Mendelian GeneticsMendelian Genetics

Introduction to GeneticsIntroduction to Genetics Introduction to GeneticsIntroduction to Genetics GENETICSGENETICS – branch of biology – branch of biology

that deals with heredity and that deals with heredity and variation of organisms.variation of organisms.

ChromosomesChromosomes carry the carry the hereditary information hereditary information (genes)(genes)

Arrangement of Arrangement of nucleotides in DNAnucleotides in DNA

DNA - RNA - ProteinsDNA - RNA - Proteins

Gregor MendelGregor Mendel

Gregor Johann MendelGregor Johann Mendel Austrian Monk, born in what is Austrian Monk, born in what is

now Czech Republic in 1822now Czech Republic in 1822

Son of peasant farmer, studiedSon of peasant farmer, studiedTheology and was Theology and was

ordainedordainedpriest O.S.A.priest O.S.A.

Went to the university of Went to the university of Vienna, where he studied Vienna, where he studied botany and learned the botany and learned the Scientific MethodScientific Method

Mendel’s Peas Mendel’s Peas

Mendel looked at 7 traits in pea plantsMendel looked at 7 traits in pea plants

MendelMendel

Mendel was the first biologist to use Mathematics Mendel was the first biologist to use Mathematics – to explain his results quantitatively.– to explain his results quantitatively.

Mendel predictedMendel predicted

Described the concept of genesDescribed the concept of genes

Determined that genes occur in pairsDetermined that genes occur in pairs

Described that one gene of each pair isDescribed that one gene of each pair ispresent in the gametes (one from the sperm and present in the gametes (one from the sperm and

one from the egg. one from the egg.

Important TerminologyImportant Terminology Homozygous Homozygous – having – having

identical genes (one from each identical genes (one from each parent) for a particular parent) for a particular characteristic (TT, tt) .characteristic (TT, tt) .HeterozygousHeterozygous – having two – having two different genes for a particular different genes for a particular characteristic (Tt)characteristic (Tt)

Dominant Dominant – the allele of a – the allele of a gene that masks or suppresses gene that masks or suppresses the expression of an alternate the expression of an alternate allele; the trait appears in the allele; the trait appears in the heterozygous condition. (Tt, TT)heterozygous condition. (Tt, TT)Recessive Recessive – an allele that is – an allele that is masked by a dominant allele; masked by a dominant allele; does not appear in the does not appear in the heterozygous condition, only in heterozygous condition, only in homozygous. (tt)homozygous. (tt)

Check for Understanding #1Check for Understanding #1

Terminology Cont’dTerminology Cont’d Phenotype – the physical appearance Phenotype – the physical appearance

of an organism.of an organism. Genotype – the genetic makeup of an organismsGenotype – the genetic makeup of an organisms

Gamete – a specialized sex cell that contains HALF of the Gamete – a specialized sex cell that contains HALF of the genetic information necessary for expression of a traitgenetic information necessary for expression of a trait

Monohybrid cross: a genetic cross involving a single pair Monohybrid cross: a genetic cross involving a single pair of genes (one trait); parents differ by a single trait.of genes (one trait); parents differ by a single trait.

P = ParentalP = Parental

F1 = First filial generation; offspring from a genetic F1 = First filial generation; offspring from a genetic cross.cross.

F2 = Second filial generation of a genetic crossF2 = Second filial generation of a genetic cross

Monohybrid CrossMonohybrid Cross Parents differ by a single trait.Parents differ by a single trait.

Crossing two pea plants that differ in stem Crossing two pea plants that differ in stem size, one tall one shortsize, one tall one short

T = allele for TallT = allele for Tall

t = allele for dwarft = allele for dwarf

TT = homozygous tall plantTT = homozygous tall plant

t t = homozygous dwarf plantt t = homozygous dwarf plant

Punnett SquarePunnett SquareA useful tool to do genetic crossesA useful tool to do genetic crosses

For a monohybrid cross, you need a square divided For a monohybrid cross, you need a square divided by four….by four….

We use the Punnett square to We use the Punnett square to predict the genotypes and predict the genotypes and phenotypes of the offspring.phenotypes of the offspring.

Using a Punnett SquareUsing a Punnett Square

Steps: Steps: 1. determine the genotypes of the parent organisms 1. determine the genotypes of the parent organisms 2. write down your "cross" (mating) 2. write down your "cross" (mating) 3. draw a p-square 3. draw a p-square

Parent genotypes:Parent genotypes:TT and TT and t tt t

CrossCrossT T ´ T T ´ t tt t

Using a Punnett Square Using a Punnett Square cont’dcont’d

4. "split" the letters of the genotype 4. "split" the letters of the genotype for each parent & put them "outside" for each parent & put them "outside" the p-square the p-square 5. determine the possible genotypes 5. determine the possible genotypes of the offspring by filling in the p-of the offspring by filling in the p-square square 6. summarize results (genotypes & 6. summarize results (genotypes & phenotypes of offspring)phenotypes of offspring)

H

h

H h

HH Hh

Hh hh

Mendels PrinciplesMendels Principles

Principle of DominancePrinciple of Dominance::One allele masked another, one One allele masked another, one

allele was dominant over the other in the F1 allele was dominant over the other in the F1 generation.generation.

