GENETICS

Gregor Mendel: “Father of Genetics"

Gregor Mendel: “Father of Genetics"

• parents were farmers

• he became ordained as a priest

• studied science and mathematics at the University of Vienna

Mendel's Experiments:

• Mendel chose pea plants as his experimental subjects, mainly because they were easy to cross and showed a variety of traits

• (purple vs. white flowers, tall vs. short stems, round vs. wrinkled seeds)

Mendel’s Experiment1. Mendel chose true-breeding lines of each

plant/trait. (true breeding lines always produced offspring of the same type)

2. He crossed a true breeding plant with a plant of the opposite trait (purple x white). He called this the Parent (P) generation.

3. He recorded data on the offspring of this cross First Filial (F1) generation.4. He self-pollinated the F1 offspring5. He recorded data on the offspring of the second

generation as the Second Filial or (F2) generation.

Cross showing Parent, F1 and F2 Generations for Flower Color

Analysis of Mendel’s Experiment • The F1 generation always displayed one trait (he

later called this the dominant trait)• The F1 generation must have within it the trait

from the original parents - the white trait• The F2 generation displayed the “hidden” trait,

1/4 of the F2 generation had it (he later called this hidden trait the recessive trait)

• Each individual has two "factors" that determine what external appearance the offspring will have. (We now call these factors genes or alleles).

Mendel established three principles (or Laws) from his research

1. The Law of Dominance and Recessiveness - one trait is masked or covered up by another trait

2. Law of Segregation - the two factors (alleles) for a trait separate during gamete formation.

3. Law of Independent Assortment - factors of a trait separate independently of one another during gamete formation.

Ex. A flower being purple has nothing to do with the length of the plants stems - each

trait is independently inherited.

MODERN GENETICS• Mendel's factors are now called alleles.

• Alleles are variations of a gene.

• For every trait a person has, two alleles determine how that trait is expressed.

• We use letters to represent alleles, since every gene has two alleles, all genes can be represented by a pair of letters.

PP = purple, Pp = purple, pp = white

(P = purple p = white)

Homozygous:

• Homozygous when the alleles are the same SIZE, the individual is said to be homozygous, or true breeding.

• Letters can be capital or lowercase, as long as they are the same. – Ex. AA, bb, EE, dd

Heterozygous:

Heterozygous: when the alleles are different SIZES, in this case the DOMINANT allele is expressed.

• Ex. Pp, Aa

Genotype:

• What the GENES are.

• Genotype: gene is represented by alleles (letters)!– Ex. BB, Pp, etc…

Phenotype:

• PHOTO

• Phenotype: what an organism physically looks like – tall, purple..

Punnett Square:• Punnett Square: used to determine the

PROBABILITY of having a certain type of offspring given the alleles in the gametes of the parents

How to Solve a Punnett Square 1. Determine the genotypes (letters) of the

parents. Bb x Bb2. Set up the punnett square with one parent on each side.3. Fill out the Punnett square middle. B b

4. Analyze the number of Boffspring of each type. b

Solve the following:

• In pea plants, round seeds are dominant to wrinkled seeds. Genotype Phenotype

RR = roundRr = roundrr = wrinkled 

Cross two plants, one that is homozygous round and one that is recessive.

Monohybrid cross:

What you have done is called a Monohybrid cross:

•a cross involving one pair of contrasting traits. •The genotypic ratio would be ___:___:___ HD He hr

The phenotypic ratio would be ____;____

Dihybrid cross:

• Dihybrid cross: a cross involving two pairs of contrasting traits. Ex. PpTt x PpTt

• For these crosses your punnett square needs to be 4x4

• In any case where the parents are heterozygous for both traits (AaBb x AaBb) you will get a 9:3:3:1 ratio.

• If you cross other combinations, you will need to do a square. Try RrYy x rryy.

Incomplete Dominance

• There is no dominant or recessive, the heterozygous condition results in a “BLEND" of the two traits. Example: Snapdragons can be red, white, or pink (heterozygous)

Codominance

BOTH traits are dominant and therefore both traits show.

Example: A cross between a white rabbit and a brown rabbit produces offspring with brown AND white spots.

Sex-Linked Traits

• The genes for these traits are on the X chromosome, because boys only receive one X chromosome they are more likely to inherit disorders passed to them from their mother who would be

a carrier.

• Ex. Hemophilia

Colorblindness

Colorblindness tests

• What number do you see?

The punnett square below shows how a woman who is a carrier

passes the trait to her son, but not her daughters.

Multiple Allele Traits

• Traits that are controlled by more than two alleles.

EX: Blood type in humans is controlled by three alleles: A, B, and O.

Phenotype Genotype

A AA or AO

B BB or BO

AB AB only

O OO only

Examples of Blood-type crosses

Blood Transfusions• Blood can only be transferred to the body of a person

who's immune system will "recognize" the blood. A and B are antigens (flags) on the blood that will be recognized. If the antigen is unfamiliar to the body, your body will attack and destroy the transfused blood as if it were a hostile invader (which can cause death).

• O is like a blank, it has no antigens. O is called the universal donor because a person can receive a transfusion from O blood without having an immune response.

• AB is the universal acceptor, because a person with AB blood has both the A and B antigens already in the body, A and B blood can be transfused to the person (as well as O) and the body will recognize it and not attack.

Polygenic Traits

• Traits controlled by many genes:

hair color, height, weight, intelligence

Sex Influenced Traits

• Traits that are influenced by gender. Pattern baldness affects men because testosterone activates the genes.

Human Genetics

• Human genetics is studied using PEDIGREES, which diagram how a trait is inherited in a family. It helps us determine genotypes of the family members.

This pedigree is for the recessive allele that causes albinism. Albinos are humans that have no pigment, their skin is very pale and all of their hair is white, including their eyebrows and eyelashes.Label each part of the pedigree below. How would you designate a carrier?

This pedigree shows how albinism can be inherited over 2 generations.

Human genetics can also be studied by looking at identical twins, which help establish whether NATURE or NURTURE influences our traits.

Human Genetic Diseases

Albinism -

• Albinism: inability to produce pigment, white hair and skin, autosomal recessive

Huntingtons -

• symptoms of mental illness appear late in life

• autosomal dominant

Huntington’s Disease effects the brain’s basal Ganglia

Sickle Cell Disease

• blood cells are

shaped abnormally

so can’t properly

carry oxygen

molecules.

*autosomal recessive

Tay Sachs -

• fat builds up in the brain of infants and causes degeneration and early death, autosomal recessive

Hemophilia

• “Bleeder's Disease”, inability of the blood to clot, sex linked recessive

Cystic Fibrosis

• - mucus builds up in lungs causing respiratory problems, autosomal recessive