Using Punnett Squares to Predict the Outcomes of Crosses

Transcription

1 Name: Block: Due Date: Usin Punnett Squares to Predict the Outcomes of Crosses The possible ene combinations in the offsprin that result from a enetic cross can be determined by drawin a diaram known as a Punnett square. A Punnett square shows the enes (represented by letters) in the parents ametes alon the top and left-hand side of a square and the possible ene combinations in the offsprin, within the square. A completed Punnett square ives the probable outcome of a iven cross. However, actual results may vary from the probable results, especially if only a few oranisms are considered. MONOHYBRID CROSSES Crosses that involve one trait, such as seed coat color, are called monohybrid crosses. For the monohybrid crosses in this activity, we will use some of the traits Mendel observed in pea plants. The expressions of the dominant and recessive alleles for the enes controllin these traits are described in the followin chart. The chart also assins letters to represent the different alleles. Trait Dominant Allele Recessive Allele Pod Shape Smooth (N) Constricted (n) Pod Color Green (G) Yellow () Flower Position Axial (A) Terminal (a) Plant Heiht Tall (T) Short (t) In the example that follows, we will predict the results of a cross between a plant that is heterozyous for reen pods and a plant that has yellow pods. Sample Problem A plant that is heterozyous for reen pods is crossed with a plant that has yellow pods. What are the probable enotypic and phenotypic ratios in the offsprin resultin from this cross? Step 1 Choose a letter to represent the alleles in the cross. In this case, the letters have already been selected G for the dominant reen allele and for the recessive yellow allele. Step 2 Write the enotypes of the parents. Since the plant with the reen pods is heterozyous for the trait, its enotype must be G. The problem does not state whether the plant with yellow pods is homozyous or heterozyous. But we know that yellow pods are a recessive character, and that recessive characters are expressed only in a homozyous recessive individual. Thus the enotype of this plant must be. The cross, therefore, is G X. Step 3 Determine the possible ametes (reproductive cells) that the parents can produce. The two alleles of any ene are sereated durin the formation of ametes. Thus the reen-pod parent (G) will produce two kinds of ametes G and. The yellow-pod parent () will produce ametes.

2 Step 4 Enter the possible ametes at the top and side of the Punnett square. At this point, the Punnett square for this problem would look like this: G Step 5 Complete the Punnett square by writin the alleles from the ametes in the appropriate boxes. This step represents the process of fertilization, in which a male amete from one parent combines with a female amete from the other parent. To predict all possible offsprin enotypes, each type of possible amete from one parent is combined with each possible type of amete from the other parent. The completed Punnett square for this problem would look like this: G G G As you can see, ½ of the offsprin are enotype G and ½ are. Step 6 Determine the phenotypes of the offsprin. Since reen (G) is dominant over yellow (), plants that have G in their enotypes have reen pods. Only plants with enotype have yellow pods. In this example, ½ of the offsprin have reen pods and ½ have yellow pods. Step 7 Usin the results of Steps 5 and 6, answer the problem. In this example, the enotypic ratio is 2 G:2, or 1:1. The phenotypic ratio is 2 reen:2 yellow, or 1:1. Practice Problems For each of the followin problems, draw a Punnett square in the space provided and fill in the information on the indicated lines. Refer to the table on pae 209 of your textbook for a description of pea traits. 1. Nn x NN 2. Aa X aa

3 3. Tt x Tt 4. Cross two plants that are heterozyous for reen pods. 5. Cross a plant that is heterozyous for axial flowers with a plant that has terminal flowers. 6. Cross a homozyous tall plant with a short plant. 7. Cross a plant that is heterozyous for smooth pods with a plant that has constricted pods. 8. When a tall plant is crossed with a short plant, some of the offsprin are short. What are the enotypes of the parents and the offsprin? What is the phenotypic ratio in the offsprin? Parent enotypes: Offsprin enotypes:

