Genetics & The Work of Mendel

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

Gregor Mendel Modern genetics began in the mid-1800s in an abbey garden, where a monk named Gregor Mendel documented inheritance in peas – used good experimental design – used mathematical analysis collected data & counted them – excellent example of scientific method

Mendel’s work Bred pea plants – cross-pollinate true breeding parents – raised seed & then observed traits – allowed offspring to self-pollinate & observed next generation

Mendel collected data for 7 pea traits

Looking closer at Mendel’s work true-breeding purple-flower peas true-breeding white-flower peas Parents 100% 1 st generation (hybrids) 100% purple-flower peas 2 nd generation 3:1 75% purple-flower peas 25% white-flower peas X self-pollinate

What did Mendel’s findings mean? Some traits mask others – purple & white flower colors are separate traits that do not blend purple x white ≠ light purple purple masked white – dominant allele functional protein – affects characteristic masks other alleles – recessive allele no noticeable effect allele makes a non-functioning protein allele producing functional protein mutant allele malfunctioning protein homologous chromosomes I’ll speak for both of us!

Genotype vs. phenotype Difference between how an organism “looks” & its genetics – phenotype description of an organism’s trait – genotype description of an organism’s genetic makeup Explain Mendel’s results using …dominant & recessive …phenotype & genotype F1F1 P X purplewhite all purple

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 F1F1 P X purplewhite all purple PP x pp PpPp

Punnett squares Pp x Pp Pp male / sperm P p female / eggs PP 75% 25% 3:1 25% 50% 25% 1:2:1 % genotype % phenotype PPPpPp PpPppp PpPp PpPp 1 st generation (hybrids)

Mendel’s Laws **Inheritable factors or genes are responsible for all heritable characteristics **Phenotype is based on Genotype **Each trait is based on two genes, one from the mother and the other from the father **True-breeding individuals are homozygous ( both alleles) are the same

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 )

Law of Segregation *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 offspring.

Law of Independent Assortment *Alleles for different traits are distributed to sex cells (& offspring) independently of one another. *This law can be illustrated using dihybrid crosses. Dihybrid Cross A breeding experiment that tracks the inheritance of two traits. Mendel’s “Law of Independent Assortment” a. Each pair of alleles segregates independently during gamete formation b. Formula: 2n (n = # of heterozygotes)

Summary of Mendel’s laws LAWPARENT CROSS OFFSPRING DOMINANCETT x tt Tall x Short 100% T t TALL SEGREGATIONTt x Tt tall x tall 75% tall 25% short INDEPENDENT ASSORTMENT RrGg x RrGg round & green x round & green 9/16 round seeds & green pods 3/16 round seeds & yellow pods 3/16 wrinkled seeds & green pods 1/16 wrinkled seeds & yellow pods