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GENETIC TRAITS…WHICH DO YOU HAVE? Widows peak vs. straight hairline Free earlobes vs. attached earlobes Dimples vs. no dimples Cleft chin vs. no cleft chin Straight thumb vs. curved thumb Mid-digit hair vs. no mid-digit hair Longer 2nd toe vs. shorter 2nd toe (compared to big toe) Can roll tongue vs. can’t roll tongue CHAPTER 11 Introduction to Genetics 11-1 THE WORK OF GREGOR MENDEL Heredity: transmitting genetic information from parent to offspring Genetics: the study of heredity Gene: a piece of DNA that codes for a trait Alleles: a different form of a gene Trait: specific characteristic that varies between individuals Gametes: specialized cells involved in sexual reproduction (eggs and sperm) MENDEL AND HIS PEAS A priest with a garden… Studied pea plants Male part = stamen Female part = pistil Fertilization: joining of male and female parts Form a new cell (seed) PEA PLANTS = TRUE BREEDING True-breeding: pass on ALL genetic traits to offspring Self-pollinating- use male and female parts from same plant Breed offspring identical to parent CROSS-BREEDING Using male and female parts from different plants Mendel attempted this with pea plants Aka: cross pollination SO WHAT DID MENDEL DO? Identified 7 traits each with 2 contrasting characteristics Crossed those characteristics Resulted in hybrid plants P generation (parents) F1 generation (offspring) WHAT DID HE FIND? Offspring only showed the traits of one parent 2 conclusions: 1. Inheritance is determined by factors passed from parent to offspring (genes and alleles) 2. Principle of dominance some alleles are dominant others recessive WHERE DID THE RECESSIVE TRAIT GO? Next, Mendel allowed self pollination to occur again Results of F2 generation (offspring’s offspring)… ¾ of the plants = showed dominant trait ¼ of plants = showed recessive trait The recessive trait was still there…just “hiding” SEGREGATION Separation of alleles during gamete formation Occurs so gametes only carry one type of allele Why we saw only one type of the trait Mom II I gametes Dad II I 11-2 PROBABILITY AND PUNNETT SQUARES Mendel repeated experiments, got same results Probability: likelihood that an event will occur Ex: coin flip (1 in 2 odds) Each turn is independent of any previous or future turns Principles of probability can be used to predict outcomes of genetic crosses PUNNETT SQUARES Used to predict/compare the genetic variations that will result from a cross Alleles: capital letter = dominant lowercase letter = recessive PUNNETT SQUARES (Tt) Parent’s alleles The boxes in the square contains every possible combination of alleles (Tt) PUNNETT SQUARES TT or tt = homozygous (2 identical alleles) Tt = heterozygous (2 different alleles) TT or Tt = dominant allele will show tt = recessive allele will show ¾ = dominant allele shown ¼ = recessive allele shown *3:1 ratio for dominant trait GENOTYPE VS. PHENOTYPE Genotype: genetic makeup What the alleles are (TT, Tt or tt) Phenotype: physical characteristics What you see (tall or short) TT or Tt tt PROBABILITY AND PREDICTIONS REMEMBER: probability only predicts averages NOT precise outcomes Larger numbers of trials means outcomes closer to the probability PUNNETT SQUARE EXAMPLE QUESTIONS In seals, whisker length is determined by two alleles. “W” codes for long whiskers and is dominant over “w” which codes for short whiskers. Draw a Punnett square to show a cross between a homozygous long whiskered dad and a heterozygous long whiskered mom. What percentage of their offspring would have long whiskers? Short whiskers? What percentage of their offspring would be homozygous? What percentage would be heterozygous? PUNNETT SQUARE EXAMPLE ANSWERS What percentage of their offspring would have long whiskers? 100% What percentage would have short whiskers? 0% What percentage of their offspring would be homozygous? 50% What percentage would be heterozygous? 