A pair of homologouschromosomesA gene locusA pair of allelesThree pairs of genes
Some terms in genetics• Some traits mask others – purple & white flower colors are separate traits that do not blend I’ll speak for • purple x white ≠ light purple both of us! • purple masked white – dominant allele Dominant allele leads to production of mutant allele leads to no pigmen – masks other alleles purple pigment – recessive allele – shows effect when dominant homologous allele is absent chromosomes
Some terms in genetics• Homozygous = same alleles on the homologous chromosomes= PP, pp• Heterozygous = different alleles on the homologous chromosomes = Pp homozygous dominant heterozygous homozygous recessive
Some terms in genetics• Phenotype = The characteristics of an organisms• Genotype = The alleles of an organism Discussion: What is the relationship between phenotype and genotype?
Phenotype vs. genotype• 2 organisms can have the same phenotype but have different genotypes purple PP homozygous dominant purple Pp heterozygous
Some terms in genetics• P - Parent generation• ♀- female, ♂- male• F1 - First-generation offspring• F2 - Second-generation offspring P F1 F2 F3
Some terms in genetics• Self-pollination• The anther opens and the pollen lands on the stigma of the same flower.• Cross pollination• Anthers open on one flower and a vector (insects, wind, or animals) moves pollen to the stigma of another flower. Discussion: why a lot of plants develop strategies to prevent self-pollination?
Some terms in genetics• True breeding plant (纯种) – If self-pollinated, the offspring will always be same as parents P F1 F2 F3
Some terms in genetics• Hybrid (杂种) – The offspring of a cross between parents with different traits father hybrid mother
Some terms in genetics• Gamete (配子) = sex cells – Sperm and egg
Gregor Mendel• Modern genetics began in the mid- 1800s in an abbey garden, where a monk named Gregor Mendel documented inheritance in peas – used experimental method – used quantitative analysis • collected data & counted them – excellent example of scientific method
Mendel’s work Pollen transferred from white flower to stigma of purple flower• Bred pea plants P – cross-pollinate true breeding parents (P) – raised seed & then anthers removed observed traits (F1) all purple flowers result – allowed offspring F1 to self-pollinate & observed next generation (F2) self-pollinate F2
Looking closer at Mendel’s work true-breeding true-breeding XP purple-flower peas white-flower peas 100%F1 purple-flower peasgeneration 100%(hybrids) self-pollinate 75% 25% purple-flower peas white-flower peas 3:1F2generation
Traits are inherited as discrete units• For each characteristic, an organism inherits 2 alleles, 1 from each parent – diploid • inherits 2 sets of chromosomes (2n), 1 from each parent • haploid 1 set of chromosomes (n) What are the advantages of being diploid?
Making crosses• Can represent alleles as letters – flower color alleles P or p – true-breeding purple-flower peas PP – true-breeding white-flower peas pp P X ___ x ___ purple white F1 all purple ___
Aaaaah, Punnett squares phenotype & genotype can have different Pp x Pp ratiosF1generation(hybrids) % % genotype phenotype male / sperm PP ___ Pp ___ ___ female / eggs Pp pp ___ ___ ____ ___
Test cross• Breed the dominant phenotype — the unknown genotype — with a homozygous recessive (pp) to determine the identity of the unknown allele x How does that work? is it pp PP or Pp?
How does a Test cross work? x xPP pp Pp pp 100% purple 50% purple:50% white or 1:1
Mendel’s 1st law of heredity P• Law of segregation PP – during production of gametes(meiosis), P alleles segregate • homologous chromosomes separate p – each allele for a trait is packaged into a separate gamete pp p P Pp p
Law of Segregation • Which stage of meiosis creates the law of segregation? Metaphase 1Whoa!And Mendeldidn’t even knowDNA or genesexisted!
Monohybrid cross• Some of Mendel’s experiments followed the inheritance of single characters – flower color – seed color – monohybrid crosses
Dihybrid cross• Other of Mendel’s experiments followed the inheritance of 2 different characters – seed color and seed shape – dihybrid crosses Mendel was working out many of the genetic rules!
Dihybrid cross true-breeding true-breedingP yellow, round peas x green, wrinkled peas Y = yellow _____ _____ y = green R = round r = wrinkled yellow, round peasF1generation 100%(hybrids) _____self-pollinate 9:3:3:1F2generation 9/16 3/16 3/16 1/16 yellow green yellow green round round wrinkled wrinkled peas peas peas peas
What’s going on here?• The genes for different characters are on same or different chromosomes? – how do they assort in the gametes? – together or independently? On same chromosome YyRr On different chromosomes YyRr YR yr YR Yr yR yr Which system explains the data?
YyRr YyRr Dihybrid cross or YR yr YR Yr yR yrYyRr x YyRr 9/16 yellow round YR yr 3/16 green roundYR 3/16 yellow wrinkled yr 1/16 green wrinkled
YyRr YyRr Dihybrid cross or YR yr YR Yr yR yr YyRr x YyRr 9/16 yellow round YR Yr yR yr 3/16YR green roundYr 3/16 yellow wrinkledyR 1/16yr green wrinkled
Mendel’s 2nd law of heredity • Law of independent assortment • non-homologous chromosomes align and assort independently during meiosis yellow • classes of gametes produced in equal amounts green – YR = Yr = yR = yr round wrinkled YyRr Yr yR yR YR YR yr yr 1 : 1 : 1 : 1Yr
Law of Independent Assortment Which stage of meiosis creates the law of independent assortment?
The chromosomalbasis of Mendel’slaws…Trace the geneticevents throughmeiosis, gameteformation &fertilization tooffspring
Review: Mendel’s laws of heredity• Law of segregation – monohybrid cross • single trait – each allele segregates into separate gametes • established by Meiosis 1• Law of independent assortment – dihybrid (or more) cross • 2 or more traits – genes on separate chromosomes assort into gametes independently • established by Meiosis 1 metaphase1
Mendel chose peas wisely• Pea plants are good for genetic research – available in many varieties with distinct heritable features with different variations • flower color, seed color, seed shape, etc. – Mendel had strict control over which plants mated with which • each pea plant has male & female structures • pea plants can self-fertilize • Mendel could also cross-pollinate plants: moving pollen from one plant to another
Mendel chose peas luckily• Pea plants are good for genetic research – relatively simple genetically • most characters are controlled by a single gene with each gene having only 2 alleles, – one completely dominant over the other