probability & genetics 11-2
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Probability & Genetics 11-2. Genetics & Probability. Mendel’s laws: segregation independent assortment reflect same laws of probability that apply to tossing coins or rolling dice. Probability is the likelihood that a particular event will occur. - PowerPoint PPT PresentationTRANSCRIPT
Genetics & Probability
Mendel’s laws: segregation independent assortment
reflect same laws of probability that apply to tossing coins or rolling dice
What is Probability? Probability is the likelihood that a particular event
will occur.
It does not ALWAYS happen (think Deal or No Deal)
Can be written as a decimal, percentage, ratio, or fraction
How do we use the principles of probability in our daily lives?
Ex.Horse racingNCAA March MadnessSuperbowl Coin TossLas Vegas $$$$$
Probability & genetics
Calculating probability of making a specific gamete is just like calculating the probability in flipping a coin probability of tossing heads?
50% probability making a gamete…
PP
P
P
Pp
P
p
50%
100%
Determining probability
Number of times the event is expected Number of times it could have happened
What is the probability that you picked an odd number when picking from 1-10? There are five odd numbers between 1 and 10.
Or you can express it as a fraction: 5/10. Since it's a fraction, why not reduce it? The probability that you will pick an odd number is 1/2.
Probability can also be expressed as a percent...1/2=50% Or as a decimal...1/2=50%=.5
Chance that 2 or more independent events will occur together probability that 2 coins tossed at the
same time will land heads up
Or .25 or 25% or 1:4 probability of Pp x Pp pp
Rule of multiplication
1/2 x 1/2 = 1/4
1/2 x 1/2 = 1/4
Terminology and Directions
ALLELES = (WARNING - THIS WORD CONFUSES PEOPLE; READ SLOW) alternative forms of the same gene.
Hair Texture Gene- Straight (S) or Curly (s)
One form of the gene codes for curly hair.
A different code for of the same gene makes hair straight.
So the gene for hair texture exists as two alleles --- one curly code (s), and one straight code (S).
Alleles
Dominant (Capital Letter) Shows up more in a
population, Expresses itself when it is present
Recesive (Small Letter) Shows up less in a
population, only shows when there is no dominant trait.
GENOTYPE = the genes present in the DNA of an organism.
We will use a pair of letters (ex: Tt or YY or ss, etc.) to represent genotypes for one particular trait.
There are always two letters in the genotype -one letter (gene) from mama organism -one letter (gene) from papa organism
Phenotypes
PHENOTYPE = how the trait physically shows-up in the organism.
Wanna know the simplest way to determine an organism's phenotype ?
Look at it. Examples of phenotypes: blue eyes, brown fur, striped fruit, yellow flowers.
VOCAB
Now, turns out there are three possible GENOTYPES – two big letters (like "TT"), one of each ("Tt"), or two lowercase letters ("tt").
Since WE LOVE VOCABULARY, each possible combo has a term for it.
Hetero/ Homo
Two capital (TT) or
Two lowercase (tt) in the GENOTYPE
Called HOMOZYGOUS ("homo" means "the same").
Sometimes the term "PUREBRED" is used instead of homozygous.
Heterozygous
When the GENOTYPE is made up of one capital letter & one lowercase letter (ex: Tt) it's called HETEROZYGOUS ("hetero" means "other"). Just to confuse you, a heterozygous
genotype can also be referred to as HYBRID.
Let's Summarize:
Genotype = genes present in an organism (usually abbreviated as two letters)
AA = homozygous = pure Aa = heterozygous = hybrid aa = homozygous = pure
Quick Review
Genotype= genes of the organism “Letters” ex. TT or Tt or tt.
Phenotype= the physical appearance of a trait in an organism What it physically looks like.
The Letters are chosen for the Dominant trait of the allele.
Homozygous and Heterozygous
When we have two capital or two lowercase letters in the GENOTYPE (ex: TT or tt) it's called HOMOZYGOUS ("homo" means "the same"). Sometimes the term "PUREBRED" is
used instead of homozygous. When the GENOTYPE is made up of one
capital letter & one lowercase letter (ex: Tt) it's called HETEROZYGOUS ("hetero" means "other"). Just to confuse you, a heterozygous
genotype can also be referred to as HYBRID.
Dominant and Recessive
Dominant (Capital Letter) Shows up more in a population, Expresses itself when it is present
Recesive (Small Letter) Shows up less in a population, only shows when there is no dominant trait.
Punnet Square
Baby steps of Punnet Squares
1. Determine and write down the genotypes of the parents
2. Draw a punnet square of appropriate size3. Split the letters of the parents and put
them on the outside4. Do the punnet square5. Summarize the results by showing
genotypic and phenotypic ratios
Punnet Square Practice Steps 1 -Determine the genotypes of the
parents and write them down Tall (T) is dominant to short pea plants (t).
