing together of male and female gametes.

Self pollinationSelf pollination – – male and female gametes come from the same plant.male and female gametes come from the same plant.

Cross pollinationCross pollination – male and female gametes come from different – male and female gametes come from different plants.plants.

Mendel’s Monohybrid Mendel’s Monohybrid CrossCross

Monohybrid crossMonohybrid cross – Reproductive cross in – Reproductive cross in which the parents differ by only one traitwhich the parents differ by only one trait

Mendel cross-pollinated a Mendel cross-pollinated a true breeding talltrue breeding tall and and true breeding shorttrue breeding short pea plant to produce a new pea plant to produce a new hybridhybrid plant. plant. True breedingTrue breeding – when crossed with itself, – when crossed with itself,

produces only offspring like itself.produces only offspring like itself. HybridHybrid – offspring of parents that have different – offspring of parents that have different

forms of a certain trait. forms of a certain trait.

All the F1 offspring of the monohybrid cross were All the F1 offspring of the monohybrid cross were tall.tall. F1 generation = 1F1 generation = 1stst offspring offspring P1 generation = parentsP1 generation = parents

Then he self-pollinated the F1 generation Then he self-pollinated the F1 generation ¾ of the F2 offspring were tall, and ¼ were short.¾ of the F2 offspring were tall, and ¼ were short.

Rule of Unit FactorsRule of Unit Factors Mendel’s conclusion from his monohybrid cross:Mendel’s conclusion from his monohybrid cross:

Rule of unit factors:Rule of unit factors: each organism has two factors for each trait. each organism has two factors for each trait. He knew this because the short factor showed up in the F2 generation.He knew this because the short factor showed up in the F2 generation.

We now know that these factors are We now know that these factors are genesgenes that are located that are located on chromosomes. on chromosomes. One gene (or factor) comes from the female parent and one comes One gene (or factor) comes from the female parent and one comes

from the male parent.from the male parent.

Different forms of genes (factors) for the same trait are Different forms of genes (factors) for the same trait are called called alleles. alleles.

Each allele exists on a chromosome from mom and dad.Each allele exists on a chromosome from mom and dad. TT, tt, TtTT, tt, Tt

Phenotype and GenotypePhenotype and Genotype PhenotypePhenotype – the way a trait is expressed, its looks or – the way a trait is expressed, its looks or

behaviorbehavior

GenotypeGenotype – allele / genetic makeup of trait – allele / genetic makeup of trait Dominant Allele –Dominant Allele – upper case, written firstupper case, written first Recessive Allele –Recessive Allele – lower case, written last lower case, written last

HomoHomozygous –zygous – both alleles are the same both alleles are the same TT (homozygous dominant)TT (homozygous dominant) tt (homozygous recessive)tt (homozygous recessive)

HeteroHeterozygous –zygous – alleles are different alleles are different TtTt

Rule of DominanceRule of Dominance Dominant traitsDominant traits

The dominant allele is the one The dominant allele is the one that is always seen in the that is always seen in the phenotype.phenotype.

Tall plant allele is dominant Tall plant allele is dominant over short plant allele.over short plant allele.

Organisms can look the Organisms can look the same but have different same but have different alleles (genes) due to alleles (genes) due to dominance.dominance. TT, TtTT, Tt

Recessive traitsRecessive traits The recessive allele is only The recessive allele is only

seen in the phenotype when seen in the phenotype when no dominant allele is present.no dominant allele is present. tttt

Law of Law of SegregationSegregation

Every individual plant has two alleles of each gene. (Tt)Every individual plant has two alleles of each gene. (Tt)

Gametes (sex cells) receive only one of these alleles, so two Gametes (sex cells) receive only one of these alleles, so two types of gametes can be produced. (Gamete 1 = T, types of gametes can be produced. (Gamete 1 = T, Gamete 2 = t)Gamete 2 = t)

Then, fertilization combines one allele from each parent Then, fertilization combines one allele from each parent randomly.randomly.

In other words, the alleles of the same trait separate (or In other words, the alleles of the same trait separate (or segregate) from one another and enter the gametes.segregate) from one another and enter the gametes. during fertilization, the gametes randomly pair to form 4 combinations of during fertilization, the gametes randomly pair to form 4 combinations of

allelesalleles

Mendel’s Dihybrid Mendel’s Dihybrid CrossesCrosses

Mendel also did crosses Mendel also did crosses involving TWO different traits, involving TWO different traits, these are called these are called dihybrid dihybrid crossescrosses..

Pea Shape – Round or wrinkledPea Shape – Round or wrinkled RR, Rr, rrRR, Rr, rr

And Pea Color – Yellow or And Pea Color – Yellow or greengreen YY, Yy, yyYY, Yy, yy

Dihybrid Dihybrid CrossCross

Parent Generation (P1)Parent Generation (P1) All true breeding pea plantsAll true breeding pea plants RRYY x rryyRRYY x rryy

Second Generation (F1)Second Generation (F1) All Round and Yellow All Round and Yellow

Third Generation (F2)Third Generation (F2) 9 Round Yellow, 3 Round Green, 3 9 Round Yellow, 3 Round Green, 3

Wrinkled Yellow, 1 Wrinkled GreenWrinkled Yellow, 1 Wrinkled Green 9:3:3:1 phenotypic ratio9:3:3:1 phenotypic ratio This ratio always occurs in the F2 This ratio always occurs in the F2

generation!generation!

