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Mendelian Genetics. Chapter 2. Phenotype and Genotype. Genotype and Phenotype. Genotype – genetic constitution of an organism Phenotype – observable characteristic Genotype and environment Contribution of environment varies between genes Can be controlled by many genes

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Genotype and phenotype l.jpg
Genotype and Phenotype

  • Genotype – genetic constitution of an organism

  • Phenotype – observable characteristic

    • Genotype and environment

      • Contribution of environment varies between genes

    • Can be controlled by many genes

    • Random developmental events



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Stamen

Carpel

Mendelian Genetics

  • Modern genetics began with Gregor Mendel’s quantitative genetic experiments

  • Austrian monk

  • Mathematician

  • Numerical and observational data

  • Several generations


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Mendelian Genetics

  • Heritable, obvious traits

  • Simple crosses at first

  • Used peas because:

    • Easy to grow and available

    • Many distinguishable characteristics

    • Self-fertilization

  • True breeding peas




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Breeding Crosses

  • Initial cross is the P generation

    • Parents

  • Progeny of parents is first filial generation

    • F1 generation

  • Inbreeding of first generation creates second filial generation

    • F2 generation


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Monohybrid Crosses

  • Cross between true-breeding individuals with one different trait

  • Mendel’s first crosses

  • Resembled only one of the parents

  • Planted progeny and allowed self-fertilization

    • Revealed both phenotypes


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GENETIC MAKEUP (ALLELES)

P PLANTS

PP

pp

Gametes

All P

All p

F1 PLANTS(hybrids)

All Pp

Gametes

1/2P

1/2p

P

P

Eggs

Sperm

PP

F2 PLANTS

p

p

Pp

Pp

Phenotypic ratio3 purple : 1 white

pp

Genotypic ratio1 PP : 2 Pp : 1 pp

Monohybrid Cross

  • Mendel determined that

    • Particulate factors for genes, each contains a set of two

    • Transmitted by both parents

    • Alternate forms called alleles

    • True breeding forms contains identical set


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Monohybrid Cross

  • F1 generation had both alleles

  • Only one expresses

  • One allele masks

    • Dominant

    • Recessive

  • Identical alleles – homozygous

  • Different alleles - heterozygous



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Principle of Segregation

  • Recessive characteristics are masked

    • Reappear in F2

  • Members of a gene pair (alleles) segregated during gamete formation


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How cells carry characteristics

  • Genes on chromosomes

    • At a specific loci

  • Homologous pairs carry the same genes at the same locus

    • Different versions

  • Separation of homologous chromosomes yields separation of alleles


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Branch Diagrams

  • Punnett squares can become messy with more than one gene

  • Use branch diagram to figure out genotype and phenotype expected frequency


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TESTCROSS:

GENOTYPES

B_

bb

Two possibilities for the black dog:

BB

or

Bb

B

B

b

GAMETES

b

Bb

b

Bb

bb

OFFSPRING

All black

1 black : 1 chocolate

Test Cross

  • Mendel did several crosses

    • Followed over several generations

  • Selfing also very important

    • Allowed plants to reveal their genotype and not just their phenotye



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Recessive Alleles

  • Wild-type allele – functional allele

    • Predominates in population

    • Dominant allele

  • Loss-of-function mutations – causes protein product to be absent, partially functional, or nonfunctional

    • Recessive

    • Function of other in heterozygote is sufficient


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Wrinkled Peas

  • SS type contains more starch and lower sucrose

    • Also more water

    • SBEI - starch-branching enzyme

    • Extra 800 bp piece in mutation



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The Principle of Independent Assortment Independent Assortment

  • Factors for different traits assort independently of one another

    • Genes are inherited independently of each other

    • Segregate randomly in gametes

  • Dihybrid Cross


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Branch Diagram of Dihybrid Cross Independent Assortment

Phenotype


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Genotype vs. Phenotype Independent Assortment


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Test Cross With Dihybrid Independent Assortment


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Trihybrid Cross Independent Assortment


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Tribble Traits Activity Independent Assortment



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Statistical Analysis Independent Assortment

  • Data from genetics is quantitative

  • Use statistics to show deviation of observed results from predicted results

    • Chance factors cause deviations

  • Null-hypothesis – no difference between the predicted and observed

    • If not accepted then have to come up with a new hypothesis for deviation


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Chi-Square Test Independent Assortment

  • Goodness of fit test

    • How much observed number deviates from the expected number


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Mendelian Genetics in Humans Independent Assortment


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Pedigree Analysis Independent Assortment

  • Inheritance patterns are studied using family trees

    • Pedigree analysis

    • Phenotypic records

      • Proband is where gene was discovered


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Examples of Human Genetic Traits Independent Assortment

  • Most genetic disorders are recessive

    • Due to lack of function

  • Homozygous recessive expression

    • Dominant usually selected out

  • Albinism


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Characteristics of Recessive Inheritance Traits Independent Assortment

  • Most have normal heterozygous parents

  • Heterozygotes have 3:1 ratio

  • When both parents have the trait then all progeny have the trait

  • Cystic Fibrosis, Sickle Cell Anemia, Tay Sachs


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Characteristics of Dominant Inheritance Traits Independent Assortment

  • Gain of function mutations

    • New property of the mutant gene

    • No loss of function

  • Must have one parent with disease

  • Does not skip generations

  • Will transmit to half its progeny

    • Huntingtons disease, Marfan syndrome, achondroplasia


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