Cross a short pea plant with one that is heterozygous for tallness. What are the parents genotypes? tt x Tt
Cross a short pea plant with one that is heterozygous for tallness. What are
the parents genotypes?
tt x Tt
T
t
t t
3. Draw a p-square4. Split the letters of the
parents and put them on the outside
5. Do the punnet square
Tt Tt
tt tt
Summarize the results by showing genotypic and phenotypic ratios
Genotypic Ratio (letters) TT- 0 or 0/4 or 0% Tt- 2 or 2/4 or 50% tt- 2 or 2/4 or 50%
Phenotypic (looks) Tall- 2 or 2/4 or 50% Short- 2 or 2/4 or 50%
Tt Tt
tt tt
T
t
t t
Decide what the genotypes of the parents are In humans, brown eyes, B, are dominant to
blue eyes, b. If the father has brown eyes and is homozygous dominant for the trait, BB, and the mother has blue eyes and is homozygous recessive for the trait, bb, what are the possible genotypes and phenotypes of their offspring.
Father-homozygous dominant-BB Mother-homozygous recessive-bb
Then, write the genotype for one parent across the top of the punnett square, and the genotype for the other parent along the left side of the square.
B B
b
b
Fill in the boxes inside with whatever letter are on top and to the left of them
B B
b
b
The genotypes inside the boxes represent the possible gene combination of their offspring
Each box is a 25% or ¼ chance
So, the genotypes of all the offspring in this genetic cross would be Bb
Because there is at least one dominant allele the phenotype (appearance) would also be dominant all offspring would have brown eyes
B B
b
b Bb
BbBb
Bb
Try another punnett square
Two brown eyed parents, who are both heterozygous for the trait, Bb, mate. What are the chances they will have a blue eyed child?
Step 1
•What are the genotypes of the 2 parents?
Step 3
B b
b
B
Step 4
B b
b
B BB
bbBb
Bb
Step 5
Genotypes of offspring BB-1/4 or 25% Bb-1/2 or 50 % Bb-1/4 or 25%
Phenotypes of offspring Brown eyes (BB and Bb)-
3/4 or 75% Blue eyes (bb)-1/4 or
25% Probability of blue eyed
child=1/4 or 25 %
B b
b
B BB
bbBb
Bb
Biology
What is a dihybrid cross?
Work 2 genes at once
Find the expected offspring for both traits
They are a little more work than the monohybrid cross, but…
So much fun!!
Steps to solving a dihybrid problem 1. Read the problem and find the two
separate traits. Sample problem: In Springfield, red hair (H) is
dominant to blue hair (h) and having four fingers (F) on your hand is dominant to having five fingers (f). If Side Show Bob and Mrs. Van Houten get married and decide to have a little brother for Millhouse, what is the chance that the newborn will have red hair and five fingers if Side Show is heterozygous for red hair and heterozygous for four fingers and Mrs. Van Houten has blue hair and is homozygous for four fingers?
Genotypes
2. Find the two parents and write down their genotypes.
HhFf x hhFF
What do they Want?
3. Read the question and see what you have to look for. Do they want you to find a ratio, percent, fraction, or do they want you to list all possibilities?
Math Class
4. Once you have found all the information, you can start to set up the problem by finding the gametes for each parent by using FOIL.
FOIL stands for First, Outside, Inside, Last
FOIL
Here is an example of how to use FOIL:
(a + b) * (c + d) First = (a + b) * (c + d) = ac Outside = (a + b) * (c + d) = ad Inside = (a + b) * (c + d) = bc Last = (a + b) * (c + d) = bd So our pairs are: ac, ad, bc, bd
Parent Genotypes
Now try it with the parents alleles Side Show = HhFf So, HF, Hf, hF, hf Mrs. Van Houten = hhFF So, hF, hF, hF, hF
DiHybrid (Double Cross)
5. Once you have the gametes, you have to line them up in a punnett square.
You are going to need a bigger square!
Put the gametes of one parent across the top of the square and the other down the side of the square
HF Hf hF hf
hF
hF
hF
hF
6. Now you have to do the actual cross.
This is a single box example. Remember to always put the
two like alleles back together, The same letters go together,
and The capital letter should go
first Keep the same sequence of
alleles (letters)
HhFF
HF
hF
Fill in the rest of the box!
