Mendel’s WorkChapter 3 Section 1

Pg 80-85

Gregor Mendel—the father of genetics

• Genetics—the scientific study of heredity

• Heredity—the passing of traits from parents to offspring

Mendel’s PeasMendel could learn a lot from peas because:

• Pea plants have many traits that exist in only 2 forms such as tall vs short plants, but no medium plants.

Mendel’s PeasMendel could learn a lot from peas because:

• Peas produce a large number of offspring (kids) in each generation

HOW CAN PEA PLANTS REPRODUCE?

Self-pollination example

Mendel’s Peas

• Normally pea plants self pollinate. This means that pollen from the stamen of a plant enters the pistil of the same plant.

Self-pollinationPollen located on the stamen moves to pistil of the same plant

Mendel’s Peas

• Mendel cross pollinated plants. This means that he took pollen from a plant and rubbed it on the pistil of a different plant from which the stamens had been removed (so they couldn’t self pollinate).

Cross Pollination example

Cross Pollination diagram

Mendel’s Experiments• Purebred—always produces offspring with

the same form of a trait as the parent.

• Ex. Purebred dogs. Like Cocker spaniels, golden retrievers, Poodles.

The opposite is hybrid.This would be a pound puppy

Mendel’s Experiments

• In his first experiment Mendel crossed purebred tall plants

(they only have tall genes) with purebred short plants (that only have short genes).

• Filial (F1) generation—name given to the offspring of the first cross.

Mendel’s Experiments

?Mendel cross-pollinated purebred tall plants with purebred short plants and got….

Tall plant (breed) short plants

P generation (think “parents”)

F1 generation

Mendel’s ExperimentsAll TALL pea plants in the F1 generation….

P generation (think “parents”) F1 generation

So we could look at it like this too…

P generation

(cross pollinated)

F1 generation

(self-pollinated)

Mendel’s Experiments

Results of F2

• Mendel allowed the F1 plants to self-pollinate.

• He found that the F2 plants were a mix of tall and short plants in a ration of ¾ tall to ¼ short.

P generation

(cross pollinated)

F1 generation

(self-pollinated)

All tall

F2 generation

¾ tall and ¼ short

Traits Mendel studied in Pea Plants• Mendel also studied _7_ traits of pea plants: They were :

Seed shape, seed color, seed coat color, pod shape, flower position, flower color, and stem height.

Dominant and Recessive Alleles

• Pairs—the factors that control each trait exist in pairs.– Female parent—contributes one factor– Male parent—contributes one factor– Together these make a pair

Dominant and Recessive Alleles

• Genes—are the factors that control traits.

• Genes consist of pairs of alleles. One that comes from the mother parent and one that comes from the father parent.

Dominant and Recessive Alleles

• Alleles—the different forms of a gene (such as tall or short, wrinkled or smooth).

• Dominant allele—one whose trait always shows up when the allele is present.

• Recessive allele—is masked (or covered up) when the dominant allele is present. Recessive alleles only show up if a dominant allele is not present.

EXAMPLESrecessive dominant

Recessive is the green box and dominant is the black box. Each of your parents has a pair of alleles that they can share. If they only give one… answer the following questions.

?

?

?

dominant dominant

recessive recessive

recessive dominant

EXAMPLESrecessive dominant

Recessive is the green box and dominant is the black box. Each of your parents has a pair of alleles that they can share. If they only give one… answer the following questions.

dominant dominant

recessive recessive

recessive dominant

?

?

?

dominant

EXAMPLESrecessive dominant

Recessive is the green box and dominant is the black box. Each of your parents has a pair of alleles that they can share. If they only give one… answer the following questions.

dominant dominant

recessive recessive

recessive dominant

?

?

?

dominant

dominant

EXAMPLESrecessive dominant

Recessive is the green box and dominant is the black box. Each of your parents has a pair of alleles that they can share. If they only give one… answer the following questions.

dominant dominant

recessive recessive

recessive dominant

?

?

?

dominant

dominant

recessive

Notice

• The only time the green box (or recessive allele) could show up is when a black box (or dominant allele) was not present.

• This will lead us into punnett squares.

recessive recessive recessive

Punnett Squares• So Mendel started out with 2 purebred

plants. One was tall and one was short.

• Capital T means “tall allele”, lowercase t means “short allel”

• But, each of those two plants (tall one and short one) has 2 alleles. They received one from their mother and one from their father.

