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Probability: A. Scientists use probability to predict the phenotypes and genotypes of offspring in breeding experiments. 1. When you use fractions, percents, ratios, or decimals to predict the outcome of an event, you’re measuring probability. 2. In biology, Punnett squares are used to help predict the results of breeding experiments a. grid to organize genetic information b. shows probabilities B. A monohybrid cross (one trait, both parents are heterozygous) will give a 3 dominant : 1 recessive ratio in the offspring for Mendelian traits. Example: P tall Tt F1 T t X T TT Tt 3 tall : 1 short t Tt tt tall Tt Key: T= allele for tall t = allele for short Genetics Problem Set-Up KEY (symbol here) (symbol here) P ! = allele for (dominant version of trait here) = allele for (recessive version of trait here) phenotype of 1st parent genotype of 1st parent F1 ! (if applicable) - X ! phenotype of 2nd parent genotype of 2nd parent ! Answer to the question in a box Also – If the sex of each parent is known (male, female) the symbols for the sex must be included in the parental generation and in the Punnett square. (see cloverleaf marks) Male: Female: Answer Set-Up for Certain Genetics Problem Questions If a genetics problem asks you to “list (give, name) the genotypes and phenotypes of the offspring” there is a specific way to arrange your answer. So that it is clear in your answer as to what is a genotype and what is a phenotype, you must make a chart. For example, if you are asked to list the genotypes and phenotypes of the offspring for the following Punnett square: T t T TT Tt t Tt tt then your answer would be: genotypes phenotypes TT tall Tt tall tt short Also, if you are asked to give the “genotypic and phenotypic ratios of the offspring” you must identify which ratio is which. Using the same Punnett square from above, you would write: Genotypic ratio - 1 TT : 2 Tt : 1 tt Phenotypic ratio - 3 tall : 1 short Notice that each is labeled and colons separate each part. Practice Single Trait Inheritance (STI) Problems 1. Cross a homozygous tall plant with a short plant. What percentage of the offspring will be tall? 2. Cross a plant hybrid for flower position with a plant that is purebred dominant for flower position. What fraction of the offspring will be heterozygous? 3. Cross two heterozygous plants using the trait pea shape. What percentage of the plants will be recessive? A dihybrid cross (2 traits, both parents are completely heterozygous for both traits) will result in a 9:3:3:1 ratio in the offspring where: 9 offspring will have both dominant traits 3 have one dominant and one recessive trait 3 have one dominant and one recessive trait 1 has both recessive traits (remember – works for Mendelian traits only) Example: P purple tall PpTt X purple tall PpTt Key: P = purple p = white T = tall t = short F1 PT Pt pT pt PT PPTT PPTt PpTT PpTt Pt PPTt PPtt PpTt Pptt pT PpTT PpTt ppTT ppTt pt PpTt Pptt ppTt pptt 9 purple tall : 3 purple short : 3 white tall : 1 white short How to Set Up a Double Trait Inheritance Problem The hard part is getting the gametes from each parent to make up the top and left side of your Punnett Square. Follow the pattern: The left side tells you which letter to take from the alleles for the first trait. 1st 1st 1st 2nd 2nd 1st 2nd 2nd The right side tells you which letter to take from the alleles for the second trait. So, following the pattern, here are the gametes you would get from a parent with the genotype AaRr. Note that “Aa” are the alleles from the first trait and that “Rr” are the alleles for the second trait. AaRr 1st 1st 1st 2nd 2nd 1st 2nd 2nd AR Ar aR ar Now you take these combinations to begin setting up your Punnett square, as is shown below. AR To get the gametes that go down this side, you do the same procedure only use the other parent. Ar aR ar Shortcuts for Punnett Squares You only need one of each different gamete from each parent for your Punnett squares. If you have 2 or more copies of the exact same gamete FROM THE SAME PARENT, eliminate the duplicates. For example, let’s say your crossing the parents RRGg and RrGG. Use the 1st 1st, 1st 2nd, etc. pattern to get all the possible gametes. 1st 1st 1st 2nd 2nd 1st 2nd 2nd RRGg RrGG RG Rg RG Rg RG RG rG rG Some of the gametes are the same for the same parent, so cross them out. 1st 1st 1st 2nd 2nd 1st 2nd 2nd RG Rg RG Rg RG RG rG rG Now, use the remaining gametes to make your Punnett square. The blue shows the resulting offspring. RG Rg RG RRGG RRGg rG RrGG RrGg Please note the following about double trait problems: • You either have to cross out ALL of the duplicates or NONE of them for a double trait problem to work out correctly. • You will NEVER have 3 gametes from a parent in a double trait problem. If you do, you either crossed out a gamete you shouldn’t have, or you didn’t cross out ALL of the duplicates. • Only a completely or fully heterozygous parent will get you 4 different gametes, which is the highest number of gametes you can have for a double trait problem.