Mendelian Genetics

Principles of Heredity

Gregor Mendel: The Father of Genetics

1. Traits are controlled by particles2. Two genes per trait3. Heterozygous vs. Homozygous4. Law of Dominance5. Law of Segregation6. Law of Independent Assortment

Basics of Heredity: Mendel’s Rules

1. Traits are controlled by “particles”

a. The “particles” are solids in the cells

b. “Particles” = genes

c. Genes are…

d. Gene means…

e. Alleles are…

2. Two genes per trait

a. Most biologists thought it was a single gene per trait (in the sperm), but really…

b. One gene from dad and one from momc. Trait: general description of what is being

controlled by the genes (e.g. seed color)d. Phenotype: visible expression of the genes

(e.g. yellow or green)e. Genotype: Actual genes present

3. Heterozygous vs. Homozygous

a. the 2 genes may be the same= homozygous e.g. EE or ee(purebred, true, non-carrier)b. …or different = heterozygous e.g. Ee(hybrid, crossed, mixed breed, carrier)

4. Law of Dominance

a. when it comes to showing up, some genes are more powerful than others = Dominant e.g. E = or

b. Others only show up if both genes are recessive e.g. e =

Antonio Alfonseca (P)

4. Law of Dominance, cont.

c. Some more traits:PolydactylySyndactylyAchondroplasia

4. Law of Dominance, cont.

d. Some traits are inherited as Recessives:Sickle-cell anemiaHitchhiker’s ThumbAlbinismPhenylketylnuria

5. Law of Segregation

a. “When gametes are made, the two genes for a trait separate and each gamete has only one gene for each trait” This happens in meiosis

b. Punnett squares: A a show the possible

offspring from a

cross

A

a

6. Law of Independent Assortment

a. “The way one pair of genes for a trait is inherited has no effect on any other trait”

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Parent 2: AaBb

Parent 1:

AaBb

b. This allows us to examine multiple traits on a single (larger) Punnett Square. This is called a dihybrid cross and predicts 2 traits at once.

c. Probability II “The chances of two separate events happening at the same time equals the product of their separate probabilities”

d. Solving a Dihybrid Cross

1. Make “Gamete Tree” for both parents

2. Create Punnett Square

3. Insert Gametes along top and side

4. Fill in Punnett Square

I. Linkage A. genes for two traits carried on the same chromosomeB. example: Freckles and alleles for red hair

II. Mutations A. random changes in the genetic codeB. may produce “unexpected” offspring that Mendel

couldn’t account for. C. Ex: achondroplastic kids (Dd) usually come from

two perfectly normal (dd) parents

Exceptions to Mendel’s Rules

III. Polygenic

A. Controlled by multiple alleles on different chromosomes.

B. Best examined at the population level

C. Shown as continuous (bell curve) distribution

D. Ex.: human height, skin and hair color.

IV. Environmental Effects

A. some genes are affected by environmental influence

B. example: BRCA1 Breast Cancer Gene

Exceptions to Mendel’s Rules

Exceptions to Mendel’s Rules

IV. Sex Linkage

A. traits specifically carried on (usually) the X chromosome

B. Work like recessive traits (needs 2 alleles to show)

C. Show up most often in males (XY versus XX genotype)

D. Examples: Hemophilia and Color Blindness

Exceptions to Mendel’s Rules

V. Codominance A. two alleles that express themselves equally in the

presence of each other B. Example: ABO blood grouping, Roan cattle

VI. Multiple Alleles A. some traits have more than two possible phenotypes

because there are more than just two alleles for the traitB. This creates multiple combinations of possibilityC. Example: ABO blood grouping

Multiple Alleles & Codominance: ABO Blood Groups

I. A and B alleles code for glycoproteins (antigens) on red blood cells which can be detected immunochemically:

A. mix blood sample with type A or type B antibodies

B. look for clumping (agglutination) of RBC’s

II. O allele carries neither antigen

ABO Blood Groups

A - A antigen only

B - B antigen only

AB - Both A and B antigens

O - Neither antigen

III. ABO Genotypes and Phenotypes

Genotype Antigen Phenotype

IAIA

IAIOAA

A

IBIB

IBIOBB

B

IAIB A, B AB

IOIO Neither O

Some Important ABO Factoids

IV. IA and IB are codominantV. Both IA and IB are dominant to IO

VI. ApplicationsA. testing compatibility of blood transfusions

A. Who can donate to who?B. What happens in case of incompatibility?

B. disproving parentage of a childC. forensic scienceD. childbirthing (Rhogam and hemolytic disease)

Pedigree Analysis

I. Introduction to Pedigrees

A. Background: What is a Pedigree?“A diagram that shows appearance of phenotypes for a single trait in a group of related individuals from one generation to the next.”

Pedigree Analysis

B. Reading a Pedigree: SymbolsMales (squares or triangles) Females (circles)Marriage/MatingOffspring and SiblingsShaded or UnshadedCrossed outGeneration Labels (Roman)Individual Labels (Arabic)Birth Order (left to right)

Father 1. Mother 2.

Daughter 1. Son 2. Daughter 3.

Marriage line

Siblings

Oldest Child Youngest ChildMarriage into family

I.

II.

Dead

Pedigree Analysis

C. Genotypes in a Recessive PedigreeThere are some rules to follow:1. Shaded people are homozygous recessive. Fill

them in as such.2. Unshaded people are either:

Homozygous Dominant OR Heterozygous So… Assign one Dominant allele to each person

3. Work one generation at a time to determine the “unknown” genotypes. NEVER “skip” generations!!!!!

I-1 I-2

II-4

III-2

II-6II-3II-2 II-5II-1

III-1 III-3

Please copy this pedigree

Pedigree Analysis, continued

II. Advanced Pedigrees: Unknown InheritanceA. First, “flip a coin” i.e. pick a mode ( Dom or Rec) B. Assign known genotypes across pedigreeC. Begin filling in unknowns

Remember to work 1 generation at a time!!! Don’t skip!!!

D. Look for anomalies (matings that don’t work)E. Try the pedigree again with the other mode of inheritanceF. Use colored pencils, different ink pens, or different letters

to help you solve