Genetics Part I

1. Genetics Part I Genetics – The study of heredity

2. What is a Chromosome? • Chromosome – Made from DNA and Protein. Found in the nucleus of cells! • DNA is wrapped around proteins called histones • Chromatin – Dense patches of lengthened chromosomes found in the nucleus • Gene- The “functional unit” of heredity • Genes are a discrete section of DNA that code for a specific protein……that influence a metabolic trait (height, eye color, hormone or enzyme production)

3. Gregor Mendel- The “father of genetics” • Born in 1822 • Published work in 1866 • Lived in an Austrian monastery • Worked with Pea plants! • Mendel used pea plants and observed a variety of traits. • He spent 2 years developing “pure breeds” for each trait BEFORE he began testing!

4. Mendel’s First Crosses • Mendel crossed a “pure breeding tall” plant with a “pure breeding short” plant. • (Pure Tall X Pure Short) • 100% of the offspring were Tall • Why? • He then crossed his F1 generation and 75% of the offspring were tall and 25% short…what happened??

5. Pea Plant Traits(monohybrid results)

6. Mendel’s First Two Principles • Principle of Dominance – One allele (gene) in a pair can prevent the other from being expressed. • Principle of Segregation – The members of each pair of genes (alleles) separate (segregate) during gamete formation. • These two principles explain the outcome of the F1 and F2 generation!! Mendel had NO knowledge of meiosis!!!

7. Mendel’s ideas continued • Today Mendel’s “factors” are called Genes! • Allele – a term used to describe either member of a pair of genes (that determine a trait). • T = tall allele • t = short allele • Three possible Combinations of alleles in offspring • TT • Tt • tt

8. Important Terms!! • If Tall is Dominant • T = dominant allele (gene) • t = recessive allele (gene) • The possible allele combinations in offspring are: • TT = Homozygous Dominant these are • tt = Homozygous Recessive genotype • Tt = Heterozygous (hybrid) terms! • Genotype – The combination of alleles or genetic “makeup” of an organisms. (homo dom, homo rec., hetero) • Phenotype – What an organism “looks” like! (tall or short)

10. Punnett Squares • A method used to help visualize crosses • Named after Reginald Punnett…early 1900’s • Uses probability to predict what is LIKELY to happen….not what will happen. • Monohybrid Cross – A cross involving one trait! • Dihybrid Cross – A cross involving two traits! • Used to explain Mendel’s 3rd principle…..soon to come!

11. Mendel’s Third Principle Principle of Independent Assortment • Two or more pair of alleles will segregate independently of one another during gamete formation. • Mendel wondered if two traits always go together! • Ex: Do all yellow seeded plants also produce round seeds? • He used a Dihybrid cross to test his question.

12. YyRr X YyRr What will the possible gametes be? YR Yr yR yr YR Yr yR yr

13. Dihybrid Cross

14. Test Cross A method used to determine an unknown genotype • Always cross with a homozygous recessive organism. • Refer to problems in genetics G1 activity! G G G g g g g g

15. Incomplete Dominance • When both alleles for a specific trait are expressed • Also called “blending” • Example: Roses (“R” stands for roses) • Rr Rr = Red Rose • RwRw = White Rose • Problem #1 - Cross a Red rose with a White rose Problem #2 – Cross two Pink Roses

16. Multiple Alleles Having three or more alleles that code for a specific trait! • Example: Human Blood Types (3 alleles) • Karl Landsteiner 1930 (Nobel Prize) • A blood = 41% IA (allele) • B blood = 10% IB (allele) • AB blood = 4% • O blood = 45% i (allele) • Any two of the three alleles combine to determine blood type • A is dominant over O and B is dominant over O

17. Blood Types IA IA and IA i = Type A blood IB IBand IB i = Type B blood IA IB= Type AB blood (Codominance – both expressed equally!!!) i i = Type O blood Problem: Cross a mom with Type A blood who carries the O allele with a father with type O blood What does the +/- mean? Which blood type is the universal donor/recipient?

18. The Chromosome Theory of Heredity History • Walter Flemming (1882) – Observed chromosomes during mitosis • Gregor Mendel’s death – 1884 • Augusta Weisman (1892)– Hypothesized that chromosomes are responsible for heredity • Scientists Rediscover Mendel’s work in 1900 and agree that chromosomes carry hereditary information!! • Walter Sutton (1903 )Columbia University – Observes meiosis in grasshoppers. • Wilhelm Johannsen (1909) – Coined the word “gene”

19. The Chromosome Theory of Heredity 1. The egg and sperm carry hereditary information. 2. Hereditary information is found in the nucleus. 3. Chromosomes behave according to Mendel’s Principles • Chromosomes only….all genes do not behave according to his principles*** • Chromosomes independently assort….not genes • Example: Linkage Groups – A group of genes that are inherited together. Crossing over can produce new combinations and break up linkages. (Sometimes crossing over can’t break up linkage groups because they are so close to each other on the same chromosome)

20. Sex Chromosomes Thomas Hunt Morgan (University of Columbia) • Worked with fruit flies: (Drosophila melanogaster) • 10-15 day life span • Only 4 pair of chromosomes • Morgan notice that one pair was different based on gender • Autosomes = Pair #1-22 • Sex Chromosome = Pair #23 • Sex-Linked Trait • A trait determined by alleles carried on the X chromosome • Example: Colorblindness, hemophilia, Duchene's muscular dystrophy. XCXc = Female carrier XcXc= Female colorblind XCXC = Female normal XCY = Male normal XcY= Male colorblind

21. Polygenic Traits • Traits determined by the presence of many genes • Examples • Eye color • Height • Hair color • Intelligence • Foot size • Etc, etc, etc! Video: X Chromosome Inactivation