Mendelian Mendelian GeneticsGenetics

How pea plants and humans How pea plants and humans mix it upmix it up

Early Ideas About Early Ideas About Heredity Heredity

People learned that it took two People learned that it took two parents to make a babyparents to make a baby

Many early beliefs about how Many early beliefs about how characteristics are transmitted.characteristics are transmitted.

Blending theoryBlending theory

Early Research Early Research Mendel was not the first to perform experiments with pea Mendel was not the first to perform experiments with pea

plantsplants

British farmers (stem height)British farmers (stem height) Performed almost the same experimentsPerformed almost the same experiments Obtained the same resultsObtained the same results Over 200 years beforeOver 200 years before

T A Knight (flower color)T A Knight (flower color) Same experimentsSame experiments Same resultsSame results 1790’s1790’s

Gregor MendelGregor Mendel Born in 1822 in the Born in 1822 in the

Chech republicChech republic

Joined an Augustinian Joined an Augustinian order in 1843order in 1843

Flunked out of college, Flunked out of college, but made some great but made some great friends who showed friends who showed him the value of good him the value of good datadata

Using pea plants, Using pea plants, found indirect but found indirect but observable evidence of observable evidence of how parents transmit how parents transmit genes to offspringgenes to offspring

The Garden Pea PlantThe Garden Pea Plant

Self-pollinatingSelf-pollinating True breeding (different alleles True breeding (different alleles

not normally introduced)not normally introduced) Can be experimentally cross-Can be experimentally cross-

pollinatedpollinated

FF11 Results of One Results of One Monohybrid CrossMonohybrid Cross

FF22 Results of Results of Monohybrid CrossMonohybrid Cross

Mendel’s Mendel’s Monohybrid Monohybrid Cross ResultsCross Results

787 tall 277 dwarf

651 long stem

207 at tip

705 purple 224 white

152 yellow428 green

299 wrinkled882 inflated

6,022 yellow 2,001 green

5,474 round 1,850 wrinkled

F2 plants showed dominant-to-recessive ratio that averaged 3:1

Mendel’s Theory Mendel’s Theory of Segregationof Segregation

An individual inherits a unit of An individual inherits a unit of information (allele) about a information (allele) about a characteristic from each parentcharacteristic from each parent

During gamete formation, the alleles During gamete formation, the alleles segregate from each other segregate from each other

Now we know about Alleles, alternative forms of a geneNow we know about Alleles, alternative forms of a gene

Punnett Square of a Punnett Square of a Monohybrid CrossMonohybrid Cross

Female gametes

Male gametes

A a

A

a Aa

AA Aa

aa

Dominant phenotype canarise 3 ways,recessive only one

Test CrossTest Cross

Individual that shows dominant Individual that shows dominant phenotype is crossed with individual phenotype is crossed with individual with recessive phenotypewith recessive phenotype

Examining offspring allows you to Examining offspring allows you to determine the genotype of the determine the genotype of the dominant individualdominant individual

Punnett Squares of Punnett Squares of Test CrossesTest Crosses

Homozygous recessive

a a

A

a aa

Aa Aa

aa

Homozygous recessive

a a

A

A Aa

Aa Aa

Aa

Two phenotypes All dominant phenotype

A Dihybrid Cross - FA Dihybrid Cross - F11 ResultsResults

AABB aabbx

AaBb

AB AB ab ab

TRUE-BREEDING PARENTS:

GAMETES:

F1 HYBRID OFFSPRING:

purple flowers, tall

white flowers,dwarf

All purple-flowered, tall

16 Allele 16 Allele Combinations in Combinations in FF22

aB

AB

AB

abAb

Ab

aB

ab

1/4

1/4

1/4

1/4

1/4 1/4 1/4 1/4

AaBb aabbAabb aaBb

AABB AABb AaBB AaBb

AABb AAbb AaBb Aabb

AaBb aaBB aaBbAaBB

1/161/161/161/16

1/161/161/161/16

1/161/161/161/16

1/16 1/16 1/16 1/16

Explanation of Mendel’s Explanation of Mendel’s Dihybrid ResultsDihybrid Results

If the two traits If the two traits are coded for are coded for by genes by genes on separate on separate chromosomes, chromosomes, sixteen gamete sixteen gamete combinations combinations are possibleare possible

aB

AB

AB

abAb

Ab

aB

ab

1/4

1/4

1/4

1/4

1/4 1/4 1/4 1/4

AaBb aabbAabb aaBb

AABB AABb AaBB AaBb

AABb AAbb AaBb Aabb

AaBb aaBB aaBbAaBB

1/161/161/161/16

1/161/161/161/16

1/161/161/161/16

1/16 1/16 1/16 1/16

Phenotypic Ratios in Phenotypic Ratios in FF22

Four Phenotypes:Four Phenotypes: Tall, purple-flowered Tall, purple-flowered (9/16) (9/16) Tall, white-floweredTall, white-flowered (3/16) (3/16) Dwarf, purple-floweredDwarf, purple-flowered (3/16) (3/16) Dwarf, white-flowered Dwarf, white-flowered (1/16) (1/16)

AaBb X AaBb

Independent AssortmentIndependent Assortment

Mendel concluded that the two Mendel concluded that the two “units” for the first trait were to be “units” for the first trait were to be assorted into gametes independently assorted into gametes independently of the two “units” for the other traitof the two “units” for the other trait

Members of each pair of homologous Members of each pair of homologous chromosomes are sorted into chromosomes are sorted into gametes at random during meiosis gametes at random during meiosis