Transcript Ch 8- genetics - MrsWrightsSciencePage

Origin of Genetics
All of your traits come from
your parents
Hair color
Eye color
Height
Weight
Heredity is the passing down
of traits from parent to child
Humans have always been
interested in how traits are
passed down
English sheep dogs were bred to be
herders of sheep, goats and cattle
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Beagles were bred to be scent
dogs to track rabbits, foxes
etc.
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Alaskan Malamutes were bred
to be sled dogs.
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Dogs have been owned and
bred by humans for thousands
of years
Humans have bred them to have many
different characteristics
Gregor Johann Mendel was
interested in heredity
Austrian Monk
Born in 1822
Parents were peasants
Studied Theology and Science and
Math
Learned to use Math to explain natural
phenomena
Mendel used T.A. Knights
previous experiments on Pea
Plants
Crossed a variety of Purple Pea plant with
a White Pea Plant
Purple Pea plant + White Pea
Plant
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When the second generation
were crossed with
themselves…
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Mendel counted his resulting
pea plants and he noticed a
pattern
After 1 Cross Purple + White
ALL OFFSPRING WERE PURPLE
After Crossing the second
generation…
3 pea plants had purple flowers
1 pea plant had white flowers
At the time counting scientific
data was considered “cutting
edge” technology
Why were Peas a good
organism to study heredity?
Had traits that were easy to tell apart
Easy to mate pea plants
Garden pea is small and grows quickly
Mendel’s first experiments
were Monohybrid crosses
A cross that involves 1 pair of
contrasting traits
Either purple or white pea plants
Mendel first formed truebreeding pea plants
PURE purple plants --- if they were
crossed with themselves they only had
purple offspring
PURE white plants --- if they were
crossed with themselves they only had
white pea plants
Once he was sure they were
true breeding
They became his parent generation or
P generation
The p generation were they first two that e
bred
Then Medel crossed a truebeeding purple plant with a
true breeding white plant
He called the offspring plants his F1
generation
He counted how many F1 were purple
and how many were white
Last he allowed his F1
generation to self pollinate
This next generation was called F2
He counted the number of purple and
white pea plants in the F2 group
Mendel’s Results:
After the first cross ALL the F1
generation were had purple flowers!!!
After the second cross 705 plants had
purple flowers and 224 plants had white
flowers… What is the ratio of
purple:white flowers?
705 = 3.15
224
We can say that this was about a 3 ratio.
1
3:1
3 purple for every 1 white
Dr. Mendel studied other pea
traits as well
Flower color ---- purple or white
Seed color----- yellow or greed
Seed shape ---- round or wrinkled
Pod color----- green or yellow
Plant height----- tall or dwarf
A monohybrid cross occurs when
two plants with one different trait
are crossed
Example: when a plant with purple
flowers are crossed with a plant with
white flowers
Mendel discovered that offspring
are not just a blend of traits
If a tall and short plant are crossed the
result is NOT a plant with medium
height
Mendel thought that each plant
held an “inheritable factor”
What do you think these inheritable
factors are?
Genes!
Because we are diploid we each have
two copies of each gene
When sex cells are formed during
Meiosis, each cell only gets one
of these copies
When the two sex cells fuse the
new zygote has two copies of
each trait
Every person has two alleles
One from mom and one from dad
There are alternative versions of
genes
Gene for flower color can be purple or
white
Each VERSION of a gene is called an
Allele
If a zygote has two different
alleles one might not be
expressed
When the purple flowers were matched
with White flowers, the resulting
offspring were all purple
The white allele did not show up
Purple flower color is said to be
Dominant
White flower color is said to be
recessive
So any plant with a Purple allele will be
purple
Dominant
The trait that is expressed
Recessive
The trait that is present but is not
expressed
An organism that has two of the
same allele is called
Homozygous
So purple flower PP or white flowers pp
Both would be considered homozygous
An organism that has two
different alleles is called
Heterozygous
Because Purple is dominant, all
heterozygous plants for plant
color are purple
Genotype
The actual alleles that an organism has
PP, Pp or pp - these are genotypes
Phenotype
What trait is expressed
So, what the plant looks like
Example - purple or white = phenotype
What is the result if a pp flower is
crossed with a pp flower?
What is the result if a PP flower is
crossed with a PP flower?
What happens if a pp flower is
crossed with a PP flower?
What happens if a Pp flower is
crossed with a Pp flower?
What happens if a PP flower is
crossed with a Pp flower?
Probability
 The likelihood that a specific event will
occur
 can be decimals, percentages or
fractions
If an event will definitely
occur…
The probablity is …
4/4 purple ; 0/4 white
100% purple offspring
Probablity = # of an outcome

