LEH U21 01 ClassicalGeneticsHonorsElwoodskh2017

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(1)

Classical Genetics

(2)

Genetics got its start as the study of inheritance.

Charles Darwin proposed that

favorable traits could be passed from generation to generation resulting in natural selection.

However, Darwin did not know how these traits were passed on.

(3)

An Austrian monk,

Gregor Mendel, in 1865, who carried out the

(4)

Gregor Johann Mendel

Gregor Johann Mendel

Austrian monk

Austrian monk

Studied

Studied

inheritance of

inheritance of

traits

traits

in pea

in pea

plants

plants

Developed the

Developed the

laws of

laws of

inheritance

inheritance

Work not

Work not

recognized until

recognized until

turn of 20th

turn of 20th

century

century

(5)

Gregor Johann Mendel, cont.

Gregor Johann Mendel, cont.

Between 1856

Between 1856

and 1863, Mendel

and 1863, Mendel

cultivated and

cultivated and

tested some

tested some

28,000 pea plants

28,000 pea plants

plants' offspring

plants' offspring

retained traits

retained traits

of

of

parents

parents

Father of

Father of

Genetics"

Genetics"

(6)

https://www.dnalc

(7)

Why was Mendel wise to choose pea plants

Why was Mendel wise to choose pea plants

as a model organism?

as a model organism?

Many different traits to study

◦ flower color, seed color, seed shape, etc.

◦ Mendel had strict control over which plants mated with which

each pea plant has male & female

structures

pea plants can self-fertilize

Mendel could also cross-pollinate

(8)

Site of

Site of

Gregor

Gregor

Mendel

Mendel

s

s

experiment

experiment

al garden

al garden

in the

in the

Czech

Czech

Republic

Republic

(9)

copyright cmassengale 9

Mendel stated that

Mendel stated that

physical traits are

physical traits are

inherited as

inherited as

particles

particles

Mendel did not

Mendel did not

know that the

know that the

particles

particles

were

were

(10)

Genetic Terminology

Genetic Terminology

Trait

Trait

- any characteristic

- any characteristic

that can be passed from

that can be passed from

parent to offspring

parent to offspring

Heredity

Heredity

- passing of traits

- passing of traits

from parent to offspring

from parent to offspring

Genetics

Genetics

- study of

- study of

heredity

heredity

(11)

Genes

Genes

Alleles

Alleles

- two forms of a

-

two forms of a

gene

gene

(dominant & recessive)

(dominant & recessive)

Dominant

Dominant

- stronger of two

-

stronger of two

genes expressed in the hybrid;

genes expressed in the hybrid;

represented by

represented by

a

a

capital letter

capital letter

(R)

(R)

Recessive

Recessive

- gene that shows

-

gene that shows

up less often in a cross;

up less often in a cross;

represented by a

represented by a

lowercase

lowercase

letter (r)

letter (r)

(12)

Genotype vs. Phenotype

Genotype vs. Phenotype

Difference between genetics & how an

organism “looks”

 phenotype the “physical”What you see!

◦ genotype

 description of an organism’s genetic makeup

Explain Mendel’s results using …dominant & recessive

…phenotype & genotype F

1

P X

purple white

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Homozygous = same allele Heterozygous = different allele

When both alleles for a trait are identical, the

organism is homozygous for that trait. When the 2 alleles are different, is heterozygous.

TT = Homozygous Td = Heterozygous tall tall

(14)

What was Mendel

What was Mendel

s first step?

s first step?

 He identified pea plants that

were true-breeding for a certain trait.

 Next, he crossed bred plants

hat had two distinct and

(15)

Mendel crossed tall and dwarf pea plants: all offspring were tall.

Tall Dwarf

P generation x

F1 generation

all tall plants

(16)

Next, Mendel crossed some of these F1 plants among themselves. Of these offspring (the F2 generation),

about 3/4 of the plants were tall and 1/4 were dwarf.

tall tall

F1 cross x

F2 tall tall tall dwarf

Dwarf plants came

(17)

Mendel tested 6 other traits of pea plants:

In each F2

generation both traits always

appeared -- and always in a

(18)

Genotype & Phenotype in Flowers

Genotype & Phenotype in Flowers

copyright cmassengale 18

Genotype of alleles:

Genotype of alleles:

R

R

= red flower

= red flower

r

r

= yellow flower

= yellow flower

All genes occur in pairs, so

All genes occur in pairs, so

2

2

alleles

alleles

affect a characteristic

affect a characteristic

Possible combinations are:

Possible combinations are:

Genotypes

Genotypes

RR

RR

R

R

r

r

rr

rr

Phenotypes

(19)

Law of Dominance

Law of Dominance

copyright cmassengale 19

In a cross of parents that are

In a cross of parents that are

pure for contrasting traits

pure for contrasting traits

, only

, only

one form of the trait will appear in

one form of the trait will appear in

the next generation.

the next generation.

