Genetics: Mendel and Beyond
Chapter 10
Gregor Mendel
• Developed the fundamental laws of
heredity
• He studied science and mathematics
• Mendel chose garden peas (Pisum sativum)
as his subjects as they are easily grown and
their pollination is easily controlled. He
controlled pollination by manually moving
pollen between plants
• Developed true-breeding plants by selfpollination
Funfact: Mendel originally
wanted to breed mice, but
wasn't allowed to because it
was considered scandalous
• He could also allow the plants to self-pollinate.
• Mendel examined varieties of peas for heritable characters and traits
for his study. (stem length, pod shape, seed shape, seed color..etc)
Mendel's Law of Segregation
(MONOHYBRID CROSS)
• Mendel crossed true-breeding plants that
differed for a given character
• A monohybrid cross involves one (mono)
character and different (hybrid) traits.
• Pollen from true-breeding pea plants with
purple flowers (one trait) was placed on
stigmas of true-breeding plants with white
flowers (another trait).
• The F1 seeds were all purple; the white flower
trait failed to appear at all.
Because the purple flower trait completely masks the
white flower trait when true-breeding plants are
crossed, the purple flower trait is called dominant,
and the white flower trait is called recessive.
• The F1 plants were allowed
to self-pollinate.
• The progeny, called F2,
were examined: roughly
1/4 were white, and 3/4
were purple.
• Mendel proposed that the
units responsible for
inheritance were discrete
particles - particulate
theory `
The same pattern occurred over and
over, for seven different traits.
As Viewed by Modern Genetics
• During production of gametes, only one of the pair
members for a given character passes to the gamete.
(LAW OF SEGREGATION)
• When fertilization occurs, the zygote gets one from each
parent, restoring the pair.
• Mendel's units of inheritance are now called genes.
• Different forms of a gene are called alleles.
• Each allele is given a symbol: In the case of purple
flowers, P represents purple and p white. By convention,
uppercase P represents the dominant; lowercase p
represents the recessive. Never mix your letters! Choose
one letter and use a capital to denote dominate and a
lowercase to denote recessive.
Parental Cross
PP x pp
purple x white
• True-breeding individuals would have two copies of the same allele: Purple
would be PP. Two copies of same allele = homozygous. Homo means "the
same"
• White true-breeding would be pp. Two copies of same allele =
homozygous. Therefore both PP and pp are considered homozygous, just one
is purple and the other is white.
• Some purple-flowered plants could be Pp, although they would not be truebreeding. Individuals that are purple, but had a white parent, are heterozygous
or hybrid: Pp. Hetero means "different".
The F1 cross
Pp x Pp
purple x purple
When an organism is studied for three different genes and has
the alleles AABbCC, it is homozygous for A and C genes but
heterozygous for the B gene.
A = antenna
a = no antenna
B = black
b = purple
C = chompers
c = no chompers
Practice picking letters.... the following traits are
found in the common Shirtus americanus.
1. Polka dots are dominant to stripes.
2. Long sleeves are dominant to short sleeves.
3. Collared shirts are recessive.
4. Buttons are dominant over snaps.
5. Pockets are recessive.
Practice with Punnett Squares
1. A round seeded plant (RR) is crossed with a wrinkle seeded
plant (rr). What are the phenotypes of the offspring?
2. Two heterozygous purple flowered pea plants are
crossed. What are the phenotypes of their offspring and in
what proportion?
3. A plant with green seeds (yy) is crossed with a
heterozygous plant. What percentage of their offspring have
yellow seeds?
In dragons...
Wings are a dominant
trait, but some dragons
are born wingless.
What are the chances that
two heterozygous
dragons have a whelp
that is wingless?
If a wingless dragon is crossed with
one that is heterozygous, how many
of its offspring will also be wingless?
What is a test cross?
Help, help! I don't know
what my genotype is!!
Am I Dd or DD?
I can help
you! Let's have
offspring!
Why does the punnett square work?
It all goes back to meiosis.. each side represents a sperm or
egg. The boxes filled out simply give you the statistical chance
that a certain sperm will fertilize a certain egg.
Consider a pea plant that is RrTt (round seed, tall)
When this plant's cells go through MEIOSIS, the alleles
segregate - each sperm receives a random combination...
Possible Gametes:
RT
Rt
rT
rt
What are the gametes possible for an
organism that is AaBbRr?
Mendel’s Three Laws
1. Dominance & Recessiveness
2. Segregation
3. Independent Assortment
Mendel's Law of Independent Assortment –
Illustrated by the DIHYBRID cross
The second law describes the outcome of dihybrid
(two character) crosses, or hybrid crosses involving
additional characters.
A dihybrid is an individual that is a double
heterozygote (e.g., with the genotype RrYy - round
seed, yellow seed).
What are the gametes that can be produced by this
individual?
Dihybrid Cross: RrYy x RrYy
BEYOND MENDEL
Conditions that Mendel didn't explore - genetic traits
that are inherited in new and interesting patterns.
Codominance / Incomplete dominance / Multiple Allele
Traits / Polygenic Inheritance / Lethal Alleles /
Epistatic Alleles
Incomplete
Dominance
Traits appear to
"blend" in offspring
RR x WW
RW (pink)
Show:
Pink x Red
Pink x Pink
White x White
This illustrates
another style of
"letters" to denote
genotypes
R1 and R2
Sickle Cell Trait in Humans
Genotypes & Phenotypes
Coloration in Rodents
Black x White = Gray
BB x WW = BW
ROAN COW What happens when you cross a white and a red cow?
Roan is codominant - both alleles R and W are expressed
What happens when two Roan Cows
are Crossed?
Blood Types - Multiple Alleles
When doing blood type crosses, you will need to know whether at type
A or B person is heterozygous or homozygous. Type O's are
automatically OO and type AB is automatically AB. Crosses are
performed the same as any other.
