Mendel and Genetics

Mendel and Genetics

Gregor Mendel• Austrian Monk• Worked with pea plants

– Small– Easier to use and control (breeding)– Produce hundreds of offspring– Grew quickly– Self pollinating– Short life cycle– Have many visible traits/characteristics

• Other organisms?– Fruit flies– Zebra fish

What do Pea Plants Have to Do with Genetics?

• “All living things have DNA”• Peas plants are living organisms; therefore,

they also have DNA.• “All living things reproduce”• Pea plants are living organisms; therefore,

they reproduce.• Pea plants reproduce sexually – combination

of two sex cells (gametes)

the next few images may be graphic in nature

Anatomy of a Flower

Fertilization in Plants

Anatomy of a Pea Flower

Pea Plant Pollination

How did he do it???

• Stamen = male reproductive part of a flower• Anther – produces pollen• Filament – holds up the

anther• Pollen – holds sperm

cells

• Carpel/Pistil = female reproductive part of a flower• Stigma – top of carpel; sticky

to catch pollen• Style – structure that leads to

ovules/ovaries

Developing Truebreeders

• Truebreeders – organisms whose offspring are always identical to the parents

• Their traits are the same in each generation.• Trait – specific characteristic or feature of an

individual that is controlled by genes• Ex. – tall plants always produce tall plants;

short plants always produce short plants; purple flowers always produce purple flowers…etc.

Crossbreeding• Mendel wanted to learn how traits are passed

on from parent to offspring.• He used a process called crossbreeding and

produced plants that has different parents.– Used pollen from one type to pollinate– Fertilized the female parts of another

flower of another type• The offspring of crosses between parents

with different traits are called hybrids.

What’s a Hybrid???

Just kidding!! These are hybrids because they have both an electric engine and a gas engine.

Animal Hybrids

Gregor Mendel’s Observations• Noticed a pattern in offspring after the P

generation, F1 generation, and F2 generation• Developed “true breeding” plants

– Offspring always resembled parents– White always gave rise to white flowers– Purple always gave rise to purple flowers

• Mendel crossed one true breeding plant with another with a different trait to produce a hybrid.

Crossed Purple Flowers with White Flowers

• True breeding purple and white flowers are known as the parent (P) generation

• Offspring of P generation are known as Filial 1 (F1) generation.

• All the offspring from the F1 generation were hybrids

• All of F1 were purple. No white flowers!

P x P F1Purple x White 100% Purple

F1 Purple X F1 Purple

• When F1 is crossed with F1, their offspring is called the second Filial or F2 generation.

• When Mendel crossed a purple F1 with another purple F1, he got both purple and white flowers!

• He kept a record of numbers and noticed a pattern.

• For every 3 purple flowers, there was 1 white. In other words, there was a 3:1 ratio.

Mendel and Genes

• Mendel did not know about DNA and genes.

• He realized there must be something that determines the traits of the pea plants and called them factors.

• Genes can have several (at least two) forms. These forms of genes are called alleles.

Genes vs. Alleles

• Genes are segments of DNA that has instructions for making proteins (traits).

• Alleles are specific forms of genes that code for a specific trait.– Mendel dealt with genes for plant height.– Mendel crossed plants with the tall allele and

short allele.– Mendel dealt with genes for pea color.– Mendel crossed plants with the yellow allele

and green allele.

Not only flower color!!!• Mendel saw the same pattern and ratio

happening with other traits as well– Pea color (green vs. yellow)– Pea texture (smooth vs. wrinkled)– Pod shape (full vs. constricted)– Plant height (tall vs. short)

• 3:1 ratio!!!• Characteristics are called phenotypes• Genotypes – genetic makeup; actual

genes/alleles

Mendel didn’t know about genes

• Since we now know about genes, we have an explanation for what Mendel observed

• Two types of traits– Dominant – trait that is always expressed– Recessive – trait that is only expressed when

dominant allele is not present• We use letters to represent the genes

– Capital letters = dominant alleles– Lowercase letters = recessive alleles

SegregationThrough meiosis, the alleles, (T) (t), are segregated into their gametes.

Through fertilization, the alleles, (T) (t), are recombined when zygotes are formed.

Summary of Mendel’s ResultsIn the P generation, truebreeding tall plants and truebreeding short plants are cross pollinated.

In the F1 generation 100% of the offspring are tall plants. Short plants seem to have disappeared.

F1 plants are crossed with F1 plants to get the F2 generation.

In the F2, short plants reappeared! 75% are tall, 25% are short. There is a 3:1 ratio of tall to short plants.

Reminder about Genes• Genes are segments of DNA that codes for protein;

protein shows up as trait• Genes are found on chromosomes (chromosome

theory)• Chromosomes are found in pairs (homologous)• Therefore, genes are also found in pairs• If both genes are alike (both dominant or both

recessive) we use the term homozygous• If genes are different (one dominant, other

recessive) we use the term heterozygous

XP P p p

P P p p

P p P pP pP p

P generation

Segregation

gametes

F1 Generation

P = purplep = white

Homozygousdominant

Homozygousrecessive

Heterozygous Heterozygous Heterozygous Heterozygous

P p P pF1 Generation

Pp p

F2 Generation

???

P

PP Pp Pp pp

segregation

Punnett Square

• Tool used by geneticists use to predict the possible genotypes/phenotypes of offspring

• Chart shows – allele of gametes of parents on the top and

side of the chart– All possible allele combinations of offspring

Punnett Square- diagram used to show the possible genetic combinations from a particular cross

P generation: PP x pp

P

P

p p

P p P p

P p P p

F1 Generation

Note: All offspring have the genotype: Ppphenotype: purple

F1 generation: Pp X Pp

P

P

p

p

PP

Pp

Pp

pp

F2 generation:

In the F2 generation the possible genotypes are :1 PP, 2 Pp, and 1 ppor a 1:2:1 ratio

Phenotypes are:3 Purple and 1 white

or a 3:1 ratio

Independent Assortment

• Mendel wondered if alleles for one trait affected the alleles for another trait.

• Mendel set up a cross that enabled him to study two different genes (dihybrid) at the same time.– Two different genes = two different traits.– Seed color: yellow vs. green– Seed shape: round vs. wrinkled

Getting the F1 Generation• Crossed round/yellow

(RRYY) with wrinkled/green (rryy)

• The gametes from RRYY all carry RY.

• The gametes from rryy all carry ry.

• The only possible combination of alleles in the F1 is RrYy.

• All are round/yellow.

Getting the F2 Generation• Crossed RrYy with RrYy• Each parent has 4

possible combination of alleles in gametes

• Not only were there round/yellow and wrinkled/green, there were other combinations

• Round/green and wrinkled/yellow

• Showed 9:3:3:1 ratio

Dihybrid Revelation

• New combinations other than parents revealed another property of genetics.

• Independent assortment – alleles of different genes segregate independently or separately during gamete formation

• In other words, traits do NOT come in specific combinations.– Round does not necessarily mean yellow– Wrinkled does not necessarily mean green