Mendelelian Genetics

Gregor Mendel ( ) Called the “Father of Genetics" Responsible for the laws governing Inheritance of Traits

Gregor Johann Mendel  Austrian monk  Studied the inheritance of traits in pea plants  Developed the laws of inheritance  Mendel's work was not recognized until the turn of the 20th century

Gregor Johann Mendel  1856 and Mendel cultivated and tested some 28,000 pea plants  He found that the plants' offspring retained traits of the parents Site of Gregor Mendel’s 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 Particulate Inheritance

Mendel’s Pea Plant Experiments

Why peas, Pisum sativum?  Can be grown in a small area  Reproduce sexually  Produce lots of offspring  Can be self-pollinated  Can be artificially cross- pollinated

Reproduction in Flowering Plants Pollen contains spermPollen contains sperm –Produced by the stamen Ovary contains eggsOvary contains eggs –Found inside the flower Pollen carries sperm to the eggs for fertilization

Gametes Sperm – 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

*Mendel’s Experimental Methods Mendel hand- pollinated flowers using a paintbrushMendel hand- pollinated flowers using a paintbrush Snipped off stamens to prevent self- pollinationSnipped off stamens to prevent self- pollination He traced traits through the several generationsHe traced traits through the several generations

*Eight Pea Plant Traits Seed shape --- Round (R) or Wrinkled (r)Seed shape --- Round (R) or Wrinkled (r) Seed Color ---- Yellow (Y) or Green (y)Seed Color ---- Yellow (Y) or Green (y) Pod Shape --- Smooth (S) or wrinkled (s)Pod Shape --- Smooth (S) or wrinkled (s) Pod Color --- Green (G) or Yellow (g)Pod Color --- Green (G) or Yellow (g) Seed Coat Color ---Gray (G) or White (g)Seed Coat Color ---Gray (G) or White (g) Flower position---Axial (A) or Terminal (a)Flower position---Axial (A) or Terminal (a) Plant Height --- Tall (T) or Short (t)Plant Height --- Tall (T) or Short (t) Flower color --- Purple (P) or white (p)Flower color --- Purple (P) or white (p)

Mendel’s Crosses His parental generation (P) were all true-breeding –All produce offspring identical to themselves when self-fertilized When Mendel crossed two “pure-bred” plants, he made hybrids –Produced by parents of different traits

Mendel’s Results When 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 Generation F1 Generation Parental Generation

Mendel’s Results When Mendel crossed his F1 generation with itself, he got a surprise! F1 Generation Parental Generation ?? F2 Generation

Mendel’s Experimental Results

Mendel’s F2 Results Every time Mendel self-fertilized his F1 generation plants, he got the same results: 3:13:1 He saw 3 plants (75%) with the “dominant” trait And 1 (25%) with the “recessive” trait!

Genetic Terminology  Trait - any characteristic that can be passed from parent to offspring  Heredity - passing of traits from parent to offspring  Genetics - study of heredity

Designer “Genes”  Alleles - 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 Terminology  Genotype - gene combination for a trait (ex: RR, Rr, rr)  Phenotype - the physical feature resulting from a genotype (ex: red, white)

Genotype & Phenotype in Flowers Genotype of alleles: R = red flower r = yellow flower All genes occur in pairs, so 2 alleles affect a characteristic Possible combinations are: GenotypesRR Rrrr PhenotypesRED RED YELLOW

Genotypes  Homozygous genotype - gene combination involving 2 dominant or 2 recessive genes (ex: RR or rr)  also called pure  also called pure  Heterozygous genotype - gene combination of one dominant & one recessive allele (ex: Rr)  also called hybrid

Mendel’s Peas Mendel’s 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 “Gap” Parental P 1 Generation = the parental generationParental P 1 Generation = the parental generation F 1 generation = the first-generation offspring (1st filial generation)F 1 generation = the first-generation offspring (1st filial generation) –From breeding individuals from the P 1 generation F 2 generation = the second-generation offspring (2nd filial generation)F 2 generation = the second-generation offspring (2nd filial generation) – From breeding individuals from the F 1 generation

Following the Generations Cross 2 Pure Plants TT x tt Results in all Hybrids Tt Cross 2 Hybrids get 3 Tall & 1 Short TT, Tt, tt

Types of Genetic Crosses  Monohybrid cross - cross involving a single trait example: flower color  Dihybrid cross - cross involving two traits example: flower color & plant height

Punnett Square  Used to help solve genetics problems

Monohybrid Crosses

Trait: Seed ShapeTrait: Seed Shape Alleles: R – Roundr – WrinkledAlleles: R – Roundr – Wrinkled Cross: Round seeds x Wrinkled seedsCross: Round seeds x Wrinkled seeds RR x rrRR x rr P 1 Monohybrid Cross R R rr Rr Genotype:Rr Genotype: Rr PhenotypeRound Phenotype: Round Genotypic Ratio:All Rr Genotypic Ratio: All Rr Phenotypic Ratio: All Round

Trait: Seed ShapeTrait: Seed Shape Alleles: R – Roundr – WrinkledAlleles: R – Roundr – Wrinkled Cross: Round seeds x Round seedsCross: Round seeds x Round seeds Rr x RrRr x Rr F 1 Monohybrid Cross R r rR RR rrRr Genotype:RR, Rr, rr Genotype: RR, Rr, rr PhenotypeRound & wrinkled Phenotype: Round & wrinkled G.Ratio:1:2:1 G.Ratio: 1:2:1 P.Ratio: 3:1

What Do the Peas Look Like?

