Chapter 5 section 1

What is heredityWhat is heredity??

Heredity is…Heredity is…

the passing of traits the passing of traits from parents to from parents to offspring.offspring.

Who first studied Who first studied heredity?heredity?

born in 1822born in 1822Served as a monk Served as a monk (a type of religious life)(a type of religious life)Known as the Known as the “father of “father of

genetics”genetics”

Worked asWorked as

A GardenerA Gardener in a monastery in a monasteryExperimented with pea Experimented with pea

plants to understand the plants to understand the process of process of

heredityheredity

Unraveling a MysteryUnraveling a Mystery

Mendel was interested in how traits Mendel was interested in how traits (physical characteristics) were passed (physical characteristics) were passed down from parents to offspringdown from parents to offspring

He saw that sometimes a trait would He saw that sometimes a trait would show up in one generation but not the show up in one generation but not the next.next.

He decided to study these patterns in He decided to study these patterns in pea plants.pea plants.

Used pea plantsUsed pea plants

Grow Grow quicklyquickly and have a and have a fast life cyclefast life cycle (from seed to (from seed to offspring)offspring)

Could self-pollinate and Could self-pollinate and cross-pollinatecross-pollinate

Pea PlantsPea Plants Come in Come in many varietiesmany varieties (round (round

vs. wrinkled seed, purple vs. vs. wrinkled seed, purple vs. white flowers)white flowers)

Each characteristic had 2 forms Each characteristic had 2 forms

Mendel’s ExperimentsMendel’s Experiments Cross-pollinated plants with contrasting traits. (Ex: Cross-pollinated plants with contrasting traits. (Ex:

tall plant x short plant)tall plant x short plant) Had to start with two purebred plants (true-Had to start with two purebred plants (true-

breeding) plants – plants that are the result of many breeding) plants – plants that are the result of many generations of the same trait. (Example: purebred generations of the same trait. (Example: purebred short plants always come from short parent plants)short plants always come from short parent plants)

Mendel’s ExperimentsMendel’s Experiments Had to control Had to control

cross-pollination:cross-pollination: Anthers of flower Anthers of flower

are removed from are removed from one plant.one plant.

Pollen from another Pollen from another plant with plant with contrasting trait is contrasting trait is used to fertilize the used to fertilize the plant with no plant with no anthersanthers

Mendel’s 1Mendel’s 1stst Experiment Experiment

Mendel crossed two purebred plants with one Mendel crossed two purebred plants with one contrasting trait. This was the contrasting trait. This was the parentalparental generation (P generation)generation (P generation)

Example: purebred tall plant x purebred short Example: purebred tall plant x purebred short plant plant

Result: All offspring were tall. (FResult: All offspring were tall. (F11 generation) generation)

X

Parent TallP generation

Parent ShortP generation

All Offspring TallF1 generation

ConclusionConclusion

There are two forms of each There are two forms of each trait. One is trait. One is dominantdominant the the other other recessiverecessive..

Dominant trait – the trait that Dominant trait – the trait that was present in the Fwas present in the F11

generation.generation.

ConclusionConclusion

Recessive trait - the trait that Recessive trait - the trait that seemed to disappearseemed to disappear

Mendel performed a second Mendel performed a second experiment to find out what experiment to find out what happened to this traithappened to this trait

Mendel’s 2Mendel’s 2ndnd Experiment Experiment Mendel allowed the FMendel allowed the F11 generation to self-pollinate. generation to self-pollinate.

Result: ¾ of the offspring were tall, ¼ were short Result: ¾ of the offspring were tall, ¼ were short (F(F2 2 generation)generation)

The recessive trait reappeared in the FThe recessive trait reappeared in the F22

generation.generation.

Always occurred in the above ratio, didn’t matter what Always occurred in the above ratio, didn’t matter what trait it was (plant height, flower color, seed shape, trait it was (plant height, flower color, seed shape, etc…)etc…)

.

TallF1 generation

X

3⁄4 Tall & 1⁄4 ShortF2 generation

Parent Plants Offspring

Mendel’s ConclusionMendel’s Conclusion

Each parent donates one set Each parent donates one set of instructions to the of instructions to the offspring.offspring.

These instructions are now These instructions are now known as GENESknown as GENES

What are genes?What are genes?

Sections of DNASections of DNA Genes are represented Genes are represented

by a letterby a letter Sometimes, one gene Sometimes, one gene

controls a traitcontrols a trait

Genes Have Two FormsGenes Have Two Forms

There are two forms of each gene - called There are two forms of each gene - called alleles. alleles.

Organisms inherit one allele from each Organisms inherit one allele from each parentparent

Some alleles are dominant over others. Some alleles are dominant over others. The dominant allele “masks” the presence The dominant allele “masks” the presence of the recessive allele.of the recessive allele.

If the recessive form of the trait shows, If the recessive form of the trait shows, then the organism must have 2 recessive then the organism must have 2 recessive alleles.alleles.

AllelesAlleles Dominant allelesDominant alleles are symbolized are symbolized

with capital letters (T, B, X, C)with capital letters (T, B, X, C) Recessive allelesRecessive alleles are symbolized are symbolized

with lowercase letters (t, b, x c)with lowercase letters (t, b, x c) An organism can be purebred An organism can be purebred

dominant (TT), purebred recessive dominant (TT), purebred recessive (tt), or hybrid (Tt) for a trait(tt), or hybrid (Tt) for a trait

A hybrid individual will show the A hybrid individual will show the dominant trait.dominant trait.

Genes and Mendel’s Genes and Mendel’s ExperimentsExperiments Parent Generation: tall x shortParent Generation: tall x short

TT x ttTT x tt FF11 Generation: All hybrids Generation: All hybrids All Tt = All TallAll Tt = All Tall FF22 Generation: Generation:

¾ inherited at least 1 dominant ¾ inherited at least 1 dominant allele (T) = Tallallele (T) = Tall

¼ inherited 2 recessive alleles (t) = ¼ inherited 2 recessive alleles (t) = short short

X

Tall parent

TT

Short parent

tt

All Offspring TallF1 generation

Tt

Mendel’s 1st experiment: Tall is dominant, short recessive

Mendel’s 2nd experiment: Tall is dominant, short recessive

X

TallF1 generation

Tt

F2 generation

¾ TT or Tt and ¼ tt