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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
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
slide5

Stamen

Carpel

Mendelian Genetics

  • Modern genetics began with Gregor Mendel’s quantitative genetic experiments
  • Austrian monk
  • Mathematician
  • Numerical and observational data
  • Several generations
mendelian genetics6
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
breeding crosses
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
monohybrid crosses
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
monohybrid cross

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
monohybrid cross12
Monohybrid Cross
  • F1 generation had both alleles
  • Only one expresses
  • One allele masks
    • Dominant
    • Recessive
  • Identical alleles – homozygous
  • Different alleles - heterozygous
principle of segregation
Principle of Segregation
  • Recessive characteristics are masked
    • Reappear in F2
  • Members of a gene pair (alleles) segregated during gamete formation
how cells carry characteristics
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
branch diagrams
Branch Diagrams
  • Punnett squares can become messy with more than one gene
  • Use branch diagram to figure out genotype and phenotype expected frequency
test cross

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
recessive alleles
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
wrinkled peas
Wrinkled Peas
  • SS type contains more starch and lower sucrose
    • Also more water
    • SBEI - starch-branching enzyme
    • Extra 800 bp piece in mutation
the principle of independent assortment
The Principle of Independent Assortment
  • Factors for different traits assort independently of one another
    • Genes are inherited independently of each other
    • Segregate randomly in gametes
  • Dihybrid Cross
statistical analysis
Statistical Analysis
  • 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
chi square test
Chi-Square Test
  • Goodness of fit test
    • How much observed number deviates from the expected number
pedigree analysis
Pedigree Analysis
  • Inheritance patterns are studied using family trees
    • Pedigree analysis
    • Phenotypic records
      • Proband is where gene was discovered
examples of human genetic traits
Examples of Human Genetic Traits
  • Most genetic disorders are recessive
    • Due to lack of function
  • Homozygous recessive expression
    • Dominant usually selected out
  • Albinism
characteristics of recessive inheritance traits
Characteristics of Recessive Inheritance Traits
  • 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
characteristics of dominant inheritance traits
Characteristics of Dominant Inheritance Traits
  • 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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