Introduction to Genetics

Remember…

• DNA RNA ProteinTraits

• DNA contains the code for proteins

(protein synthesis…remember?)

• Proteins determine our traits

Remember…

Gregor Mendel 1822-1884

• Father of Genetics

– Studied characteristics garden plants

– Curious about inheritance patterns

Mendel’s pea plants

• Pea plants can:

– Self fertilize

– Cross fertilize

• Have easily observed contrasting traits (ex. tall or short)

Pea traits

Mendel’s experiments

Parent generation (P generation starting point)

Pure Green Pea X Pure Yellow Pea

Hypothesis: yellow-green offpsring

Genetic Crossing Parent (green) X Parent (yellow)

First Generation (F1 - offpsring)

**ALL YELLOW**

P P

F1 F1 F1 F1

Mendel’s Second Generation – F2

Mendel self-fertilized the F1:

F1 F1

F2 F2 F2 F2 F2 Green trait re-

appeared in F2

(second)

generation

3:1 ratio

Mendel’s First Law: The Law of Segregation

All individuals have two

copies of each allele (i.e.

Ss); each gamete receives

one copy of every allele

during gamete formation.

Hybrid Cross - mating between individuals

who have different alleles at one locus of interest

SS Ss

Ss ss

S

s

S s

Parent(mom) plant:

Ss

Parent

(dad)Plant:

Ss

Possible gametes passed down

Principle of Dominance

DOMINANT TRAIT – Traits that is

expressed; what you see (ex. T)

RECESSIVE TRAIT – Trait that is hidden;

(ex. t)

Each parent will either pass down an allele

that is dominant or an allele that is

recessive

Each parent

passes down only

1 of 2 possible

alleles

• Offspring will inherit two alleles, one

from mom and one from dad

• These alleles will either be dominant

or recessive

• Offspring inherit 2 alleles, one ‘S’

from each parent

– Possibilities:

• SS

• Ss or sS

• ss

Common Vocabulary

• Genotype

• Phenotype

• Homozygous

• Heterozygous

Common Vocabulary cont.

• Genotypic description:

– Homozygous dominant: SS

– Homozygous recessive: ss

– Heterozygous: Ss

• Remember: the phenotype is ‘what

you see’… SS and Ss - same

phenotype, different genotype

Test cross – to determine what

parent genotype was

• Cross between a dominant

phenotype and a recessive individual

S ? X ss

• Parent genotype can be determined

by looking at the offspring

Monohybrid Cross • Cross involving only one trait

Dihybrid Cross

• Cross involving two traits – Ex. Shape and Color

• Round (R) X Yellow (Y) (for peas)

• RrYy x RrYy

Each allele sorts independently from other alleles

in gamete formation

Mendel’s Second Law: The Law of Independent

Assortment

The two alleles for one gene segregate

(assort) independently of the alleles for

other genes during gamete formation

(meiosis).

2nd Law only refers for dihybrid crosses

involving 2 traits – the alleles for EACH trait will

separate independent of each other.

Summer squash – dihybrid ex.

• W (white) is dominant to w (yellow)

• D (disk shape) is dominant to d (sphere shape)

WWDD (white, disk-shaped fruit) X wwdd (yellow, sphere-shaped fruit)

Genotype

Phenotype

P (parent) Generation

All offspring for F1 : WwDd X WwDd

white, disk-shaped yellow, sphere-shaped

F1

P

Cross all the possibilities for the

offspring to determine F2

F1 possibilities

F1 possibilities

F1

F2 possibilities

What would the

genotype and

phenotype ratios

be?

You try it…monohybrid test cross

• In humans, brown eyes (B) are dominant

over blue (b)*. A brown-eyed man marries

a blue-eyed woman and they have three

children, two of whom are brown-eyed and

one of whom is blue-eyed. Draw the

Punnett square that illustrates this

marriage. What is the man’s genotype?

What are the genotypes of the children?

Try again.

