intro to mendelian genetics
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Intro to Mendelian Genetics
1.28.13
What is genetics?
What is genetics?
• The science of heredity
What is genetics?
• The science of heredity• How traits are passed to offspring
What is genetics?
• The science of heredity• How traits are passed to offspring
• Gene – A section of DNA that codes for a specific protein
• Gene – The unit of heredity
Gregor Mendel
• Born in Czech Republic, settled in Austria
• In 1842, entered monastery at age of 21, tended to a garden of peas
Gregor Mendel
• In 1851 he went to the U. of Vienna to study science and statistics
• Returned to monastery to teach high school and study heredity through his peas. (OMG, this is like sooooo exciting! I can’t wait to hear what happens next. Seriously, I’m about to pea my pants!)
Current thoughts in 1850
• Blended inheritance– Offspring are intermediate in appearance
Current thoughts in 1850
• Blended inheritance– Offspring are intermediate in appearance• Ex: skin tone in humans
Current thoughts in 1850
• Blended inheritance– Offspring are intermediate in appearance• Ex: skin tone in humans
– Ink in water1. Combined 2. Inseparable
Current thoughts in 1850
• Blended inheritance – (Incorrect)– Offspring are intermediate in appearance• Ex: skin tone in humans
– Ink in water1. Combined 2. Inseparable
Used peas to study inheritance
• Why peas?
Used peas to study inheritance
• Each plant has various traits to study
• Cheap• Simple to grow• Seeds easily stored for
later study• Short generation time
• Self-pollinators (flowers have male and female parts)
• Cloned flowers can serve as control group
Statistics was the key!
• Used math to explain living systems (one of the first to do so)
• Published his work in 1866, but mostly ignored because he didn’t know the mechanism of heredity (DNA)
Mendel’s 1st Experiment
• Studied 7 characteristics with two traits each• Plant height, flower color, etc.
Mendel’s 1st Experiment
• Studied 7 characteristics with two traits each• Plant height, flower color, etc.
– Focus on plant height:• Tall or short:
Mendel’s 1st Experiment
• Studied 7 characteristics with two traits each• Plant height, flower color, etc.
– Focus on plant height:• Tall or short: T or
Mendel’s 1st Experiment
• Studied 7 characteristics with two traits each• Plant height, flower color, etc.
– Focus on plant height:• Tall or short: T or t
Mendel’s 1st Experiment
• Studied 7 characteristics with two traits each• Plant height, flower color, etc.
– Focus on plant height:• Tall or short: T or t
– All studied traits were heritable, not environmental
– Used purebreds (clones) achieved through many generations of self-pollination
Mendel’s 1st Experiment
• Crossed a purebred tall with a purebred short– Paternal: T x t– Filial (F1): ??
Mendel’s 1st Experiment
• Crossed a purebred tall with a purebred short– Paternal: T x t– Filial (F1): 100% tall (T)
• Same result with all 7 characteristics• Trait that showed up in F1 generation =
dominant• Disappearing trait = recessive
Mendel’s 1st Experiment
• Where did the recessive trait go?
Mendel’s 1st Experiment
• Where did the recessive trait go?• To find out, he self-pollinated the F1 plants– F1: T x T
Mendel’s 1st Experiment
• Where did the recessive trait go?• To find out, he self-pollinated the F1 plants– F1: T x T
– F2: 75% T, 25% t
Law of segregation
• Law of segregation : Traits are discrete units that occur in pairs and retain their integrity
Law of segregation
• Law of segregation : Traits are discrete units that occur in pairs and retain their integrity
• Characteristics are now called genes (one or more genes lead to the appearance of a certain characteristic)
Mendelian Genetics Continued…• The location (address) of a gene on a
chromosome is called the locus• Genes have alternate versions called alleles
• ex: locus for flower color
• Genotype: The collection of alleles in an individual– Represented by letters
• Genotype: The collection of alleles in an individual– Represented by letters
• P: TT x tt (T = dominant allele –> only one needed for expression)
• F1:
• Phenotype:
• Phenotype: Physical expression of genotype + environment– Represented by
• Phenotype: Physical expression of genotype + environment– Represented by descriptive words
• F2 : Tt x Tt
Genotypes:
Phenotypes:
Genotype terminology
• TT = Homozygous dominant or purebred dominant
• Tt =
• tt =
Genotype terminology
• TT = Homozygous dominant or purebred dominant
• Tt = Heterozygous dominant or hybrid
• tt = Homozygous recessive or purebred recessive
• How can you tell the difference between a homozygous dominant (DD) and a heterozygous dominant (Dd)?
• How can you tell the difference between a homozygous dominant (DD) and a heterozygous dominant (Dd)?
• Test Cross: Breed the unknown with a homozygous recessive to bring out any hidden recessive traits
• Test Cross: Breed the unknown with a homozygous recessive to bring out any hidden recessive traits
• Law #1: Law of segregation
• Law #2: Law of independent assortment– Genes located at different loci (locations) on the
same chromosome assort (cross-over) independently of each other
• Law #1: Law of segregation
• Law #2: Law of independent assortment– Genes located at different loci (locations) on the
same chromosome assort (cross-over) independently of each other• (Whether one gene crosses over during meiosis has no
influence over whether another gene on the same chromosome crosses over)
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