MENDEL AND THE GENE IDEA

Gregor Mendel Austrian monk Brought experimental and quantitative approach to genetics Bred pea plants to study inhertance Why peas? Control mating (self- vs. cross-pollination) Many varieties available Short generation time

P (parental) generation = true breeding plants F1 (first filial) generation = offspring F2 (second filial) generation = F1 offspring

7 characters in pea plants (observed 3:1 ratios) Dominant vs. Recessive (expressed) or (hidden)

Mendel’s Model Tried to explain the 3:1 inheritance pattern he saw among the F2 offspring using 4 basic concepts

First- alternative versions of genes accounts for variations in inherited traits Alleles: alternate versions of a gene

Second- an organism inherits two alleles, one from each parent Second- an organism inherits two alleles, one from each parent. A genetic locus is represented twice

dominant (P), recessive (p) THIRD-IF two alleles at a locus differ then the dominant allele determines the organisms appearance dominant (P), recessive (p) homozygous = 2 same alleles (PP or pp)l; fully epxressed heterozygous = 2 different alleles (Pp); no noticeable effect; gene make a non-functional protein

Fourth-Law of Segregation The two alleles for a heritable character separate (segregate) during gamete formation and end up in different games

MENDEL’S PRINCIPLES Alternate version of genes (alleles) cause variations in inherited characteristics among offspring. For each character, every organism inherits one allele from each parent. If 2 alleles are different, the dominant allele will be fully expressed; the recessive allele will have no noticeable effect on offspring’s appearance. Law of Segregation: the 2 alleles for each character separate during gamete formation.

Law of Independent Assortment: Each pair of alleles segregates (separates) independently during gamete formation Eg. color is separate from shape

Testcross: determine if dominant trait is homozygous or heterozygous by crossing with recessive (pp)

Phenotype: expressed physical traits Genotype: genetic make-up

Punnett Square Device for predicting offspring from a cross Example: Bb x Bb (B=purple, b=white) Genotypic Ratio: 1 BB, 2 Bb, 1 bb Phenotypic Ratio: 75% or 3/4 purple and 25 % or ¼ white

Monohybrid cross: study 1 character eg. flower color Dihybrid cross: study 2 characters eg. flower color & seed shape

Dihybrid Cross (video) Example: AaBb x AaBb F: Take the FIRST allele of each gene (left eyebrow) to make one possible gamete. O: Take the OUTER alleles (big smile) to make one possible gamete. I: Take the INNER alleles (nose curve) to make one possible gamete. L: Take the LAST allele of each gene (right eyebrow) to make one possible gamete.

Segregation of alleles and fertilization as chance events Probability Rules: AND= multiply ½ x ½ = ¼ OR=add ¼ + ¼= 2/4 =1/2