Gregor Mendel: The Father of Genetics

Mendelian Genetics Principles of Heredity

Basics of Heredity: Mendel’s Rules 1. Traits are controlled by particles 2. Two genes per trait 3. Heterozygous vs. Homozygous 4. Law of Dominance 5. Law of Segregation 6. Law of Independent Assortment

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 mom c. 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

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…

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)

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4. Law of Dominance 4. Law of Dominance, cont. 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 =

c. Some more traits: Polydactyly Syndactyly Achondroplasia

Antonio Alfonseca (P)

5. Law of Segregation 4. Law of Dominance, cont. d. Some traits are inherited as Recessives: Sickle-cell anemia Hitchhiker’s Thumb Albinism Phenylketylnuria

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:

cross

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

a

show the possible offspring from a

6. Law of Independent Assortment

A

A a

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”

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Parent 2: AaBb ?? ??

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Parent 1: ?? ???? ?? ???? AaBb ?? ????

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Exceptions to Mendel’s Rules

d. Solving a Dihybrid Cross 1. 2. 3. 4.

I. Linkage A. genes for two traits carried on the same chromosome B. example: Freckles and alleles for red hair II. Mutations A. random changes in the genetic code B. may produce “unexpected” offspring that Mendel couldn’t account for. C. Ex: achondroplastic kids (Dd) usually come from two perfectly normal (dd) parents

Make “Gamete Tree” for both parents Create Punnett Square Insert Gametes along top and side Fill in Punnett Square

Exceptions to Mendel’s Rules

Exceptions to Mendel’s Rules

III. Polygenic

IV. Sex Linkage

A.

Controlled by multiple alleles on different chromosomes.

B.

Best examined at the population level

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

C.

Shown as continuous (bell curve) distribution

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

D.

Ex.: human height, skin and hair color.

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

IV. Environmental Effects

D. Examples: Hemophilia and Color Blindness

A. some genes are affected by environmental influence B. example: BRCA1 Breast Cancer Gene

Multiple Alleles & Codominance: ABO Blood Groups

Exceptions to Mendel’s Rules V. Codominance

I.

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 trait B. This creates multiple combinations of possibility C. Example: ABO blood grouping

II.

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 O allele carries neither antigen

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ABO Blood Groups III. ABO Genotypes and Phenotypes A - A antigen only

Genotype

Antigen

Phenotype

IAIA IAIO IBIB IBIO IAIB

A A B B A, B

A

AB

IOIO

Neither

O

B - B antigen only

AB - Both A and B antigens

B

O - Neither antigen

Some Important ABO Factoids IV. V. VI.

IA and IB are codominant Both IA and IB are dominant to IO Applications A. testing compatibility of blood transfusions A. B.

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

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.”

B. disproving parentage of a child C. forensic science D. childbirthing (Rhogam and hemolytic disease)

Pedigree Analysis

Father 1.

Mother 2.

B. Reading a Pedigree: Symbols Males (squares or triangles) Females (circles) Marriage/Mating Offspring and Siblings Shaded or Unshaded Crossed out Generation Labels (Roman) Individual Labels (Arabic) Birth Order (left to right)

I. Dead Marriage line Siblings Marriage into family

Oldest Child

Youngest Child

II. Daughter 1.

Son 2.

Daughter 3.

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Pedigree Analysis C. Genotypes

Please copy this pedigree

I-1

I-2

in a Recessive Pedigree

There 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!!!!!

II-1

II-2

III-1

II-3

II-4

II-5

II-6

III-2 III-3

Pedigree Analysis, continued II.

Advanced Pedigrees: Unknown Inheritance A. B. C.

First, “flip a coin” i.e. pick a mode ( Dom or Rec) Assign known genotypes across pedigree Begin filling in unknowns

D. E. F.

Look for anomalies (matings that don’t work) Try the pedigree again with the other mode of inheritance Use colored pencils, different ink pens, or different letters to help you solve

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

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