Gregor Mendel (1822-1884)Called the Father of Genetics"
Responsible for the laws governing Inheritance of Traits
Gregor Johann MendelAustrian monkStudied the inheritance of traits in pea plantsDeveloped the laws of inheritanceMendel's work was not recognized until the turn of the 20th century
Gregor Johann Mendel1856 and 1863 - Mendel cultivated and tested some 28,000 pea plantsHe found that the plants' offspring retained traits of the parents
Site of Gregor Mendels experimental garden in the Czech Republic
Mendel used words like particles or units or factors to refer to inherited traits.
Mendel did not know that these were actually Chromosomes & DNA
Mendels Pea Plant Experiments
Why peas, Pisum sativum?Can be grown in a small area
Produce lots of offspring
Can be self-pollinated
Can be artificially cross-pollinated
Reproduction in Flowering Plants
Pollen contains spermProduced by the stamen
Ovary contains eggsFound inside the flower
Pollen carries sperm to the eggs for fertilization
GametesSperm male sex cell
Egg female sex cell
Normal body cells have 2 copies of chromosomes
Gametes only have 1 so that when put together, they have 2 for the new cell
Self-fertilization can occur in the same flower -or-
Cross-fertilization can occur between flowers
*Mendels Experimental MethodsMendel hand-pollinated flowers using a paintbrush
Snipped off stamens to prevent self-pollination
He traced traits through the several generations
*Eight Pea Plant TraitsSeed shape --- Round (R) or Wrinkled (r)Seed Color ---- Yellow (Y) or Green (y)Pod Shape --- Smooth (S) or wrinkled (s)Pod Color --- Green (G) or Yellow (g)Seed Coat Color ---Gray (G) or White (g)Flower position---Axial (A) or Terminal (a)Plant Height --- Tall (T) or Short (t)Flower color --- Purple (P) or white (p)
Mendels CrossesHis parental generation (P) were all true-breedingAll produce offspring identical to themselves when self-fertilizedWhen Mendel crossed two pure-bred plants, he made hybridsProduced by parents of different traits
Mendels ResultsWhen Mendel crossed a plant with round seeds (R) and a plant with wrinkled seeds (r), all the plants produced had round (R) seeds!
The first generation of plants produced is called the F1 GenerationF1 GenerationParental Generation
Mendels ResultsWhen Mendel crossed his F1 generation with itself, he got a surprise!
F1 GenerationParental Generation??F2 Generation
Mendels Experimental Results
Mendels F2 ResultsEvery time Mendel self-fertilized his F1 generation plants, he got the same results:
3:1He saw 3 plants (75%) with the dominant traitAnd 1 (25%) with the recessive trait!
Genetic TerminologyTrait - any characteristic that can be passed from parent to offspring
Heredity - passing of traits from parent to offspring
Genetics - study of heredity
Designer GenesAlleles - two forms of a gene (dominant & recessive)
Dominant - stronger of two alleles; represented by a capital letter (R)
Recessive weaker of the two alleles; represented by a lowercase letter (r)
More TerminologyGenotype - gene combination for a trait (ex: RR, Rr, rr)
Phenotype - the physical feature resulting from a genotype (ex: red, white)
Genotype & Phenotype in FlowersGenotype of alleles: R = red flower r = yellow flowerAll genes occur in pairs, so 2 alleles affect a characteristicPossible combinations are:
GenotypesRR RrrrPhenotypesRED RED YELLOW
GenotypesHomozygous genotype - gene combination involving 2 dominant or 2 recessive genes (ex: RR or rr) also called pure
Heterozygous genotype - gene combination of one dominant & one recessive allele(ex: Rr) also called hybrid
Mendels PeasMendels peas were all true breeding.
What would the genotype of a true-breeding tall plant (T) plant look like?
What would the genotype of a true-breeding short plant (t) look like?
