Chapter 5: Heredity

http://www.youtube.com/watch?v=6OPJnO9W_rQ

Chapter 5 – Heredity

• An organism is a collection of traits, all inherited from its parents

• Heredity: the passing on of characteristics (hair color, skin color, eye color, height, body shape… overall appearance inside and out) from one organism to its offspring

• How are these characteristics passed on?– DNA – contains the ability to make and control every

part of an organism• The different forms of a trait that make up a gene pair

are called alleles.• When a pair of chromosomes separates during

meiosis, alleles for each trait also separate into different sex cells.

• Every sex cell has one allele for each trait

Genetics: the study of how traits are inherited through the interactions of alleles.

Discovery of DNA – 100 years ago by Gregor Mendel• Experimented with garden pea plants and could explain how traits

were passed on from one generation to another.

Why pea plants? • Grew quickly, • Many generations could be seen in a short period of time, • Crossed and bred easily

Why not use animals?• Multiple generations take too long

• Mendel’s Experiments studied many different characteristics of pea plants

• First Characteristic – Plant Height• Crossed Short X Short plants – Parental generation = P1

– This Produced all short plants – 1st generation = First Filial generation= F1

– Crossed seeds form those plants with each other• Produces all short plants – 2nd generation = Second Filial

generation= F2

• Conclusion = All generations resembled parents• Called this = True Breeding Plants… we now call this Purebred

• Crossed Tall X Tall plants – P1– Some tall x tall crosses produces all tall

plants – F1– Some tall x tall crosses produced both

tall and short plants – F1• Conclusion = Two kinds of tall plants =

True Breeders, and some were NOT True Breeders (Hybrids)

• Crossed Tall X Short plants = P1– Produced all Tall plants = F1

• Crossed F1 plants with its own pollen• Expected all Tall plants… But…• Produced some Tall plants and some Short

plants in the F2– Shortness had reappeared!– Plants did Not breed true

• Conclusion = Plants had to contain factors for both tallness and shortness

– Factors = Genes = unit of heredity

• Mendel knew when you crossed a Tall plant with a Short plant, the next generation was Tall

• The stronger Tall characteristic had covered the weaker short characteristic

• This strong characteristic is called Dominant• The weaker trait that is covered or hidden is called Recessive• Today = the Dominant trait is symbolized by capital letters, the

Recessive trait is symbolized by lower case letters– IE = Dominant = T– Recessive = t

• (Explain crosses in form of dominant and recessive letters.)• (Show IE on board.)

• From multiple experiments with many characteristics, Mendel created a hypothesis: • each pea plant had

a pair of factors (Genes)

• Also – each parent contributed only one of those factors to each plant in the next generation

• One gene from each parent created the gene pair in the next generation

• Punnett Squares:• chart that shows possible

combinations of the cross between two organisms

• Guinea pigs – color• Black crossed with White• Black = BB• White = bb• (Show on board)• Percentages Black?• Percentages White?

• BB (2 capital Letters)= Homozygous Dominant – Purebred

• bb (2 lower case letters)= Homozygous Recessive – Purebred

• Bb (capital and lower case letters)= Heterozygous

• Phenotype = Visible characteristics

- looks like…• Genotype = Genetic makeup

• genes look like…

• Now cross two of F1 Generations from guinea Pigs…

• Bb X Bb• (Show on board)• Percentages?

– White?– Black?• Homogeneous Black?• Heterogeneous Black?• Homogeneous White?

• Mendel’s work remained unnoticed until around 1900

• Scientists found his work… still gave him credit!

• Led to Crick and Watson description of DNA.