mendel and the gene idea

Many diseases are caused by

genetic mutations

Understanding inheritance helps scientists

identify disease genes, predict inheritance patterns, develop genetic tests and treatments

Sickle-cell disease

red blood cells become rigid and sickle shaped, cells cannot easily move through capillaries, reduced oxygen delivery to tissues

Gregor Mendel (1822–1884)

Austrian monk and scientist, performed inheritance experiments in the

mid-1800s, studied thousands of plant crosses

Mendel applied mathematical analysis to biological experiments

His discoveries form the foundation of modern genetics

Mendel studied the ____(____)

pea plant (Pisum sativum)

Pea plants were ideal because they

grow quickly, produce many offspring, have easily distinguishable traits, reproduce through self-pollination

HOW TO CROSS A PEA PLANT- step 1

remove the stamens from the purple flower so it cannot self-pollinate

HOW TO CROSS A PEA PLANT- step 2

transfer pollen from the white flower’s stamens to the purple flower’s carpel

HOW TO CROSS A PEA PLANT- step 3

the pollinated carpel develops into a pod

HOW TO CROSS A PEA PLANT- step 4 and 5

plant the seeds and examine the offspring

Complete control over who mates with whom. That is what makes this an ______

experiment, not an observation

Mendel tracked only characters with two distinct alternative forms

purple or white, never in between

Mendel started with true-breeding varieties

self-pollinated, they always produce offspring like themselves

Crossing two contrasting true-breeding varieties is ___

hybridization

The true-breeding parents are the P generation

their hybrid offspring are the F₁ generation

F₁ plants crossed with each other give the

F₂ generation

Before Mendel, the standard view was blending inheritance

parental traits mix in the offspring, like paint

Blending makes a hard prediction___

variation should disappear. Mix long enough and every individual is the same shade

Darwin had a real problem with this

natural selection needs variation, and blending destroys it

Cross true-breeding purple with true-breeding white

every F₁ plant is purple

But let the F₁ self-pollinate, and white comes back

pure white: 705 purple, 224 white

That is 3.15 to 1

approximately 3 to 1- Whatever carries whiteness survived a generation completely hidden

SEVEN CHARACTERS, SAME RATIO

Mendel repeated the experiment with six more characters

Every one gave an F₂ ratio close to

3 to 1

Seven independent experiments, one number.

That is what tells you there is a mechanism underneath.

MENDEL’S FOUR-PART MODEL- 1

Alternative versions of genes account for variation. We now call them alleles

MENDEL’S FOUR-PART MODEL- 2

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

MENDEL’S FOUR-PART MODEL- 3

If the two differ, the dominant allele determines appearance and the recessive one has no noticeable effect.

MENDEL’S FOUR-PART MODEL- 4

The two alleles segregate during gamete formation and end up in different gametes.

Different versions of a gene are called

allele

Each organism carries

two alleles per gene

One allele is inherited from

each parent

Dominant allele

expressed when present

Recessive allele

expressed only when both alleles are recessive

Pp

purple phenotype

Genotype

the genetic makeup of an organism

Genotype describes the

allele combination

Phenotype

Observable characteristics

Phenotype is influenced by

genotype and environmental factors

Homozygous

Two identical alleles

Heterozygous

Two different alleles

The P generation

PP purple crossed with pp white

Each parent makes gametes carrying one allele, so every

F₁ plant is Pp

Pp plants are purple, because P is dominant

the white allele is present but masked

When an F₁ plant makes gametes, the two alleles separate

half carry P, half carry p

A Punnett square

lays out every possible combination of sperm and egg

Gametes from one parent across the top

gametes from the other down the side

Pp × Pp gives

1 PP : 2 Pp : 1 pp

A testcross

mates the unknown individual with a homozygous recessive — pp

If the unknown is PP

all offspring are purple

If the unknown is Pp

about half the offspring are white

The recessive parent contributes nothing visible

so the offspring report the unknown genotype directly

Mendel’s fourth point

the two alleles separate during gamete formation — is a description of

anaphase I

The two alleles sit at the same locus on two

homologous chromosomes

When the homologues are pulled to opposite poles

the alleles are pulled apart with them

Mendel inferred the mechanism of meiosis from counting flowers

about 40 years before anyone watched it happen

Everything today followed one character

Those F₁ plants are monohybrids

Mendel’s second question:

what happens if you track two characters at the same time?

Cross true-breeding YYRR (yellow, round seeds) with yyrr (green, wrinkled).

The F₁ are all YyRr — dihybrids

Law of segregation

the two alleles for a character separate into different gametes — that is anaphase I

Mendel

crossed true-breeding peas, counted the offspring, and found 3:1 in the F₂ for seven characters