11-2 Probability and Punnett Squares
11-2 Probability and Punnett Squares Genetics and Probability Probability: the likelihood that a particular event will occur The probability of a particular event occurring from a coin toss is ½ or 50% Past outcomes do not affect future ones Principles of probability can be used to predict outcomes of genetic crosses Genetic Terminology Homozygous: organisms with two identical alleles for a trait (ex. TT or tt) Heterozygous: organisms with two different alleles for a trait (ex. Tt) Phenotype: the manner in which the genes for a trait are expressed (what we physically see) Genotype: the actual genetic makeup of the individual (the “code” we do not see) For example in pea plants T- Tall-Dominant Trait t- Short- Recessive Genotype TT Tt tt Phenotype Tall Tall (T is dominant over t) short Back to the focus question. Is TT homozygous of heterozygous Genetic Terminology Punnett squares: a grid used for determining the genetic combinations that would result from a genetic cross Reginald Punnett parents gametes predicted offspring A heterozygous Tall Plant is cross-pollinated with a homozygous tall plant. What are the possible genotypes and phenotypes. Step 1: Write down the cross in words (heterozygous tall x homozygous tall) Step 2: Write down which trait is dominant and which is recessive Dominant-Tall Recessive- Short Step 3: Choose a logical letter to represent the gene. This is usually the dominant trait. Dom -Tall T Rec - Short t Step 4:Write down the possible genotype and phenotype combinations Genotypes Phenotypes TT Tall Tt Tall tt Short Step 5: Write down the cross in genetic symbols Tt x TT Step 6 : Use a punnett square to calculate the probabilities of the offspring t T T TT Tt T TT Tt Step 7: Write down the genotype and phenotype ratios Genotypes Phenotypes TT:Tt:tt Tall:Short 2:2:0 4:0 2:2 100% tall Step 8: Answer the question in a sentence One half of the offspring will be TT and one half will be Tt, and all the plants will be tall. (genotypes) (phenotypes) Round face is dominant over a square face. A homozygous round faced man marries a square faced woman. What are the possible genotypes and phenotypes of their children. Round- dominant- R Square- Recessive-r R R Genotypes r r Rr Rr Rr Rr all Rr Phenotypes All Round Face Probability and Segregation The principle of segregation (separation of alleles (R or r) during gamete formation) is valid since the phenotypic ratio of 3:1 is consistently shown in each F2 generation Probabilities Predict Averages In order to get the predicted average for a particular event, the event would have to occur many times In a genetics cross, you need many offspring in a generation to come close to the predicted ratio Parents First Generation Second Generation Long stems short stems All long 787 long: 277 short Red flowers white flowers All red 705 red: 224 white Green pods yellow pods All green 428 green: 152 yellow Round seeds wrinkled seeds All round 5474 round: 1850 wrinkled Yellow seeds green seeds 6022 yellow: 2001 green All yellow Results All traits have a 3:1 ratio in F2 generation Last photo of a moron!
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