HhFF HhFf hhFF hhFf
HhFF HhFf hhFF hhFf
HhFF HhFf hhFF hhFf
HhFF HhFf hhFF hhFf
HF Hf hF hf
hF
hF
hF
hF
Genotypic Ratio
7. Find the genotypic ratio
HhFF = 4/16 (25%) HhFf = 4/16 (25%) hhFF = 4/16 (25%) hhFf = 4/16 (25%)
Phenotypic Ratio
8. Find the phenotypic ratio
Red hair, four fingers = 8/16 (50%)
Blue hair, four fingers = 8/16 (50%)
Answer the Question
9. Now, don’t forget to answer the question!
If Side Show Bob and Mrs. Van Houten get married and
decide to have a little brother for Millhouse, what is the chance that the newborn will have red hair and five fingers if Side Show is heterozygous for red hair and heterozygous for four five fingers and Mrs. Van Houten has blue hair and is homozygous for four fingers?
0%
Review: Dominant/Recessive
One allele is dominant over the other (capable of masking the recessive allele)
PP = purple pp = white Pp = purple
Review Problem: Dominant/Recessive
In pea plants, purple flowers (P) are dominant over white flowers (p) show the cross between two heterozygous plants.
P
p
P p
pp
Pp
Pp
PP- PP (1); Pp (2); pp (1)- ratio 1:2:1
- purple (3); white (1)- ratio 3:1
GENOTYPES:
PHENOTYPES:
Incomplete Dominance
Incomplete dominance is when one allele is not completely dominant over the other.
The alleles do not blend, but the phenotype is a blending of the two traits.
Incomplete Dominance
Ex. Flower Color in 4 O’clocks
RR = red rr = white Rr = pink
Problem: Incomplete Dominance Show the cross between a pink and a
white flower.
- Rr (2); rr (2)- ratio 1:1
- pink (2); white (2)- ratio 1:1
R r
r
r
rrRr
rrRr
GENOTYPES:
PHENOTYPES:
Codominance
Codominance are when the alleles are neither dominant or recessive.
In cases of codominance, both traits are expressed in the offspring
Symbols are always written as capital letters.
Example of Codominance
In chickens-they are either white, black or black and white
The colors are comdiminant
Another Example of Codominance
•White and red are codominant.
•Cows are white, red or red and white
Codominance
The heterozygous condition, both alleles are expressed equally
Sickle Cell Anemia in Humans
NN = normal cells
SS = sickle cells NS = some of each
Problem: Codominance
Show the cross between an individual with sickle-cell anemia and another who is a carrier but not sick.
N S
S
S
NS
NS
SS
SS
- NS (2) SS (2)- ratio 1:1
- carrier (2); sick (2)- ratio 1:1
GENOTYPES:
PHENOTYPES:
Multiple Alleles
There are more than two alleles for a trait
Blood type in humans Blood Types?
Type A, Type B, Type AB, Type O
Blood Alleles? A, B, O (in book – IA, IB, I)
Rules for Blood Type
A and B are codominant AA = Type A BB = Type B AB = Type AB
A and B are dominant over O AO = type A BO = type B OO = type O
Blood Also Shows Codominance
Problem: Multiple Alleles Show the cross between a mother who has type
O blood and a father who has type AB blood.
- AO (2) BO (2)- ratio 1:1
- type A (2); type B (2)- ratio 1:1
GENOTYPES:
PHENOTYPES:
O O
A
B
AO
BO
AO
BO
Problem: Multiple Alleles Show the cross between a mother who is
heterozygous for type B blood and a father who is heterozygous for type A blood.
-AB (1); BO (1); AO (1); OO (1)- ratio 1:1:1:1
-type AB (1); type B (1) type A (1); type O (1)- ratio 1:1:1:1
GENOTYPES:
PHENOTYPES:
A O
B
O
AB
OO
BO
AO
Sex-Linked Inheritance
Traits that are only found on the X or Y chromosome
Colorblindness and Hemophilia are examples of sex-linked traits.
These genes are recessive and found only on the X chromosome.
Sex-linked Punnet Squares
Constructed the same way as a regular Punnet Square.
Alleles are placed next to the X chromosome.
Sex-linked Punnet Squares
XB Xb
XBXb XbXb
XBY XbY
Xb
Y
Definition
Some traits are determined by the combined effect of two or more pairs of alleles. These traits are called polygenic traits.
Each gene contributes a small but additive effect to the trait.
Other names for polygenic traits are multi-factorial traits, or quantitative traits.
Polygenic traits are continuous
Because so many alleles contribute to the final phenotype, a variety of phenotypes can occur!
For example, height is a polygenic trait. If you look around, you will notice there are not two set heights, but rather a continuum of height among your classmates.
Another example of a polygenic trait: Hair Color
Hair color is controlled by alleles on chromosomes 3, 6, 10, and 18.
The more dominant alleles that appear in the genotype, the darker the hair!