• Options are T(tall) or t(short)

• Tall plant was TT –purebred (top of ps)

• Short plant was tt—purebred (left of ps)

• What would Tt be? A tall/medium/or short plant?

• You take this information and put it in a Punnett Square.

Punnett Square

.

1st take letters across….

. .

. .

Then take letters down…

• In a cross between two purebred plants (TT x tt), the probability of the offspring having the dominant characteristics is 100% (or 4 out of 4 which is the same as 4/4)

But what does it all mean?

Represents 4 possible offspring; the probability of children’s

height.

Probability and Genetics

Probability is

The likelihood that a particular event will occur.

Tossing a coin—landing head us is ?

The larger the sample size (more tosses of a coin), the closer the actual results predicted by probability.

You said 50%?

• Lets go to the virtual coin toss…• http://pbskids.org/cyberchase/games/probability/cointoss.html

• What about children. What is the likelihood that a woman would have a boy instead of a girl?

• Mendel used probability in genetics.

Mendel and Probability• Mendel was the first person to realize that

probability can be used to predict the results of genetic crosses.

• In other words he could use probability to “know” that all the offspring of the first generation would be tall… without even seeing them.

• Can be used to calculate the probability that offspring will have a certain combination of alleles.

• Can also be used to predict the probability of an offspring possessing a certain trait.

• Get used to this… we will do it a lot.

Punnett Squares

• In a cross between 2 hybrid tall plants (? x ?), the probability of offspring having the dominant characteristic is 75% (or 3 out of 4 or ¾) while the probability of the offspring having the recessive characteristic is 25% (or 1 out of 4 or ¼)

Punnett Square for 2 Hybrid plants to determine height of

offspring.

What letters should go on the top and to the side? You have to know what hybrid/ heterozygous means.

Punnett Square for 2 Hybrid plants to determine height of

offspring.

Right! But…

What do we do now?

Punnett Square for 2 Hybrid plants to determine height of

offspring.

We put each allele in a spot on the top and left. It doesn’t matter if the t is above or below the T as long as they are both there.

Punnett Square for 2 Hybrid plants to determine height of

offspring.Right! Now you move the alleles over…

Punnett Square for 2 Hybrid plants to determine height of

offspring.Right! Now you move the alleles over…and DOWN!!!

Day 2

• REMINDER: Science fair projects are DUE today and tomorrow!

But what does this square mean?First we have to learn about phenotype and genotypes.

Phenotype and Genotype

• Phenotype—The physical appearance of the offspring. (ex. Tall or short)

• Genotype—The genetic makeup or allele combination of the offspring.– Homozygous (purebred)—2 of the same alleles

• TT or tt

– Heterozygous (hybrid)—2 different alleles• Tt or tT which are the same thing.

What are the genotypes present?

Heterozygous/ Hybrid

Homozygous/ Purebred

Remember

• If there is one dominant (capital letter) trait, then that is the physical characteristic that shows up.

• Recessive traits only show up if there are no dominant traits present.

What are the phenotypes present?

Which ones are tall plants?

Which ones are tall plants?

Which ones are short plants?

Which ones are short plants?

Codominance• Alleles are neither dominant nor recessive.

Neither allele is covered up. Both show up.

• Write them as capital letters with superscripts. (F or F = black / white feathers).

• This means that F F heterozygous chickens have both black and white feathers.

W

WB

B

Codominance

• Red hair and White hair are codominant in cattle. Heterozygous ( H H ) cattle have red hairs and white hairs. They are called roan.

R W

Yellow (yy) is recessive, Green (YY) is dominant

Yellow (yy) is recessive, Green (YY) is dominant

Y is green, y is yellow

Y is green, y is yellow

F=feathers; W-white B-black

F F

F

F

W W

W

B

F=feathers; W-white B-black

F F

F FF F

F F

F F

F

F

W W

W

B

W

W W

W W WW

B B

What color are they?

F F

F FF F

F F

F F

F

F

W W

W

BW W

W W WW

B B

R=round, r=wrinkled Y=yellow, y=green

Activities for them

• Pea Soup

• Animated Cross

• Princess and Wrinkled Pea, Biologica

• Dragon Genetics, Biologica

• Mendel’s Peas, Biologica

• Cloning, Brain Pop

• DNA, Brain Pop (digestion, avian flu too)

Future activities

Resources for me

• Serendipity Labs

• Holy moly animations