Genetics&

The Work of Mendel

Genetic Terminology Trait - any characteristic thatcan be passed from parent tooffspringHeredity - passing of traitsfrom parent to offspring Genetics - study of heredity

Gregor Mendel• Modern genetics began in

the mid-1800s in an abbey garden, where a monk named Gregor Mendel documented inheritance in peas– used good experimental design– used mathematical analysis• collected data & counted them

– excellent example of scientific method

Mendel’s work• Bred pea plants– cross-pollinate

true breeding parents

– raised seed & then observed traits

– allowed offspring to self-pollinate & observed next generation

Mendel collected data for 7 pea traits

Looking closer at Mendel’s worktrue-breedingpurple-flower peas

true-breeding white-flower peasParents

100%1st

generation(hybrids)

100%purple-flower peas

2nd

generation

3:175%purple-flower peas

25%white-flower peas

X

self-pollinate

What did Mendel’s findings mean?• Some traits mask others – purple & white flower colors are separate traits

that do not blend • purple x white ≠ light purple• purple masked white

– dominant allele • functional protein

– affects characteristic• masks other alleles

– recessive allele • no noticeable effect• allele makes a

non-functioning protein

allele producingfunctional protein

mutant allele malfunctioningprotein

homologouschromosomes

I’ll speak for both of us!

Genotype vs. phenotype• Difference between how an organism

“looks” & its genetics– phenotype • description of an organism’s trait

– genotype • description of an organism’s genetic makeup

Explain Mendel’s results using…dominant & recessive …phenotype & genotype

F1

P X

purple white

all purple

Making crosses• Can represent alleles as letters– flower color alleles P or p– true-breeding purple-flower peas PP– true-breeding white-flower peas pp

F1

P X

purple white

all purple

PP x pp

Pp

Punnett squares

Pp x Pp

P pmale / sperm

P

pfem

ale

/ eg

gs

PP

75%

25%

3:1

25%

50%

25%

1:2:1

%genotype

%phenotype

PP Pp

Pp pp pp

Pp

Pp

1st

generation(hybrids)

Mendel’s Laws**Inheritable factors or genes areresponsible for all heritablecharacteristics**Phenotype is based on Genotype**Each trait is based on two genes, one from the mother and the other from the father**True-breeding individuals are homozygous ( both alleles) are the same

Law of DominanceIn a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation.All the offspring will be heterozygous and express only the dominant trait.RR x rr yields all Rr (round seeds)

Law of Segregation*During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other.*Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring.

Law of Independent Assortment*Alleles for different traits are distributed to sex cells (& offspring) independently of one another.*This law can be illustrated using dihybrid crosses.

Dihybrid CrossA breeding experiment that tracks the inheritance of two traits.Mendel’s “Law of Independent Assortment”a. Each pair of alleles segregatesindependently during gamete formationb. Formula: 2n (n = # of heterozygotes)

Summary of Mendel’s laws

LAW PARENTCROSS

OFFSPRING

DOMINANCE TT x ttTall x Short

100% T t TALL

SEGREGATION Tt x Tttall x tall

75% tall25% short

INDEPENDENT ASSORTMENT

RrGg x RrGground & greenxround & green

9/16 round seeds & green pods3/16 round seeds & yellow pods3/16 wrinkled seeds & green pods1/16 wrinkled seeds & yellow pods