MENDEL AND GENETICS

What is genetics?

• Gregor Mendel, an Austrian monk who studied botany. He made important progress in understanding heredity.

• Mendel studied the traits of ordinary garden peas.• During Mendel’s time, the belief was that the traits of the parents blended.

Genetics, a branch of biology, is the scientific study of heredity and variation of organisms.

Gregor Mendel’s work with plants• Most plants have female and male parts on the same

flower.

**Male part of the flower produces pollen (sperm).

**Female part of the flower produces ova (eggs)

Gregor Mendel’s work with plants

• Two options for the same flower:• Self-pollinating: pollen and

ovum come from the same flower.

• Cross-pollinating: ovum on one flower and pollen from flower on a different plant..

• Plants reproduce sexually.• Pollen (male gamete) and ovum (female gamete) join making a

new diploid cell.• Fertilization produces a new diploid cell.

Mendel’s peas• Mendel had pea plants that

were true breeding.• Seed from purple flowered plants

had only purple flowered offspring.

• True breeding plants are ones that produce offspring like themselves.

• Mendel experimented with cross pollination between different parents.

Gregor Mendel’s work with peas• A trait is a specific observable characteristic that varies from

one individual to another.

• Seven traits of peas studied by Gregor Mendel: dominant version of trait is on the left, recessive on the right.

Mendel’s Cross pollinationP two different parents, example: trait = height; alleles = tall or short

F1 offspring are all heterozygous plants. He used F1 as parentsHeterozygotes have different alleles.Homozygotes have the same alleles.F2 ‘grandchildren’ have ratios of 3 out of 4 dominant trait or 75% and 1 out of 4 recessive trait or 25%.

• P = parental generation• F1 = first filial generation

(offspring or children)• F2 = second filial generation

(grandchildren)

Crossing different pea plants

Mendel—Father of Genetics• Mendel’s application of scientific method and mathematics to heredity in peas led to his discovery of ‘factors’.

• Mendel concluded that biological inheritance is determined by factors passed from one generation to the next.

Today, scientists call the factors that determine traits genes.

Mendel—Father of Genetics• Each trait that Mendel studied was controlled by one gene that occurred in two contrasting forms.

• The different forms of a gene are called alleles• Example—trait is flower color• Gene controls flower color• Two forms are purple and white• Alleles are P-purple, p-white

Mendel and his workMendel predicted:• The role of genes in heredity• Alleles for a gene occur in pairs for an organism• One allele for a gene is in each gamete

Mendel had three key concepts:DominanceSegregationIndependent Assortment (Day 3)

Two of Mendel’s Principles• 1. Law of Dominance:

One allele does the ‘talking’. One allele is expressed. One allele was dominant over the other in the F1 generation.

• 2. Principle of Segregation:When gametes are formed, the pairs of hereditary

factors (alleles) become separated. Each sex cell (egg/sperm) receives only one version of the gene.

• 3. Principle of Independent Assortment:Day 3 (Hint: multiple chromosomes divide in

meiosis)

Gregor Mendel’s ideas• Mendel observed that some traits appeared to dominate.• Dominant allele of gene is always expressed in classic

Mendelian genetics.• Other traits skipped a generation. Recessive trait.• A trait is an observable characteristic that varies between

individuals.• Mendel counted the results of cross pollination. He

calculated the probability of outcomes from his hypothesis.• Probability is how likely something will happen. Mendel

assumed the forms of a trait happened as random events with equally likely probabilities.• Consider flipping a coin. Two equally likely outcomes.

Using a Punnett SquareSTEPS:

1. determine the genotypes of the parent organisms 2. write down your "cross" (mating) 3. draw a Punnett square

Parents are tall and short.Parent genotypes:TT and t t

CrossT T t t

Punnett square4. "split" the letters of the genotype for each parent

& put them "outside" the p-square 5. determine the possible genotypes of the offspring by

filling in the p-square 6. summarize results (genotypes & phenotypes of

offspring)

T t T t

T t

T t

T T

t

t

Genotypes:100% T t

Phenotypes:100% Tall plants

T T t t

Monohybrid cross: F2 generation• If you let the F1 generation self-fertilize, the next monohybrid cross would be:

T t T t (tall) (tall)

T T T t

T t

t t

T t

T

t

Genotypes:1 TT= Tall2 Tt = Tall1 tt = dwarf Genotypic ratio= 1:2:1Label your ratio TT:Tt:tt

Phenotype:3 Tall1 dwarf Phenotypic ratio= 3:1

Label your ratios tall: short

Secret of the Punnett Square• Key to the Punnett Square:• Determine the gametes of each parent…• How? By “splitting” the genotypes of each parent:

If this is your cross

T T t t

T T t tThe gametes are:

Once you have the gametes…

T T t t

T t T t

T t

T t

T

T

t t

Another example: Flower color

For example, flower color:P = purple (dominant)

p = white (recessive)

If you cross a homozygous Purple (PP) with a homozygous white (pp):

P P p p

P p ALL PURPLE (Pp)

Cross the F1 generation:

P p P p

P P P p

P p

p p

P

p

P pGenotypes:1 PP2 Pp1 pp

Phenotypes: 3 Purple 1 White

Stations• Each group will visit all stations.• Station 1—Vocabulary A • Station 2—Punnett Squares• Station 3—Analysis of Punnett Square for Parents• Station 4—Vocabulary B• Station 5—Segregation• Station 6—Phenotype/Genotype

• Use your handout to record your work at each station.

Resources• Dr. Dennis O’Neill, 2013, Basic Patterns of Genetics,

Palomar College http://anthro.palomar.edu/mendel/Default.htm

• Paul Anderson, 2013, Mendelian Genetics, Bozeman Science, http://www.youtube.com/watch?v=NWqgZUnJdAY

• CrashCourse, 2013, Heredity, Crash Course in Biology, http://www.youtube.com/watch?v=CBezq1fFUEA

• Staff, 2013, Glossary of Terms, Palomar College, http://anthro.palomar.edu/mendel/glossary.htm