The Work The Work of of

Gregor Gregor MendelMendel

An Introduction to An Introduction to GeneticsGenetics

GeneticsGenetics Genetics is Genetics is

everywhere these everywhere these days – and it will days – and it will continue as a continue as a dominant force in dominant force in Biology and society for Biology and society for decades to come!decades to come!

DefinedDefined: as the : as the scientific study of scientific study of heredityheredity

It is now the core of a It is now the core of a revolution in revolution in understanding biologyunderstanding biology

Gregor MendelMonk at St Thomas Abbey in Brnoof the Czech Republic

Studied garden peas from 1856-63

Published his 'paper' in 1866

Pretty much ignored until 1900

That year 3 scientists independently came to many of the same ideas

His experiments were special because:1) Quantitative2) carefully documented3) elegantly designed

Gregor Mendel

Breeding for traits was long established.

Cross (or mating) -- sexual reproduction between 2 organisms

Hybrid-- cross between 2 parents which differ in some trait

Phenotype-- observable difference between 2 members of the species

Genotype-- relationship between genes; the genetic make-up

True breeding-- only produce their own phenotype when bred to self

2 key features of Mendel's work: could pollinate from specific parents chose unambiguous phenotypes that were easy to identify

Genes and DominanceGenes and Dominance Mendel studied Mendel studied

7 different pea 7 different pea plant traitsplant traits

TraitTrait – specific – specific characteristic characteristic that varies from that varies from one individual one individual to anotherto another

Example: seed Example: seed color, or plant color, or plant heightheight

Mendelian GeneticsSome terminology:

Parental generations (P0 or F0)-- originally crossed organisms

F1 generation-- “first filial”, the offspring of the F0 generation (parents)

Hybrid– offspring of crosses between parents with different traits

F2 generation-- offspring of F1 generation crossed to itself

Dominant-- phenotype visible in the F1 generation

Recessive-- trait which reappears in the F2 generation after self cross

Genes, Alleles, and Chromosomes GenesGenes – are the chemical factors that – are the chemical factors that

determine traitsdetermine traits AllelesAlleles – the different forms of a gene – the different forms of a gene HomozygousHomozygous – organisms carrying two – organisms carrying two

copies of the same allele( i.e. copies of the same allele( i.e. TTTT or or tt tt )) HeterozygousHeterozygous – organism carrying different – organism carrying different

alleles of the same gene (i.e. alleles of the same gene (i.e. Tt Tt ))

x

Mendelian Genetics

F1 generation 227 0

F2 generation 593 193 3.07

In this case, green is dominant, yellow is recessive

green yellow ratio

P0

Mendelian Genetics

Importantly, Mendel went another step further looking at F3 plants

193 crosses 193 yellow offspring only 0 green pea pods

593 crosses 201 green offspring only 392 green and yellow F3

3:1 ratio green: yellow

Overall

1/4 F2 matings green, 1/2 F2 matings mixed, 1/4 F2 matings yellow1:2:1 ratio for all F2 crosses

Mendelian GeneticsMore terminology:

Gamete -- reproductive cell

Zygote-- fertilized egg

From the ratios above Mendel determinedeach plant has 2 genetic determinantsthese determinants go to each offspringrandomly

Mendel's Principle of Segregation: In the formation of gametes, thepaired hereditary determinants separate such that each gamete is equallylikely to contain either one.

note: gametes have 1 "determinant" zygotes and adults have 2

Capitalized traits = dominant phenotypeslowercase traits= recessive phenotypes ** most mutations are recessive

adult

gamete

zygote

Mendelian GeneticsClassical Punett's Square is a way to determine ways traits can segregateThey can be used to predict and compare the genetic variations that will result from the cross.

Parental P0 cross

A Aa Aa Aaa Aa Aa

F1 cross

A aA AA Aaa Aa aa

Test cross: mating to a homozygous recessive individual

Mendelian Genetics

dihybrid cross-- parental generation differs in two traits example-cross violet/axial flowers with

white/terminal ones

Violet/axial is dominant, so F1 is all violet axial.

What about F2?

VA Va vA vaVAVavAva

VV AA

VV Aa

Vv AA

Vv Aa

VV Aa

VV aa

Vv Aa

Vv aa

Vv AA

Vv Aa

vv AA

vv Aa

Vv Aa

Vv aa

vv Aa

vv aa

Mendelian GeneticsPrinciple of Independent Assortment: segregation of any pair of alleles is independent of other pairs in the formation of gametes; genes for different traits can segregate independently during the formation of gametes

Accounts for the many genetic variations observed in plants, animals and other organisms.

adult

gametes

Mendelian Genetics

va va va vaVAVavAva

Vv Aa

Vv aa

vv Aa

vv aa

Vv Aa

Vv aa

vv Aa

vv aa

Vv Aa

Vv aa

vv Aa

vv aa

Vv Aa

Vv aa

vv Aa

vv aa

test cross: F1 generation crossed to double homozygous recessive

What are the expected phenotype ratios in the testcross generation?violet, axial = violet, terminal =white, axial = white, terminal =

1/41/4

1/41/4

A test cross gives the ratio of gametes in the F1 generation