Genetics

Gregor Mendel: Father of Genetics

Genetics: study of heredity

Heredity: passing traits from parent to offspring

Used peas to study heredity

Why Peas?

1. Several Traits 2. Control mating 3. Small, grow easily, and matures

quickly

Alleles: Different versions of a geneEx: White and Purple Flowers

Mendel’s Experiment

Monohybrid Cross: One pair of contrasting traits. Ex: Mating a plant with purple flowers with white

flowers

True-breeding: All offspring would have only one form of the trait Ex: Purple-flowering plant only has purple-

flowered offspring

Mendel’s Experiments

Vocabulary

Dominant: Expressed in appearance (A)

Recessive: Not noticeable in appearance (a)

Homozygous: AA or aa

Heterozygous: Aa

More Vocabulary

Genotype: Genetic make up (ex: Pp)

Phenotype: Appearance (ex: Purple flowers)

Laws of Heredity Law of Segregation

The two alleles for a trait separate when gametes are formed

Law of Independent Assortment Alleles of different genes separate independently of

each other during meiosis

Law of Segregation

Separation of alleles into separate gametes

50% chance that one allele will go into a gamete

Law of Independent Assortment

Punnett Square

Ratios

Test Cross

Dihybrid Cross: Cross that involve two traits

Incomplete DominanceIncomplete Dominanceandand

CodominanceCodominance

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Incomplete DominanceIncomplete Dominance

F1F1 hybridshybrids have an appearance somewhat inin betweenbetween the phenotypesphenotypes of the two parental varieties.

Example:Example: snapdragonssnapdragons ( (flowerflower))red (RR) x white (rr)

RR = red flowerRR = red flower rr = white flower

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R

R

r r

Incomplete DominanceIncomplete Dominance

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RrRr

RrRr

RrRr

RrRr

RR

RR

rr

All Rr All Rr == pink pink(heterozygous pink)(heterozygous pink)

produces theproduces theFF11 generation generation

r

Incomplete DominanceIncomplete Dominance

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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

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Codominance ProblemCodominance Problem

Example: homozygous male Type B (IBIB)

x heterozygous female Type A (IAi)

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IAIB IBi

IAIB IBi

1/2 = IAIB

1/2 = IBi

IB

IA i

IB

Another Codominance ProblemAnother Codominance Problem

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

IAi IBi

IAi IBi

1/2 = IAi1/2 = IBi

i

IA IB

i

Sex-linked TraitsSex-linked Traits

Traits (genes) located on the sex Traits (genes) located on the sex chromosomeschromosomes

Sex chromosomes are X and YSex chromosomes are X and YXX genotype for femalesXX genotype for femalesXY genotype for malesXY genotype for malesMany sex-linked traits carried on Many sex-linked traits carried on

X chromosomeX chromosome24

Sex-linked TraitsSex-linked Traits

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Sex ChromosomesSex Chromosomes

XX chromosome - female Xy chromosome - male

fruit flyeye color

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

Sex-linked Trait ProblemSex-linked Trait ProblemExample: 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 eyedRr = red eyed rr = white eyedXY = maleXX = female

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XR

Xr Xr

Y

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

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XR Xr

Xr Y

XR Xr

Xr Y

50% red eyed female

50% white eyed male

XR

Xr Xr

Y

Female CarriersFemale Carriers

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