Genetics

What is Genetics?

• The branch of biology that seeks to explain biological variation

Heredity

• Transmission of characteristics (traits) from parent to offspring

Vocab You Should Recall:

• Diploid-– a cell containing 2 sets of

homologous chromosomes (2n)• Haploid-

– a cell containing 1 set of chromosomes without a homologous pair (n)

• Gamete- – a male or female sex cell (n)

• Zygote- – a fertilized egg cell (2n)

More to Recall:• Chromosome

– a long piece of DNA containing many alleles

• Autosome– All chromosomes not

involved in determining sex• Gene

– The length of DNA that codes for a trait.

– Genes come in pairs that separate in the formation of gametes.

New Vocab• Genotype

– An organisms genetic make-up (DNA code passed on from parents to offspring)

•Phenotype- The physical appearance of a gene (visible trait)

What is the relationship between genes (genotype) and observable characteristics (phenotype)?

Phenotype = Genotype + Environment.

Genetically identical hydrangeas growing in soils of different acidity (different environments).

The phenotype = genotype + environment principle applies equally to human traits.

Different Genotypes Can Produce the Same Phenotype

Classroom GeneticsCharacteristic: d=dominant / r=recessive Number w/trait % of Class with Trait

Hairline widow's peak (d) continuous hairline (r)

Earlobes unattached (d) attached (r)

Hitchhiker's thumb top segment of thumb straight (d) top segment of thumb can be bent back (r)

U-shaped tongue able to roll tongue in trough-like shape (d) unable to roll tongue (r)

Nondisjunction

• Failure of homologous chromosomes to separate in meiosis

• Web Karyotyping activity

• Allele– Alternative versions of a

gene (one from each parent 2 alleles = one gene)

• Homozygous– Having a pair of identical

alleles for a characteristic (pure)

• Heterozygous– Having 2 different alleles

for a characteristic (hybrid)

For example, the gene for seed shape in pea plants exists in two

forms:•one form or allele for round seed shape (R) •the other for wrinkled seed shape (r).

•A homozygous plant would contain the following alleles for seed shape:

• RR or rr.•A heterozygous plant would contain the alleles Rr

• Dominant-– In heterozygote, the allele that is

expressed in phenotype• Recessive

– In heterozygote, the allele that is completely masked in the phenotype

Earlobes:

Free ear lobes (dominant trait)

Attached ear lobes (recessive trait)

Dimples:

Dimples (dominant trait)

No dimples (recessive trait)

Tongue-Rolling:

Rolling up edges (dominant trait)

not rolling (recessive)

Mid- Didget Finger Hair:

Present – even if just one hair (dominant trait)

Not present (recessive)

Freckles:

Freckles (dominant trait)

No freckles (recessive)

Widow’s Peak

Present- (dominant trait)

Not present- (recessive)

• Monohybrid Cross– a genetic cross

between individuals differing in one trait

•Dihybrid Cross- a cross between individuals differing in two traits

• F1 Cross- – The first generation of

hybrid offspring in a genetic cross

• F2 Cross– Offspring resulting from

interbreeding of the hybrid F1 generation.

• Test Cross– Breeding of recessive

homozygote with dominant phenotype, but unknown genotype

• Punnett square-– diagram used by

biologists to predict the possible outcome of a genetic cross

Mendelian Genetics• Gregor Mendel (1823- 1884)

– Known as the “Father of Modern Genetics”

– Augustinian Monk who wondered how plants obtain atypical characteristics

– Wrote “Experiments with Hybrid Plants”

Why Peas?

• Easy to grow• Fast growing

– could grow many generations during his 8-year experiment

• Self-fertilizing– don’t need a mate

Traits he looked at:

• He chose traits with an either-or form. For example:– Short or tall– Smooth or wrinkled peas– Green or yellow pods and seeds

•He DID NOT study traits with a continuous range of variation (like seed size)

Mendel’s Experiments• 1st, he needed true-breeding varieties (homozygous

dominant) for each of the traits --this took years.• He then found an “atypical” variety of plant

(homozygous recessive) and transplanted them next to typical varieties and recorded which progeny arose.

• He found that all offspring retained the essential traits from parents.

