1

Mendelelian Mendelelian GeneticsGenetics

2

Gregor Gregor MendelMendel

(1822-1884)(1822-1884)•Austrian monkAustrian monk•Studied the Studied the inheritanceinheritance of of traits in traits in pea pea plantsplants•Developed the Developed the laws of laws of inheritanceinheritance

3

Site of Site of Gregor Gregor Mendel’s Mendel’s experimentexperimental garden al garden in the in the Czech Czech RepublicRepublic

4

5

6

Mendel Performed “crosses” on different pea plants:

Parents: Yellow Seeds X Green Seeds

F1: All offspring had yellow seeds

7

Parents:X

F1: X

F2: 3: 1

8

Generation “Gap”Generation “Gap”Parental PParental P11 Generation Generation = the parental = the parental

generation generation

FF11 generation generation = the first-generation = the first-generation

FF22 generation generation = the second-= the second-generationgeneration

9

Mendel found plants Mendel found plants were able to pass on were able to pass on traitstraits

He said these He said these physical traits are physical traits are inherited as inherited as “factors”“factors”

Mendel’s “factors” Mendel’s “factors” are actually are actually genesgenes

InheritanceInheritance

10

What we know now:

Genes code for traits

We have two copies of each gene – because we have two copies of each chromosome

11

Genetics TermsGenetics Terms

Gene – codes for Gene – codes for a trait (tallness)a trait (tallness) AllelesAlleles - - forms of a forms of a gene:gene: DominantDominant – “ – “stronger” of two stronger” of two alleles; always expressed;alleles; always expressed;(T)(T) RecessiveRecessive - - allele that shows only if allele that shows only if dominant is NOT there; dominant is NOT there; (t)(t)

Dominant MASKS the recessiveDominant MASKS the recessive

12

HomozygousHomozygous – SAME alleles – SAME alleles (TT or tt);(TT or tt); also called also called pure pure 

HeterozygousHeterozygous DIFFERENT DIFFERENT alleles (alleles (Tt);Tt); also called also called hybridhybrid

13

More TerminologyMore Terminology

GenotypeGenotype – – what what GENEGENE is is therethere (e.g. TT, Tt, tt) (e.g. TT, Tt, tt) PhenotypePhenotype - - the the PHPHysical ysical featurefeature (e.g. tall, short) (e.g. tall, short)

14

Genotype & Phenotype in Genotype & Phenotype in FlowersFlowers

Genotype of alleles:Genotype of alleles:TT = tall plant= tall plant

tt = short plant= short plant

GenotypesGenotypes TTTT TtTt tttt

PhenotypesPhenotypestall tall tall tall shortshort

15

Punnett SquarePunnett Square

Used to Used to

predictpredict outcomes of outcomes of genetic crossesgenetic crosses

16

Genetic Practice Genetic Practice ProblemsProblems

17

Breed the PBreed the P11 generation generation

tall (TT) x dwarf (tt) pea tall (TT) x dwarf (tt) pea plantsplants

t

t

T T

18

Solution:Solution:

t

t

T T

Tt

Tt

Tt

Tt All Tt = tall(heterozygous tall)

produces theFF11 generation generation

tall (TT) vs. dwarf (tt) pea tall (TT) vs. dwarf (tt) pea plantsplants

19

Breed the FBreed the F11 generation generation

tall (Tt) vs. tall (Tt) pea plantstall (Tt) vs. tall (Tt) pea plants

T

t

T t

20

Solution:Solution:

TT

Tt

Tt

tt

T

t

T tFF22 generation generation

1/4 (25%) = TT1/2 (50%) = Tt1/4 (25%) = tt1:2:1 genotype1:2:1 genotype 3:1 phenotype3:1 phenotype

tall (Tt) x tall (Tt) pea tall (Tt) x tall (Tt) pea plantsplants

21

Monohybrid Monohybrid CrossesCrosses

22

Trait: Seed ShapeTrait: Seed Shape

Alleles: Alleles: RR – Round – Round rr – Wrinkled – Wrinkled

Cross: Cross: RoundRound seedsseeds xx Wrinkled Wrinkled seedsseeds

RRRR xx rr rr

PP11 Monohybrid Cross Monohybrid Cross

R

R

rr

Rr

RrRr

Rr

Genotype:Genotype: RrRr

PhenotypePhenotype: RoundRound

GenotypicGenotypicRatio:Ratio: All alikeAll alike

PhenotypicPhenotypicRatio:Ratio: All alike All alike

23

PP11 Monohybrid Cross Monohybrid Cross ReviewReviewHomozygous dominant x Homozygous dominant x

Homozygous recessiveHomozygous recessiveOffspringOffspring allall HeterozygousHeterozygous (hybrids)(hybrids)Offspring calledOffspring called FF11 generation generation

Genotypic & Phenotypic ratio Genotypic & Phenotypic ratio isis ALL ALIKEALL ALIKE

24

Trait: Seed ShapeTrait: Seed Shape

Alleles: Alleles: RR – Round – Round rr – Wrinkled – Wrinkled

Cross: Cross: RoundRound seeds seeds xx Round Round seedsseeds

RrRr xx Rr Rr

FF11 Monohybrid Cross Monohybrid Cross

R

r

rR

RR

rrRr

Rr

Genotype:Genotype: RR, Rr, RR, Rr, rrrr

PhenotypePhenotype: Round Round && wrinkled wrinkled

G.Ratio:G.Ratio: 1:2:11:2:1

P.Ratio:P.Ratio: 3:1 3:1

25

FF11 Monohybrid Cross Monohybrid Cross ReviewReviewHeterozygous x heterozygousHeterozygous x heterozygousOffspring:Offspring:

