Genetics & The Work of Mendel
Gregor Mendel Modern genetics began in the mid-1800s in an abbey garden, where a monk named Gregor Mendel documented inheritance in peas u used experimental method u used quantitative analysis collected data & counted them u excellent example of scientific method
Mendel s work Bred pea plants u cross-pollinate true breeding parents (P) P = parental u raised seed & then observed traits (F 1 ) F = filial (kids) u allowed offspring to self-pollinate & observed next generation (F 2- grandkids ) Pollen transferred from white flower to stigma of purple flower P all purple flowers result F 1 F 2 self-pollinate anthers removed
P Looking closer at Mendel s work true-breeding purple-flower peas X true-breeding white-flower peas F 1 generation (hybrids) 100% purple-flower peas 100% F 2 generation 75% purple-flower peas self-pollinate 25% white-flower peas 3:1
What did Mendel s findings mean? Traits come in alternative forms = alleles u purple vs. white flower color u different alleles vary in the sequence of nucleotides at the specific locus of a gene w some difference in sequence of A, T, C, G purple-flower allele & white-flower allele are two DNA variations at flower-color locus different versions of gene at same location on homologous chromosomes
What did Mendel s findings mean? Some traits mask others u purple & white flower colors are separate traits that do not blend purple x white light purple purple masked white u dominant allele functional protein masks other alleles u recessive allele wild type allele producing functional protein mutant allele producing malfunctioning protein allele makes a malfunctioning protein homologous chromosomes
Genotypes Homozygous = same alleles = PP, pp Heterozygous = different alleles = Pp homozygous dominant heterozygous homozygous recessive
Genotype vs. phenotype Difference between how an organism looks & its genetics u phenotype description of an organism s trait the physical u genotype description of an organism s genetic makeup P X Explain Mendel s results using dominant & recessive phenotype & genotype F 1 purple all purple white
Mendel s 1 st law of heredity Law of segregation PP u during meiosis, alleles segregate homologous chromosomes separate u each allele for a trait is packaged into a separate gamete pp P P p p Pp P p
Law of Segregation Which stage of meiosis creates the law of segregation? Anaphase 1
Monohybrid cross Some of Mendel s experiments followed the inheritance of single characters u flower color u seed color u monohybrid crosses
Making crosses Can represent alleles as letters u flower color alleles P or p u true-breeding purple-flower peas PP u true-breeding white-flower peas pp P X PP x pp purple white F 1 all purple Pp
Punnett squares Pp x Pp F 1 generation (hybrids) female / eggs P p male / sperm P PP p Pp PP Pp Pp Pp pp pp % genotype 25% 50% 25% % phenotype 75% 25% 1:2:1 3:1
Phenotype vs. genotype 2 organisms can have the same phenotype but have different genotypes purple PP homozygous dominant purple Pp heterozygous How do you determine the genotype of an individual with with a dominant phenotype?
Test cross Breed the dominant phenotype the unknown genotype with a homozygous recessive (pp) to determine the identity of the unknown allele x is it PP or Pp? pp
How does a Test cross work? x x PP pp Pp pp p p p p P Pp Pp P Pp Pp P Pp Pp p pp pp 100% purple 50% purple:50% white or 1:1
Dihybrid cross Other of Mendel s experiments followed the inheritance of 2 different characters u seed color and seed shape u dihybrid crosses
Dihybrid cross P true-breeding yellow, round peas Y = yellow R = round YYRR x yyrr true-breeding green, wrinkled peas y = green r = wrinkled F 1 generation (hybrids) yellow, round peas YyRr 100% F 2 generation 9/16 yellow round peas 3/16 green round peas self-pollinate 3/16 yellow wrinkled peas 1/16 green wrinkled peas 9:3:3:1
What s going on here? If genes are on different chromosomes u how do they assort in the gametes? u together or independently? YyRr Is it this? Or this? YyRr YR yr YR Yr yr yr
Is this the way it works? YyRr x YyRr YR YR YYRR yr YyRr NO! yr YyRr yyrr YyRr YR yr or YR Yr YyRr yr yr 9/16 yellow round 3/16 green round 3/16 yellow wrinkled 1/16 green wrinkled
Dihybrid cross YyRr x YyRr YES! YyRr YR yr YR Yr yr yr or YR Yr YyRr yr yr 9/16 yellow round YR Yr yr yr YYRR YYRr YyRR YyRr YYRr YYrr YyRr Yyrr YyRR YyRr yyrr yyrr YyRr Yyrr yyrr yyrr 3/16 green round 3/16 yellow wrinkled 1/16 green wrinkled
Mendel s 2 nd law of heredity yellow green round wrinkled Law of independent assortment u different loci (genes) separate into gametes independently non-homologous chromosomes align independently classes of gametes produced in equal amounts w YR = Yr = yr = yr only true for genes on separate chromosomes or on same chromosome but so far apart that crossing over happens frequently YyRr Yr Yr yr yr YR YR yr yr 1 : 1 : 1 : 1
Law of Independent Assortment Which stage of meiosis creates the law of independent assortment? Metaphase 1 EXCEPTION! If genes are on same chromosome & close together will usually be inherited together rarely crossover separately linked
The chromosomal basis of Mendel s laws Trace the genetic events through meiosis, gamete formation & fertilization to offspring
Review: Mendel s laws of heredity Law of segregation u monohybrid cross single trait u each allele segregates into separate gametes established by Metaphase 1 Law of independent assortment u dihybrid (or more) cross 2 or more traits u genes on separate chromosomes assort into gametes independently established by Metaphase 1 EXCEPTION! linked genes metaphase1