50% 11-3 EXPLORING MENDELIAN GENETICS Mendel’s next question: do alleles separate independently? Tried crossing 2 traits at once Found that round (R) peas and yellow (Y) peas were dominant traits Would they stay together or separate? LAW OF INDEPENDENT ASSORTMENT Genes for different traits will segregate independently of each other during gamete formation Accounts for all genetic variation! SUMMARY OF MENDEL’S PRINCIPLES Inheritance of traits is determined by genes (alleles) passed from parent to offspring When 2 or more alleles exist, some will be dominant and some will be recessive In sexually reproducing organisms, each adult has 2 copies of each gene which separate when gametes are formed Alleles for different traits usually separate independently EXCEPTIONS TO THE RULES Genetics is very complicated Some alleles are not simply dominant/recessive Some traits controlled by multiple genes INCOMPLETE DOMINANCE One allele is not completely dominant over another A “mixing” occurs Ex: Mirabilis plant- crossing red flowers with white flowers gives you pink flowers X CODOMINANCE 2 dominant alleles (both will contribute to the phenotype) Both alleles are present and can be “seen” Ex: erminette chicken- has both black and white feathers MULTIPLE ALLELES 3 or more alleles available Gives more options for the combination of alleles Ex: blood types POLYGENIC TRAITS Traits controlled by 2 or more genes Ex: skin color LOTS of variation APPLYING MENDEL’S PRINCIPLES Thomas Hunt Morgan Used fruit flies All principles held true GENETICS AND THE ENVIRONMENT Genes act as the “blue print” plan for living things Environment will influence how the plan works Ex: sunflower Height and color determined by genes Influenced by water, light, temperature, soil, etc.) 11-4 MEIOSIS Mendel’s principles require 2 things: 1. Organisms inherit single copy of genes from each parent 2. Therefore, when gametes are formed, those copies must separate But…how? CHROMOSOMES Remember: genes are found on chromosomes Half come from mom and half come from dad Each chromosome has a “corresponding chromosome” (1 from mom and 1 from dad) Known as homologous chromosomes CHROMOSOMES Diploid- cells that contain both sets of homologous chromosomes Haploid- cells that contain only a single set of homologous chromosomes Gametes! MEIOSIS Meiosis: the process where chromosome # is cut in half by separating homologous chromosomes in diploid cells (makes gametes) Turns 1 diploid cell into 4 haploid cells Involves meiosis I and meiosis II MEIOSIS I Must go thru interphase first to replicate chromosomes Prophase I- chromosomes pair with homologous chromosomes (these are called tetrads) Crossing over- exchange parts of chromatids Very important for genetic variation Get new allele combinations MEIOSIS 1 (CONTINUED) Metaphase I- line up across middle, attach to spindle Anaphase I- spindle pulls homologous chromosomes apart Telophase I- two new nuclear membranes form Cytokinesis- two new diploid cells are formed IMPORTANT! After Meiosis 1, the 2 new diploid cells have “shuffled” sets of chromosomes From “crossing over” Different from each other Different from original cell Genetic variation MEIOSIS II Prophase II- sister chromatids condense Metaphase II- sister chromatids line up Anaphase II- sister chromatids separate Telophase II and cytokinesis = 4 new haploid cells GAMETE FORMATION Males = sperm Females = eggs usually, only 1 of the 4 produced are used in reproduction (actually become an egg) MITOSIS VS. MEIOSIS Mitosis End with 2 genetically identical diploid cells Role: growth and replacement of cells Asexual reproduction Meiosis End with 4 genetically different haploid cells Role: make gametes 11-5 LINKAGE AND GENE MAPS Linkage: if genes are close to each other on a chromosome, they may be inherited “linked” together chromosomes follow the Law of Independent Assortment…NOT individual genes GENE MAPS Gene maps: shows the relative locations of genes on a chromosome How can we tell this? The closer together genes are, the more likely they are to be “linked” Human Genome Project ANNEMARIE PARISI… Will you go to prom with me? -EJ