Mendel crossed plants with wrinkled peas and plants with smooth peas

• Would the offspring plants produce smoothish-rough pods?

• Would some of the pods be smooth and some be rough?

•Mendel found that the next generation of plants produced only smooth peas.•However, when those smooth pod plants self-fertilized, some plants in the next generation produced rough pods.

A Color Cross:

Punnett Square:• Predicts offspring genotype from parental

gamates in a visual form:

Tongue Rolling• Dolly cannot roll her tongue, so we know

she is homozygous recessive for that trait.– Phenotype: non roller– Genotype: tt

• Dolly’s dad cannot roll his tongue either– Phenotype: non roller– Genotype: tt

• Dolly’s mom can roll her tongue.– Phenotype: roller– Genotype: TT or Tt

• In order to have a daughter who cannot roll her tongue, what does Dolly’s mom genotype have to be?

t t

T

?

Tt Tt

-t -t

In order for Dolly to be a non-tongue roller, her mom has to be heterozygous for tongue rolling

tt ttt

Some Alleles Are Related Through

Incomplete Dominance

•pattern of gene expression in which the phenotype of a heterozygous individual is intermediate between those of the parents.

Some generalizations for genetics problems:• Two alleles are necessary for a trait• The genes are symbolized by the first

letter of the dominant gene.• The letter for the dominant gene is always

capitalized.• The letter for the recessive trait is always

lower case (make sure you can tell the difference between the two)

• Wild Type is the typical form of the organism, strain, or gene

• Pure traits are those with identical genes (homozygous).

• Hybrids have mixed genes for the same trait (heterozygous).

• Gametes only carry one allele for each trait (they are haploid)

Gregor Mendel’s Rules of Inheritance• Rule of Dominance and Recessiveness:

– The allele that expresses itself in the phenotype when a gene is heterozygous (hybrid) is the dominant allele. The allele that is masked is the recessive allele. Ex. Tongue rolling.

• Rule of Incomplete Dominance: – When a gene is heterozygous, incomplete dominance (or

codominance) results when the phenotype is a mixture of the two genotypes. Ex. Red, pink and white snapdragons.

• Rule of Segregation: – During Meiosis, two alleles of a gene separate during the formation of

gametes (egg and sperm).• Rule of Independent Assortment:

– Alleles of one gene separate independently of the alleles of any other gene. In other words, the way in which one pair of alleles segregates has nothing to do with the way any other pair of alleles segregate.

Probability:• It’s the mathematical tool used for predicting the

likelihood of events in everyday life• The expected frequency of a particular event

when an experiment is repeated an infinite number of times is the probability of the event.

• Probabilities in genetics are often predicted based on certain hypotheses and then the predictions are used to test the hypothesis using real data.

• Mendel actually did this with pea plants. He predicted outcomes and then tested them thousands of times over 8 years!

Probability = # times event is expected to happen# opportunities (trials)

• It is usually expressed as a fraction.– Ex.

• The chance of a coin landing heads up is one out of two or ½

• The chance of drawing an ace out of a deck of cards is 4 out of 52 or 1/13

• Or explained as a percentage chance.– There is a 50% chance of a coin landing heads up when

you toss it.– There is a 7.7% chance of picking an ace up from a deck

of cards.

• Why can the principles of probability be used to predict the outcomes of genetic crosses?

The way in which the alleles segregate is completely random, like a coin flip.

Segregation of Alleles

Principle of Segregation:• Genes come in pairs that

separate in the formation of sex cells (and these sex cells unite randomly at fertilization).

• Cystic fibrosis is an autosomal recessive disorder. – In order to have the disease, an individual has to be

homozygous recessive (ff) • A man and woman are both carriers for cystic fibrosis

(Ff). What are the chances of them having a child with the disorder? F f

F

f

FFFf

Ff ff

First Law of Probability

• the results of one chance event have no effect on the results of subsequent chance events.

• ex. My cousin already has 3 boys. His wife is pregnant. What is the probability of his fourth child being a boy?

½

If my friend Victoria and her husband Peter are carriers of cystic fibrosis and have already had a son with the disorder, what are their chances of having another child with the disorder?

