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Fundamentals of Genetics

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Patterns of Inheritance• The History of

Genetics

–Genetics - scientific study of heredity

–Trait - characteristic that can be passed from parents to offspring

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Blending of Traits

• Once accepted hypothesis

• did not explain appearance of unexpected traits in offspringOops…blond baby boy. Where did that come from?

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Gregor Mendel - 1860’s

• Studied pea plants• cross-fertilization -

cross of 2 different plants; offspring receives different forms of genetic trait from each parent

•self-fertilization - cross of the same plant; offspring receives genetic information from one parent only

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• Purebred - organism receives the same genetic traits from both of its parents

• Hybrid - organism receives different forms of a genetic trait

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Mendel’s Observations

• Crossed two different purebreds (P generation = parent generation)

• resulting offspring is F1 (1st filial) -offspring

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• crossed these, resulting in F2 (2nd filial) generation

• Found that there were always two types of traits - Dominant and Recessive

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Mendel’s Conclusions:

• Disproved blending hypothesis

• offspring receives one of two “factors” from each parent - “factors” are genes– genes - sections of a chromosome that

code for a trait

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• Allele - distinct form of a gene

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• dominant allele - expressed when two different alleles are present; represented with a capital letter: A

• recessive allele - form of a gene that is not expressed when paired with a dominant allele; represented with a lower case letter: a

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Mendel is the Father of Modern Genetics

Gregor Gregor MendelMendel

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Cellular Basis of Inheritance• Chromosome

Theory of Heredity states that the material of inheritance is carried by the genes in the chromosomes

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Genes represent traits

• Genotype - genes that make up an organism

–includes both genes in a homologous pair

• Phenotype - outward expression of the trait

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• homozygous - two alleles are identical (AA or aa)–also known as purebred organism

• heterozygous - two alleles are different (Aa)–also known as a hybrid

organism

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Mendel’s Laws

• Law of Segregation - gene pairs separate when gametes form

–one gamete carries one gene and the other carries the other gene

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• Law of Independent Assortment - gene pairs segregate into gametes randomly and independently of each other

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• Law of Dominance - dominant allele is expressed, recessive allele will be hidden unless in homozygous form

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Genetics and Prediction

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Predictions for One Trait

• Probability - predict likelihood of an event or outcome

““heads”heads” ““tails”tails”

“Heads” is one possible outcome out of a total of 2 possible outcomes.

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• Punnett square - grid for organizing genetic information

–can be used to make predictions about a cross between two organisms

–monohybrid cross - cross between two parents involving one trait

P

P

p

p

pP

pP

P

p

P

p

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AA x aa (A = normal, a = albino)

A A

a

a

Aa Aa

A a aA

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• Genotypic ratio = AA 0/4 or 0% Aa 4/4 or 100%

aa 0/4 or 0%

• Phenotypic ratio = Normal 4/4 or 100%

Albino 0/4 or 0%

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Predictions for Two Traits• Two trait cross – cross between two

parents and two traits

• Dihybrid cross - cross between two heterozygous parents –use a 4 X 4 punnett square–sixteen possible outcomes

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HhTt x hhtt (H = long hair, h = short hair; T = tail, t = no

tail)

FOIL!

HT Ht hT ht

ht

ht

ht

ht

HhTt Hhtt hhTt hhtt

HhTt Hhtt hhTt hhtt

HhTt Hhtt hhTt hhtt

HhTt Hhtt hhTt hhtt

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• Genotypic ratio =

– HHTT 0/16 or 0%

– HHTt 0/16 or 0%

– HHtt 0/16 or 0%

– HhTT 0/16 or 0%

– HhTt 4/16 or ¼ or 25%

– Hhtt 4/16 or ¼ or 25%

– hhTT 0/16 or 0%

– hhTt 4/16 or ¼ or 25%

– hhtt 4/16 or ¼ or 25%

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• Phenotypic ratio:–25% Long hair, tail

–25% Long hair, no tail

–25% short hair, tail

–25% short hair, no tail

JK!

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Test Cross

• Test cross - breed an organism whose genotype is questionable with a homozygous recessive organism

– results determine genotype of questionable organism

G?

GG or Gg

gg G?XIf all offspring

then it’s probably GG

If any offspring

then it’s Gg

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Incomplete Dominance

• heterozygous offspring show a phenotype that is in-between the phenotypes of the two homozygous parents

– blending of traits

– color in snapdragons, hypercholestolemia, instead of white or red – color is pink

X

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r wr

w

rr rw

rw ww

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Codominance• Both alleles are

expressed equally– Ex. Sickle Cell

Disease

• HA = normal• HS = sickle• both are dominant

and equally expressed – both normal red blood cells and sickle-shaped blood cells present in heterozygote

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Polygenic Trait (Epistasis)

• Trait controlled by more than one gene

–ex. Eye color

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Multiple Alleles

• Three or more alleles for a trait in a human population

–ex. Blood types: IA, IB, i (type O)

–Karl Landsteiner discovered antigens which are responsible for different blood types

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Multiple Alleles - Blood Types

• IA - contains antigen A

• IB - contains antigen B

• IAIB - contains antigens for both A & B

• i - contains neither A nor B

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Blood Groups

Phenotypes Genotypes Antigen onRed Blood

Cell

Can receiveblood from:

Can giveblood to:

A IAIA or IAi A A, O A, ABB IBIB or IBi B B, O B, AB

AB IAIB A, B A, B, O, AB ABO ii none O A, B, O, AB

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• Universal donor: O

• Universal recipient: AB

• Dominant Trait in Blood Groups

– Rh antigen = named after Rhesus Monkey

• people who have the Rh antigen = Rh+ (positive)

• people who do not have it = Rh- (negative)

• Rh-, Rh+ person = positive

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Pleiotropy• Single gene affects

more than one trait

–ex. Sickle cell anemia - effects include blood cell shape, anemia, weakness, brain damage, spleen damage, & heart damage

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Environmental Effects• Phenotype is a

combination of genetic and environmental influences, epigenetics

Ex. Identical twins share the same DNA but may look and act differently

ex. Himalayan rabbit - fur color depends on body temperature – dark where cooler, white where warmer

Yes, they are

identical!

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Issues in Biology

• “Nature vs. Nurture” Issue– studies have been

done on identical twins that have been separated at birth

• Up to a 3 inch height difference observed in studies where twins were raised in two different countries/environments

Twins raised in different adoptive homes showed similar interests and careers when they reconnected as adults

VS