4 9. Three-fourths (¾) of the plants produced by a cross between two unknown pea plants have axial flowers and ¼ have terminal flowers. What are the enotypes of the parent plants? Parent enotypes: 10. What cross would result in ½ of the offsprin havin reen pods and ½ of the offsprin havin yellow pods? Cross: DIHYBRID CROSSES Crosses that involve two traits, such as pod color and pod shape, are called dihybrid crosses. Predictin the outcome of dihybrid crosses requires basically the same procedure as that for crosses involvin one trait. Keep in mind that in dihybrid crosses the enes controllin the two different traits are located on nonhomoloous chromosomes. Durin meiosis, nonhomoloous chromosomes assort independently. This means that each of the chromosomes of any pair of homoloous chromosomes has an equal probability of endin up in a amete with either chromosome from any other pair of homoloous chromosomes. The enes that are located on nonhomoloous chromosomes also assort independently, as you can see in the followin diaram. Because of independent assortment, a plant that is heterozyous for two traits (enotype AaBb) will produce equal numbers of four types of ametes AB, Ab, ab, and ab. In the example that follows, we will predict the results of a cross between two plants that are heterozyous for both pod color and pod shape. Sample Problem What are the enotypic and phenotypic ratios in the offsprin resultin from a cross between two pea plants that are heterozyous for pod color and pod shape? What is the phenotype of the parents in this cross? Step I Choose letters to represent the enes in the cross. Let s use the letters we used in the monohybrid crosses G for reen., for yellow, N for smooth, and n for constricted.

5 Step 2 Write the enotypes of the parents. Since the parents are heterozyous for both traits, their enotype must be GNn. The cross can be written as GNn x GNn. Step 3 Determine the possible ametes that the parents can produce. Each parent produces four types of ametes GN, Gn, N, and n. Step 4 Enter the possible ametes at the top and side of the Punnett square. GN Gn N n GN Gn N n Step 5 Complete the Punnett square by writin the alleles from the ametes in the appropriate boxes. The alleles from the amete above the box and the alleles from the amete to the side of the box are combined inside each of the boxes. Write the capital letter first for each pair of alleles. The letters inside each box represent the probable enotypes of the offsprin resultin from the cross. GN Gn N n GN Gn N n GGNN GGNn GNN GNn GGNn GGnn GNn Gnn GNN GNn NN Nn GNn Gnn Nn nn Step 6 Determine the phenotypes of the offsprin. In this example, 9/16 have reen smooth pods, 3/16 have yellow smooth pods, 3/16 have reen constricted pods, and 1/16 have yellow constricted pods. Step 7 Usin the results of Steps 5 and 6, answer the problem. Note that in this example, as in many of the enetics problems you will encounter, you are asked for more than just the ratios resultin from the cross. This is one reason why it is important to read enetics problems carefully. In this example, the enotypic ratio is 1/16:2/16:1/16:2/16:4/16:2/16:1/16:2/16:1/16 = 1:2:1:2:4:2:1:2:1.The phenotypic ratio is 9/16:3/16:3/16:1/16 = 9:3:3:1. The phenotype of the parent is reen smooth pods.

6 Practice Problems In mice, the ability to run normally is a dominant trait. Mice with this trait are called runnin mice (R). The recessive trait causes mice to run in circles only. Mice with this trait are called waltzin mice (r). Hair color is also inherited in mice. Black hair (B) is dominant over brown hair (b). For each of the followin problems, draw a Punnett square in the space provided and fill in the information on the indicated lines. 1. Cross a heterozyous runnin, heterozyous black mouse with a homozyous runnin, homozyous black mouse. Parental enotypes: 2. Cross a homozyous runnin, homozyous black mouse with a heterozyous runnin, brown mouse. Parental enotypes: 3. Cross a waltzin brown mouse with a waltzin brown mouse. Parental enotypes:

7 4. Cross a homozyous runnin, heterozyous black mouse with a waltzin brown mouse. Parental enotypes: 5. Cross a heterozyous runnin, brown mouse with a heterozyous runnin, homozyous black mouse. Parental enotypes: Prentice-Hall, Inc.