Show Crosses:
AB x AO
AO x BO
AA x AB
A woman who is type A is married to a man who is type B,
what are ALL of the possible blood types of their children?
Many Genes Have Multiple Alleles
• A population might have more than two alleles for a given gene.
• In Labrador retriever, coat color is determined by one gene with four
different alleles. Five different colors result from the combinations of these
alleles. (More on labradors later)
Eye color is also controlled by multiple alleles
Try this activity on Eye Color
Mix Those Genes
If you played "Mix Those Genes" you saw how the eye color of the
parents determines the eye color of their offspring.
The simulation is a bit simplified, but the idea is that MULTIPLE
ALLELES control a single trait (eye color)
It is likely that more than 2 alleles
control eye color, the simulation just
made it simple to understand.
Determine the genotypes of these
aliens given that the trait is controlled
by multiple alleles.
Polygenic Traits
• Individual heritable characters are often found to be controlled by
groups of several genes, called polygenes.
• Each allele intensifies or diminishes the phenotype.
• Variation is continuous or quantitative (adding up) - also called
quantitative inheritance
• Seed Color in wheat - aabbcc, Aabbcc, AaBbcc, AaBbCc, AABbCc,
AABBCC (light, intermediate colors, dark)
• In humans - hair color, height, skin color
Polygenic Inheritance
AABBCC x aabbcc (P)
AaBbCc x AaBbCc (F1)
Seven Possible Phenotypes in the F2
Environment and Phenotype
Temperature, water, food sources can have an affect on how a gene is
expressed
Rabbits have a gene that codes for darker
pigments - this gene is more active at low
temperatures. Parts of the body that are colder
will develop the darker pigmentation - ears and
feet
• SIAMESE CATS
Multiple Alleles control the combs of
chickens.
Assignment:
Multiple Alleles in
Chickens
LEGHORN
CHICKEN –
SINGLE COMB
Lethal Genes
Some genes are lethal when both alleles
are present. Lethality can occur before or
after birth
• Huntington's disease in humans is
caused by a lethal allele, death
occurs later in life
• Other examples: Mouse coat color
(yellow), Creeper legs in chickens,
Manx Cats (no tails)
An example is the "creeper" allele in
chickens, which causes the legs to be
short and stunted.
LABRADOR RETRIEVERS AND EPISTATIC ALLELES
• A population might have more than two alleles for a given gene.
• In labrador retriever, coat color is determined by one gene with four different
alleles. Five different colors result from the combinations of these alleles.
• Even if more than two alleles exist in a population, any given individual can
have no more than two of them: one from the mother and one from the
father.
Black is dominant to chocolate B or b
Yellow is recessive epistatic (when present, it
blocks the expression of the black and
chocolate alleles) E or e
Yellow Lab
Black Lab
Chocolate Lab
BBee
Bbee
bbee
BBEE
BbEE
BBEe
BbEe
bbEE
bbEe
1. A black lab (BBEe) x
yellow lab (bbee)
2. A chocolate lab (bbEe) x black
lab (BbEe)
3. Two black labs (BBEE x BbEe)
Linkage Groups &
Chromosome Maps
When Mendel crossed his F1 generation: PpRr x PpRr, he
got a 9:3:3:1 ratio. He would have not seen this pattern if
the alleles had been located on the same chromosome and
inherited together.
A return to Drosophila....
• Different alleles exist because any gene is subject to
mutation, or change, to a stable, heritable new form
• Alleles can randomly mutate to become a different allele
depending on DNA sequence changes.
• Wild type is a term used for the most common allele in the
population.
• Other alleles, often called mutant alleles, may produce a
phenotype different from that of the wild-type allele.
• An alternate form of designating alleles. Alleles that are wild
type are expressed with a +
• Ex. Red eye color (w+) is dominant to white eye color (w).
The red eye is the wild type. Don't let this confuse you, its
just a different way to express alleles.
Check out:
http://www.exploratorium.edu/exhibits/mutant_flies/mutant_flies.html
Many genes are located on the same chromosome, and
they do not assort independently, instead, they are inherited
together
Consider the following chromosome map of the fruit fly: All
the alleles are located on chromosome 2 of the fruit fly, and
are inherited together.
Punnett Squares with Linkage Groups
When performing crosses with linkage groups, I find it best to draw a little picture
of the chromosomes to show how they are inherited.
Example: A fly that is heterozygous for long wings (Ll) and heteroyzygous for long
aristae (Aa) is crossed with another fly of the same type. AaLl x AaLl. In both
cases the dominant allele is located on the same chromosome.
Before you set about making a 4x4 square, you need to consider the linkage
groups. Sketch them!
The results of the cross will change if the alleles are arranged
in a different way. Show the cross if the dominant alleles were
located on DIFFERENT chromosomes (don't forget to sketch)
How Chromosome Maps Are Determined
Alleles that are farther apart, like the gray body and the brown
eye color allele are more likely to cross-over and exchange
than ones closer together.
Biologists use the percent of crossing over to determine the locus of
alleles on a chromosome.
The distance between alleles is measures in MAP UNITS, or MU. On
the diagram the long wing allele is 13MU from the aristae allele. The
image above is a linkage map because it shows the distance between
the alleles
Practice Questions (assume no crossing over occurs)
1. A dumpy winged (ww) fruit fly with long aristae (AA) is
crossed with a long winged (Ww) short aristae (aa). Show the
cross and the phenotypic proportions.
2. A fruit fly with short legs (ll) and vestigial wings (ww) is
crossed with one that is heterozygous for both traits. Assuming
the dominant alleles are on separate chromosomes, show the
cross and the expected phenotypic proportions.