Genes and Environment Determine Characteristics
Generation GapParental P1 Generation = the parental generation F1 generation = the first-generation offspring (1st filial generation)From breeding individuals from the P1 generationF2 generation = the second-generation offspring (2nd filial generation) From breeding individuals from the F1 generation
Following the GenerationsCross 2 Pure Plants TT x ttResults in all Hybrids TtCross 2 Hybrids get 3 Tall & 1 Short TT, Tt, tt
Types of Genetic CrossesMonohybrid cross - cross involving a single trait example: flower color Dihybrid cross - cross involving two traits example: flower color & plant height
Punnett SquareUsed to help solve genetics problems
Trait: Seed ShapeAlleles: R Roundr WrinkledCross: Round seeds x Wrinkled seedsRR x rrP1 Monohybrid CrossRRrrRrRrRrRrGenotype: RrPhenotype: RoundGenotypic Ratio: All RrPhenotypic Ratio: All Round
Trait: Seed ShapeAlleles: R Roundr WrinkledCross: Round seeds x Round seedsRr x RrF1 Monohybrid CrossRrrRRRrrRrRrGenotype: RR, Rr, rrPhenotype: Round & wrinkledG.Ratio: 1:2:1P.Ratio: 3:1
What Do the Peas Look Like?
Results of Monohybrid CrossesGENES are responsible for all heritable characteristics Phenotype is based on Genotype Each trait is based on two alleles, one from the mother and the other from the father True-breeding individuals are homozygous ( both alleles are the same)Heterozygous individuals have one of each allele
Law of Dominance
In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation.All the offspring will be heterozygous and express only the dominant trait.RR x rr yields all Rr (round seeds)
The Law of Segregation
Law of SegregationDuring the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other.Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring.
Law of Independent Assortment
Alleles for different traits are distributed to sex cells (& offspring) independently of one another.
This law can be illustrated using dihybrid crosses.A breeding experiment that tracks the inheritance of two traits.
Dihybrid CrossTraits: Seed shape & Seed colorAlleles: R round r wrinkled Y yellow y green
RrYy x RrYyRY Ry rY ryRY Ry rY ryAll possible gamete combinations
Dihybrid CrossRound/Yellow: 9 Round/green: 3 wrinkled/Yellow: 3 wrinkled/green: 19:3:3:1
Summary of Mendels laws
Test CrossA mating between an individual of unknown genotype and a homozygous recessive individual.Example: bbC__ x bbcc
BB = brown eyesBb = brown eyesbb = blue eyes
CC = curly hairCc = curly haircc = straight hair
Test CrossPossible results:
Incomplete DominanceF1 hybrids have an appearance somewhat in between the phenotypes of the two parental varieties.Example: snapdragons (flower)red (RR) x white (rr)
RR = red flowerrr = white flower
CodominanceBOTH alleles are expressed (multiple alleles) in heterozygous individuals.Example: blood type
1.type A= IAIA or IAi2.type B= IBIB or IBi3.type AB= IAIB4.type O= ii
Codominance ProblemExample:homozygous male Type B (IBIB)x heterozygous female Type A (IAi)
Another Codominance ProblemExample: male Type O (ii) x female type AB (IAIB)
CodominanceQuestion: If a boy has a blood type O and his sister has blood type AB, what are the genotypes and phenotypes of their parents?
boy - type O (ii) X girl - type AB (IAIB)
CodominanceAnswer:Parents:genotypes = IAi and IBiphenotypes = A and B
Sex-linked TraitsTraits (genes) located on the sex chromosomesSex chromosomes are X and YXX genotype for femalesXY genotype for malesMany sex-linked traits carried on X chromosome
Sex-linked TraitsSex ChromosomesExample: Eye color in fruit flies
Sex-linked Trait ProblemExample: Eye color in fruit flies (red-eyed male) x (white-eyed female) XRY x XrXrRemember: the Y chromosome in males does not carry traits.
RR = red eyedRr = red eyedrr = white eyedXy = maleXX = female
Sex-linked Trait Solution:50% red eyed female50% white eyed male