Monohybrid Crosses Usin a Punnett Square A cross between individuals that involves one pair of contrastin traits is called a monohybrid cross. Bioloists use a diaram called a punnett square to help them predict the probability that certain traits will be inherited by offsprin. Vocabulary: Allele: Alternative forms of a ene. Homozyous: When both alleles for a pair are alike. Heterozyous: When two alleles for a pair are different. Genotype: The enetic make-up of an oranism. Phenotype: The physical appearance of an oranism as a result of its enotype. The basic punnett-square looks like a windowpane When iven enouh info about two parents, we can use a punnett-square to predict the enotypes & phenotypes of their offsprin. For example, in pea plants, reen pods are the dominant trait and yellow pods are recessive. Green pods and yellow pods are two forms of the same ene, or alleles. We abbreviate the two alleles with the same letter. We use G for the dominant allele and r for the recessive allele. The possible enotypes & phenotypes would be like so: Genotype GG G Phenotype Green Pods Green Pods Yellow Pods Note: Remember, we don't use "G" for reen & "Y" for yellow because alleles for the same trait are always represented by the same letter. One more note: A very helpful thin to memorize is that the ONLY way for a recessive trait to show up in an oranism is if that oranism's enotype is homozyous recessive (two little letters, like "rr"). Here are the basic steps to usin a Punnett Square when solvin a enetics question. After you et ood at this you should never miss a enetic question involvin the cross of two oranisms. STEPS: 1. Determine the enotypes of the parent oranisms 2. Write down your "cross" (matin) 3. Draw a punnett-square 4. "Split" the letters of the enotype for each parent & put them "outside" the p-square 5. Determine the possible enotypes of the offsprin by fillin in the p-square 6. Summarize results (enotypes & phenotypes of offsprin) Step #1: Determine the enotypes of the parent oranisms Sometimes this is already done for you in the question. If the question says "Cross two parents with the followin enotype: Tt & tt", it's all riht there in the question already. More likely, however, is a question like this: "Cross a short pea plant with one that is heterozyous tall". Here, you have to use your understandin of the vocabulary to fiure out what letters to use in the enotypes of the parents. Heterozyous always means one of each letter, so we'd use "Tt" (where "T" = tall, & "t" = short). The only way for a pea plant to be short is when it has 2 lowercase "t's", so that short parent is "tt". So the cross ends-up the same as in the first example: Tt x tt.
Step #2: Write down your "cross" (matin) Write the enotypes of the parents in the form of letters (ex: Tt x tt). Step #3: Draw a punnett-square. Step #4: "Split" the letters of the enotype for each parent & put them "outside" the punnett-square. For an example cross we'll use these parental enotypes: G x Take the enotype letters of one parent, split them and put them on the left, outside the rows of the punnett-square. Then take the two letters of the second parent's enotype, split them up, and place them above each of the two columns of the punnett-square. G Step #5: Determine the possible enotypes of the offsprin by fillin in the punnett-square. To determine the enotypes of the offsprin all you have to do is fill in the boxes of the punnett-square. Take the letters from the left and match them with the letters from the top. Like so: G G G Step #6: Summarize the results (enotypes, phenotypes and ratios of the offsprin). Offsprin enotypes: G and Phenotypes: Green and Yellow Genotypic Ratios: 2G:2 or 1G:1 Phenotypic Ratios: 2reen:2yellow or 1reen:1yellow
Use the information if the chart below to answer the followin questions: Trait Dominant Recessive Position of Flowers Axial (A) Terminal (a) Heiht of Plant Tall (T) Short (t) Pod Appearance Inflated (C) Constricted (c) Pod Color Green (G) Yellow () Seed Texture Smooth (S) Wrinkled (s) Seed Color Yellow (Y) Green (y) Flower Color Purple (P) White (p) 1. Cross two parents with the followin enotypes: Ss x SS 2. Cross two heterozyous purple plants.
3. Cross a homozyous smooth seed plant with a heterozyous seed texture plant. 4. Cross two constricted pod plants. 5. Cross a heterozyous axial plant with a homozyous axial plant.
6. In mice, black fur (B) color is dominant to brown fur color (b). Cross a brown mouse with a heterozyous black mouse. 7. Cross a homozyous black mouse with a homozyous black mouse. 8. Cross two homozyous black mice.