Unit 4 – Lecture 5

MendelGregor Mendel – father of genetics /

hereditary scigenetics – branch of biology which studies

heredityheredity – passing on of traits from parent

to offspringmonk; mid-1800s

Before Mendel…blending hypothesis – idea that all offspring

traits are a blend of the parent traits ex: tall + short = mediumrejected by Mendel’s work

Mendel’s Crossesto “cross” organisms means to breed them

together“test-cross” = graphic representation of

offspring probabilities

Mendel carried out his experiments on pea plants.

Mendel’s CrossesSteps to Cross Pea Plants

remove stamen [male parts] w/ pollen from one plant

use paintbrush to transfer pollen to pistil [female parts]of a plant with a different version of that trait[ex: flower color – white/purple]fertilization = fusion of

two gametes [1n +1n = 2n]

Mendel’s CrossesSteps to Cross Pea Plants

wait for results…then repeat…many times.

Mendel’s Crosses – cont’dExample:

Purple Flower1+ White Flower 1 Purple Flowers2

…mix two of those purple 2nd generation flowers…

Purple2 + Purple2 Purple and White!!So….what happened?First, we have to get some vocabulary…

Discuss

What is a test cross?

Before Mendel’s work, what was the name of idea that most people believed regarding

offspring inheriting traits equally from their parents?

Helpful VocabularyAllele – a version of a gene / trait

ex: trait = eye color; alleles = brown, blue, etc.

Traits are represented by two alleles one from each parentalleles are shown by

combinations of letters; each trait you work with will have a different letter.

Helpful Vocabulary – cont’dDominant Allele – is expressed when combined

with another allele represented by a capital letter [A, B, C, D, etc]ex: purple flowers, yellow seeds, round seeds

Recessive Allele – can be repressed/hidden when combined with another allelerepresented by a lower-case letter [a, b, c, d,

etc]ex: white flowers, green seeds, wrinkled seeds

Discuss

In Mendel’s work with the pea plants, which was the dominant flower allele? the recessive?

In review… what does “homo” mean? “hetero”?

Helpful Vocabulary – cont’dGENotype – set of alleles that an organism

has for a trait. [it’s genetic combination]homozygous = having two of the same

alleles for a traitex: AA or aa

heterozygous = having two different alleles for a traitex: Aa

Helpful Vocabulary – cont’dPHenotype – physical traits an organism has.

ex: brown hair, blue eyes, etc.

if genotype is homozygous, you will see phenotype of that gene if genes A = purple & a = whiteAA = purple phenotype / aa = white phenotype

if genotype is heterozygous, you will see the phenotype of the dominant geneAa = purple [recessive is hidden/repressed]

Helpful Vocabulary – cont’d

Law of Segregation of alleles– you have two alleles per traiteach of those alleles will separate so each

gamete gets one copy

[This is similar to the Law of Independent Assortment – which is basically the same thing, but talking about chromosomes instead of the trait alleles]

Mendel’s CrossesMendel controlled his experiments by:

studying ONE trait at a time.choosing plants which showed the same

trait through several generations true breeder = plant, when self-fertilized,

only produces offspring with the same traitsaka homozygous [ex: AA or aa]

Discuss

What is the difference between a genotype and a phenotype? Give an example of both.

What is the difference between Mendel’s Law of Segregation and the Law of Independent

Assortment?

What is the difference between homozygous and heterozygous? Give an example of each.

More Vocabulary P [Parent] Generation –

original, typically true-breeding organisms producing offspring

F1 [first filial/offspring] Generation – offspring of P generation

F2 [second filial/offspring] Generation – offspring of F1 generation

Discuss

If you are the F2 generation, who in your family would be:

the original P generation?

the F1 generation?

Mendel’s Crosses – cont’dGoing back to our original plants: Parent 1 = Parent 2 =

Purple Flower x White Flower [true breeder] [true breeder]

Which trait was hidden? Which was expressed?

aa AA[white flower] [purple flower]

Mendel’s Crosses – cont’dRecall – each parent donates a single allele for

each trait to their offspring.

Parent 1 = AA can only donate ‘A’ alleles

Parent 2 = aacan only donate ‘a’ alleles

DiscussIf you were to cross the parents AA and aa,

all offspring have…what genotype [allele combination]?what phenotype [physical appearance]?

All F1 offspring are Aa genotypeAll F1 offspring are purple phenotype

Mendel’s Crosses – cont’dAa = hybrid

hybrid = heterozygous offspring of two true-breeding parents

What happens if we cross the F1 generation?

monohybrid cross = test cross involving one traitex: eye color OR hair coloraka: Punnett Square

dihybrid cross = test cross involving two traits ex: eye color AND hair color [at the same time]

Creating a Test-crossFirst, let’s go back and model the

Parent to F1 generation Test-Cross

draw the test-cross boxes

Creating a Test-crossFirst, let’s go back and model the

Parent to F1 generation Test-Cross

determine the alleles of each parent, then place each allele next to or above the boxes:

A A

a

a

Creating a Test-crossFirst, let’s go back and model the

Parent to F1 generation Test-Cross

copy alleles above each column into the boxes of that column A A

a

a

A

A

A

A

Creating a Test-crossFirst, let’s go back and model the

Parent to F1 generation Test-Cross

copy alleles next to each row into the boxes of that row A A

a

a

A

A

A

A

a

a

a

a

Creating a Test-crossFirst, let’s go back and model the

Parent to F1 generation Test-Cross

interpret results:[genotypes & phenotypes]genotype ratios:

4/4 or 100% = Aaphenotype ratios:

4/4 or 100% = purple flowers

A A

a

a

A

A

A

A

a

a

a

a

Creating a Test-cross – cont’dNow, let’s model the F1 generation test-

cross.

draw the test-cross boxesdetermine the alleles of each parent,

then place each allele next to or above the boxes A

a

A a

Creating a Test-cross – cont’dNow, let’s model the F1 generation test-

cross.

copy alleles above each column into the boxes of that column

copy alleles next to each row into the boxes of that row A

a

A a

A

A

a

a

A A

a a

Creating a Test-cross – cont’dNow, let’s model the F1 generation test-

cross.

interpret results [genotypes and phenotypes]:genotype ratios: 1/4 or 25% = AA

[homozygous dominant]¼ or 25% = aa

[homozygous recessive]½ or 50% = Aa

[heterozygous]

A

a

A a

A

A

a

a

A A

a a

Creating a Test-cross – cont’dNow, let’s model the F1 generation test-

cross.

interpret results [genotypes and phenotypes]:phenotype ratios:3/4 or 75% =

purple flowers¼ or 25% =

white flowers

A

a

A a

A

A

a

a

A A

a a

HomeworkComplete the Super Peas Worksheet

title on worksheet: Genetics Practice Problems – Simpl(er)

Worksheet