Ch. 11 Introduction to Genetics

Experiments of Gregor Mendel 11.1, Work of Gregor Mendel Experiments of Gregor Mendel Individual’s looks determined by traits passed to offspring from parents heredity: delivery of characteristics from parent to offspring genetics: scientific study of heredity

11.1, work of Mendel Mendel worked w/ ordinary garden peas peas = “model system” Model system because they have few traits and they grow fast! Mendel did experiments in short time that would take years to do. w/ humans

11.1, work of Mendel Role of Fertilization male part of each flower makes pollen, which contains sperm (male reproductive cells) female part of each flower makes eggs (female reproductive cells) pea flowers normally “self-pollinating” (sperm fertilize eggs from w/in same flower)

11.1, work of Mendel fertilization: in sexual reproduction, egg & sperm join to produce new cell in peas, new cell develops into tiny embryo (the peas in the pod). Because peas self-pollinate, they inherit all its characteristics from its single parent plant

11.1, work of Mendel Mendel had peas that were “true-breeding” (produced offspring w/ identical traits to themselves) trait: specific characteristic of individual (seed color, plant height, etc.) that may vary from 1 individual to another Mendel decided to “cross-pollinate” his stocks (transfer pollen to cause 1 plant to reproduce w/ another plant)

11.1, work of Mendel cross-pollination let Mendel breed plants w/ traits different from their parents & study results hybrids: offspring of crosses between parents w/ different traits Genes & Alleles original organisms studied = P (parental) generation offspring of P = F1 generation

11.1, work of Mendel For all 7 traits Mendel studied, every offspring looked identical to the trait of 1 of their parents (no “blended” traits) Other parent’s trait “disappeared”

11.1, work of Mendel Mendel’s 1st conclusion: an individual’s characteristics are determined by factors passed from 1 generation to next factors = genes alleles: genes for different forms of a given trait In the picture, the Alleles are “R” and “r” F1 recieves 1 allele from each parent.

11.1, work of Mendel Mendel’s 2nd conclusion: principle of dominance some alleles are dominant & others are recessive dominant allele= organism will show that form of trait (represented with : CAPITAL LETTER) recessive allele= organism will exhibit that form only if no dominant allele is present (represented with: lowercase letter)

11.1, work of Mendel Segregation Each parent has 2 alleles, the genes segregate from each other, so each sex cell only carries one allele. WHERE DID THE RECESSIVE GO?!? Solution: Mendel let his F1 hybrids self-pollinate, making F2 hybrids

Results of crossing F1 Generation 11.1, work of Mendel Results of crossing F1 Generation recessive alleles reappeared in F2 THEY CAME BACK! (1/4 had recessive trait) Dominant allele hid recessive allele in F1 Reappearance of recessive trait indicated that, at some point, short allele separated from tall allele.

11.1, work of Mendel Formation of Gametes Assume each F1 plant has inherited 1 tall allele from its tall parent & 1 short allele from its short parent. each F1 plant in Mendel’s cross produced 2 kinds of gametes — ½ w/ tall allele (T) & ½ w/ short allele (t) This could go two ways…. 1. If each 2 gametes with “t” allele paired to produce F2 plant, that plant was short (tt) 2. If either of the two gametes in F2 plant was T, that plant was tall (TT or Tt).

http://www.youtube.com/watch?v=GTiOETaZg4w

11.2, applying Mendel’s principles Probability & Punnett squares Punnett squares use mathematical probability to help predict genotypes & phenotypes in genetic crosses probability: likelihood that particular event will occur past outcomes DO NOT affect future ones

11.2, applying To find probability of multiple events, multiply probabilities of each example: probability of flipping head is 1/2, so probability of 3 heads in a row is: 1/2 × 1/2 × 1/2 = 1/8 How the alleles segregate during gamete formation is just as random as coin flip

11.2, applying How alleles segregate during gamete formation is just as random as coin flip ex: F1 parent w Tt genotype has 1/2 chance of t in gamete, so chance of F2 w/ tt is 1/2 × 1/2 = 1/4, & Mendel had 1/4 short

11.2, applying homozygous: w/ 2 identical alleles for trait (ex.: TT or tt) Homozygous dominant: TT (2 dominant) Homozygous recessive: tt (2 recessive) heterozygous: w/ 2 different alleles for trait (ex.: Tt)

11.2, applying Genotype & Phenotype genotype: genetic makeup for trait phenotype: physical trait (what you see) genotype determines phenotype, but organisms with the same phenotype may have a different genotype ex.: TT or Tt both look tall

