Chapter 12Mendel & Heredity

Section 12.1Genetics – study of heredity; how traits are passed from parents to offspringI. Gregor Mendel “Father of Genetics”

A. History of1. Lived in a monastery in Austria in 18422. In 1851, moved to the University of Vienna

where he studied science and mathematics3. Returned to monastery, taught, & kept a

garden (most famous for studies on peas)

B. Initial Observation1. Mendel crossed (mated) purple flowered

peas plants with white flowered pea plants.

2. All the offspring had purple flowers.3. Two of these first offspring were crossed,

and some of their offspring had purple flowers, but a few had white flowers!

II. Features of Pea PlantsA. Vocabulary

1. Characters – physical features that are inherited• Flower color in peas is a character

2. Trait – one of several possible forms of a character• The flower colors of purple or white are traits

3. Hybrid – offspring of a cross between parents with different traits• If a purple flowered peas crosses with a white

flowered pea, its offspring would be a hybrid

B. Pea Plant Characters – Mendel observed 7 characters (do not need to write down)

1. Height of plant’s stem (tall or short)2. Position of flower on stem (mid-stem or end

of stem)3. Pod Color (green or yellow)4. Pod Appearance (smooth or bumpy)5. Seed Texture (round or wrinkled)6. Seed Color (green or yellow)7. Flower Color (purple or white)

C. Pollination – Mendel manually took pollen from an anther (male) & transferred it to the stigma (female).

1. Self-Pollination: a flower will pollinate itself or another flower on the same plant

2. Cross Pollination: a flower will pollinate a flower on a different plant

***Mendel uses both forms of pollination in is experiments.

III. Mendel’s Experiment

A. Mendel first used true-breeding plants.1. True-Breeding – a plant that is pure for a

trait; when self-pollinated, its offspring will always have that same trait• A true-breeding purple flowered plant will always

produce purple flowered plants!

2. Mendel formed 14 strains that were true-breeding.

3. These true-breeding plants were labeled the P1 generation since they were the original parents.

B. Two P1 generation plants of contrasting traits were cross-pollinated. Their offspring were labeled the F1 generation.

C. An F1 generation plant was allowed to self-pollinate. Its offspring were labeled the F2 generation.

D. An Example of Mendel’s Results P1 = True-Breeding Purple X True-Breeding White

F1 = 100% were Purple Flowers

F2 = 75% Purple flowers & 25% White flowers * = 3 Purple : 1 White Ratio

**Mendel crossed all 14 traits and always received a 3:1 ratio in the F2 generation

E. Calculating Ratios1. Probability – chance something will happen

Probability = # of times an event happens

# of times it could of happenedEx: In an F2 generation, Mendel recorded 705 purple flowered plants and 234 white flowered plants.Purple: happened 705 times out of 939; 705/939 = 75.1 %White: happened 234 times out of 939; 234/939 = 24.9%

Section 12.2I. Explaining Mendel’s Experiments

A. Alternate Versions of Genes = alleles1. For each character, there are 2 possible

traits2. Each trait is controlled by a specific gene– Purple flowers is controlled by one gene

for flower color & white flowers is controlled by a different gene for flower color

– The purple gene is called the purple allele

B. Dominant & Recessive Alleles1. Dominant Allele – the allele whose trait is

always expressed2. Recessive Allele – not expressed when a

dominant allele is present.

P = purple allelep = white allele

2 Dominant alleles (PP) = purple flowers1 Dominant 1 Recessive (Pp) = purple

flowers2 Recessive alleles (pp) = white flowers

3. We receive one allele for each character from each parent.• So 2 alleles are needed to determine our

traits

II. Mendel’s LawsA. Law of Segregation

1. A pair of alleles are segregated (separated) during meiosis.

2. This means one allele on one chromosome will be separated from the other allele on the other homologous chromosome during meiosis to make gametes.

Law of Segregation

B. Law of Independent Assortment1. Alleles are separated randomly and

independently into gametes

Section 12.3

I. VocabularyA. Genotype – all the alleles/genes inherited;

an individual’s genetic information• Example –Pp is the genotype for a purple

flowered plant

B. Phenotype – the physical expression of one’s genotype• Example – the color purple is the phenotype of

a plant with a genotype of Pp.

C. Homozygous – genotype with 2 identical alleles• Example: PP = homozygous dominant

pp = homozygous recessive

D. Heterozygous – genotype with 2 different alleles• Example: Pp = heterozygous

II. Punnett Squares – model/tool that predicts the outcomes of a genetic cross

A. Monohybrid Cross – cross between individuals involving one trait (just flower color)

1. Example: In pea plants, purple is dominant over white flowers. Determine the probabilities of possible outcomes for a cross between a homozygous dominant plant and a homozygous recessive plant.

B. Types of Dominance1. Incomplete Dominance – an allele does not

completely dominate another; phenotype will be a blend.

Ex: In snapdragons, the red allele (R) does not completely

dominate the white allele. A heterozygote (Rr) will have pink flowers.

2. Codominance – both alleles for a trait are fully expressed (spots, AB blood type in humans)

A heterozygous plant(Rr) would display a phenotype of spots

3. Multiple Alleles – a trait has 3 or more possible alleles (an individual still has only 2 alleles for the trait).

Ex: Blood type in humans. There are 3 alleles a person can inherit: the

A allele, B allele, and O allele. Type A = AA or AOType B = BB or BOType AB = AB (codominance)Type O = OO