Genetics

How are traits passed from parents to offspring?

• Heredity– the passing of traits from parents to offspring.

Gregor Mendel

• Gregor Mendel is known as the father of genetics—the study of heredity.

Mendel studied genetics by doing controlled breeding experiments with pea plants… Why peas?

• Grow quickly

• Come in many varieties

• Mendel could control pollination

• There are two types of pollination:

Self pollination

Cross pollination

Self-Pollination

Cross-Pollination

Mendel’s Experiments• Used plants that he knew were true-

breeding

• True breeding—when a plant self-pollinates it will always produce more plants with the same traits; (is homozygous)

(a.k.a. purebred)

X =

• Mendel crossed (mated) a true-breeding purple flowered pea plant with a true-breeding white flowered pea plant

• All of the offspring had purple flowers!

X =

(Parents) (Offspring)

What happened to the white flowers?

• (Transparency demo)

• Dominant trait—the trait that appears if two traits are mixed (capital letter)

• Recessive trait—trait that is hidden if 2 traits are mixed (lowercase)

• Purple flowers are dominant over white flowers

Mendel’s Experiments continued…

• Mendel also cross-pollinated hybrid plants.

X =

Parents

Offspring

75%

25%

Mendel’s Experiments continued…

• The trait for white flowers showed up again!

• He observed that offspring of hybrid crosses always showed traits in a 3:1 ratio.

75%

25%

Mendel’s Experiments continued…

Mendel concluded that two factors, one from each sperm and one from each egg, control each inherited trait.

• The factors that Mendel talked about are called genes

• A gene is a section on a chromosome that has genetic information for one trait.

Gene

Genes continued…

• One trait/gene may have a few different varieties

• Example: hair color

• The different forms of a gene are called alleles

brown

red

blonde

http://connected.mcgraw-hill.com/

Genotype• The GENetic code (or combination of

alleles) of an organism is called its GENotype

• Shown as 2 letters per trait

• Example: someone with brown hair might be BB

One allele from mom

One allele from dad

• When the two alleles of a gene are the same, the genotype is true breeding, or homozygous.

• If the two alleles of a gene are different, the genotype is hybrid, or heterozygous.

What Controls Traits (cont.)

• The PHenotype is the PHysical appearance of the organism

• Example: brown hair, purple flowers, blue eyes

Phenotype

Check this out…

• B= brown hair b=blonde hair

• BB

• Bb

• bb

Brown Hair

Blonde Hair

Brown Hair

Homozygous (true-breeding)

Heterozygous (hybrid)

Homozygous (true-breeding)

Lets see how quick you are….• P = purple flowers in pea plants, p= white flowers in pea

plants

• PP

• Pp

• pp

Purple

Purple

White

What letters do you use?

• Choice of letter code is up to you…

• Usually the first letter of the trait

• Capital letter means the trait is dominant

• Lower case means it is recessive

A Punnett Square is a model used to predict possible genotypes and phenotypes of offspring.

•Bb x Bb

B

B

b

b

BB

Bb

Bb

bb

Probability- the mathematical chance an event will occur

• Bb x Bb

• Probability of Brown: 3 out of 4; 75%

• Probability of blue: 1 out of 4; 25%

B

B

b

b

BB

Bb

Bb

bbbrownbrown

brown blue

Phenotypic Ratio

•Bb x Bb

•Probability of Brown: 3 out of 4; 75%

•Probability of blue: 1 out of 4; 25%

•Phenotypic Ratio is 3:1 (Brown:blue)

B

B

b

b

BB

Bb

Bb

bb

brownbrown

brown blue

Genotypic RatioBb x Bb

•Probability of BB: 1 out of 4; 25%

•Probability of Bb: 2out of 4; 50%

•Probability of bb: 1 out of 4; 25%

•Genotypic Ratio is 1:2:1 (BB:Bb:bb)

B

Bbb bb

BB

B

Bb

b

brown

brown

brown

blue

A pedigree shows phenotypes of genetically related family members.

Modeling Inheritance (cont.)

• Sometimes traits appear to be blends of alleles.

• Alleles show incomplete dominance when the offspring’s phenotype is a blend of the parents’ phenotypes.

Complex Patterns of Inheritance

• Red, white and pink flowers

• R=red

• r=white

• How do you get pink flowers?

• RR

• Rr

• rr

Red

White

Pink

• Mixing of two traits (blending)

• One dominant allele is only “half-strong”

• Red + white = pink

R=red

r=white

• Codominance occurs when both alleles can be observed in a phenotype.

Complex Patterns of Inheritance

Cattle• Fur can be red or white

• Mixing the two gives roan fur

Codominance• Both alleles are expressed at the same

time in the phenotype

• Red fur + white fur = roan fur (red and white mixed)

R=red

r=white

Websites

• Incomplete & Codominance