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Mendelian Inheritance

BIO 2215

Oklahoma City Community College

Dennis Anderson

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Gregor Mendel

• Father of Genetics• 1823-1884• Monk in Austria• Experimented with

garden peas

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Short

Tall

Tall

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Mendel’s Hypotheses

• Each parent has two factors (alleles)

• Each parent gives one of those factors to the offspring

• Tall has TT

• Short has tt

• Tall is dominant

• Short is recessive

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TT tt

T t

Tt

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TT TT

T T

TT

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tt tt

t ttt

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Law of Segregation

• Alleles separate during gamete production

• Gametes have one allele for each trait

• During fertilization gametes combine at random to form individuals of the next generation

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Discovery of Chromosomes in 1900 Confirmed Law of Segregation

• Chromosomes are in pairs

• Each chromosome has one of the allele pair

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Chromosomes line up in a double row.

Meiosis Metaphase

Assume a T allele on each red chromatid and a t allele on each blue chromatid

T T t t

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Chromosomes separate

Each each daughter cell gets doubled chromosomes

T T t t

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Doubled Chromosomes Separate in Second Meiotic Division

T T t t

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Each gamete will have a T allele or a t allele

T T t t

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Allele

• Member of a paired gene– One allele comes from each parent

• Represented by a single letter

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Dominant & Recessive Alleles

• Dominant alleles are expressed

• Recessive alleles are not expressed in the presence of a dominant allele– Recessive alleles are only expressed if both

recessive alleles are present

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Gene

• A unit of heredity that controls the development of one trait

• Made of DNA

• Most genes are composed of two alleles

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Homozygous

• Both alleles alike

• AA or aa

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Heterozygous

• Alleles are different

• Aa

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Genotype

• Genetic make up

• Represented by alleles

• TT & Tt are genotypes for tall pea plants

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Phenotype

• A trait

• Genotype determines the phenotype

• Tall is a phenotype

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Homologous Chromosomes

• Chromosomes of the same pair

• Each homologue will have one allele for a paired gene

• Homologous chromosomes pair up during meiosis

• Only one of each homologue will be in each gamete

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Sickle Cell Anemia

• RBCs sickle shaped• Anemia• Pain• Stroke• Leg ulcers• Jaundice• Gall stones• Spleen, kidney & lungs

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Sickle Cell Anemia

• Recessive allele s, codes for hemoglobin S – Long rod-like molecules– Stretches RBC into sickle shape

• Homozygous recessive ss, have sickle cell anemia

• Heterozygous Ss, are carriers

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Albinism

• Lack of pigment– Skin

– Hair

– Eyes

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Amino Acids Melanin PigmentEnzyme

A a

AA = Normal pigmentation

Aa = Normal pigmentation

aa = Albino

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PKU Disease

• Phenylalanine excess• Mental retardation if

untreated

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Phenylalanine TyrosineEnzyme

P p

PP = Normal

Pp = Normal

pp = PKU

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A man & woman are both carriers (heterozygous) for albinism. What is the chance their children will inherit albinism?

Monohybrid Cross or One Trait

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AA = Normal pigmentation

Aa = Normal pigmentation (carrier)

aa = Albino

Man = Aa Woman = Aa

A

a a

A

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A

a

a

A AA

Aa

Aa

aa

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AA

Aa

Aa

aa

Genotypes1 AA, 2Aa, 1aa

Phenotypes

3 Normal

1 Abino

Probability

25% for albinism

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A man & woman are both carriers (heterozygous) for PKU disease. What is the chance their children will inherit PKU disease?

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p

p

P PP

Pp

Pp

pp

P

PP = Normal

Pp = Normal (carrier)

pp = PKU disease

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PP

Pp

Pp

pp

Genotypes1 PP, 2Pp, 1pp

Phenotypes

3 Normal

1 PKU disease

Probability

25% for PKU disease

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A man with sickle cell anemia marries a woman who is a carrier. What is the chance their children will inherit sickle cell anemia?

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s

s

s Ss

Ss

ss

ss

S

SS = Normal

Ss = Normal (carrier)

ss = Sickle Cell

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Ss

Ss

ss

ss

Genotypes2 Ss, 2ss

Phenotypes

2 Normal (carriers)

2 Sickle cell

Probability

50% for Sickle cell

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Dwarfism = D

Normal height = d

DD = Dwarfism

Dd = Dwarfism

dd = Normal height

Dwarfism

Dwarf Band

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A man with heterozygous dwarfism marries a woman who has normal height. What is the chance their children will inherit dwarfism? Dwarfism is dominant.

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d

d

D Dd

dd

Dd

dd

d

DD = Dwarf

Dd = Dwarf

dd = Normal

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Dd

dd

Dd

dd

Genotypes2 Dd, 2dd

Phenotypes

2 Normal

2 Dwarfs

Probability

50% for Dwarfism

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Law of Independent Assortment

• The inheritance of one gene does not influence the inheritance of another gene if they are on separate chromosomes.

• The gene for albinism does not affect the gene for dwarfism

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Dihybrid Cross or Two Traits

• A heterozygous tall plant that is also heterozygous for yellow seeds is crossed with another plant with the same genotype

• Tall and yellow seeds are dominant to short and green seeds.

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Tall Yellow

TtYy

TY

What gametes can each parent produce?

TytYty

TtYy

TYTytYty

Tall Yellow

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9 Tall-Yellow

Match gametes on a Punnent Square

TY

Ty

tY

ty

TY Ty tY ty

TtYy

TtYy

TTYY TTYy TtYY

TTYy TTyy Ttyy

TtYY TtYy ttYY ttYy

TtYy Ttyy ttYy ttyy

3 Tall-Green

3 Short-Yellow

1 Short-Green

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A man with blue eyes and normal height marries a woman with heterozygous brown eyes and heterozygous dwarfism. What are the possible phenotypes of their children? Dwarfism & brown eyes are dominant.

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Normal height-Blue

ddbb

db

What gametes can each parent produce?

DdBb

DBDbdBdb

Dwarf-Brown

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DB

Db

dB

db

Match gametes on Punnent Square

db

DdBb

Ddbb

ddBb

ddbb

Dwarf-Brown eyes

Dwarf-blue eyes

Normal height-Brown eyes

Normal height-Blue eyes

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The End