Introduction to Genetics

Remember…

• DNA RNA ProteinTraits

• DNA contains the code for proteins

(protein synthesis…remember?)

• Proteins determine our traits

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 (P) generation (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

Before we can figure out what

happened….

• We need to learn some genetics!

Mendel’s First Law: The Law of Segregation

1. All individuals

have two copies

of each allele (i.e.

Ss)

2. Each gamete

receives one copy

of every allele

during gamete

formation (S or s)

In other words, they

SEGREGATE

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 allele from gamete

passed down to offspring

Possible

combinations for

alleles of offspring

after fertilization

Vocabulary

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, either

dominant or

recessive

Which allele is dominant? Which is recessive?

SS Ss

Ss ss

S

s

S s

Parent(mom) plant: Ss

Parent

(dad)Plant:

Ss

Possible

combinations for

alleles of offspring

after fertilization

In each square (offspring

trait), which parent is

passing down the

dominant trait? The

recessive trait?

• 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

From Mom (only 1 gamete)

From Dad (only 1 gamete)

Common Vocabulary

• Phenotype

• Genotype

Phenotype

• The observable traits of an organism

(what you see!)

Genotype – the genetic make up

combination of alleles located on

homologous chromosomes that determines

a specific characteristic or trait.

Offspring can have genotypes

that are:

Homozygous dominant: SS

Homozygous recessive: ss

Heterozygous: Ss

**Remember on allele from mom and one

from dad**

You Try It

• Answer #7 on your PowerPoint Notes

Monohybrid Cross • Cross involving only one trait

• Used to find traits of offspring

• Can be used to find the traits of

parents

You Try It

• In humans, brown eyes (B) are

dominant over blue (b). What are the

chances that a blue eyed man and a

homozygous brown eyed woman will

have children with blue eyes?

You try it…monohybrid cross

• 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?

You Try It

• If a heterozygous purple flower is mated

with another heterozygous purple

flower, what ratio of offspring will be

white in color? (Use P for purple and p

for white)

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 (Dd x dd and DD x dd). 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?

You try it…

• Genetics Protocol

• Monohybrid practice problems

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, so, color and shape sort

independently. i.e. round doesn’t necessarily go

to the same gamete as yellow

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 more than one trait– the alleles for

EACH trait will separate independent of each

other.

Summer squash – dihybrid example

• W (white) is dominant to w (yellow)

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

• Cross a white, disk shape with a yellow,

sphere shape

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

Genotype

Phenotype

Let’s try it together: Cross WwDd x

wwdd

What are all the possible allele

combinations for WwDd? wwdd?

What would the genotype and

phenotype ratios be for this cross?

(WwDd x wwdd)

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.

•

• Part A: 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?

• Part B: 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.

You try it…

Non- Mendelian Genetics

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

• For male, xry , xRy

• For female, xrxr , xRxr , xR xR

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.

Male Pattern Baldness

• Male-pattern baldness is a sex-linked

trait in which affected people become

bald.

• XB = No baldness (dominant)

• Xb = Male-pattern baldness (recessive)

• If a Male with XbY genotype mated with

a Female with XB Xb genotype.

• What percent of male children will

have male-pattern baldness?

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.

? ?

?

?

• There are three alleles of this gene:

A, B, and O.

• A person’s blood type is determined

by which allele he/she inherits from

each parent.

Blood Type – a type of

codominance

• A and B genes are “co-dominant”

• if both an A and B allele are inherited, both are expressed

• O is a recessive allele

• If an A or B gene is inherited along with the O gene, the A

or B gene determines the person’s blood type.

• A person is type O only if he/she inherits two O genes.

AB, A, B, O type blood

Polygenic Inheritance – many

genes affecting one trait

• 3 genes affect skin color

Epistasis

• Occurs when two or more genes are

involved in determining traits (i.e.

skin color)

• When gene at one locus affects the

expression of a gene at another

locus

Epistasis example - Mice

• Black coat (B) is dominant to brown

(b)

• Must have bb to have brown coat

• A color gene (C) must be present for

the color to be deposited on the hair

• Must have Cc or CC for color, if cc,

no color and mouse is white

Linked Genes

• Genes that are inherited together

• Located near each other on the same

chromosome

• Chances of them crossing over to

another chromosome during meiosis

is small

• Ex. Eye color and wing color in fruit

flies are inherited together, not

independently sorted during meiosis

Environmental Factors

• Can affect whether or not a trait is

expressed (turn on the trait or turn it

off)

– Chemicals

– Radiation

– Diet

– Stress

– Why twins look different – due to their

environment!