Gregor Mendel• Gregor Mendel:

– Austrian monk lived from 1822-1884

– Mendel developed principles of heredity without any knowledge of genes or chromosomes

– His principles were established through experiments with pea plants

Why was Mendel successful with the pea?

• Used pure breeding, 7 contrasting traits• Studied characteristics one at a time for many

generations• Used mathematics in analyzing his results• Obtained large numbers of offspring• Chose pea plants which normally self-fertilize• Inexpensive • Used scientific method • Easy to pollinate (transfer of male pollen to egg)

Mendel’s 7 contrasting traits

Genetics Terms• Define genes:

factors that control organism’s traits.

the part of chromosome that contains the genetic code.

• Every organism requires a set of coded instructions for specifying its traits

• For offspring to resemble their parents, their must be a reliable way to transfer hereditary information from one generation to the next

Genetics Terms• homozygous (pure): the alleles on homologous

chromosomes are the same • heterozygous: (hybrid): the alleles on

homologous chromosomes are different • parental generation (P): the two original

organisms being crossed - usually pure

• first filial generation (F1) : the first generation of offspring from the parents

• second filial generation (F2) : generation of offspring arising from the first filial generation

Genetics Terms

• Genotype: the genetic makeup of an organism– Homozygous Dominant: TT– Homozygous Recessive: tt– Heterozygous: Tt

• Phenotype: the appearance of an organism– Describes what it looks like

TT - Tall tt - short Tt - Tall

Punnett Squares• a model used to predict the results of a

genetic cross

• BB X bb

b

b

BB

B B

BB b

b b

b

Three Laws by Mendel1. Law of Dominance: a pattern of heredity

in which one allele of a gene may express itself by masking the presence of the other allele

Dominant Trait: the trait or allele that is expressed (capital letter) R

Recessive Trait: the trait or allele that is present but that is not expressed (lowercase letter) r

Three Laws by Mendel

1. Law of Dominance:

Example:red flower (RR) X white flower (rr) red flower (Rr)

X

Example of Dominance

• R = red

• r = white

• RR x rr

R R

r

r

Rr

Rr

Rr Rr

Results:Phenotype:

Genotype:

Problem:Cross homozygous dominant with homozygous recessive

100% red

100% heterozygous

2. Law of Segregation Mendel’s second law

– When gametes are formed during meiosis:• There is a random segregation of homologous

chromosomes

• Random segregation of sister chromatids & alleles

• The result: new gene combinations are likely to be produced

• Segregation means separation and can lead to genetic recombination.

Phenotype:

____% red, ____% white

___ red : ___ white Genotype: ___% homozygous dominant, ___% homozygous recessive, ___% heterozygous

Example of Segregation

• R = red

• r = white

• Rr x Rr

R r

R

r

RR Rr

Rr rr

Problem:Cross two offspring from 1st cross (2 heterozygous parents)

75 25

13

252550

3. Law of Independent Assortment

Mendel’s third Law – Scenario: Two different traits located on two different

chromosomes

• They segregate randomly during meiosis

• May be inherited independently of each other

• The cross of two organisms heterozygous for a trait is known

as a dihybrid cross

Law of Independent Assortment

Dihybrid CrossProblem:Cross homozygous tall and homozygous wrinkled seeds with homozygous short and homozygous smooth seeds

T = tallt = short

Q = wrinkledq = smooth

What are the genotypes for these plants?