• In dogs, there is an hereditary deafness caused by a

recessive gene, “d.” A kennel owner has a male dog that

she wants to use for breeding purposes. The dog can hear,

so the owner knows his genotype is either DD or Dd. If the

dog’s genotype is Dd, the owner does not wish to use him

for breeding so that the deafness gene will not be passed

on. This can be tested by breeding the dog to a deaf female

(dd). Draw the Punnett squares to illustrate these two

possible crosses. In each case, what percentage/how many

of the offspring would be expected to be hearing? deaf?

How could you tell the genotype of this male dog?

Dihybrid Cross • In humans, there is a gene that controls formation (or lack thereof)

of muscles in the tongue that allow people with those muscles to

roll their tongues, while people who lack those muscles cannot

roll their tongues. The ability to roll one’s tongue is dominant over

non-rolling. The ability to taste certain substances is also

genetically controlled. For example, there is a substance called

phenylthiocarbamate (PTC for short), which some people can

taste (the dominant trait), while others cannot (the recessive trait).

To people who are tasters, the paper tastes very bitter, but to non-

tasters, it just tastes like paper.

• Let’s let R represent tongue-rolling, r represent a non-roller, T

represent ability to taste PTC, and t represent non-tasting.

•

• Suppose a woman who is both a homozygous tongue-roller and a

non-PTC-taster marries a man who is a heterozygous tongue-

roller and is a PTC taster, and they have three children: a

homozygous tongue-roller who is also a PTC taster, a

heterozygous tongue-roller who is also a taster, and a

heterozygous tongue-roller who is a non-taster. If these parents

would have a bunch more children so that they had 12 in all, how

many of those 12 would you expect to be non-tasters who are

homozygous for tongue-rolling?

• If the first child (the homozygous tongue-roller who is also a PTC

taster) marries someone who is heterozygous for both traits, draw

the Punnett square that predicts what their children will be.

Sex linked Traits

• Found only on the X

chromosome

• Women - 2 X

chromosomes (XX)

• Men - 1 X

chromosome (XY)

• Any genes found on

the X chromosomes

are referred to as

sex-linked genes.

Designating Sex linked Traits

• Xr - Recessive trait

• XR - Dominant trait

What is the genotype for a female that is homozygous

dominant?

What is the genotype for a woman that is Homozygous

recessive?

What is the genotype for a woman that is Heterozygous?

What is the genotype for a male that is has the dominant trait?

What is the genotype for a man with the recessive trait?

Fruit Flies – gene for eye color • Eye color gene is carried on the X

chromosome (sex-linked)

• Red eyes – dominant - XR

• White eyes – recessive - Xr

• Try it…

If a white-eyed female

fruit fly is mated with a

red-eyed male, predict

the possible offspring.

Incomplete Dominance -

Blending • two different phenotypes produce a

third phenotype that is a blending of

the parental traits.

4 o’clock flowers, Red is

not completely

dominant over white RR – Red flowers

RW – PINK flowers

WW– White flowers

Incomplete Dominance

? ?

?

?

Cross a Red flower with a

White flower. Predict the

ratio of offspring?

Incomplete Dominance

? ?

?

?

Cross a Red flower with a

Pink flower, what is the

ratio of offspring?

Codominance - Together

• two different phenotypes produce a third

phenotype where both parental traits

appear together.

• For Example: Cattle can be red (RR =

all red hairs), white (WW = all white

hairs), or roan (RW = red & white

hairs together).

Codominance

• Cross a Red cow with

a white cow. Predict

the ratio of offspring.

? ?

?

?

Multiple Alleles – trait controlled

by more than two alleles

• Blood Type –

controlled by 3

alleles, not 2

• A, B, O alleles

• 4 blood types

– A, B, AB, O

Polygenetic Inheritance – many

genes affecting one trait

Environmental Factors

• Can affect whether or not a trait is

expressed (turn on the trait or turn it

off)

– Chemicals

– Radiation

– Diet

– Why twins look different – due to their

environment!