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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


• Blended inheritance– Offspring are intermediate in appearance• Ex: skin tone in humans


• Blended inheritance– Offspring are intermediate in appearance• Ex: skin tone in humans

– Ink in water1. Combined 2. Inseparable


• 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.



– Focus on plant height:• Tall or short:



– Focus on plant height:• Tall or short: T or



– Focus on plant height:• Tall or short: T or t



– 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


• Crossed a purebred tall with a purebred short– Paternal: T x t– Filial (F1): ??


• 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


• Where did the recessive trait go?


• Where did the recessive trait go?• To find out, he self-pollinated the F1 plants– F1: T x T


• 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)