probability and genetics. objectives predict possible outcomes of various genetic combinations such...

Probability and Genetics

Objectives

Predict possible outcomes of various genetic combinations such as monohybrid crosses, dihybrid crosses and non-Mendelian inheritance (TEKS 6F)

• Explain how geneticists use the principles of probability

• Describe how to use Punnett squares

• Explain the principle of independent assortment with dihybrid crosses

• Explain non-Mendelian inheritance (inheritance patterns other than simple dominance)

Probability

The likelihood that a particular event will occur is called probability.• The principles of probability can be used to predict

the outcomes of genetic crosses.

If you flip a coin 4 times in a row, what is the probability it will land on heads every time?

½ probability of getting heads each time you flip the coin

½ x ½ x ½ x ½ = 1/16

ProbabilityThe “AND” “OR” rule of probability:

What’s the probability of getting 3 tails in a row? Tail AND Tail AND Tail

This is called an independent event, which means the outcome of the first event (the coin flip) has no influence over the outcome of any of the following events.

Whenever you have a probability question with “AND” you multiply

½ x ½ x ½ = 1/8

What is the probability of drawing a 3 of spades OR a 3 of clubs in a deck of cards?

This is a mutually exclusive event, which means the two events cannot occur at the same time. If you’re only drawing one card out of a deck of 52, you can’t draw a 3 of spades and 3 of clubs; you can only draw one OR the other.

Whenever you have a probability question with “OR” you add

1/52 + 1/52 = 2/52 or 1/26

Probabilities

• Probabilities predict the average outcome of a large number of events.

• Probability cannot predict the precise outcome of an individual event.

• In genetics, the larger the number of offspring, the closer the resulting numbers will get to expected values.

Larger amount of data = more accurateLet’s try it!

Flip your penny 10 times and record how many times it lands on heads and how many times it lands on tails

Now, combine your numbers with your group. How many heads did you get? How many tails?

Now, let’s combine your group numbers with the class to get a larger amount of data.

Objectives




• Explain the principle of independent assortment with dihybrid crosses


Punnett SquaresNamed after biologist Reginald Crundall Punnett who

created the first ‘Punnett square’

Punnett squares can be used to predict and compare the genetic variations that will result from a cross.

Before we set up a Punnett square we need to learn some terminology:

Organisms that have two identical alleles for a particular trait (gene) are said to be homozygous. • Example: A true-breeding tall pea plant’s genotype is TT

A true-breeding short pea plant’s genotype is tt• These are BOTH examples of homozygous alleles• Homo = sameOrganisms that have two different alleles for a particular trait (gene) are said to be heterozygous• Example: A tall pea plant with the genotype Tt is

heterozygous• Hetero = different

Setting up a Punnett Square

Note:Dominant alleles are upper case: FRecessive alleles are lower case: f

Freckles on skin is a dominant trait. A man with no freckles marries a woman who is homozygous for the freckle gene. What are the possible genotypes and

phenotypes of their children?

What is the man’s genotype?ff

What is the woman’s genotype?FF

P generation: ff x FF

What gametes would the man produce?

f and f (or only f)

What gametes would the woman produce?

F and F (or only F)


Freckles on skin is a dominant trait. A man with no freckles marries a woman who is homozygous for the freckle gene. What are the possible genotypes and

phenotypes of their children?

P generation: ff x FF

f

f

F F

Ff Ff

Ff Ff

What is the genotype of the offspring?

100% Ff

What is the phenotype of the offspring?

All their children will have freckles

Gametes from the parents

This is the F1

generation


Now let’s say a daughter from the previous couple meets and marries a man who is also heterozygous for the freckled gene. What are the possible genotypes and

phenotypes for their children and in what proportion ?

F1 generation: Ff x Ff

F

f

F f

FF Ff

Ff ff

What is the genotype of the offspring?

¼ FF, ½ Ff, ¼ ff

or a 1:2:1 ratio

What is the phenotype of the offspring?

¾ will have freckles ¼ will not have

freckles

or a 3:1 ratio

This is a monohybrid

cross

Objectives




• Explain the principle of independent assortment with dihyrbid crosses


Mendel’s Laws

Law of segregation:During the production of gametes the two copies of each hereditary factor segregate so that offspring acquire one factor from each parent.

Law of Independent Assortment:The principle of independent assortment states that genes for different traits can segregate independently (random chance) during the formation of gametes.

Independent assortment helps account for the many genetic variations observed in plants, animals, and other organisms.

Independent AssortmentAfter showing that alleles segregate during the formation of gametes, Mendel wondered if they did so independently. • For example, does the gene that determines if a

pea is round or wrinkled have anything to do with the gene for seed color (yellow vs green)?

In order to test his hypothesis for independent assortment he set up a dihybrid cross

Di= two hybrid = heterozygote

Example: Cross a true-breeding (homozygous) Round and yellow pea plant to a true-breeding (homozygous) wrinkled and green pea plantRound ( R ) is dominant to wrinkled ( r )Yellow ( Y ) is dominant to green ( g )

X

RRYY rryy

X

RRYY rryy

Dihybrid Cross

P

RrYy

F1

All F1 offspring had round and yellow peas.

He then crossed these peas to themselves to create a dihybrid cross

Dihybrid Cross

RrYy x RrYyIf independent assortment is true for these traits, then Mendel should get a combination of 4 different gametes for each parent.

Each gamete would have the same probability of occurring (like flipping a coin)

Possible gametes if independently assorting:

RY Ry rY ry

Each gamete would have a ¼ or 25% chance of occuring

Dihybrid Cross

Results from Mendel’s dihybrid cross:9 round and yellow3 round and green3 wrinkled and yellow1 wrinkled and green

This phenotypic ratio of 9:3:3:1

Due to the ratio and phenotypes not seen in the parents but observed in this F2 generation, Mendel proved independent assortment WAS occurring between these two genes.

Objectives




• Explain the principle of independent assortment with dihyrbid crosses


Non-Mendelian Inheritence

EduSmart video in class

See pages 272-273 in your book

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