Chapter 12 Patterns of Inheritance. BACK IN THE DAY…. Theories on Inheritance –Bloodlines –Heridity occurs within species –Traits are transmitted directly.

Chapter 12

Patterns of Inheritance

BACK IN THE DAY….

• Theories on Inheritance– Bloodlines– Heridity occurs within species– Traits are transmitted directly from parents to

offspring

Josef Kolreuter

• Hybridizations– First generation looked like neither parents– Second generation was variable

T.A. Knight

• Worked with yellow and green peas

• Observed all F1 generation were yellow, but F2 generation varied between green and yellow

Mendel

• Worked with pea plants • Round vs wrinkled• Yellow vs green seed• White vs purple flower• Green vs yellow pod• Inflated vs constricted pod• Terminal vs axial flower• Tall vs short height

Mendel’s experimental design

• Allowed plants to self cross for many generations to ensure purity

• Crossed pollen of plants true breeding for specific characteristics

• Permitted hybrid offspring to self fertilize for several generations

Monohybrid Crosses

Always showed similar results

• F1 generation exhibited only one of the two traits without blending….what theory did this disprove?

• F2 generation exhibits both traits in a 3:1 ratio– Actually a 1:2:1 ratio…why?– Genotype vs Phenotype

What was Mendel able to understand based on his

observations?• 1. Plants did not follow blended inheritance,

traits were discrete characteristics• 2. For each pair of alternatives for a trait the one

that was not shown in the F1 generation was still present, but it was latent

• 3. Pairs of alternative traits were segregated among the offspring

• 4. Mendelian Ratio ALWAYS seen

Mendel’s Model

• 1. Parents transmit discrete information to offspring not direct physiological traits now called genes

• 2. Each individual receives two genes that encode each trait. ( where are the genes found?)

• 3. Not all genes are identical- alternative forms called alleles– Homozygous– Heterozygous

• 4. Two alleles remain discrete• 5. The presence of an allele does not ensure that the

trait it encodes will be expressed.

Principle of Segregation

• Two alleles for a gene segregate during gamete formation and are rejoined at random, one from each parent during fertilization

Punnett Squares

Dominant Traits in Humans

• Middigital hair

• Brachydactyly

• Huntington Disease

• PTC sensitivity

• Polydactyly

• Hypercholesterolemia

Recessive Traits

• Albinism

• Red-green color blindness

• Cystic fibrosis

• Duchenne muscular dystrophy

• Hemophilia

• Sickle cell anemia

Pedigree

Independent Assortment

• Alleles of each gene assort independantly

• What does this mean?– Pea color does not necessarily connected to

plant height

• Demonstrated in Dihybrid Crosses

Dihybrid Cross

• You are looking at 4 different traits

• F1 generation only shows 2 of the 4 traits…Why?

• F2 generation 9:3:3:1 ratio

Probability

• Rule of addition– Probability of tow mutually exclusive events– This OR that– = Probability of event 1 + probability of even 2– Suppose a high school consists of 25% juniors, 15%

seniors, and the remaining 60% is students of other grades. The relative frequency of students who are either juniors and seniors is 40%. We can add the relative frequencies of juniors and seniors because no student can be both junior and senior.

– P(J or S) = 0.25 + 0.15which equals 0.40

Probability

• Rule of Multiplication• Probability of This AND that occuring• =Probability of Event A * probability of Event B• Suppose we have two dice. A is the event that 4

shows on the first die, and B is the event that 4 shows on the second die. If both dice are rolled at once, what is the probability that two 4s occur?

• P(A) = 1/6P(B) = 1/6P(A and B) = P(A) . P(B) = 1/6 . 1/6 = 1/36

Rules can be applied to Dyhibrid cross

• Example

• Look at F1 generation

• What are the odds of Wrinkled Seeds?

• What are the odds of Green Seeds?

• Multiply them together to get the odds of wrinkled green seeds.

Test Cross

• If you do not know what genotype an organism has you can cross it with an individual who is homozygous recessive for the trait to find out.

Breaking the Rules

• Instances when Medel’s Laws may not be followed.– Polygenic Inheritance– Pleiotropy– Multiple alleles – Dominance is not always complete– Environmental Factors– Gene Interaction

Polygenic Inheritance

• More than one gene affecting a single trait

• Example Height or skin color

Pleiotropy

• A single gene affects more than one trait

• Example Cystic Fibrosis, albinism

• Mouse Study- Yellow Mice Die

Multiple Alleles for One Gene

• More than one allele controlling the phenotype

• Example ABO blood types

Incomplete Dominance

• Incomplete dominance is when the heterozygote is intermediate in appearance between the two homozygote.

Codominance

• When the heterozygote shows some piece of the phenotype of both homozygotes

• Ex. ABO blood types fur color in some animals

ABO Blood Types

Genes can be affected by the environment

• Example Temperature effects on Siamese Cats

• Temp above 33 degrees no pigment

• Temp Below 33 degrees pigment

Epistasis

• An interaction between genes in which one gene is altered by another gene

• Example Labrador Retrievers

• Black is dominant to chocolate B or bYellow is recessive epistatic (when present, it blocks the expression of the black and chocolate alleles) E or e