TTQQ x ttqq

TTQQ x ttqq

TtQq

TtQq

TtQq

TtQq TtQq TtQq TtQq

TtQq

TtQq

TtQqTtQqTtQq

TtQq

TtQq TtQq

TtQq

Phenotype: 100% Tall & Wrinkled

TQ TQ TQ TQtq

tq

tq

tq

Dihybrid Cross

• What is the phenotype from this cross?– 100% Tall and Wrinkled

• What is the genotype from this cross?– We don’t worry about genotype for dihybrid

crosses

Complete the following Dihybrid cross

Step 1 - set up gamettes(sex cells)

(1 3, 1 4, 2 3, 2 4)

TtQq x TtQq

Complete the following Dihybrid cross TtQq x TtQq T-Tall t-short Q-Wrinkled q-smooth

TTQQ

TTQq

TtQQ

TtQq Ttqq ttQq ttqq

ttQq

Ttqq

TtQqTtQQTTQq

TTqq

TtQq ttQQ

TtQq

TQ tQTq tq

TQ

Tq

tQ

tq

What are the phenotypes for the above cross???

• - Tall & Wrinkled

• - Tall & smooth

• - Short & wrinkled

• - Short & smooth

9

3

3

1

Incomplete Inheritance• Two examples of Incomplete Inheritance:

– Incomplete Dominance & Codominance

• Incomplete Dominance: – A case where one allele is partially dominant over the other

– Examples of Incomplete Dominance:

red snapdragons X white snapdragons pink snapdragons

cross between black and white Andulusian

fowl gives blue (gray) fowl

Example of Incomplete Dominance Pink Snapdragons

X

Results:Phenotype: ___% red, ___% pink, ___% white

Genotype: ___% homozygous dominant ___% heterozygous___% homozygous recessive

Example of Incomplete Dominance

• R = red

• r = white

• Rr x Rr

R r

R

r

RR Rr

Rr rr

Problem:Cross offspring from 1st cross (2 heterozygous parents)

5025 25

255025

Ratio ___ : ___ : ___1 2 1

Ratio ___ : ___ : ___1 2 1

Intermediate Inheritance• Codominance: a case in which neither allele is

dominant over the other– Alleles have equal power

• Examples:– Cross between red and white short horned cattle gives

roan cattle

– Checkered black & white chicken

– Sickle-cell Anemia - a blood disease where RBCs are sickle shaped or half moon. Most common African.

– Heterozygous - half normal half sickle shape

Roan Cattle

Red CattleX

White Cattle

Roan Cattle

Results:Phenotype: ___% red, ___% red & white, ___% white

Genotype: ___% homozygous dominant ___% heterozygous___% homozygous recessive

Example of Codominance

• R = red

• r = white

• Rr x Rr

R r

R

r

RR Rr

Rr rr

Problem:Cross offspring from 1st cross (2 heterozygous parents)

5025 25

255025

Ratio ___ : ___ : ___1 2 1

Ratio ___ : ___ : ___1 2 1

Problem 1

Homozygous dominant

Phenotype of tt -------------------------

Genotype of tt---------------------------

Phenotype of TT -----------------------

Genotype of TT-------------------------

Phenotype of pure dominant--------

Genotype of pure dominant---------

Phenotype of pure recessive--------Genotype of pure recessive---------

Short

Homozygous recessive

Tall

Tall

TT

Shorttt

__ __ x __ __

X Y

X

X

XX XY

XX XY

Results:

Phenotype: 50% male, 50% female Phenotype ratio: 2 male : 2 female

Problem 2:A married couple want to know their chances of having girl

X Y X X

Phenotype: ___% red, white ___%

Phenotypic Ratio: __ : ___

Genotype: ___% heterozygous

___% homozygous dominant

____% homozygous recessive

• R = red

• r = white

• __ __ x __ __

R r

R

r

RR Rr

Rr rr

Problem 3: Cross two

heterozygous parents

R r rR

75

25

502525

25

3 red 1 white

Results:

Phenotype: ___% red, white ___%

Phenotype Ratio: __ : _ _ __ Genotype: ___% heterozygous

___% homozygous dominant ___% homozygous recessive

• R = red

• r = white

• __ __ x __ __

R R

R

r

RR RR

Rr Rr

Problem 4: Pure dominant crossed with hybrid

R R rR

100

50500

0

4 red 4 white

Phenotype: ___% red, white ___%

Phenotypic Ratio: __ : ___ Genotype: ___% heterozygous

___% homozygous dominant ___ % homozygous recessive

• R = red

• r = white

• __ __ x __ __

r r

R

r

Rr Rr

rr rr

Problem 5: The male’s genotype is homozygous recessive. The female is phenotypically dominant but does carry the recessive allele.