25% Homozygous dominant25% Homozygous dominant RRRR50% Heterozygous50% Heterozygous RrRr25% Homozygous Recessive25% Homozygous Recessive rrrrOffspring calledOffspring called FF22 generation generationGenotypic ratio isGenotypic ratio is 1:2:11:2:1Phenotypic RatioPhenotypic Ratio is 3:1 is 3:1

26

Mendel’s LawsMendel’s Laws

27

Law of DominanceLaw of Dominance

Some alleles are dominant and other alleles are recessive. (T or t)

An organism with at least one dominant allele for a trait will always have that trait. (TT, Tt)

Recessive traits are only seen in homozygous recessive organisms. (tt)

28

Law of SegregationLaw of Segregation

During the During the formation of formation of gametesgametes (eggs or sperm), the (eggs or sperm), the two allelestwo alleles responsible for a responsible for a trait trait separateseparate from each other: from each other:

Tt can make two gametes:Tt can make two gametes: T gameteT gamete

t gametet gamete

29

Applying the Law of Applying the Law of SegregationSegregation

30

Law of Independent Law of Independent AssortmentAssortment

Alleles for Alleles for differentdifferent traits are traits are distributed to gametes distributed to gametes independently of one independently of one another.another.

This law can be illustrated This law can be illustrated using using dihybrid crossesdihybrid crosses..

31

Dihybrid CrossDihybrid CrossTraits: Seed shape & Seed colorTraits: Seed shape & Seed color

Alleles:Alleles: R round Y yellow r wrinkled y green

What gametes are possible in an individual that is heterozygous for both traits?

32

Dihybrid CrossDihybrid CrossTraits: Seed shape & Seed colorTraits: Seed shape & Seed colorAlleles:Alleles: R round Y yellow

r wrinkled y green

RrYy x RrYy

RY Ry rY ryRY Ry rY ry RY Ry rY ryRY Ry rY ry

All possible gamete combinationsAll possible gamete combinations

33

Dihybrid CrossDihybrid Cross

RYRY RyRy rYrY ryry

RYRY

RyRy

rYrY

ryry

34

Dihybrid CrossDihybrid Cross

RRYY

RRYy

RrYY

RrYy

RRYy

RRyy

RrYy

Rryy

RrYY

RrYy

rrYy

RrYy

Rryy

rrYy

rryy

Round/Yellow: 9

Round/green: 3

wrinkled/Yellow: 3

wrinkled/green: 1

9:3:3:1 phenotypic ratio

RYRY RyRy rYrY ryry

RYRY

RyRy

rYrY

ryry

35

Dihybrid CrossDihybrid Cross

Round/Yellow: 9Round/green: 3wrinkled/Yellow: 3wrinkled/green: 1

9:3:3:1

36

Incomplete DominanceIncomplete Dominanceandand

CodominanceCodominance

37

Incomplete DominanceIncomplete Dominance

F1 hybrids F1 hybrids have an appearance somewhat in betweenin between the phenotypes phenotypes of the two parental varieties.

Example:Example: snapdragons (flower)snapdragons (flower) RR = red flowerRR = red flower

WW = white flowerRW = pink

Cross a red x white on your 1st box

38

Incomplete DominanceIncomplete Dominance

RWRW

RWRW

RWRW

RWRW

WW

WW

RR RR

All RW =All RW = pink pink(heterozygous pink)(heterozygous pink)

produces theproduces theFF11 generation generation

39

Now cross two pink snapdragons on your paper.

40

CodominanceCodominance

Two allelesTwo alleles are expressed at same are expressed at same timetime

Example:Example: blood type has 3 alleles: blood type has 3 alleles:A, B, O (this is called MULTIPLE A, B, O (this is called MULTIPLE

alleles)alleles)

41

CodominanceCodominance

3 alleles: A, B, O3 alleles: A, B, O

1.1. type Atype A = AA or AO (_______)= AA or AO (_______)2.2. type Btype B = BB or BO (____)= BB or BO (____)3.3. type ABtype AB= AB = AB (codominant)(codominant)4.4. type Otype O = OO (______)= OO (______)

42

Codominance ProblemCodominance Problem

• Example:Example: male Type O (OO) x female type AB

AO BO

AO BO

1/2 = IAi1/2 = IBi

O

A B

O

43

Sex-linked TraitsSex-linked Traits

Traits (genes) located on the Traits (genes) located on the sex chromosomes – mostly X sex chromosomes – mostly X chromosomechromosome

44

Sex-linked TraitsSex-linked Traits

Sex ChromosomesSex Chromosomes

XX chromosome - female Xy chromosome - male

fruit flyeye color

Example: Example: Eye color in fruit Eye color in fruit fliesflies

45

Sex-linked Trait Sex-linked Trait ProblemProblem

Example: Eye color in fruit flies

Cross a (red-eyed male) x (white-eyed female)

46

Sex-linked Trait Sex-linked Trait ProblemProblem

(red-eyed male) x (white-eyed female) XRY x XrXr

.

Xr

XR y

Xr

47

Sex-linked Trait Solution:Sex-linked Trait Solution:

XR Xr

XR Xr

Xr y

Xr y

50% red eyed female

50% white eyed male

Xr

XR y

Xr

48

Female CarriersFemale Carriers

49

Polygenic Traits

Most traits have more than one gene involved

Example: height in humans