Basic Mendelian Genetics

What lovely things we pass onto our children!

Mendel’s Laws• Law 1: Law of Unit Characteristics

– Inherited traits are controlled by “factors” which occur in “pairs”.

Factors = __________ Pairs = __________

• Law 2: Law of Dominance– One allele hides another allele in a heterozygote

Stronger allele = _____________ Weaker allele = _____________

Mendel’s Laws Continued

• Law 3: Law of Segregation– Alleles of a gene are separated during gamete

formation

Each cell only gets 1 allele for a gene

A a

B b

Mendel’s Laws Continued

• Law 4: Law of Independent Assortment of Alleles– When alleles separate during meiosis, they do

so independently of alleles of other genes

Placement of alleles is random

Not influenced by other genes

A A

BB

a a

bb

Mendel’s Laws Continued

• Law 5: Law of gene pricing– Lee genes are usually less expensive than

Levi’s, except when Levi’s are on sale

Life Beyond the Law of Dominance

Not all traits are governed by the law of dominance

• Incomplete Dominance– Neither allele is completely dominant over the

other in the heterozygote Blending/Mixing

Ex) Snap dragons

WW – White flowers

RR – Red Flowers

RW – Pink Flowers

Incomplete Dominance

Life Beyond the Law of Dominance

• Codominance– Both genes express themselves FULLY at the

same time

Example: Roan cattle

RR = Red Hair

WW = White Hair

RW = Roan (both red + white)

Example 2: Blood type in humans (AB)

Life Beyond the Law of Dominance

• Multiple Alleles– A gene possessing more than 2 alleles

Example: Blood types

Blood types differ in antigens found on RBC’s

AA – Homozygous A Type A

BB – Homozygous B Type B

OO – Homozygous O Type O

AO – Heterozygous A Type A

BO – Heterozygous B Type B

AB – Heterozygous AB Type AB

Life Beyond the Law of Dominance

• Polygenic Inheritance– When a trait is controlled by more than 2

genes

Example: Eye color (3 genes)

Amount of pigment

Location of pigment

Tone of pigment

Cowboys denied home field by Eagles

A Review of Mendelian Genetics

1) Neither allele is dominant a) Complete Dominance

2) More than 2 alleles b) Codominance

3) Both alleles are dominant c) Incomplete dominance

4) Many genes code for 1 trait d) Multiple alleles

5) One allele is dominant e) Polygenic inheritance