Genetics 2

GeneticsGenetics

Heredity and GeneticsHeredity and Genetics• Heredity is the passing of

traits from parents to offspring

• Traits are controlled by genes, so GENETICS is the study of how traits are inherited through the action of alleles

• Austrian monk born in 1822 who is responsible

for the laws governing the inheritance of traits

• Between 1856 and 1863, Mendel cultivated and tested over 28,000 pea plants

• Mendel performed cross-pollination by transferring pollen from one plant to selected ova of other plants, thereby controlling which plants mixed

Gregor Mendel – “Father of Genetics”

Mendel’s ExperimentsMendel’s Experiments• Mendel produced pure strains by allowing

plants to self-pollinate, I.e., pollen (male gamete) from one plant fertilizes ova (female gamete) of same plant

Mendel’s Peas….Mendel’s Peas….

Mendel’s Results….Mendel’s Results….(F2 from crossing to heterozygous F1s)

Genetics TerminologyGenetics Terminology• Traits – any characteristic that can be passed from

parents to offspring• Heredity – the passing of traits from parents to

offspring• Alleles – one form (dominant or recessive) of a gene

• Sex cells have ONE form of a gene on their chromosomes

• Body cells have TWO forms or alleles for a single gene (you get one from mom and the other from dad)

• Dominant – is always expressed; masks a recessive trait

• Recessive – can only be expressed if there are no dominant alleles present

Genetics TerminologyGenetics Terminology• Dominant alleles are represented by a capital letter• Recessive alleles are represented by a lower case letter

Example: B = Brown eye color (dominant) b = Blue eye color (recessive)

• “Purebred” species have two of the same alleles; this is also called HOMOZYGOUS, e.g., BB (homozygous dominant) or bb (homozygous recessive)

• Species with two different alleles are called HETEROZYGOUS, e.g., Bb

• Genotype: the alleles present in the organism, i.e., BB, Bb, or bb

• Phenotype: the expression of the genes; what is observed

Genotype and Genotype and Phenotype in FlowerPhenotype in Flower

• All genes occur in pairs, so TWO alleles affect a trait.

• Possible combinations if:R = Red flowerr = Yellow flower

• Genotypes RR Rr rr• Phenotypes Red Red Yellow

Genetic CrossesGenetic Crosses• Monohybrid Cross: cross involving ONE

trait, e.g., eye color • Dihybrid Cross: cross involving TWO traits,

e.g., eye color and hair color• Offspring’s genotype and phenotype is

determined using a Punnett square

BrainPop - Heredity

Punnett Square, contPunnett Square, cont.

Generations in CrossesGenerations in Crosses• Parental Generation (P1) = the parental

generation in a breeding experiment• First Filial Generation (F1) = the first

generation of offspring in a breeding experiment

• Second Filial Generation (F2) = the second generation of offspring in a breeding experiment

TT Tttt TtTTTtTt tt

P1 Monohybrid CrossP1 Monohybrid Cross• Trait: Seed Color• Alleles: Y – Yellow y – Green• Cross: Yellow seeds X Green seeds

YY X yy Crossing two true-breeding (pure)

plants

Y YGenotype: Yy

Phenotype: Yellow

Genotypic Ratio: 100% Yy

Phenotypic Ratio: 100% Yellow

F1 Monohybrid CrossF1 Monohybrid Cross• Trait: Seed Color• Alleles: Y – Yellow y – Green• Cross: Yellow seeds X Green seeds

Yy X YyCrossing to heterozygotes (hybrids)

Y yGenotype: YY, Yy, yy

Phenotype: Yellow and Green

Genotypic Ratio: 25% YY, 50% Yy, 25% yy (1:2:1)

Phenotypic Ratio: 75% Yellow, 25% Green (3:1)

Dihybrid CrossesDihybrid Crosses• A breeding experiment that tracks the

inheritance of two traits• Mendel’s “Law of Independent Assortment”