Principle of SegregationPrinciple of Segregation::When gametes are formed, the When gametes are formed, the

pairs of hereditary factors (genes) become pairs of hereditary factors (genes) become separated, so that each sex cell separated, so that each sex cell (egg/sperm) receives only one kind of gene.(egg/sperm) receives only one kind of gene.

Real Life Case: Cystic Real Life Case: Cystic FibrosisFibrosis

IF two parents carry the recessive IF two parents carry the recessive gene of Cystic Fibrosis (gene of Cystic Fibrosis (cfcf), that is, ), that is, they are heterozygous (Cf they are heterozygous (Cf cfcf), one in ), one in four of their children is expected to four of their children is expected to be homozygous for be homozygous for cfcf and have the and have the disease: disease:

Cf Cf = normal

Cf cf = carrier, no symptoms

cf cf = has cystic fibrosis

Cf

cf

Cf cf

CfCf Cfcf

Cfcf cfcf

Dihybrid CrossDihybrid Cross Dihybrid crossesDihybrid crosses Matings that involving parents that differ in Matings that involving parents that differ in twotwo genes (two genes (two

independent traits)independent traits) For example flower colorFor example flower color

A = purple (dominant)A = purple (dominant)a a = white (recessive)= white (recessive)and stem length:and stem length:B = tallB = tall

b b = dwarf= dwarf

Dihybrid Cross RatiosDihybrid Cross Ratios

11 AABBAABB 22 AABbAABb 22 AaBBAaBB 44 AaBbAaBb 11 AAbbAAbb 22 AabbAabb 11 aaBBaaBB 22 aaBbaaBb 11 aabbaabb

Tall; Purple Flowers (9)

Tall; White Flowers (3)

Dwarf; Purple Flowers (3)Dwarf; White Flowers (1)

Principle of Independent Principle of Independent AssortmentAssortment

Based on these results, Mendel postulated Based on these results, Mendel postulated the the Principle of Independent Principle of Independent AssortmentAssortment::

“Members of one gene pair “Members of one gene pair segregate independently from other gene segregate independently from other gene pairs during gamete formation”pairs during gamete formation”

Genes get shuffled – these many Genes get shuffled – these many combinations are one of the advantages of combinations are one of the advantages of sexual reproduction sexual reproduction

Incomplete DominanceIncomplete Dominance

Snapdragon flowers Snapdragon flowers come in many colors.come in many colors.

If you cross a red If you cross a red snapdragon (RR) with snapdragon (RR) with a white snapdragon a white snapdragon (rr)(rr)

You get PINK flowers You get PINK flowers (Rr)(Rr)

Incomplete DominanceIncomplete Dominance

When F1 generation (all When F1 generation (all pink flowers) is self pink flowers) is self pollinated, the F2 pollinated, the F2 generation is 1:2:1 generation is 1:2:1 red, pink, whitered, pink, white

Other ExamplesOther Examples

•Hair Texture:  Curly hair is incompletely dominant (HH) to straight hair (SS).  Those who have wavy hair are intermediate (HS).• Inter-eye Distance:  Close-set eyes are incompletely dominant (DD) to eyes set far apart (FF).  Medium-set eyes are DF•Lip Protrusion:  Protruding lips are incompletely dominant (PP) to nonprotruding lips (NN).  Slightly protruding lips are intermediate (PN).

CodominanceCodominance

Two alleles that are equally Two alleles that are equally dominantdominant

One allele that is recessive to One allele that is recessive to bothboth

When both dominants are When both dominants are present, both show, not a present, both show, not a mid-range.mid-range.Roan coat color in ungulates Roan coat color in ungulates (cows!!)(cows!!)

-giving both -giving both brown and white HAIRS!brown and white HAIRS!

Codominance in Human Codominance in Human BloodBlood

Two allelesTwo alleles are expressed ( are expressed (multiple multiple allelesalleles) in ) in heterozygous heterozygous individualsindividuals..Example: bloodExample: blood

1.1. type Atype A = IAIA or IAi= IAIA or IAi2.2. type Btype B = IBIB or IBi= IBIB or IBi3.3. type ABtype AB= IAIB= IAIB 4.4. type Otype O= ii= ii

Polygenic TraitsPolygenic Traits

More than one allele exists within a More than one allele exists within a population population Example: human blood groups A, B, Example: human blood groups A, B, AB, OAB, O

Summary of GeneticsSummary of Genetics Chromosomes carry hereditary info (genes)Chromosomes carry hereditary info (genes)

Chromosomes (and genes) occur in pairsChromosomes (and genes) occur in pairs

New combinations of genes occur in sexual reproductionNew combinations of genes occur in sexual reproduction

Monohybrid vs. Dihybrid crossesMonohybrid vs. Dihybrid crosses

Mendel’s Principles:Mendel’s Principles:

Dominance: one allele masks anotherDominance: one allele masks another

Segregation: genes become separated in gamete formationSegregation: genes become separated in gamete formation

Independent Assortment: Members of one gene pair segregate Independent Assortment: Members of one gene pair segregate independently from other gene pairs during gamete formationindependently from other gene pairs during gamete formation