Introduction to Genetic Variation Or, lame photo montage thinly disguised as illustration of genetic variation

Meiosis Key Haploid (n) Diploid (2n) Haploid gametes (n = 23) Egg (n) Sperm (n) MEIOSIS FERTILIZATION Ovary Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46)

Contributors to Genetic Variation Independent assortment Which chromosome does a gamete get? Crossover events (“recombination”) Chimeric alleles (remember chiasma formation?) Random fertilization Any sperm can fertilize any egg

Independent assortment Whose chromosome did I get in Meiosis I? 50-50 shot at maternal or paternal per gamete Independence of pairs Each homologous pair is sorted independently from the others For humans (n = 23) there are about 8 million possible combinations of chromosomes! Maternal Paternal n=2 chromosomes M1/M2 P1/P2 M1/P2 P1/M2

Separation of Homologs Example: individual who is heterozygous at two genes Allele that contributes to red hued hair Allele that contributes to dark hair Allele that contributes to green eyes Allele that contributes to blue eyes Hair color gene Eye color gene During meiosis I, tetrads can line up two different ways before the homologs separate. OR Green eyes Red hues Blue eyes Dark hair Green eyes Dark hair Blue eyes Red hues

Crossing Over- Genetic Recombination Recombinant chromosomes combine genes from each parent. Prophase I Chromosomes pair up gene by gene Chiasma Homologous portions of two nonsister chromatids traded In Humans two to three times per chromosome pair. New combinations of alleles combinations that did not exist in each parent. Independent assortment builds on this variability

Key Events in Prophase of Meiosis I Centromere Sister chromatids Chromosomes One homolog Synaptonemal complex Second homolog Prophase I 2 pairs of sister chromatids are held tightly together Crossing over can occur at many locations Swapping of segments between maternal and paternal chromosomes. Non-sister chromatids Protein complex

Recombinant chromosomes Fig. 13-12-5 Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Anaphase I Figure 13.12 The results of crossing over during meiosis Anaphase II Daughter cells Recombinant chromosomes

Random Fertilization Any sperm can fuse with any egg. Humans (n=23) Each ovum is one of 8 million possible chromosome combinations Successful sperm is one of 8 million different possibilities Zygote (diploid offspring) is 1 of 70 trillion possible combinations of chrms Amazing how similar siblings/offspring can look! Recombination adds even more variation to this. Independent assortment builds on recombination Mutations- ultimately create a population’s genetic diversity …or not!

Gregor Mendel (1822-1884) Lots of training Monastery garden Augustine monk Beekeeper Physics teacher Meteorologist Monastery garden Pea plants

“Experiments on Plant Hybridization” Published in 1866 Before 20th century, cited 3 times NOT cited in “The Origin of Species” (1859) Rediscovered Hugo de Vries Better publicity

Mendel and the Gene Idea What he knew: Heritable variations exist Traits are transmitted from parents to offspring Two main theories existed Blending (mixing of traits) Particulate inheritance (direct passage of one trait over another) Where he started: documented particulate inheritance with garden peas (Pisum sativum).

Why Peas are Awesome Genetic Models for 1860s Trait Phenotypes Seed shape Seed color Pod shape Pod color Flower color Flower and   pod position Stem length Tall Dwarf Terminal (at tip) Axial (on stem) Purple White Yellow Green Inflated Constricted Round Wrinkled Lots of visible traits (“phenotypes”) flower color, seed shape, pod shape, etc. Controlled mating Hermaphroditic sperm-producing organs (stamens) and egg-producing organs (carpels) Cross-pollination (fertilization between different plants) can be done intentionally

Mendel Focused on Particulate Inheritance True-breeding varieties Offspring of the same variety when they self-pollinate Hybridization mate two contrasting, true-breeding varieties True-breeding parents P generation Hybrid offspring of the P generation are called the F1 generation F1 individuals self-pollinate, the F2 generation is produced

How was this Technically Done? TECHNIQUE 1 Peas normally self-fertilize This is a problem… Cut the stamen Removes male gametes Prevents selfing Manually add pollen Carpels fertilized by non-self plants Forced outcrossing 2 Parental generation (P) Stamens Carpel 3 4 RESULTS First filial gener- ation offspring (F1) 5

Cross-Pollination (“Forced outcrossing”) Control over matings Allows observations and predictions Great approach for genetics at large Self-pollination Female organ (receives pollen) Eggs Male organs (produce pollen grains, which produce male gametes) Cross-pollination CROSS- POLLINATION 3. Transfer pollen to the female organs of the individual whose male organs have been removed. 2. Collect pollen from a different individual. 1. Remove male organs from one individual. SELF-

Particulate Inheritance: Dominant and Recessive Traits Mendel’s outcrossed plants Seed shapes were either round or wrinkled No “chimeric” version NOT 50-50; round seeds were more common Dominant trait Round seeds Recessive trait Wrinkled seeds Writing convention for alleles: R vs. r Capital letter = dominant allele; lowercase = recessive allele Individuals with two copies of the same allele (RR or rr) are homozygous, and those with two different alleles (Rr) are heterozygous. RR or Rr If Dominant gene is present, offspring WILL have the trait without exception always rr