Gregor Mendel

Mendelian genetics explains the principles of heredity

Why Pea Plants???•have easy to identify reproductive organs

•easy to grow

• have a short generation time

•could be cross-pollinated (produces seeds that are the offspring of two different plants)

Genes vs. Alleles

Law of Segregation

• each organism contains two factors for each trait (ex: widow’s peak and no widow’s peak)

• factors segregate, or separate during Meiosis; each gamete contains one factor for each trait.

Punnett Squares!!!

Dihybrid Crosses

• A cross that examines two pairs of contrasting traits

• Example: Mendel looked at Color (yellow or green) and seed shape (round or wrinkled)

• Law of Independent Assortment stated that traits are inherited independently of each other

Dihybrid Crosses

• Mendel crossed two plants from the F1 generation

• RRYY x rryy – F1 generation

• What are the possible genotypes?

Dihybrid Practice

• In Guinea pigs, black hair (B) is dominant to white hair (b) and short hair (L) is dominant to long hair (l)

• Cross a guinea pig that is heterozygous for both traits with guinea pig that is white and heterozygous for hair length.

Dihybrid Crosses

• TtYY x Ttyy what are the possible genotypes?

P1

P2

Dihybrid Practice

• In mice, the ability to run normally is a dominant trait called running (R). The recessive trait causes mice to run in circles which is called waltzing mice (r). Hair color is also inherited in mice. Black hair (B) is dominant over brown hair (b).

• Cross a heterozygous running, heterozygous black mouse with a homozygous running, homozygous black mouse

Dihybrid Practice

• Cross a waltzing brown mouse with a running brown mouse

• Remember Running (R), waltzing (r) Black fur (B), brown fur (b)

• 2 answers are possible!!

Dihybrid Bellringer

In Kentucky wildcats, long tails are dominant (T) to short tails (t) and stripes (S) are dominant to spots (s). Cross two wildcats that are heterozygous for both traits. What are the possible phenotypes?

Other Patterns Of Inheritance

• Most alleles are not simply dominant or recessive

• What determines dominance? Dominant genes code for polypeptides (enzymes) that work

Incomplete Dominance

• when one allele is not completely dominant or recessive

• The heterozygous phenotype is somewhere in between the two homozygous phenotypes

• RED Flower x WHITE Flower ---> PINK Flower

Incomplete Dominance• Curly and

Straight Hair

• What would wavy hair crossed with wavy hair look like?

Codominance

• More than one allele for the same gene are fully expressed

• Two traits appear at the same time

• black x white ---> red & white spotted

Polygenic Inheritance

Traits which are the result of many gene combinations.

Example of human traits: height, body weight, and skin color.

http://www.youtube.com/watch?v=gouqTq5p168

Eye Color Inheritance

• OCA2 is one of the genes and is dominant. • This is why brown eyes are said to

be dominant over blue. It takes just one good OCA2 gene to end up with brown eyes.

Multiple Alleles

• Genes that are controlled by more than two alleles are said to have multiple alleles• An individual can’t have more than

two alleles. However, more than two possible alleles can exist in a population.

Multiple Alleles

sometimes caused by mutations. Some traits have up to 100

alleles. Example: rabbit fur color fruit fly

eye-color, and blood typing.

C = full color; dominant to all other alleles

cch = chinchilla; partial defect in pigmentation; dominant to ch and c alleles

ch = Himalayan; color in certain parts of the body; dominant to c allele

c = albino; no color; recessive to all other alleles

Pleiotropy• a single gene influences many

phenotypic traits• Individuals with Marfan

syndrome tend to be tall and thin with long legs, arms, and fingers; are nearsighted; and the wall of their aorta is weak.

Marfan Syndrome

Gene Linkage• Two genes that occur on the same

chromosome are said to be linked, and those that occur very close together are tightly linked

• Example: a fly with reddish-orange eyes most always have miniature wings

Environmental factorsInternal· Age· Gender (hormone

differences).

External factors can all influence gene expression.

· Temperature (rabbit fur color, bacteria)

· Nutrition· Light· Chemicals, Infectious diseases

Environment and Phenotype

• Himalayan Rabbits - Enzyme coding for black fur is active only at low temperatures.– Black fur only occurs on extremities.

Environment and Phenotype:Himalayan Rabbits