All the offspring will be

All the offspring will be

heterozygous and express only the

heterozygous and express only the

dominant trait.

dominant trait.

RR x rr

(20)

Law of Segregation

Law of Segregation

During the

During the

formation of

formation of

gametes

gametes

(eggs or sperm), the

(eggs or sperm), the

two alleles

two alleles

responsible for a

responsible for a

trait

trait

separate

separate

from each other.

from each other.

Alleles for a trait are then

Alleles for a trait are then

"recombined" at fertilization

"recombined" at fertilization

,

,

producing the genotype for

producing the genotype for

the traits of the offspring

the traits of the offspring

.

(21)

Law of Independent Assortment

Law of Independent Assortment

Alleles for

Alleles for

different

different

traits

traits

are distributed to sex cells

are distributed to sex cells

(& offspring) independently

(& offspring) independently

of one another.

of one another.

This law can be illustrated

This law can be illustrated

using

using

dihybrid crosses

dihybrid crosses

.

.

(22)

How are monohybrid and dihybrid

How are monohybrid and dihybrid

crosses different?

crosses different?

Monohybrid cross: follows the inheritance of

single traits

Dihybrid cross: follows the inheritance of

(23)

Dihybrid cross: two methods

Dihybrid cross: two methods

YyRr

YyRr

YR

Yr

yR

yr

YR

Yr

yR

yr

YYRR

x

YYRr YyRR YyRr

YYRr

YYrr

YyRr

Yyrr

YyRR YyRr yyRR yyRr

YyRr

Yyrr

yyRr

yyrr

(24)

Test cross

Test cross

What cross can the farmer make

to prove that his tomato seeds

are pure breeding and not

hybrid?

Cross with

homozygous

(25)

How do punnett squares determine

How do punnett squares determine

probability?

probability?

P

p

x P

p

P

p

male / sperm

P

p

fe m al e / eg g s

PP

75% 25% 3:1 25% 50% 25% 1:2:1 % genotype % phenotype

PP

P

p

P

p

pp

pp

P

p

P

p

F

1

generation (hybrids)

phenotype & genotype can have different

(26)

Dihybrid cross

Dihybrid cross

: better method

: better method

YyRr

x

YyRr

9/16 yellow

round

3/16 green round

3/16 yellow wrinkled

(27)

Dihybrid cross

Dihybrid cross

 Mendel’s Law of Independent

Assortment “when gametes are formed, one allele from each trait goes into each gamete, and any combination is possible”

 If traits assort independently, the

dihybrid cross expected ratio is

9:3:3:1

 9/16 dom/dom 3/16 dom/rec

(28)

Are there any exceptions to

Are there any exceptions to

Mendel

(29)

R = allele for red flowers W = allele for white flowers

red x white ---> pink RR x WW ---> 100% RW

Incomplete dominance (Blended):

Incomplete dominance (Blended):

 cross between organisms with two different phenotypes

produces offspring with a third phenotype that is a

blending of the parental traits.

(30)

Codominance:

Codominance:

 common example is roan fur in cattle. Cattle can

(31)

Codominance, cont

Codominance, cont

.

.

 Another example is human blood type AB, in which

(32)

Sex determination

Sex determination

 Chromosomes of

males and females are different

 Two types:

Autosomes =

homologous

chromosomes that match up

Sex chromosomes =

(33)

Sex-linked Traits

Sex-linked Traits

Traits (genes) located on the

Traits (genes) located on the

sex chromosomes

sex chromosomes

Sex chromosomes are

Sex chromosomes are

X and

X and

Y

Y

XX

XX

genotype for females

genotype for females

XY

XY

genotype for males

genotype for males

Many

Many

sex-linked traits

sex-linked traits

carried

carried

on

on

X

X

chromosome

chromosome

(34)

Sex-linked traits

Sex-linked traits

Colorblindness

XX= normal vision female

XX’ = female with recessive allele

(carrier, but normal vision)

(35)

Colorblindness Test

(36)

Female Carriers

Female Carriers

Figure

Updating...

References

Related subjects : round 3