Mendel and his Peas

Mendel and his Peas

Chapter 9

State Objectives CLE 3210.4.5 Recognize how meiosis and sexual

reproduction contribute to genetic variation in a population.

CLE 3210.4.3 Predict the outcome of monohybrid and dihybrid crosses. SPI 3210.4.4 Determine the probability of a

particular trait in an offspring based on the genotype of the parents and the particular mode of inheritance.

CLE 3210.4.4 Compare different modes of inheritance: sex linkage, codominance, incomplete dominance, multiple alleles, and polygenic traits.

Sub-Objectives Explain the experiments of

Gregor Mendel Explain how genes and alleles

are related to genotypes and phenotypes

Use a Punnett square to predict genotypes and phenotypes

Gregor Mendel• A monk• Worked in the garden at the

monestary• Wrote Experiments in Plant

Hybridization in 1866.•   Experiments unnoticed until 1900.

Gregor Mendel Wanted to know how traits are

passed from one generation to the next

How some traits seem to skip a generation and show up in the next

Chose the pea plant to study

Why Peas?

Grow quickly Self pollinating Each flower contained both

male and female parts

Image from: http://www.jic.bbsrc.ac.uk/germplas/pisum/zgs4f.htm

Sexual Reproduction in Peas Pollen from the anther of one

plant is transferred to the stigma of another. Pollen travels down to to egg cell

Image from: http://anthro.palomar.edu/mendel/mendel_1.htm

Mendel’s Peas Mendel began by studying one

trait at a time. That way, he could understand

the results Some of the traits he observed

Plant height, seed shape, flower color

How did he start? He crossed pure-bred tall plants

with pure bred tall plants Results: All tall plants

He crossed pure-bred short plants with pure bred short plants Results: All short plants

Cross-pollination Anthers of one plant are

removed so it can not self pollinate

Pollen from another plant is used to pollinate the flower

Results from cross-pollination• When the peas were cross

pollinated, they produced offspring.

• These offspring are the first generation

• In the case of tall and short plants, all the offspring came out tall

Dominant and recessive traits The trait that appeared in that first

generation was called the dominant trait Dominant trait: masks the

presence of other traits The trait that did not show up he

called the recessive trait

Mendel’s second experiment Mendel crossed the individuals

from the first generation with each other

The next offspring are called the 2nd generation

In the second generation, the recessive trait reappeared

Counting the offspring Mendel counted the offspring

in the second generation with each trait to determine the ratio of individuals with the dominant trait to those with the recessive trait.

Mendel’s peas

Results of Mendel’s experiment

Results of Mendel’s experiment

Of Genes and Alleles Mendel looked at the math and

decided for each trait offspring had to have two “factors” one from their mother and one from their father

These factors that coded for the same trait are called genes

So what’s a trait? A specific characteristic that

varies from one individual to another

Mendel looked at seven Seed shape, seed color, seed coat

color, pod shape, pod color, flower position and plant height

Of Genes and Alleles For each gene, there may be

more than one form These different forms of genes

are called alleles

Terms•      Gene: a unit of heredity on a

chromosome.•      Allele: alternate state of a gene.•      Dominant: an allele that masks the

expression of other alleles.•      Recessive: an allele whose

expression is masked by dominant alleles.

Linking with Meiosis How does the information of

two alleles for each gene compare with what we know from meiosis?

What does each zygote get when sperm and egg fertilize?

What we know from meiosis Principle of segregation:

Alleles on homologous chromosomes separate during the process of meiosis

Only one allele from each parent is passed to the offspring

Segregation

Image from: http://anthro.palomar.edu/mendel/mendel_1.htm

Probability The mathematical chance that an

event will occur If you flip a coin, what’s the

chance it will come up heads? What the chance of three tails in a

row? Biologists use probability to

predict the outcome of genetic crosses

Punnett Square To understand Mendel’s

conclusions, we use a diagram called a Punnett square

Dominant alleles are symbolized with capital letters

Recessive alleles are symbolized with lower case letters

Remember for each trait there are two alleles So a cross from a true breeding tall

plant will produce a tall offspring whose alleles are written, TT

A true breeding short plant would be tt

Genotype The actual letters represent the

alleles this combination of alleles is called the genotype

Genotype: alleles present in the organism

TT, tt, or Tt

Back to our pea plants

Genotype Two possibilities

Homozygous: contains identical alleles (TT or tt)

Heterozygous: contains different alleles (Tt)

Phenotype An organisms appearance,

what the gene looks like is the phenotype

Phenotype: the physical appearance of the trait what it looks like

Making a Punnett Square

Baby Steps to a Punnett square 1. determine the genotypes of the

parent organisms 2. write down your "cross" (mating) 3. draw a p-square 4. "split" the letters of the genotype for each parent & put them "outside" the p-square

Baby Steps to a Punnett square 5. determine the possible

genotypes of the offspring by filling in the p-square 6. summarize results (genotypes & phenotypes of offspring) 7. bask in the glow of your accomplishment !

Making gametes Remember the principle of

segregation, especially with dihybrid crosses, Each gamete only gets one allele for each trait!!!!

Image from: http://arbl.cvmbs.colostate.edu/hbooks/pathphys/reprod/fert/gametes.html

Are all wrinkled peas yellow?? Once Mendel found how traits are

passed, he wanted to know if the segregation of one pair of alleles had anything to do with the segregation of another pair

In other words, are all wrinkled peas yellow???

Dihybrid cross Use a Punnett square to track

two traits at once Works like a monohybrid

cross, but you have to take care in forming your gametes

Principle of Independent Assortment Genes for different traits do not

affect each other in segregation Works for most traits unless they

are linked: close together on the same chromosome

Other exceptions to Mendel Incomplete dominance Codominance Multiple Alleles Polygenic traits

Incomplete DominanceWith incomplete dominance, a

cross between organisms with two different phenotypes produces offspring with a third phenotype that is a blending of the parental traits.

Incomplete dominance Neither allele is completely

dominant over the other Example: White and red four

o’clocks White:W Red: R

A Classic Example: Snapdragons

R = allele for red flowers W = allele for white flowers

red x white ---> pink RR x WW ---> 100% RW

Recognizing Incomplete DominanceTwo steps:

1) Notice that the offspring is showing a 3rd phenotype.  Not shown in the parents 2) Notice that the trait in the offspring is a blend (mixing) of the parental traits.

Codominance In Codominance, traits appear

together in the phenotype of hybrid organisms.

red x white ---> red & white spotted

Another classic example: Cows In cows if you cross a pure bred

red cow with a pure bred white cow, the offspring are roan

The color difference in their coats is because they have both red and white hairs together

Practice problems Try problems three through

five on your practice problems sheet

Multiple alleles Genes that have more than two alleles for a

trait Each individual can only have two, but in

the population more than two exist

Another classic example: Blood TypeHumans have three alleles for

blood typeIA: Type AIB: Type BI: Type OA and B are both dominant over

O, but are codominant with each other

Image from: http://www-micro.msb.le.ac.uk/MBChB/bloodmap/Blood.html

Polygenic traits Some traits are determined by the

interaction of many traits Examples: Height

Hair color Skin color

Studying genetics Thomas Hunt Morgan

Uses fruit flies Drosophila melanogaster

Breed a new generation every 14 days

Used because short generation time allows production of many generations

Image from: http://www.ceolas.org/fly/intro.html

Environmental InfluencesOur genes aren’t all of what we

areOur environment has influences

as wellExample: Heart disease

People with poor diets have higher incidences of heart disease