Chapter 5 Lesson 1

Mendel and His Peas

Gregor Mendel

Young priest that worked in the garden at a monastery in Vienna.

Considered the “Father of Genetics”

Observed traits in hundreds of pea plants.

Mendel’s Work - Vocabulary

Traits Physical characteristics of organisims

Heredity Passing of traits from parent to offspring

Genetics Scientific study of heredity

Mendel’s Peas

Why was Gregor Mendel lucky he chose peas to study? Many traits exist only in two forms Peas produce many offspring in one

generation.

How did Mendel make his crosses?

White Plant Purple Plant

Mendel’s Experiments

True Breeding (Purebred) Plant One that always produces offspring with

the same form of a trait as the parent i.e. purebred short plants will only

produce short offspring.

Purebred purple plants will only produce purple plants

X

White flower would mean the parents are not purebred

X

Mendel’s Experiments

Mendel crossed plants with opposite forms of a trait i.e. purple flowers crossed with white flowers

Look at Figure 3 on page 152 Explain why all offspring are purple in the

first generation (F1) What happens in the second generation

(F2)

Mendel’s Experiments

Mendel studied flower color followed by six other “opposite” traits.

Look at table 1 on page 154. What do you notice about the ratio of

traits in the second generation (F2)

Let’s Look at another example

Genes and Alleles

Genes Factors that control traits

Alleles Different forms of a gene One allele is inherited from each parent. Alleles can be dominant or recessive

Dominant and Recessive Alleles

Dominant allele Always seen if present Indicated by a capitol letter (T)

Recessive allele Masked or hidden if dominant allele is

present Indicated by a lower case letter (t)

Hybrid vs PurebredHybrid vs Purebred

Hybrid Hybrid Organism has two different alleles for the trait – Organism has two different alleles for the trait –

one dominant and one recessive.one dominant and one recessive. PurebredPurebred

Organism has two identical alleles for a trait – Organism has two identical alleles for a trait – two recessive OR two dominant.two recessive OR two dominant.

Alleles are represented with capitol Alleles are represented with capitol and lower case letters.and lower case letters.

T = Tall allele (dominant)T = Tall allele (dominant) t = Short allele (recessive)t = Short allele (recessive)

PurebredsPurebreds TT – two dominant alleles – plants will be tallTT – two dominant alleles – plants will be tall tt – two recessive alleles – plants will be shorttt – two recessive alleles – plants will be short

Hybrid Hybrid Tt – one dominat allele and one recessive allele – Tt – one dominat allele and one recessive allele –

plants will be tall.plants will be tall.

Phenotypes and GenotypesPhenotypes and Genotypes

PhenotypePhenotype Physical appearance or visible traitPhysical appearance or visible trait Example is flower color, stem height etc.Example is flower color, stem height etc.

GenotypeGenotype The genetic makeup or allele The genetic makeup or allele

combinationcombination Examples are TT or TtExamples are TT or Tt

Genotypes determine phenotypesGenotypes determine phenotypes

Homozygous and Homozygous and HeterozygousHeterozygous

HomozygousHomozygous An organism that has two identical alleles for a An organism that has two identical alleles for a

traittrait TT – homozygous dominantTT – homozygous dominant tt – homozygous recessivett – homozygous recessive

HeterozygousHeterozygous An organism that has two different alleles for a An organism that has two different alleles for a

traittrait Tt – heterozygous – will show the dominant traitTt – heterozygous – will show the dominant trait

ProbabilityProbability

ProbabilityProbability The likelihood that a particular The likelihood that a particular

event will occurevent will occur Example: coin tossExample: coin toss

Punnett SquaresPunnett Squares

Punnett SquaresPunnett Squares chart that shows all possible chart that shows all possible

combinations of alleles that combinations of alleles that can result from a genetic can result from a genetic cross. Used to determine the cross. Used to determine the probabilityprobability of a particular of a particular outcome.outcome.

Using a punnett squareUsing a punnett square Tt

Tt

T

t

T

t

TT

Tt

Tt

tt

(Tall)

(Tall)

Tall

Tall Tall

Short

CodominanceCodominance

Codominance – alleles are neither Codominance – alleles are neither dominant or recessive therefore both dominant or recessive therefore both alleles are expressed in the offspring alleles are expressed in the offspring

CodominanceCodominance

The heterozygous offspring will express (show) both phenotypes

What is the rabbit’s phenotype for fur color? What is the rabbit’s phenotype for fur color? __________________

What is the rabbit’s genotype for fur color? What is the rabbit’s genotype for fur color? ____________________

Is the rabbit heterozygous or homozygous for Is the rabbit heterozygous or homozygous for the fur color trait? _______________the fur color trait? _______________

If black is dominant over white, what genotype If black is dominant over white, what genotype would produce a white rabbit? _______would produce a white rabbit? _______

If fur color in rabbits was a codominant trait If fur color in rabbits was a codominant trait instead of dominant/recessive trait, what color instead of dominant/recessive trait, what color fur would the (Bb) rabbit have? fur would the (Bb) rabbit have? ________________________________

Consider a rabbit with black fur and the allele combination (Bb).

Black

(Bb)

heterozygous

(bb)

White and black