Gregor Mendel
• Genetics is the field of Biology that
studies how characteristics
(traits) are passed from parent
to child
• Gregor Mendel, the Father of
Genetics, was an Austrian
Monk…gardened and grew
peas in the Mid 1800’s
• Looked at 7 different traits in pea
plants
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Height
Pod appearance
Seed texture
Position of flowers
Flower color
Pod color
Seed color
Gregor Mendel
• Mendel noticed that pea plants with purple flowers
always had seeds that grew plants with purple
flowers, when they self pollinate. He called these
plants true breeding.
• Self pollinate means the egg and pollen are from the same
plant.
• The opposite is cross pollinate where egg and pollen come
from different plants. Usually we just shorten this term
and say “cross” and we use it refer to other living things,
not just plants.
• When Mendel cross-pollinated purple flowered plants with
white flowered plants, he got plants that all had purple
flowers! The white trait had disappeared!
• We call the ones he started with the P generation (parental)
• We call the ones he got from those crosses the F1 generation
(filial 1)
Gregor Mendel
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Mendel then let that F1 generation self pollinate
to make an F2 generation
He found 3/4 of the plants had purple flowers, ¼
of the plants had flowers. This had never
happened before. Never before had purple
flowered plants made babies with white
flowers!
For the other traits, he always came up with the
same ratio in the F2: 3:1
Mendel explained his results by saying
– Something controlled traits (and called them factors)
– There must be two factors
– One factor prevents the other from happening
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Mendel concluded that the pair of factors
separate during the formation of
reproductive cells. This is called the Law of
Segregation.
Mendel also crossed different traits, (seed color
flower color) and concluded that the factors
are not connected. He called this the Law
of Independent Assortment
Today…
• We know that there are segments of DNA
on chromosomes called genes that occur
in pairs (because chromosomes do). Each
has 1 or more forms called alleles.
• We use capital letters for dominant alleles
and lowercase letters for recessive alleles.
• Dominant means that it does not let the
other allele be expressed. Recessive
means it is not expressed.
• We also know they separate during a process
called meiosis (specifically, at anaphase I)
Other terms…
• Genotype: the alleles present or genetic
makeup; uses letters like PP, Pp or
pp
– If some thing has the same two alleles, we
call it homozygous or pure bred. e.g. PP,
pp, BB, bb (note that this doesn’t tell
whether something is dominant or
recessive)
– If some thing has two different alleles, we
call it heterozygous or a hybrid. e.g. Pp, Bb
• Phenotype: the appearance; uses
words like purple or white
Punnett Squares
• One way to show the
possibilities of a genetic
cross is to use a Punnett
square
• Laws of probability state that
there is a 50/50 chance of
either allele being passed
on, so a Punnett square
doesn’t show what will
happen. It shows you what
can happen and how often.
• A Punnett Square that shows
one trait is called a
monohybrid cross.
Punnett Squares
• When giving the results, we
use ratios or percentages
for both the genotype and
phenotype
• In this example, the
genotypic ratio is 2BB:
2Bb or 50% Bb and
50% Bb
• The phenotpyic ratio is all
(100%) black fur (note
that all 4 will look exactly
the same…the Bb are not
a little “less black” than
the BB)
Complete (simple) dominance
• Complete or simple
dominance is when the
phenotype is one thing or
the other, like all of
Gregor Mendel’s … yellow
or green seeds, purple or
white flowers
• You can tell because there is
a 3:1 phenotypic ratio in F2.
• It’s not always that simple.
Other cases…
• In a flower called 4
o’clocks, red and white
flowers make pink in
the F1.
• This is called incomplete
dominance … you get a
new phenotype, a
mixture of dominant
and recessive
• You see a 1:2:1 phenotypic
ratio in the F2
Other cases…
• What if both alleles are
expressed? For the flowers
above they wouldn’t be
pink, they’d be red and
white at the same time
• That’s called Codominance …
you get both dominant and
recessive at the same time
• What would the what the rabbits
to the left look like in the F2?
• They’d have black and brown spotted fur
Dihybrid Cross
• When two traits are looked at, it is
called a Dihybrid cross.
• Notice there are 2 letters by each
row/column outside the
Punnett Square, one of each
allele (e.g. R and Y, not R and r)
• To figure out the combination, use
FOIL…first, outer, inner last.
• Dihybrid crosses have a 9:3:3:1
phenotypic ratio in the F2:
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9:
3:
3:
1:
Round, yellow (both dom.)
Round, green (1 dom.,1 rec.)
Wrinkled, yellow (1 rec.,1 dom.)
wrinkled, green (both rec.)
Sex Linked Traits
• Recall that the sex chromosomes (X and Y) determine the sex
of organism. XX is female and XY is male (in humans)
• All other chromosomes are called autososmes
• In genetics, genes on the X or Y chromosome are written
on an X or Y, still using capital letters for dominant
and lowercase for recessive, e.g. XR XR or Xr Y
• Phenotypes mention both sex and trait, e.g. red eyed female.
• Males tend to show the recessive traits more, since they only
need 1 allele, which they get from their Mom.
But most of the time…
• Traits we see are very complex. They
are controlled by more than one
allele (polygenic traits), like blood
type
• Or are controlled by more than one
gene, like eye color or skin color
Inheritance of traits
• A pedigree shows traits inherited through
generations
• You can USUALLY tell a trait is:
– Sex linked trait if only males have it (or majority
males)
– Autosomal if roughly equal number males and
females
– Dominant if every child with trait has at least 1
parent with trait
– Recessive if child with trait may have parents
without
• Not everyone shows the trait. Some have it
and pass it on without showing it. They
are called carriers.
Reading a pedigree
• squares are males,
circles are females
• colored in show the trait,
blank do not, half
colored carry it
• trait is sex linked or
autosomal?
• trait is dominant or
recessive?
• Kay’s genotype is….
• Tom’s genotype is…
• the chances of Jen
passing the disease
on to their child is…
• the chances of Tim
passing the disease
on to their child is…
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sex linked …more males than females
dominant … every kid that has it, has
a parent with it.
Kay’s genotype is XA Xa (carrier)
Tom genotype is XA Y (Sara does not
have it)
Jen … is a carrier, so 50/50
Tim … will pass it to his daughters,
who will become carriers, but
not to his sons, who get a Y
chromosome