Punnett square for 1-factor cross 11.2, applying Punnett square for 1-factor cross write genotypes of parents & possible alleles from each write one parent’s alleles across top & other down side fill boxes count ratios, from most to least dom. Genotype ratio = 1:2:1 Phenotype ratio = 3:1

11.2, applying principle of independent assortment: genes for different traits can segregate independently during formation of gametes monohybrid cross: genes for 1 trait dihybrid cross: 2 different traits trihybrid cross: 3 traits

11.2, applying Mendel used true-breeding parent plants to create F1 that were heterozygous for 2 traits (dihybrid) crossing F1 produces 4 combo phenotypes (G & Y, G & y, g & Y, or g & y)

11.2, applying, cont since seed color & pod color didn’t affect each other, Mendel concluded that 1 trait had no effect on another during gamete formation (independent assortment)

11.3, other patterns of inheritance beyond dominant & recessive alleles some alleles are neither dominant nor recessive ( called incomplete dominance or codominance)

11.3 Other Patterns Many genes exist in several different forms (have multiple alleles) Many traits are produced by the interaction of several genes (polygenic traits)

11.3, other patterns beyond dominant & recessive incomplete dominance: 1 allele is not completely dominant over another; phenotype is intermediate (ex: red & white alleles making pink flowers) codominance: both alleles show up full strength (A & B blood) multiple alleles: more than 2 possible genes (A, B, or O blood)

http://www.youtube.com/watch?v=G_-9_CF02qI

Let’s try it out.. (Type A) (Type AB) IA i IA IAIA IAi (Type A) IB IAIB (Type AB) Ibi (Type B)

11.3, other patterns genes & environment environmental conditions can affect gene expression & influence genetic traits temperature, nutrition, diseases during development. Remember Jurassic Park??

11.4, meiosis chromosome # diploid cells of most adult organisms have 2 sets of inherited chromosomes w/ 2 complete sets of genes chromosomes: strands of DNA & protein inside cell nucleus genes located in specific positions on chromosomes

11.4, meiosis, cont chromosome #, cont diploid cell: any cell w/ 2 sets of chromosomes, 1 from each parent homologous chromosomes: same type; have genes for same traits haploid cell: cell w/ 1 set of chromosomes, generally gametes

11.4, meiosis, cont phases of meiosis 2 divisions, each w/ stages similar to mitosis meiosis I (a.k.a. reduction division) separates homologous chromosomes into 2 haploid cells, still w/ 2 chromatids for each chromosome meiosis II (a.k.a. mitotic division) splits sister chromatids in each cell from meiosis I, forming 4 haploid gametes

11.4, meiosis, cont phases, cont meiosis I prophase I like mitosis, chromosomes, spindle fibers, centrioles form, nuclear envelope breaks down homologous chromosomes pair up, forming tetrads (4 similar chromatids)

11.4, meiosis, cont phases, cont meiosis I, cont prophase I, cont crossing-over: homologous chromosomes “swap” genes chromatids overlap sections crossed sections cut & spliced, exchanging genes produces new combinations of alleles

11.4, meiosis, cont phases, cont meiosis I, cont metaphase I: tetrads line up across cell’s center w/ spindle attached to chromosomes anaphase I: spindle fibers pull each homologous chromosome pair toward opposite ends of cell telophase I & cytokinesis: nuclear membranes form & cytoplasm splits

11.4, meiosis, cont phases, cont meiosis II prophase II: chromosomes & spindles form metaphase II: chromosomes line up & spindles attach to chromatids anaphase II: chromatids separate telophase II & cytokinesis: nuclear membranes form & cytoplasm splits

11.4, meiosis, cont phases, cont gametes to zygotes haploid cells produced by meiosis II are gametes in males, these gametes are called sperm (plant sperm enclosed in pollen) in female animals, usu. only 1 cell from meiosis becomes egg (other 3 form small cells called polar bodies) in female plants, might make 4 eggs or 1 egg & 3 polar bodies

11.4, meiosis, cont phases, cont gametes to zygotes, cont fertilization: fusion of male & female gametes zygote: cell formed by fertilization; forms new organism w/ new combination of genes diff. from parents gene linkage & gene maps alleles of diff. genes tend to be inherited together when those genes are on same chromosome

11.4, meiosis, cont phases, cont gene linkage & maps, cont Thomas Hunt Morgan studied genetic traits & chromosomes in fruit flies, w/ 2 conclusions: each chromosome is actually a group of linked genes chromosomes assort independently, not individual genes gene map: Alfred Sturtevant figured out that crossing-over was more frequent when genes were closer together on chromosome