Chater 14. Mendel & Genetics Gregor Mendel Modern genetics began in the mid-1800s in an abbey garden, where a monk named Gregor Mendel documented inheritance in eas used eerimental method used quantitative analysis collected data & counted them ecellent eamle of scientific method Mendel s work Bred ea lants cross-ollinated true breeding arents () raised seed & then observed traits ( ) filial allowed offsring to cross-ollinate & observed net (F 2 ) Mendel collected data for 7 ea traits Looking closer at Mendel s work (hybrids) self-ollinate F 2 X 75% white-flower eas white-flower eas 3:1 What did Mendel s findings mean? Traits come in alternative versions urle vs. white flower color alleles different alleles vary in the sequence of nucleotides at the secific locus of a gene urle-flower allele & white-flower allele are 2 DNA variations at flower-color locus different versions of gene on homologous chromosomes 1
Traits are inherited as discrete units For each characteristic, an organism inherits 2 alleles, 1 from each arent diloid organism inherits 2 sets of chromosomes, 1 from each arent homologous chromosomes like having 2 editions of encycloedia Encycloedia Britannica Encycloedia Americana What are the advantages of being diloid? What did Mendel s findings mean? Some traits mask others urle & white flower colors are searate traits that do not blend urle white light urle urle masked white dominant allele fully eressed recessive allele no noticeable effect the gene makes a non-functional rotein Genotye vs. henotye difference between how an organism looks & its genetics henotye descrition of an organism s trait genotye descrition of an organism s genetic makeu Elain Mendel s results using dominant & recessive henotye & gentotye Making crosses using reresentative letters flower color alleles or white-flower eas Looking closer at Mendel s work (hybrids) self-ollinate 75% F 2??? X white-flower eas white-flower eas? henotye 3:1 unnett squares female / eggs male / serm % genotye 50% 1:2:1 % henotye 75% 3:1 2
Genotyes Homozygous = same alleles =, Heterozygous = different alleles = henotye vs. genotye 2 organisms can have the same henotye but have different genotyes homozygous dominant urle homozygous dominant urle heterozygous homozygous recessive Dominant henotyes It is not ossible to determine the genotye of an organism with a dominant henotye by looking at it.?? So how do you figure out the genotye? Test cross Cross-breed the dominant henotye unknown genotye with a homozygous recessive () to determine the identity of the unknown allele is it or? Test cross 50%:50% 1:1 Mendel s laws of heredity (#1) Law of segregation when gametes are roduced during meiosis, homologous chromosomes searate from each other each allele for a trait is ackaged into a searate gamete 3
Law of Segregation What meiotic event creates the law of segregation? And Mendel didn t even know DNA or genes eisted! Meiosis 1 Monohybrid cross Some of Mendel s eeriments followed the inheritance of single characters flower color seed color monohybrid crosses Dihybrid cross Other of Mendel s eeriments followed the inheritance of 2 different characters seed color and seed shae dihybrid crosses This heled Mendel understand other genetic rules Dihybrid cross yellow, eas Y = yellow YYRR yyrr R = (hybrids) self-ollinate F 2 9/16 yellow eas yellow, eas 3/16 green eas green, wrinkled eas y = green r = wrinkled 3/16 1/16 yellow green wrinkled wrinkled eas eas 9:3:3:1 What s going on here? How are the alleles on different chromosomes handed out? together or searately? Dihybrid cross YR Yr yr yr 9/16 yellow YR yr YR Yr yr yr YR Yr yr YYRR YYRr YyRR YYRr YYrr Yyrr YyRR yyrr yyrr 3/16 green 3/16 yellow wrinkled Which system elains the data? yr Yyrr yyrr yyrr 1/16 green wrinkled 4
Mendel s laws of heredity (#2) Law of indeendent assortment each air of alleles segregates into gametes indeendently 4 classes of gametes are roduced in equal amounts YR, Yr, yr, yr only true for genes on searate chromosomes Can you think of an ecetion to this? Law of Indeendent Assortment What meiotic event creates the law of indeendent assortment? Remember Mendel didn t even know DNA or genes eisted! Meiosis 1 Yr Yr yr yr YR YR yr yr The chromosomal basis of Mendel s laws Trace the genetic events through meiosis, gamete formation & fertilization to offsring Review: Mendel s laws of heredity Law of segregation monohybrid cross single trait each allele segregates into searate gametes established by Meiosis 1 Law of indeendent assortment dihybrid (or more) cross 2 or more traits each air of alleles for genes on searate chromosomes segregates into gametes indeendently established by Meiosis 1 Mendel chose eas wisely ea lants are good for genetic research available in many varieties with distinct heritable features with different variations flower color, seed color, seed shae, etc. Mendel had strict control over which lants mated with which each ea lant has male & female structures ea lants can self-fertilize Mendel could also cross-ollinate lants: moving ollen from one lant to another Mendel chose eas luckily ea lants are good for genetic research relatively simle genetically most characters are controlled by a single gene each gene has only 2 alleles, one of which is comletely dominant over the other 5
Any Questions?? 6