r r rR

50

50 050

50

2 red 2 white

Results:

Phenotype: ___% red, ___ % red and white, white ___%

Phenotypic Ratio: __ red : __ red and white : ___ white Genotype: ___% heterozygous

___% homozygous dominant ___ % homozygous recessive

• R = red

• r = white

• __ __ x __ __

R r

R

r

RR Rr

Rr rr

Cross two heterozygous parents

R r rR

25

50 2525

25

1 2

Problem 6: Law of Codominance

50

1

Results:

Phenotype: ___% red, ___ % pink, white ___%

Phenotype Ratio: __ red : __ pink : ___ white Genotype: ___% heterozygous

___% homozygous dominant ___ % homozygous recessive

• R = red

• r = white

• __ __ x __ __

R r

R

r

RR Rr

Rr rr

Cross two heterozygous parents

R r rR

25

502525

50

1 2

Problem 7: Law of Incomplete Dom

25

1

Test Cross:

It will determine if a horse/dog in question is pure or carrying a recessive gene.

• B = brown

• b = white

__ __ x __ __

or

__ __ x __ __

B B

b

b

Bb Bb

Bb Bb

Problem 8: A test cross uses an individual that is homozygous recessive. It will determine if the dog in question is pure or carrying a recessive gene.

B B bb

B b bb b

b

B b

Bb

Bb

bb

bb

Multiple alleles

• Traits that are controlled by more than 2 alleles• Results in multiple phenotypes• Examples:

– Pigeons BA dominant over BBA and B are dominant over b

– Blood groups in humansFour blood types A B AB & O

X-linked InheritanceExamples: Hemophilia, Color Blindness, Loss of Hearing & Muscular Dystrophy

X-linked Recessive Inheritance

___% NORMAL HEARING of TOTAL OFFSPRINGS,

___ % HEARING LOSS of TOTAL OFFSPRINGS

___% NORMAL HEARING OF FEMALES

___ % HEARING LOSS OF MALES

XDXd

XD

XdYXDY

XDXD

Y

XD

Xd

XDXd XDY

XdYXDXdXDYXDXD

75

10050

25

Polygenic Inheritance

A pattern of a trait that is controlled by 2 or more genes. Phenotype express a range of variability.

• Examples:– Stem length, human height, eye color & skin colorStem length for a totally recessive plant is____ cm.

aabbcc = 4 cm AABBcc = cm Aabbcc = cm AABBCc = cmAAbbcc = cm AABBCC = cmAABbcc = cm

Gene Expression• Influence of External Environment:• Examples: Temp., nutrition, light, chemicals

– Color of rabbit in the summertime: brown– Color of rabbit in the winter: white– The temperature effects what color fur (or

what proteins) are expressed– Temp also determines the sex of a gator – Light determines color of bacteria

Gene Expression

• Influence of Internal Environment:

• Examples: Hormonal influences – Horn size in mountain sheep– Male pattern baldness– Peacock feathers

Gene Expression

• Influence of Internal Environment:

• Examples: Hormonal influences – Horn size in mountain sheep– Male pattern baldness– Peacock feathers

Nature vs. Nurture

• In many cases it is not only the genes that we have that determine what we look like

• Scenario: If identical twins (same DNA) were separated at birth and lived in 2 different environments and then brought together 25 years later would they look the same? Why or why not?

Nature vs. Nurture

• Answer:

The identical twins would have similar features (eye color, size of nose, etc.) but may look very different. What they did throughout their lives effects what they look like

– For example: sun exposure, diet, hygiene, injuries, etc.