• Each pair of alleles segregates independently during gamete formation

• Formula: 2n (n = # of heterozygotes)Example:

1. RrYy: 2n = 22 = 4 possible gametes RY Ry rY ry

2. AaBbCCDd: 2n = 23 = 8 gametes ABCD ABCd AbCD AbCd

aBCD aBCd abCD abCD

Dihybrid CrossesDihybrid CrossesTraits: Seed shape & Seed colorTraits: Seed shape & Seed colorAlleles:Alleles: R round r wrinkled Y yellow 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

Dihybrid CrossDihybrid Cross

RYRY RyRy rYrY ryry

copyright cmassengale

Round/Yellow: 9

Round/green: 3

wrinkled/Yellow: 3

wrinkled/green: 1

9:3:3:1 phenotypic ratio

RYRY RyRy rYrY ryry

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

9:3:3:1

Test CrossTest Cross

Y YYy yy

•Test crosses involve breeding the individual in question with another individual that expresses a recessive version of the same trait. If all offspring display the dominant phenotype, the individual in question is homozygous dominant; if the offspring display both dominant and recessive phenotypes, then the individual is heterozygous yY

Offspring all yellow!

½ Offspring yellow; ½ Offspring green!

Test CrossTest Cross• A mating between an individual of unknown

genotype and a homozygous recessive individual.• Example: bbC__ x bbcc

• BB = brown eyes• Bb = brown eyes• bb = blue eyes

• CC = curly hair• Cc = curly hair• cc = straight hair

bCbC b___b___

Test CrossTest Cross

Possible results:Possible results:bCbC b___b___

bcbc bbCc bbCc

C bCbC b___b___

bcbc bbCc bbccor

Incomplete DominanceIncomplete Dominanceandand

CodominanceCodominance

Incomplete DominanceIncomplete Dominance• F1 hybrids have an appearance somewhat in

between the phenotypes of the two parental varieties.

• Example: snapdragons (flower)red (RR)red (RR) x x white (rrwhite (rr)

RR = red flowerRR = red flowerrr = white flower

Incomplete DominanceIncomplete Dominance

AllAll Rr Rr = = pink pink(heterozygous pink)(heterozygous pink)

produces theproduces theFF11 generation generation

Incomplete DominanceIncomplete Dominance

CodominanceCodominance• Two alleles are expressed (multiple Two alleles are expressed (multiple

alleles) in heterozygous individuals.alleles) in heterozygous individuals.• Example: blood typeExample: blood type

• 1.1. type Atype A = I= IAAIIAA or I or IAAii• 2.2. type Btype B = I= IBBIIBB or I or IBBii• 3.3. type ABtype AB= I= IAAIIBB

• 4.4. type Otype O = ii= ii

Codominance ProblemCodominance Problem• Example:homozygous male Type B (IBIB)

xheterozygous female Type A (IAi)

IAIB IBi

1/2 = IAIB

1/2 = IBi

Another Codominance ProblemAnother Codominance Problem

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

IAi IBi

1/2 = IAi1/2 = IBi

•Question:If a boy has a blood type O and his sister has blood type AB, what are the genotypes and phenotypes of their parents?

boy-type O (ii) X girl-type AB (IAIB)

• Answer:Answer:

Parents:Parents:genotypesgenotypes = IAi and IBiphenotypesphenotypes = A and B

Sex-linked TraitsSex-linked Traits• Traits (genes) located on the sex

chromosomes• Sex chromosomes are X and Y• XX genotype for females• XY genotype for males• Many sex-linked traits carried

on X chromosome

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

Sex-linked Trait ProblemSex-linked Trait Problem

• Example: Eye color in fruit flies• (red-eyed male) x (white-eyed female)

XRY x XrXr

• Remember: the Y chromosome in males does not carry traits.

• RR = red eyed• Rr = red eyed• rr = white eyed• XY = male• XX = female

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

50% red eyed female

50% white eyed male

Female CarriersFemale Carriers

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