Lesson 8 Mendelian inheritance

LESSON # 8If my father is blond, why am I not?

MENDELIAN GENETICSInheritance

Gregor Mendel

Father of modern genetics

Experimented with pea plants (Pisum sativum) 1856-1863

Was a monk in the Czech Republic

His work passed unrecognized until his paper was discovered

in 1900 “Experiments in plant hybridization.” (1865)

This paper showed that each organism has physical traits that

correspond to invisible elements (genes) within the cell.

These invisible elements, which we now call genes, exist in

pairs (alleles).

Mendel showed that only one member of this genetic pair is

passed on to each progeny.

Mendel's laws form the theoretical basis of our understanding

of the genetics of inheritance.

What he did right

Picked the pea plant which is easy self or cross pollinate and grows fast.

Chose to study traits with only two possible phenotypes :

Pea texture: round or wrinkled seed phenotype

Pea color: yellow or green seed phenotype

Flower color: red or white flower phenotype

Pea plant size: tall or dwarf plant phenotype

Pea pod color: green or yellow

Genetic studies innovations

Mendel made two innovations to the science of genetics:

developed pure lines

Pure Line - a population that breeds true for a particular trait. All individuals of

the population have the same phenotype for a specific trait they are pure for.

When self-crossing or crossing with individuals of the same pure line population,

the offspring shows the same phenotype.

Counted his results and kept statistical notes

Law of Dominance or Uniformity of

Hybrids.

When two pure line individuals of different phenotypes are crossed, all the

resulting offspring show one unique phenotype (the dominant phenotype).

All the offspring that result from interbreeding pure line individuals of

different phenotypes, have the same phenotype and the same genotype.

All tall

First Filial (F1) generation

P: Parental generation

Representing the cross

The trait that is maintained in the F1

generation is considered the

Dominant trait.

The trait the disappears is

considered the Recessive trait.

The first letter of the dominant

phenotype is used to indicate the

alleles.

Capital letter for the dominant allele

Lower case for the recessive allele

P: Parental generation

First Filial (F1) generation

How to begin

Gene studied: Pea stem

Possible alleles:

Possible genotypes and corresponding phenotypes:

Tall (T)

Short (t)Tall > Short

Genotype TT Tt tt

Phenotype Tall stem Tall stem Short stem

P: Purebred Tall stem x Purebred Short stem

TT tt

Gametes T T t t

p(T) = 1 p(t) = 1

Punnet square:

Gametes

T

T

t t

Tt Tt

Tt TtGam

ete

s

F1

Genotype 100% Tt

Phenotype 100% Tall stem

F1 Genotypes F1 Phenotypes

p (Tt) = 1 100 % Tall stem

Solved Exercise 1

A homozygous brown eyed woman and a man with blue eyes want to have

children. What is the probability of the child having blue eyes?

Gene studied: Eye color

Possible alleles:


Blue (b)

Brown (B)

Brown > Blue

Genotype BB Bb bb

Phenotype Brown eyes Brown eyes Blue eyes

P: Purebred Brown eyes x Purebred Blue eyes

BB bb

Punnet square:

Gametes

B

B

b b

Bb Bb

Bb BbGam

ete

s

Answer: The possibility of having a child with blue eyes is 0.


p (Bb) = 1 = 100% 100 % Brown eyes

Gametes B B b b

p(B) = 1 p(b) = 1

Exercise 2

What is the probability of having of getting a pea plant with wrinkled peas if we

cross two hybrids?

Gene studied: Pea texture

Possible alleles:


Smooth ___

Wrinkled ___Smooth > Wrinkled

Genotype

Phenotype

Exercise 2

What is the probability of having of getting a pea plant with wrinkled peas if we

cross two hybrids?

Gene studied: Pea texture

Possible alleles:


Smooth (S)

Wrinkled (s)Smooth > Wrinkled

Genotype SS Ss ss

Phenotype Smooth peas Smooth peas Wrinkled peas

P: _____________________ x ______________________

___ ___

Gametes __ __ ___ __

Punnet square:

Gametes

Gam

ete

s

Answer:_________________________________________


P: Smooth pea plant x Smooth pea plant

Ss Ss

Gametes S s S s

p (S) = p(s) = ½ p (S) = p(s) = ½

Punnet square:

Gametes

Gam

ete

s SS Ss

Ss ss

S

S

s

s F1 Genotypes F1 Phenotypes

p (SS) = ¼ = 25%¾ Smooth peas or 75 % Smooth peas

p (Ss) = ½ = 50%

p (ss) = ¼ = 25% ¼ Wrinkled peas or 25% Wrinkled peas

Possible answers:

There is a 25% chance of getting a wrinkled pea plant.

The chance of getting wrinkled pea plants is 1 out of 4

For every 3 smooth pea plants we get, we could get 1 wrinkled pea plant.

Developing the

Law of Segregation

When two hybrids are crossed Mendel observed the

phenotype that disappeared in the F1 generation

reappears in the F2 generation.

The F2 generation shows 2 different phenotypes in

a 3:1 ratio

Seeing this he came to the conclusion that two

alleles that the parents carry for a gene segregate

when gametes are formed, and the offspring only

receives one allele from each parent.

Law of Segregation

Mendel’s law of Segregation states:

The two members of a gene pair segregate from each other into

the gametes, so that one-half of the gametes carry one member

of the pair and the other one-half of the gametes carry the other

member of the pair.

We now know that segregation occurs during meiosis I

Exercise 3

How can I find out if a black hen is purebred or heterozygous?

Gene studied: Color feathers in hens

Possible alleles:


Black ___

White ___Black > Black

Genotype

Phenotype

P: Black feathered hen x White feathered rooster

(homozygous recessive)

BB or Bb bb

Gametes ___ ___ or ___ ___ ___ ___

Punnet square:

Gametes

Gam

ete

s

Answer: If the black feathered hen is purebred (BB) ________________________________________

Test cross

Gametes

Gam

ete

s

or

Answer: If the black feathered hen is hybrid (Bb) _______________________

Therefore, if when we cross a black feathered hen with a white feathered hen, we get a white

feathered hen we know that ___________________



1st Possibility

2nd Possibility

1st Possibility 2nd Possibility

1st Possibility

2nd Possibility

Education

Lesson 8 Mendelian inheritance