B. A monohybrid cross (one trait, both parents are heterozygous) will give a 3 dominant : 1 recessive ratio in the offspring for Mendelian traits.

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1 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: Key: T= allele for tall P tall X tall t = allele for short Tt Tt F1 T t T TT Tt t Tt tt 3 tall : 1 short

2 Genetics Problem Set-Up KEY (symbol here) = allele for (dominant version of trait here) (symbol here) = allele for (recessive version of trait here) P! phenotype of 1 st parent X! phenotype of 2 nd parent genotype of 1 st parent genotype of 2 nd parent F 1!! (if applicable) - 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:

3 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 TT Tt tt phenotypes tall tall 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.

4 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?

5 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: Key: P purple tall X purple tall P = purple PpTt PpTt 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

6 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. 1 st 1 st 1 st 2 nd 2 nd 1 st 2 nd 2 nd 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. A a R r 1 st 1 st AR Now you take these combinations to 1 st 2 nd Ar begin setting up your Punnett 2 nd 1 st ar square, as is shown below. 2 nd 2 nd ar To get the gametes that go down this side, you do the same procedure only use the other parent. AR Ar ar ar

7 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 1 st 1 st, 1 st 2 nd, etc. pattern to get all the possible gametes. RRGg RrGG 1 st 1 st RG RG 1 st 2 nd Rg RG 2 nd 1 st RG rg 2 nd 2 nd Rg rg Some of the gametes are the same for the same parent, so cross them out. 1 st 1 st RG RG 1 st 2 nd Rg RG 2 nd 1 st RG rg 2 nd 2 nd 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

8 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.