Mendelian Inheritance

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Mendelian Inheritance

... the outward physical manifestation of internally coded, inheritable, information.

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Genetics: expression of genes

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Gregor MendelFather of

Genetics1823-1884

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Gregor MendelAustrian monk

– Studied science and mathematics at University of Vienna

– Conducted breeding experiments with the garden pea Pisum sativum

– Carefully gathered and documented mathematical data from his experiments

Formulated fundamental laws of heredity in early 1860s– Had no knowledge of cells or

chromosomes– Did not have a microscope

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One-Trait InheritanceMendel performed cross-breeding

experiments with pea plants– Used “true-breeding” (homozygous)

plants– Chose varieties that differed in only

one trait (monohybrid cross)– Performed reciprocal crosses

•Parental generation = P•First filial generation offspring = F1 •Second filial generation offspring = F2

– Formulated the Law of Segregation

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Why Peas?Either or

traitsEasy to growMany

offspringEasy to

regulate pollination

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Mendel’s method

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ShortTall

Tall

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Mendel’s HypothesesEach parent has two factors

(alleles)Each parent gives one of

those factors to the offspringTall has TTShort has ttTall is dominantShort is recessive

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Law of SegregationEach individual has a pair of factors

(alleles) for each trait

The factors (alleles) segregate (separate) during gamete (sperm & egg) formation

Each gamete contains only one factor (allele) from each pair

Fertilization gives the offspring two factors for each trait

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TT tt

T tTt

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TT TT

T TTT

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tt tt

t ttt

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Modern Genetics ViewEach trait in a pea plant is controlled by

two alleles (alternate forms of a gene)Dominant allele (capital letter) masks the

expression of the recessive allele (lower-case)

Alleles occur on a homologous pair of chromosomes at a particular gene locus– Homozygous = identical alleles– Heterozygous = different alleles

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Law of Segregation and Random Fertilization…

genetic variationAlleles separate during

gamete productionGametes have one allele for

each traitDuring fertilization gametes

combine at random to form individuals of the next generation

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Discovery of Chromosomes in 1900 Confirms Law of Segregation

Chromosomes are in pairs

Each chromosome has one of the allele pair

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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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Chromosomes line up in a double row.

Meiosis I Metaphase

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

T T t t

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Chromosomes separateEach 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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AlleleMember of a paired gene

– One allele comes from each parent

Represented by a single letter

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Dominant & Recessive AllelesDominant alleles are

expressedRecessive alleles are not

expressed in the presence of a dominant allele– Recessive alleles are only

expressed if both alleles are present

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HomozygousBoth alleles alikeAA or aa

A A a a

or

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HeterozygousAlleles are differentAa

A a

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GenotypeGenetic make upRepresented by allelesTT & Tt are genotypes for

tall pea plantsThis is the "internally coded, inheritable information" carried by all living organisms.

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PhenotypeA traitGenotype

determines the phenotype

Tall is a phenotype

“Think adjective!”

Descriptive

This is the "outward, physical manifestation" of the organism.

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Phenotype: red flowers

Cells contain red granules

Enzymes help convert colorless pigment into red pigment

Most enzymes are proteins

Most traits are produced by the action of proteins.

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Summary: Genotype Versus PhenotypeGenotype

– Refers to the two alleles an individual has for a specific trait

– If identical, genotype is homozygous

– If different, genotype is heterozygousPhenotype

– Refers to the physical appearance of the individual

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Punnett SquareTable listing all possible genotypes

resulting from a cross

– All possible sperm genotypes are lined up on one side

– All possible egg genotypes are lined up on the other side

– Every possible zygote genotypes are placed within the squares

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Monohybrid TestcrossIndividuals with recessive phenotype always

have the homozygous recessive genotypeHowever, Individuals with dominant

phenotype have indeterminate genotype

– May be homozygous dominant, or

– HeterozygousTest cross determines genotype of

individual having dominant phenotype

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One-Trait Test CrossUnknown is Heterozygous

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One-Trait Test CrossUnknown is Homozygous Dominant

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Human Genetic DisordersAutosome - Any chromosome other than a sex

chromosomeGenetic disorders caused by genes on

autosomes are called autosomal disorders – Some genetic disorders are autosomal dominant

• An individual with AA has the disorder• An individual with Aa has the disorder• An individual with aa does NOT have disorder

– Other genetic disorders are autosomal recessive• An individual with AA does NOT have disorder• An individual with Aa does NOT have disorder, but is a

carrier• An individual with aa DOES have the disorder

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AlbinismLack of

pigment– Skin– Hair– Eyes

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Amino Acids

Melanin Pigment

Enzyme

A a

AA = Normal pigmentationAa = Normal pigmentationaa = Albino

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

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AA = Normal pigmentationAa = 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

Phenotypes3 Normal1 Abino

Probability25% for albinism

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Dwarfism = DNormal height = d

DD = DwarfismDd = Dwarfismdd = 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 = DwarfDd = Dwarf dd = Normal

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Dd

dd

Dd

dd

Genotypes2 Dd, 2dd

Phenotypes2 Normal2 Dwarfs

Probability50% for Dwarfism

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Polydactyly (more than the normal amount of fingers or toes) is a dominant trait!

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Law of Independent AssortmentThe 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

When in the Course of human events it becomes necessary for one genome to dissolve the temporary bonds which have connected them with another and to assume among the powers of the earth, the separate and equal station to which the Laws of Nature and of Nature's God entitle them, a decent respect to the variation that mankind requires that they should declare the causes which impel them to the separation.

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Two Trait ProblemA 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 YellowTtY

y

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-Green3 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

DbdB

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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