Download Probability and Genetics - Conackamack Middle School

Probability and Genetics
Pages 118-123
What are the chances?
• Probability is the likelihood that a
particular event will occur.
Flip A Coin!
• When you flip a coin
there are only two
possible outcomes that
could occur.
– Heads
– Tails
Each side has an equal
chance of showing up
every time you flip the
coin.
Roll a Die
• How many sides
are on a die?
• What is the
chance that when
you roll the die it
will land on 4?
On 2?
Mendel and Probability
• Mendel carefully counted the offspring of
his pea plants and the traits they showed.
– ¾ dominant trait
– ¼ recessive trait
• Mendel was the first scientist to recognize
that probability can be used to predict the
results of genetic crosses.
Punnett Squares
• A Punnett square is a chart
that shows all the possible
combinations of alleles that
can result from a genetic
cross.
• Created by Reginald C.
Punnett, a geneticist, who
used Mendel’s experiments
to show the passing of
traits from one generation
to the next.
Punnett Squares
• Each parent can pass either of their two
alleles to their offspring.
• The possible alleles are written across the
top and left hand side of the box.
– Each allele gets its own box.
Punnett Squares
• To figure out the
possible allele
combinations of
the offspring the
traits are followed
from the top of
the box and the
side of the box.
t
t
T
t
In a hybrid the Dominant
trait goes first.
What are the chances?
• The allele that will be passed from parent
to offspring is based on chance. Just like
the flip of a coin.
• There are four possible allele combinations.
t
Purebred Tall - TT
t
Tt
Tt
tt
tt
Hybrid Tall – Tt/tT
Purebred Short - tt
What is the probability?
t
t
Tt
Tt
tt
tt
2 out of 4 – tt
2 out of 4 – Tt
What is the
percentage that each
allele combination will
occur?
What is the Probability?
• What is the chance
(in percent) of
getting:
– A hybrid black guinea
pig?
– A purebred black
guinea pig?
– A purebred white
guinea pig?
BB
bb
Phenotypes and Genotypes
• Phenotype: an organisms’ physical
appearance, or its visible traits.
– Example: tall/short, purple/white,
yellow/green
• Genotype: an organisms’ genetic
makeup, or allele combinations
– Example: TT, tt, Tt
Homozygous and
Heterozygous
• Homozygous: When an organism has
two identical alleles for a trait.
– Example: Purebred – TT or tt
• Heterozygous: An organism that has
two different alleles for a trait.
– Example: Hybrid – Tt, Rr, Ll, Bb
Co-dominance
• In pea plants there were only
two possible alleles.
– Ex. Green OR Yellow
– Ex. Smooth OR Pinched
• Sometimes, genes have alleles
that are NEITHER dominant nor
recessive.
• This is a co-dominant trait. In
co-dominance, neither trait is
masked which means that both
traits are equally as dominant
and the hybrid will show both.
– Ex. Feather color in chickens
Co-dominance
• When crossing co-dominant traits we use
superscripts to tell us the different allele
for the trait.
– Example: Feather color
• purebred black = FBFB
• purebred white = FWFW
• mixed = FBFW
– Example: Flower color
• purple = CPCP
• white = CWCW
• striped = CPCW
Example #1
• Cross a purebred black rooster with a
purebred white hen.
• FBFB X FWFW
• Report all genotypes and phenotypes
Example #2
• In petunias, petal color is a co-dominant
trait. Flowers that are heterozygous and
have BOTH alleles (one for purple and one
for white) will phenotypically look striped.
• Cross a purple petunia with a striped
petunia.
• Report all genotypes and phenotypes.
• Is it probable to grow any white petunias
from this cross?
Incomplete dominance
• There are times when traits show a BLEND of
both the dominant and the recessive form. This
is called INCOMPLETE DOMINANCE.
• An organism that is a hybrid will show a
phenotype that is in the “middle” of the dominant
or the recessive form.
– Ex. Hair texture in humans:
– HH = curly, hh = straight, Hh = wavy
Example #1
• In humans, lip size shows
incomplete dominance.
There are large lips, small
lips, and hybrids, will have
medium-sized lips. Cross
two parents; one with small
lips and one with mediumsized lips. Use the letter “L”
for all alleles. Report all
genotypes and phenotypes.
ll X Ll
Example #2
• In humans, lip size shows
incomplete dominance.
There are large lips, small
lips, and hybrids, will have
medium-sized lips. Cross
two parents; one with large
lips and one with mediumsized lips. Use the letter “L”
for all alleles. Report all
genotypes and phenotypes.