Heredity Molecular Biology Sumner High School. Sometimes looks like traits are “blending” The idea that genetic material contributed by two parents mixes.

HeredityMolecular Biology

Sumner High School

• Sometimes looks like traits are “blending”

• The idea that genetic material contributed by two parents mixes like blue and yellow paints blending to make green.

Explanations of Heredity

Explanations of Heredity• An alternative to the blending

model is the “particulate” hypothesis of inheritance: the gene idea

• Parents pass on discrete heritable units = genes

• Traits = characteristics that can be inherited with such variations as purple or white flowers

• Heredity = the transmission of traits from one generation to the next

• Genetics = the scientific study of genes, heredity, variation amongst organisms

Key Vocabulary

• Documented a particulate mechanism of inheritance through his experiments with garden peas (1857)

Figure 14.1

Gregor Mendel

• Gregor Mendel begins breeding pea plants in 1857

• His work was largely ignored until it was “rediscovered” in the 1920’s.

A piece of trivia

FYI: Prior to Mendel

• Preformationism

• “Homonucleus”

• Do you see any problems with this hypothesis?

• Mendel chose his specimen very well.

• Mendel chose to work with pea plants...

• Because they are available in many varieties

• Because he could strictly control which plants mated with which

• Because they produce many offspring

Mendel’s Methods

• Mendel chose to track only those characters that varied in an “either-or” (yes/no) manner

• Mendel also made sure that he started his experiments with varieties that were “true-breeding” or only showed one trait option over many generations.

• Mendel kept meticulous records of his work.

Mendel’s Methods

• Early on, Mendel crosses (breeds) true-purple with true-white flowered plants.

• What are the options for flower color?

• What did he get in the next generation?

• All purple plants.

Mendel’s Mystery

What the heck!!!Where did the

white flowers go?!?

• Then Mendel crosses this 2nd generation of plants with each other

• What are the options for flower color?

• What did he get in the next generation?

• Mostly purple but some white.

Mendel’s Mystery, Cont.

WAIT!!!Where did the white

flowers come from?!?

• Next step was to check for this pattern in other traits.

• Mendel observed the same pattern in many other pea plant characteristics

• Ratio is always close to 3:1 in the 3rd generation.

Other Traits

• When Mendel crossed contrasting, true-breeding white and purple flowered pea plants all of the offspring were purple.

The Mechanics of the Mystery

• Hybridization = crossing two true-breeding varieties with different traits

• The true-breeding parents are called the P generation

• The hybrid offspring of the P generation are called the F1 generation

The Mechanics of the Mystery

• When Mendel crossed the F1 plants, he found 3:1 purple:white flowers.

• When F1 individuals are crossed with each other, the F2 generation is produced

The Mechanics of the Mystery

• Mendel observed that...

• In the F1 plants, only the purple flower factor was affecting flower color in these hybrids

• Purple flower color was dominant, and white flower color was recessive

• In the F2 plants, the recessive trait was allowed to be expressed again.

• It took 4 ideas to help resolve all of these observations.

Mendel’s Observations

Allele for purple flowers

Locus for flower-color gene

Homologouspair of

chromosomes

Allele for white flowers

• There are alternative versions of genes which determine the variations in traits

• The different versions of the genes are called alleles

Model Concept 1

• For each trait an organism inherits two alleles, one from each parent

Model Concepts 2

• A “gene factor” or allele is actually represented twice

• If the two alleles for a gene differ:

• Then one, the dominant allele, determines the organism’s appearance

• The other allele, the recessive allele, has no noticeable effect on the organism’s appearance

• If the dominant allele is present in 1 or 2 alleles, that is the trait that will be expressed.

• The recessive allele is only expressed if 2 recessive alleles are present.

Model Concepts 3

• The law of segregation states that

• The two alleles for a trait separate (segregate) during gamete formation and end up in different gametes (haploid cells)

• This allows for “mixing” of the traits and their reappearance in later generations.

Model Concept 4

• For each trait, an organism inherits two alleles, one from each parent.

• If the two alleles differ, then…

• the dominant allele determines the organism’s appearance.