Law of Independent Law of Independent AssortmentAssortment

Explains the 9:3:3:1 ratio in the F2 generation Explains the 9:3:3:1 ratio in the F2 generation observed from Mendel’s dihybrid crosses.observed from Mendel’s dihybrid crosses.

Genes for different traits are inherited Genes for different traits are inherited independently of each other. They don’t stick independently of each other. They don’t stick together.together.

RrYy produces gametes of any of the following type, RrYy produces gametes of any of the following type, RyRy RYRY rYrY ryry

Punnett SquaresPunnett Squares Shorthand method of Shorthand method of

finding possible genotypes finding possible genotypes developed in 1905 by developed in 1905 by Reginald Punnett.Reginald Punnett.

Used as an easy way to Used as an easy way to predict the phenotypic predict the phenotypic ratios of possible offspring.ratios of possible offspring.

Classwork – Due Today!Classwork – Due Today! Section Assessment (1-5), pg. 262Section Assessment (1-5), pg. 262

Do these monohybrid cross Punnett squares:Do these monohybrid cross Punnett squares: 1.) Homozygous tall x homozygous short1.) Homozygous tall x homozygous short 2.) Homozygous tall x heterozygous tall2.) Homozygous tall x heterozygous tall 3.) Heterozygous tall x homozygous short3.) Heterozygous tall x homozygous short

Tell whether each of these representsTell whether each of these represents genotypegenotype or or phenotypephenotype. . If it If it represents genotype, tell whether it isrepresents genotype, tell whether it is heterozygous, homozygous heterozygous, homozygous dominant, or homozygous recessive.dominant, or homozygous recessive.

a.) LLa.) LL b.) blond hairb.) blond hair c.) dimpled chinc.) dimpled chin d.) blue eyesd.) blue eyes e.) Dde.) Dd f.) ssf.) ss

MeiosisMeiosis

Section 2 Section 2

Chromosome NumbersChromosome Numbers

DiploidDiploid A cell with two of each kind of chromosomeA cell with two of each kind of chromosome One set of chromosomes from each parentOne set of chromosomes from each parent 2 sets of instructions (Tt)2 sets of instructions (Tt) 2n2n

HaploidHaploid Gamete cells contain ONE of each kind of Gamete cells contain ONE of each kind of

chromosomechromosome 1 set of instructions (T) or (t)1 set of instructions (T) or (t) 1n1n

Homologous ChromosomesHomologous Chromosomes Two chromosomes of Two chromosomes of

each pair (one from each each pair (one from each parent)parent)

Genes arranged in same Genes arranged in same order on each order on each chromosomechromosome

Genes come in different Genes come in different versions called alleles!versions called alleles!

Meiosis produces haploid gametesMeiosis produces haploid gametes

MitosisMitosis = reproduction of body cells = reproduction of body cells MeiosisMeiosis = production of sex cells (gametes) = production of sex cells (gametes)

A cell undergoes a normal cell cycle before a A cell undergoes a normal cell cycle before a meiotic division - DNA is duplicatedmeiotic division - DNA is duplicated

Meiosis involves two separate divisions, producing Meiosis involves two separate divisions, producing four haploid cellsfour haploid cells

Each gamete produced is genetically uniqueEach gamete produced is genetically unique

Crossing OverCrossing Over Occurs during Prophase Occurs during Prophase

II

Two Chromatids, one Two Chromatids, one from each pair, wrap from each pair, wrap around each otheraround each other

Chromatids often break Chromatids often break as they twist and the as they twist and the broken ends may switch broken ends may switch placesplaces

Meiosis provides Meiosis provides variationvariation 7 pairs of chromosomes (Pea plant) 7 pairs of chromosomes (Pea plant)

128 Possible sperm, 128 possible eggs128 Possible sperm, 128 possible eggs 16,384 possible offspring16,384 possible offspring

23 pairs of chromosomes (Human)23 pairs of chromosomes (Human) 8 million possible sperm, 8 million eggs8 million possible sperm, 8 million eggs 70 trillion different zygotes possible70 trillion different zygotes possible

NondisjunctionNondisjunction

Failure of homologous chromosomes to Failure of homologous chromosomes to separate properlyseparate properly TrisomyTrisomy – extra chromosome – extra chromosome MonosomyMonosomy – missing chromosome – missing chromosome

Sometimes entire sets don’t separateSometimes entire sets don’t separate Triploid,Tetraploid, HexaploidTriploid,Tetraploid, Hexaploid Called Called polyploidspolyploids

Frequently occurs in plantsFrequently occurs in plants

Gene Linkage and MapsGene Linkage and Maps

Sometimes genes seem to be inherited Sometimes genes seem to be inherited togethertogether

These genes lie very close on a These genes lie very close on a chromosome to each other – chromosome to each other – LinkedLinked genesgenes

Crossing over can separate these linked Crossing over can separate these linked genes – frequency of new gene genes – frequency of new gene combinations help map genescombinations help map genes