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