1. Why do organisms need haploid cells?

2. What would happen if “crossing over” didn’t occur during Meiosis?

3. What are gametes?

Heredity• Heredity is the passing on of characteristics

from parents to offspring.

These characteristics are called

traits

• It was not until the mid-nineteenth century that Gregor Mendel, an Austrian monk, carried out important studies of heredity.

Gregor Mendel

• Mendel argued that parents pass on to their offspring factors (now called genes) that are responsible for inherited traits.

• Mendel was the first person to succeed in predicting how traits are transferred from one generation to the next.

• Mendel chose to use the garden pea in his

experiments.

• Garden pea plants reproduce sexually, which means that they produce male and female sex cells, called gametes.

Mendel chose his subject carefully

• When Mendel wanted to breed, or cross, one plant with another, he opened the petals of a flower and

removed the male organs.

Remove male parts

Mendel wanted to pick his own parent plants

• He then dusted the female organ with pollen from the plant he wished to cross it with.

Pollen grains

• He only wanted to study one trait at a time at first.

• So he picked two plants that were the same in every way, except height.

SHORT TALL

• The tall pea plants he worked with were from populations of plants that had been tall for many

generations and had always produced tall offspring.

• They were said to be pure-bred tall- “pure-bred” means that it comes from two parents with the same form of a trait.

• Likewise, the short plants he worked with were

pure-bred for shortness.

• A hybrid is the offspring of parents that have different forms of a trait, such as tall and short height.

Parents

Hybrid Offspring

Monohybrid crosses• Mendel’s first experiments are called mono-hybrid crosses

• Mono means “one”

• Hybrid means “parents with different forms of a trait”

• So we are crossing two parent plants that differ from each other by a one trait—

which in this case is height.

• He cross-pollinated the tall pea plant with pollen from a short pea plant.

• All of the offspring grew to be as tall as the taller parent.

The First Generation

• Mendel allowed the tall plants in this first generation of offspring to pollinate.

What happened?

The Second Generation

P1- Parents

F1- first generation of offspring

F2- second generation of offspring

• Mendel concluded that each organism has two genes that control each of its traits.

• We now know that these genes are located on chromosomes.

Gene

• Genes exist in alternative forms. We call these different gene forms alleles.

Chromosome # 1

Inherited from mom

Gene for Hair Texture

Chromosome # 1

Inherited from dad

Gene for Hair Texture

Mom’s allele codes for Straight hair

Dad’s allele codes for Curly hair

• An organism’s two alleles are located on their homologous chromosomes—one inherited from the female parent and one from the male parent.

B b

Bb

• Mendel called the over-powering allele “dominant” and the trait that sometimes disappeared “recessive”.

• Mendel concluded that the allele for tall plants is dominant to the allele for short plants.

Rule of Dominance

Bb

• It is customary to use the same letter for different alleles of the same gene.

T T

T

T

t t

t

t

Tall plant Short plant

All tall plants

F1

Tfor Tall

tfor Short

The letter “T” equals height.

Big T is Tall

Little T is Short

• An uppercase letter is used for the dominant allele and a lowercase letter for the

recessive allele.

• The dominant allele is always written first for each trait.

T T

T

T

t t

t

tAll tall plants

F1

Tall plant Short plant

Tt

• The law of segregation states:

every individual has two alleles of each gene. (one inherited from mom, one inherited from dad)

But, when gametes (or sex cells) are produced, each gamete only receives one of these alleles to pass on.

The Law of Segregation

Bb

B

b

bb

b b

Phenotypes and GenotypesPhenotypes and Genotypes

• The way an organism looks and behaves is called its phenotype. (brown eyes)

• The allele combination an organism contains is known as its genotype. (Bb)

• An organism’s genotype can’t always be known by its phenotype.

Bb BB

• An organism is homozygous for a trait if its two alleles for the trait are the same.

• The pure-bred tall plant that had two alleles for tallness (TT) would be homozygous for the trait of height.

TT

• An organism is heterozygous for a trait if its two alleles for the trait differ from each other.

• Therefore, the tall plant that had one allele for tallness and one allele for shortness (Tt) is heterozygous for the trait of height.

Tt

What is the phenotype of the pea plant below?

What is the genotype of the pea plant ?

ttSo therefore, what would you call this trait?

Homozygous short

• A Punnett square is a quick way to find the possible allele combinations in offspring.

• Lets say: one parent has a genotype of Tt

the other parent also has Tt

Punnett Squares

Remind me of what the Law of

Segregation states.

TT Tt

Tt tt

T t

T

t

Punnet Square Activity

Question 1

What is the relationship between TRAITS and ALLELES?

DISCUSS WITH THOSE AROUND YOU:

WHAT ARE TRAITS AND ALLELES, AND HOW THEY ARE PASSED DOWN

Question 2What is a Mono-hybrid cross?

DISCUSS WITH THOSE AROUND YOU:

WHAT DOES “MONO-HYBRID CROSS” MEAN

Question 3

In your own words, explain the law of segregation?

How are alleles segregated and passed to offspring?

DISCUSS WITH THOSE AROUND YOU:

MENDEL’S LAW OF SEGREGATION

• Remember, Mendel performed experiments on pea plants that only differed in one trait: HEIGHT

Di-hybrid crosses

• Mendel then decided to perform another set of crosses in which he used peas that differed from each other only in TWO traits rather than one.

Di-hybrid crossesThese experiments were called Di-hybrid crosses:

• Di means “two” - because there were only two differences

• Hybrid because the parents were different from each other (in two ways)

• Cross because he chose the parents to breed with each other

The pea plants were the same in every way, EXCEPT

PEA SHAPE (round, wrinkled) and PEA COLOR (yellow, green)

Mendel took pure-bred plants with round yellow peas and crossed them with pure-bred plants with

wrinkled green seeds.

• In pea plants:

Yellow is Dominant “Y” and Green is Recessive “y”

Round is Dominant “R” and Wrinkled is Recessive “r”

THE TWO DIFFERENCES

RR YY rr yy

• What happened with the first generation of offspring?

What happened with the second generation of offspring?

P1- Parents

F1- first generation of offspring

F2- 2nd generation of offspring

• The Law of Independent Assortment states:

genes for different traits—for example, eye shape and eye color—

are inherited independently of each other.

The Law of Independent Assortment

Bb bb

rr RRBb

Rr

• A Punnett square for a dihybrid cross will need to be four boxes on each side for a total of 16 boxes.

Dihybrid crossesDihybrid crosses

Punnett Square of Dihybrid Cross

Gametes from RrYy parentRY Ry rY ry

Ga

me

tes

from

RrY

y p

are

nt

RY

Ry

rY

ry

RRYY RRYy RrYY RrYy

RRYy RRYy RrYy Rryy

RrYY RrYy rrYY rrYy

RrYy Rryy rrYy rryy

Head, C-curly, c-straight

Eyes, S-slanted, s-straight

Father, CcSs (Curly hair, Slanted eyes)M

oth

er,

CcS

s (C

urly

hai

r, S

lant

ed e

yes)

CS Cs cS cs

CS

Cs

cS

cs

CCSS CCSs CcSS CcSs

CCSs CCss CcSs Ccss

CcSS CcSs ccSS ccSs

CcSs Ccss ccSs ccss