Mendel’s Laws

Results of Monohybrid Crosses GENES are responsible for all heritable characteristicsGENES are responsible for all heritable characteristics Phenotype is based on GenotypePhenotype is based on Genotype Each trait is based on two alleles, one from the mother and the other from the fatherEach 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)True-breeding individuals are homozygous ( both alleles are the same) Heterozygous individuals have one of each alleleHeterozygous 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 Segregation During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other.During 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 offspringAlleles 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.Alleles for different traits are distributed to sex cells (& offspring) independently of one another. This law can be illustrated using dihybrid crosses.This law can be illustrated using dihybrid crosses. –A breeding experiment that tracks the inheritance of two traits.

Dihybrid Cross Traits: Seed shape & Seed color Alleles: Alleles: R round r wrinkled Y yellow y green RrYy x RrYy RY Ry rY ry All possible gamete combinations

Dihybrid Cross RYRyrYry RYRy rY ry

RRYY RRYy RrYY RrYy RRYy RRyy RrYy Rryy RrYY RrYy rrYY rrYy RrYy Rryy rrYy rryy Round/Yellow: 9 Round/green: 3 wrinkled/Yellow: 3 wrinkled/green: 1 9:3:3:1 phenotypic ratio RYRyrYryRY Ry rY ry

Dihybrid Cross Round/Yellow: 9 Round/green: 3 wrinkled/Yellow: 3 wrinkled/green: 1 9:3:3:1

Summary of Mendel’s laws LAW PARENT CROSS OFFSPRING DOMINANCE TT x tt tall x short 100% Tt (tall) SEGREGATION Tt x Tt tall x tall 75% tall 25% short INDEPENDENT ASSORTMENT RrGg x RrGg round & green x round & green 9/16 round & yellow 3/16 round & green 3/16 wrinkled & yellow 1/16 wrinkled & green

Test Cross A mating between an individual of unknown genotype and a homozygous recessive individual.A mating between an individual of unknown genotype and a homozygous recessive individual. Example: bbC__ x bbccExample: bbC__ x bbcc BB = brown eyesBB = brown eyes Bb = brown eyesBb = brown eyes bb = blue eyesbb = blue eyes CC = curly hairCC = curly hair Cc = curly hairCc = curly hair cc = straight haircc = straight hairbCb___bc

Test Cross Possible results:Possible results:bCb___bcbbCc CbCb___bc bbccor c

Incomplete Dominance and Codominance

Incomplete Dominance F1 hybrids in betweenphenotypesF1 hybrids have an appearance somewhat in between the phenotypes of the two parental varieties. Example:snapdragons (flower)Example: snapdragons (flower) red (RR) x white (rr) RR = red flower rr = white flower r r RR

Incomplete Dominance RrRrRrRr r rRR All Rr = pink (heterozygous pink) F 1 generation

Incomplete Dominance

Codominance BOTH alleles are expressed (multiple alleles) in heterozygous individuals.BOTH alleles are expressed (multiple alleles) in heterozygous individuals. Example: blood typeExample: blood type 1.type A= I A I A or I A i 2.type B= I B I B or I B i 3.type AB= I A I B 4.type O= ii

Codominance Problem Example:homozygous male Type B (I B I B ) x heterozygous female Type A (I A i) IAIBIAIB IAIBIAIB IBiIBiIBiIBi 1/2 = I A I B 1/2 = I B i IAIA IBIB IBIB i

Another Codominance Problem Example: Example: male Type O (ii) x female type AB (I A I B ) IAiIAiIBiIBi IAiIAiIBiIBi 1/2 = I A i 1/2 = I B i i IAIA IBIB i

Codominance Question: If a boy has a blood type O and his sister has blood type AB, what are the genotypes and phenotypes of their parents?Question: 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 (I A I B )

Codominance Answer:Answer: IAIBIAIB ii Parents: genotypes genotypes = I A i and I B i phenotypes phenotypes = A and B IBIB IAIA i i

Sex-linked Traits Traits (genes) located on the sex chromosomesTraits (genes) located on the sex chromosomes Sex chromosomes are X and YSex chromosomes are X and Y XX genotype for femalesXX genotype for females XY genotype for malesXY genotype for males Many sex-linked traits carried on X chromosomeMany sex-linked traits carried on X chromosome

Sex-linked Traits Sex Chromosomes XX chromosome - femaleXy chromosome - male fruit fly eye color Example: Eye color in fruit flies

Sex-linked Trait Problem Example: Eye color in fruit flies (red-eyed male) x (white-eyed female) X R Y x X r X r Remember: the Y chromosome in males does not carry traits. RR = red eyed Rr = red eyed rr = white eyed Xy = male XX = female XRXR XRXR y XrXr

Sex-linked Trait Solution: X R X r X r y 50% red eyed female 50% white eyed male XrXr XRXR y XrXr

Female Carriers