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Probability & Genetics What are the chances of flipping a head? tail? 3 heads in a row? probability • What is probability? – The chance an event occurs • Coin flipping – Chances of heads? Tails? – Two coins • Chances of two heads? Two tails? Heads & tails? • Rule of Multiplication – “And” Rule • Past events do not affect future probabilities • INDEPENDENT! 11-2 Probability and Punnett Squares • Punnett squares can be used to show the possible outcomes for a trait according to the traits of the parents • Also show the probability of the outcome E e Ee x Ee • Squares represent possible offspring • Each offspring gets one gene from “dad” (from the top) and one gene from “Mom” (from the side) • What % of the offspring will have free earlobes? • What % will have attached ears? • What % will be heterozygous for the trait? E E e e EE Ee Ee ee Fast Patterns to Know… Practicing with Punnett Squares Together, then assigned one to share! Monohybrid Cross TT x tt Tt x TT TT x TT tt x tt Tt x Tt tT x tT Genotype Ratio Phenotype Ratio Dom.Hom : Het : DomRec Dom: Rec • So what about your “kids”? • Complete yesterday’s analysis. • What are the chances your “kid” will have a widow’s peak? Curly blond hair? Attached earlobes? What are the chances? • Do people with brown hair always have brown eyes? • Do people with brown hair have a higher chance of having brown eyes? • Is a round pea seed always yellow? Can a short plant have purple flowers? INDEPENDENT ASSORTMENT • The inheritance of one gene doesn’t influence inheritance of another. • Mendel named this: The Law of Independent Assortment • In meiosis, the chromosomes line up randomly on the equator to be separated. • If your parents are heterozygous for any traits, this leads to lots of possibilities! How did Mendel figure this out? • • • • Through experiments… He crossed two different plants Each was true-breeding for 2 different traits P gen: rryy (wrinkled & green) x RRYY (ROUND & YELLOW) • F1 gen: RrYy • F2: a bunch of possibilities! Aligning the Punnett Square • F1 gen now has: RrYy • Distribute the 1st letter of the 1st set to each of the 2 letters in the 2nd set. • Distribute the 2nd letter of the 1st set to each of the 2 letters in the 2nd set RrYy RY, Ry, rY, ry Patterns A Het x Het dihybrid cross yields the typical genotypic ratio of: 9: 3: 3: 1 Now You Try! In humans, the gene that causes a unibrow (u) is recessive to not connected eyebrows (U); the gene for thick lips (T) is dominant over the gene for thin lips (t). If a male that is homozygous for normal eyebrows and heterozygous for thick lips mates with a woman who is unibrowed and heterozygous for thick lips, what is the genotypic and phenotypic ratio of the offspring? How do you set it up? • • • • • • • Dad’s phenotype: Mom’s phenotype UUTt uuTt Distribution of alleles UT – Ut – UT – Ut uT – ut – uT – ut Genotype Ratio (should add up to 16) UuTT : UuTt : Uutt = 4 : 8 : 4 Phenotypic Ratio = 12 : 4 : 0 : 0 Summing It Up: Mendel’s Principles 1. Parents pass on characteristics, sexually, through genes to their offspring 2. When there are multiple alleles (appearances) for one gene, some are dominant & some are recessive 3. During formation of parental gametes, alleles are segregated into separate gametes. Each parent is then able to pass ONE allele to the child. The child therefore gets ONE allele from EACH parent 4. The chromosomes (and therefore alleles) from each parent arrange themselves independently during meiosis 11-3 Exploring Mendelian Genetics Going Beyond Simple Dominance: 1. 2. 3. 4. 5. 6. Independent Assortment Incomplete Dominance Codominance Multiple Alleles Polygenic Traits X- linked (or sex linked) Traits • In some cases, neither allele truly dominates over the other. • No allele is really dom. or rec. • The heterozygous genotype shows a MIX of the two traits. • Example- Four O’Clocks – R- gene for red flowers, W- gene for white flowers: – RR- red, WW- white, RW- pink Codominance • In some cases, both alleles are dominant. – No allele is really recessive. • The heterozygous genotype shows BOTH of the two traits. • Example- Chicken feathers – B- gene for black feathers, W- gene for white feathers: – BB- black, WW- white, BW- “erminette” Black and White! Multiple Alleles • Many genes have more than just two alleles for a trait – in the population. – Remember, you can still only have 2 alleles at a time, in a single organism – It is still just ONE gene, but lots of possibilities • Example: Alleles for rabbit fur – C-full color, dominates over Cch, Ch, c – Cch – chinchilla, dominates over Ch and c – Ch- himalayan, dominates over c – c- albino, recessive to all Practice Cross C chCh x cc • What are the phenotypes of the parents? • Fill in the Punnett Square. • What is the probability that an offspring will be albino? • What is the probability that an offspring will be himalayan? Polygenic Traits • Many traits result from the interaction of several genes. – Multiple genes, perhaps on different chromosomes even, produce one phenotype • Polygenic traits can produce a large range of phenotypes • Examples: human skin color (at least 4 genes), human eye color, human height Capital letters = dark; small letters = light; more dark alleles = darker!! Gradation of Human Skin Color Height in Humans • Range of phenotypes resulting from polygenic trait Human Blood Types • Multiple alleles, Polygenic and Codominance! • Multiple alleles- A, B, O • Polygenic- one gene controls type, another gene controls rH factor (+, -) • Codominance- A and B are codominant but both dominate over O What does your blood type actually mean? Linkage • Really it is the chromosomes that are segregated independently, not necessarily individual genes. • Some genes are LINKED if they are on the same chromosome – Ex: you get all of the genes on chromosome 1 from your mom if you get her chr.1 • Is that always the case though? What do you know might happen? Crossing Over • Depending on how FAR APART genes are on chromosomes, they may be switched during meiosis –Prophase I –Must be homologous Actual location of genes on Gene chromosomes. Discovered by a student working in molecular lab at Columbia in 1931. By looking at statistical inheritance patterns, you can calculate the “recombination frequency” of alleles. If genes are far apart, crossing over is more likely Maps Sex-linked Traits • Often called X- linked traits • Trait can be dominant or recessive • Probability of inheritance is altered because the trait is on the X chromosome • Females- XX Males- XY Examples of X-linked traits • Colorblindness • Hemophilia • Duchenne Muscular Dystrophy