Classical Genetics

The Work and Conclusions of Gregor Mendel

Father of genetics

Monk in Austria

1865

Studied patterns of inheritance in peas

Why Peas?

Why Peas?

Self-fertilization produces true breeding generations

•Prevented self-fertilization

•Manipulated reproduction

•Began monohybrid crosses

•Carefully examined results

Why Peas?

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Why Peas?

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Mendel’s Work

Some traits are dominant and some are recessive

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Mendel’s Work

1. Traits are passed on by factors (genes)

2. Factors (genes) have more than 1 form called alleles

3. There are at least 2 alleles for each trait

Mendel’s Work

Alleles are represented by the first letter of the dominant trait

Pea plant flower color (trait)

Purple or white

Purple is dominant over white

Alleles are represented as

P=purple p=white

You try a few…

1. Round seeds are dominant over wrinkled seeds

2. Yellow seeds are dominant over green seeds

3. Tall plants are dominant over short plants

1. R=round r=wrinkled

2. Y=yellow y=green

3. T=tall t= short

Mendel’s Work

Mendel’s WorkDescribing Traits (Tall or short plants)

Genotype

The actual genetic make-up of an organism

the “genes”

Phenotype

The physical appearance or form observed

the “physical”

Mendel’s WorkPossible Genotypes (TT Tt tt)

Homozygous

The two alleles for the trait are identical

TT homozygous dominant (purebred dominant)

tt homozygous recessive (purebred recessive)

Heterozygous

The two alleles for the trait are different

Tt heterozygote (hybrid)

Mendel’s Work

1. Law of Dominance

Alleles for a trait are either dominant or recessive

The dominant form is expressed and the recessive form is hidden

The only way to express a recessive trait is if there are two copies of the recessive allele

Mendel’s Conclusions

Mendel’s Conclusions

2. Law of Segregation

The two alleles for a trait separate during the formation of gametes

Test Cross

Dihybrid Cross

Mendel’s 3rd Law of Independent Assortment

• Alleles of different genes are assorted independently of one another during the formation of gametes

• This means that calculating the probability of several traits appearing together is the product of the probability of each trait taken separately

• The Rule of Multiplication

Incomplete Dominance

“Dominant” gene does not fully express itself to mask the effect of the recessive gene

A pattern of inheritance in which the phenotype of a heterozygous individual is intermediate between those of the parents

Genotypic ratio is equal to phenotypic ratio

Co-DominanceTwo or more alleles are fully dominant

When present together they are both expressed

ABO Blood Groups exhibit co-dominace

Type A Blood

•Individuals with group A blood have red blood

cells with antigen A on their surface. •Produce antibodies against antigen B,

antibody B.

•Therefore, a group A person can only receive

blood from people in groups A or O

Type B Blood• Antigen B is on their cells, and

antibodies A are produced in their serum.

• A group B person can only receive blood from people in groups B or O, preferably B.

Type AB

• Individuals with group AB blood have red blood cells with both antigens A and B.

• Do not produce antibodies A or B against either antigen in their serum.

• A person with type AB blood can receive blood from any group (preferably AB) but cannot donate blood except to another AB (universal recipient).

Type O

• Individuals with group O blood have red blood cells with neither antigen A or B.

• Produce antibodies A and B against both types of antigens.

• A group O person can only receive blood from group O (universal donor).

Sex Inheritance

Sex Chromosomes X or Y

XX is female

XY is male

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Sex-Linked Inheritance

Genes linked to sex chromosomes

GENETICGENETIC DISORDERS

Sources of Genetic Disorders• Chromosome Mutations

• Change in the genetic material on the chromosome

• Four types of chromosome mutations– Deletion, duplication, translocation and

inversion

• Change in chromosome number• Failure of homologous chromosomes to

separate during meiosis–Non-Disjunction

Deletion

• One or more genes are lost from a chromosome during division

• Occurs if homologous chromosomes cross over unequally during meiosis

Duplication

• Chromosome receives an extra piece, which duplicates some genes

Translocation

• A whole chromosome or piece of chromosome attaches to a chromosome in a different pair

• Results in extra or lost genes

• May break up important gene arrangements and change the phenotype of the organism

Inversion

• Part of chromosome breaks off and reattaches itself in reverse order

• May cause no change because it is not losing genes

• Changes the arrangement and may upset important gene interactions

Nondisjunction

• Failure of chromosome to separate during division– Mitosis

• Cell dies, organism is not harmed

– Meiosis

• Results in an abnormal gamete that will produce abnormal offspring

Turner Syndrome• XO

• phenotype-female

• small functioning ovaries

• short

• no breast development

Down Syndrome• Extra chromosome at the 21st position

• Enlarged tongues

• Small, round ears • Heart defects

• Stubby fingers and toes

• Mental retardation

Klineflelter Syndrome

• XXY at 23rd position

• small testes, no sperm development

• enlarged breast development

• tall stature

Cri-du-chat• Partial deletion from

chromosome # 5• Baby’s cry sounds like a

cat• Severe mental

retardation• Multiple anatomical

malformations

Sex-linked Disorders

Hemophilia

• mental retardation

• enlarged liver

• flat broad nose, large tongue

• inability to clot blood properly

• bruise easily

• many patients have been infected

with AIDS

Hunters Syndrome

Autosomal Dominate Disorders

Neurofibromatosis

• Disorder of connective tissue

• Affects bone, eyes, heart and blood vessels

• Long legs and hands

• Blood vessels lack elasticity

• Can be slight to severe• Tumors on head, neck, and spine

Marfan Syndrome

Huntington Disease• Progressive degeneration of brain cortex • Symptoms begin around the age of 30-50• Symptoms include

• depression• forgetfulness• clumsiness• twitching• mood swings

Autosomal Recessive Disorders

Porphyria

• Prophyrin build-up(molecules formed during the synthesis of hemoglobin)• “werewolf”• emotionally unstable• sensitive to light• self mutilation

Cystic Fibrosis

• very common, 40% of population are carriers

• disease of exocrine glands, build up of thick mucus in the lungs that makes breathing difficult

• causes respiratory infections

Leprechanism

• Short, elf-like appearance

• Hypoglycemia

Progeria• Premature aging

• Die of artery disease at age 10-15