Unit 4 – Lecture 5
Mendel
 Gregor Mendel – father of genetics / hereditary sci
 genetics – branch of biology which studies heredity
 heredity – passing on of traits from parent
to offspring
 monk; mid-1800s
Before Mendel…
 blending hypothesis – idea that all offspring traits are a
blend of the parent traits
 ex: tall + short = medium
 rejected by Mendel’s work
Mendel’s Crosses
 to “cross” organisms means to breed them together
 “test-cross” = graphic representation of offspring
probabilities
 Mendel carried out his experiments on pea plants.
Mendel’s Crosses
 Steps 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 Crosses
 Steps to Cross Pea Plants
 wait for results…
 then repeat…many times.
Mendel’s Crosses – cont’d
 Example:
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 Vocabulary
 Allele – a version of a gene / trait
 ex: trait = eye color; alleles = brown, blue, etc.
 Traits are represented by two alleles


one from each parent
alleles are shown by
combinations of letters;
each trait you work
with will have a different letter.
Helpful Vocabulary – cont’d
 Dominant 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 allele
 represented 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’d
 GENotype – set of alleles that an organism has for a
trait. [it’s genetic combination]
 homozygous = having two of the same
alleles for a trait
 ex: AA or aa
 heterozygous = having two different
alleles for a trait
 ex: Aa
Helpful Vocabulary – cont’d
 PHenotype – 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 = white
 AA = purple phenotype / aa = white phenotype
 if genotype is heterozygous, you will see the phenotype
of the dominant gene
 Aa = purple [recessive is hidden/repressed]
Helpful Vocabulary – cont’d
 Law of Segregation of alleles–
 you have two alleles per trait
 each 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 Crosses
 Mendel 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 traits
 aka 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’d
 Going back to our original plants:
Parent 1 =
Purple Flower
[true breeder]
x
Parent 2 =
White Flower
[true breeder]
 Which trait was hidden? Which was expressed?
aa
[white flower]
AA
[purple flower]
Mendel’s Crosses – cont’d
 Recall – each parent donates a single allele for each trait to
their offspring.
 Parent 1 = AA
 can only donate ‘A’ alleles
 Parent 2 = aa
 can only donate ‘a’ alleles
Discuss
 If 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 genotype
 All F1 offspring are purple phenotype
Mendel’s Crosses – cont’d
 Aa = hybrid
 hybrid = heterozygous offspring of two true-
breeding parents
 What happens if we cross the F1 generation?
 monohybrid cross = test cross involving one trait
 ex: eye color OR hair color
 aka: Punnett Square
 dihybrid cross = test cross involving two traits
 ex: eye color AND hair color [at the same time]
Creating a Test-cross
 First, let’s go back and model the
Parent to F1 generation Test-Cross
 draw the test-cross boxes
Creating a Test-cross
 First, 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-cross
 First, 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-cross
 First, 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 Aa
Aa
a Aa
Aa
Creating a Test-cross
 First, let’s go back and model the
Parent to F1 generation Test-Cross
 interpret results:
[genotypes & phenotypes]
 genotype ratios:
4/4 or 100% = Aa
 phenotype ratios:
4/4 or 100% =
purple flowers
A
A
a Aa
Aa
a Aa
Aa
Creating a Test-cross – cont’d
 Now, let’s model the F1 generation test-cross.
 draw the test-cross boxes
 determine the alleles of each parent,
then place each allele next to or
above the boxes
A
a
A
a
Creating a Test-cross – cont’d
 Now, 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
Aa
a Aa
aa
Creating a Test-cross – cont’d
 Now, 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
Aa
a Aa
aa
Creating a Test-cross – cont’d
 Now, let’s model the F1 generation test-cross.
 interpret results
[genotypes and phenotypes]:
 phenotype ratios:
 3/4 or 75% =
purple flowers
 ¼ or 25% =
white flowers
A
a
A
A
A
Aa
a Aa
aa
Homework
 Complete the Super Peas Worksheet
 title on worksheet:

Genetics Practice Problems – Simpl(er) Worksheet