total number of outcomes
What if a person wanted to
cross a plant with two traits
that are different?
A tall plant with purple flowers
is crossed with a short plant
white flowers
How do the different alleles get split up?
Alleles from one plant are not
linked to each other
Crosses that involve two traits
Called a dihybrid cross
Mendel’s Ideas
Called Laws of Heredity
The Law of Segregation
Two Alleles for a trait separate when the
gametes are formed during meiosis
A person Heterozygous for
Trait G = Gg
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Mendel’s 2nd Law
Mendel wanted to know if crossing a
plant with two different alleles affects
the outcome of the offspring
The Law of Independent
Assortment
Mendel determined that different genes
separate independantly from each other
I.e. the gene that codes for eye color
separates separately from the gene that
codes for dimples
Mendel’s first Law
The Law of Segregation
When daughter cells divide in Meiosis,
each daughter cell gets a separate
Allele
Mendel wanted to know if the
inheritance of one trait
affected the inheritance of
another trait
Did flower color influence Plant height
If Sponge Bob is
heterozygous for two traits,
Pant shape and color
His GENOTYPE is: SsYy
S - square pants
 s- round pants
Y - Yellow
 y- Blue
When sponge bob makes gametes, how do his
alleles separate?
SsYy
4 Possible Gametes:
1) SY
2) Sy
3) sY
4) sy
The Law of Segregation:
Cells go from 46 chromosomes to 23
Each daughter cell made in Meiosis will get 1
Allele instead of two for each trait
The Law of Independent
Assortment
SsYs
When Sponge Bob makes gametes, the
S’s and the Y’s separate independantly
-
What is the probability that
Sponge Bob will pass on a
Dominant gene for Square
pants?
His genotype is SsYy
If Sponge Bob is going to
marry Rosie
Roundpants…(ssYY)
What is the probability that Rosie will
pass on an allele for round pants?
How do we solve Dihybrid
word problems
Step 1 - What are the genotypes of the
parents
AaBb
aaBB… etc
Step 2 - Determine the
possible gamete combinations
for each parent
Mom is AABb
1) AB
2) Ab
3) AB
4) Ab
Step 3 - Make your Punnet
Square
 Put your Possible Gamete
combinations on top or on the side to
determine your offspring
Not all traits are simply Dominant
or Recessive
In humans many traits are controlled by
more than 1 gene
Height
Eye Color
Hair color
Weight
Polygenic Trait
A trait controlled by more than one gene
Factors that affect polygenic
traits
Genes for a trait may not be located on
the same chromosome
Independent assortment
Crossing over
There are many ways to make combinations
of genes… that is why you are such a good
combination of your mom and your dad
Mendel’s peas
In a pea plant, one plant was
completely dominant over another
This is called Complete Dominance
Incomplete Dominance
Occurs in individuals when they display
a trait that is a mixture of their two
parents
Red snapdragons and White
Snapdragons will produce Pink
snap dragons

=
In Caucasians, Curly and Strait
hair are both DOMINANT
So wavy hair is the result
Codominance
Occurs when BOTH traits are
expressed at the same time
When a red horse and a white
horse are crossed… a Roan
horse results
Sex Linked Traits
Some traits are passed down on the sex
chromosomes
X or Y
If a trait is passed down on
any other chromosome it is
called “Autosomal”
All people receive two allele’s for
autosomal traits
If a trait is passed down on the
X chromosome…
Then any male who inherits that
Chromosome will express that trait
Males have only 1 X chromosome
So if they receive a recessive trait (I.e. color
blindness) then they will express that trait
Pedigree
A family history that shows how a trait is
inherited over generations
Squares are Male ; Circles are Female
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This is a pedigree of Color
blindness, which is passed
down on the X chromosome
Which sex show color blindness more,
male or female?
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A carrier is a person who does
not exhibit a trait, but has that
allele
Carriers are heterozygous
If a person is heterozygous for color
blindness Rr
They will not be colorblind, but they will
potentially pass the trait on to their children
Color blindness is carried on
the X chromosome
 If a mother who is Rr passes the
recessive allele ‘r’ down to her son - he
will be color blind.
Color blindness is carried on
the X chromosome
 If a mother who is Rr passes the
recessive allele ‘r’ down to her son - he
will be color blind.
If the same mother passes
down the recessive allele ‘r’ to
her daughter
Her daughter will ONLY be color blind if
she also receives a recessive allele ‘r’
from her father
 to make her “rr”
If she receives a dominant allele R from
her father, she will be a carrier - Rr
Males have a much higher
chance of inheriting a sex linked trait
Male pattern balding
Male - pattern baldness is X linked
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Male pattern baldness is
passed down on the X
chromosome
Men get this trait from their mothers
Hemophilia
 Rare bleeding disorder
People cannot produce Platelets or
clotting factors
People with Hemophilia cannot form
clots, they could bleed to death if they
are cut.
Because Hemophilia is
recessive, it occurs almost
always in Males
This is a pedigree of a family
that carries the recessive
gene for Hemophilia
Notice only males have hemophilia
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British Royal Family
After generations of inbreeding (royals
marrying royals)
Hemophilia
Hemophilia in the British
Royal Family
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