• the recessive allele has no noticeable effect on the organism’s appearance.

• Mendel’s law of segregation states that the two alleles for a trait are segregated during gamete production and end up in different gametes.

Reviewing heredity so far…

• Symbols

• Capital letters for dominant alleles

• Lower-case letters for recessive alleles

• For the pea-plant flower

• P = purple (dominant)

• p = white (recessive)

Some different genetic “codes”

• We can now represent Mendel’s crosses with these symbols.

• True-breeding varieties have the same alleles

• PP or pp

PP x pp

Breeding “P’s”

• An organism that is homozygous for a particular gene has a pair of identical alleles for that gene

• PP = homozygous dominant

• pp = homozygous recessive

• Exhibits true-breeding

Useful Genetic Vocabulary

• An organism that is heterozygous for a particular gene has a pair of alleles that are different for that gene

• Pp = heterozygous

Useful Genetic Vocabulary

• The two alleles are separated when gametes (sex cells) are formed.

• For the P (parent) generation

• True-breeding purple flowers produce only P gametes (PP)

• True-breeding white flowers produce only p gametes (pp)

Splitting “P’s”

RRRR

RR RR

R RRR

RR

Start: Meiosis I

Start: Meiosis II

End: Meiosis II

Homozygous Homozygous DominantDominant

rrrr

rr rr

r rrr

rr

Start: Meiosis I

Start: Meiosis II

End: Meiosis II

Homozygous Homozygous RecessiveRecessive

RRrr

RR rr

r rRR

Rr

Start: Meiosis I

Start: Meiosis II

End: Meiosis II

HeterozygoteHeterozygote

• The only possible combination of crossingPP x pp is Pp.

• One parent contributes a P

• The other parent contributes a p

• Fusion leads to Pp (the F1 generation)

• Purple flower is dominant, so all offspring have purple flowers.

Crossing “P’s”

p pppP PPP

Heterozygous(Purple)

Sperm from Parent 1 Eggs from Parent 2

• It’s easier to track the changes using a Punnett square.

• The cross is represented with

• one parent’s alleles across the top

• the other parent’s alleles down the side

PP x pp P(dominant)

P(dominant)

p(recessive)

p(recessive)

P Generation Cross

Sorting “P’s”

• If we allow the F1 plants to self-pollinate = Pp x Pp (the F2 generation)

Crossing “P’s” Again

Pp x PpP

(dominant)

p(recessive

)

P(dominant

)PP Pp

p(recessive

)Pp pp

The Punnett square

A 3 to 1 ratio!

• There are four possible gamete combinations

• PP = purple flower

• Pp = purple flower

• Pp = purple flower

• pp = white flower

Crossing “P’s” Again

• An organism’s phenotype is its physical appearance

• An organism’s genotype is its genetic makeup

More Genetic Vocabulary

• Two organisms can have the same phenotype but have different genotypes

• PP = homozygous dominant = purple flower

• Pp = heterozygous = purple flower

• Quick questions:

• What is the phenotype?

• What is the genotype?

Tricky!

3

1 1

2

1

Phenotype

Purple

Purple

Purple

White

Genotype

PP(homozygous)

Pp(heterozygous)

Pp(heterozygous)

pp(homozygous)

Ratio 3:1 Ratio 1:2:1

Phenotype vs. Genotype

• The rules for these crosses are governed by probability.

• Sorry...this means MATH.

Rules

Success comes in cans,not cant's

• The odds of an egg having a particular allele are 1/2.

• Same odds for the sperm & eggs.

• The odds of any particular genotype are the product of the odds from the parents

• 1/2 x 1/2 = 1/4

Probability

Enough!Enough!

Time to Practice!

Enough!Enough!

Time to Practice!

Definitions - Let’s make sure we have the same understanding of:

• Gene • Traits • Heredity • Genetics • P generation• F1 generation• F2 generation • Dominant• Recessive• Allele

• Law of Segregation

• Genotype• Phenotype• Heterozygous

(aka Hybrid )• Homozygous

(aka True-breeding)

• Punnett square• Probability