¼ or 25%

Two Other Rules of Probability you’ll need to understand

• Rule of Multiplication- – the probability of a compound event is equal to the

PRODUCT of the separate probabilities of the independent single events

– Probability of event a and b happening = p(a)p(b)

Ex. The probability of flipping a coin and getting 2 tails in a row: The probability of flipping tails on the 1st penny=

x

The probability of flipping tails on the 2nd penny=

=

The probability of getting tails on both pennies=

½ ½

¼

Rule of Addition-

• The probability of an event that can occur in two or more alternative ways is the SUM of the separate probabilities of the different ways.

Ex. The probability of flipping 1 head and 1 tail in a toss with 2 coins:

The probability of flipping tails on the 1st penny and heads on the 2nd penny = (½)(½) =

+

The probability of flipping heads on the 1st penny and tails on the 2nd penny= (½)(½) =

=

The probability of getting one tail and one head in a toss with 2 coins =

¼ ¼ 1/2

Practice Problems

• What is the probability of any couple having 4 boys in a row?

½ x ½ x ½ x ½ = 1/16 or 6.25%

What is the probability of my friend Victoria and her husband having two kids with cystic fibrosis?

¼ x ¼ = 1/16

Is the following sentence true or false?

False

The past outcomes of coin flips greatly affect the outcomes of future coin flips.

How can you be sure of getting the expected 50 : 50 ratio from flipping a coin?

You must flip the coin many times.

Therefore, the _____ number of offspring from a genetic cross, the closer the resulting numbers will get to expected values.

Larger

What Works for Peas Also Works for Humans

Albinism is a autosomal recessive condition.

• In order to show the disorder, an individual has to be homozygous recessive (aa)

• Refer to the following punnett squares to see the possible offspring for different scenarios:

Stop

• The Coin Lab

Are Different Characters Like Color and Shape Inherited Together or Inherited Independently?

• Mendel performed dihybrid crosses to find out.

•Mendel’s conclusion:• Different characters are

inherited independently.

Incomplete Dominance

•pattern of gene expression in which the phenotype of a heterozygous individual is intermediate between those of the parents.

A a

B b

A A

b b

How to make a two gene cross:

Meiocytes - diploid cells that form haploid gametes– a.k.a. germ cells

Egg meiocyte Sperm meiocyte

Determine the types of gametes that each parent forms, according to

Mendel’s laws: a) The Law of Equal Segregation

• the 2 members of a gene pair segregate equally into gametes

b) The Law of Independent Assortment:different gene pairs assort independently from one another (unless otherwise noted)

How do I do this? Perform a cross.1) Align the 2N genotype horizontally (one locus) and

vertically (the second locus) on a square. Do this for each meiocyte.

egg meiocyte sperm meiocyteA a A A

B bb b

2) Perform a cross, fill in each cell of the square. Each cell is one of the types of gametes formed by the meiocyte.

A a A AB AB aB b Ab Abb Ab ab b Ab Ab

Figure out what the gametes are that the parents can make.

Using the F.O.I.L. method from mathAaBb :

First-AB

Outer-Ab

Inner-aB

Last-ab

Or:

Next, combine the male and female gametes.

These combine at random upon fertilization.

AB Ab aB ab

Ab

Ab

Ab

Ab

AB Ab aB ab

Ab AABb AAbb AaBb Aabb

Ab AABb AAbb AaBb Aabb

Ab AABb AAbb AaBb Aabb

Ab AABb AAbb AaBb Aabb

4/16 4/16 4/16 4/16 25% 25% 25% 25%

The Dihybrid Cross: a cross between 2 individuals that are heterozygote at the same two genes

Using Mendel’s strategy consider A=yellow, a=green. B=round, b=wrinkled

P: AA BB x aa bb (cross two pure lines)F1: Aa Bb x Aa Bb (the dihybrid cross, between 2 F1 progeny)

F2: genotypes phenotypes

AB Ab aB ab

AB

Ab

aB

ab

9 A- B- (yellow, round)3 A- bb (yellow, wrinkled)3 aa B- (green, round)1 aa bb (green, wrinkled)

AABB AABb AaBB AaBb

AABb AAbb AaBb Aabb

AaBB AaBb aaBB aaBb

AaBb Aabb aaBb aabb