Corn Genetics - Lab and write up PDF

Preview

Summary

Lab and write up...

Description

LABORATORY: Inheritance of Traits in Corn Materials:

Ears of corn representing F2 generations (or test crosses) Pu s hPi n s Ca l c u l a t o r

Ob j e c t i v e s :

Hy p o t h e s i sTe s t i n go fGe n e t i cBa s i sf o rPh e n o t y p i cTr a i t s Un d e r s t a n d i n go fMe n d e l ’ sPr i nc i p l eo fSe g r e g a t i o n Un d e r s t a n d i n go fCh i Sq u a r eAn a l y s i s

INTRODUCTION Zea mays, commonly called corn in the United States, is one of the world’s most important food crops. Because of its economic importance, the genetics of corn has been studied extensively. The corn ear, also known as a cob, is covered with kernels. Each kernel represents a different fertilization (fusion of egg and sperm). This is a fortunate feature for geneticists, because it means that a single ear may be conveniently used to evaluate the hereditary basis for traits of the offspring (kernels). Kernel color Yo uma yh a v en o t i c e dt h a tc o r nc h i p sa r ea v a i l a b l e i nav a r i e t yo fc o l or s , s u c ha swh i t e , y e l l o w, b l u e ( p u r pl e ) , a ndr e d.Ma n yg e n e sd e t e r mi n et he p h e n o t y p e so ft h et h r e et i s s u e st h a tc o nt r o lt h e c o l o ro fac o r nk e r n e l . Th e s et i s s u e sa r et he Figure 1. The tissues of a corn kernel p e r i c a r p ,t h ea l e u r o n e( o ut e rl a y e ro ft h e involved in producing color phenotypes. e n do s p e r m) ,a n dt h ee n d o s p e r mp r o p e r( s e eFi g u r e 1 ) .I nt h et y p eo fc o r nwewi l lb eu s i n gi no u r c l a s s ,t h epe r i c a r pi sa l wa y sc o l o r l e s s , b u tt h e a l e ur o nema yb ec o l o r l e s s , pu r p l e , o rr e d , a n dt h e e ndo s pe r my e l l o wo rwh i t e .Ke e pi nmi ndt h a t b e c a u s et h ee n do s p e r mi su n d e r n e a t ht h ea l e u r on e , ap u r p l eo rr e da l e u r on ewi l lme a nt h a tt h e e n do s p e r m wi l lno tb evi s i b l e( r e g a r d l e s so f wh e t h e ri ti sy e l l o wo rwh i t e ) .Th ec o l o ro ft h e a l e u r o n ec o me sf r om t h ep r o d u c t i o no f a nt ho c y a ni npi g me nt s( a l s or e s p o n s i b l ef o rt h e c o l o ro fr e do n i o n sa n db l u e b e r r i e s ) .Twos e p a r a t e g e n e s ,Ca n dR, p l a yar o l ei nt h epr o d u c t i o no f a n t h oc y a n i n .Ge n e sCa n dRa r el o c a t e do n s e p a r a t ec h r omo s o me sa n ds e g r e g a t e . Figure 2. A corn ear showing purple, yellow, and white kernels. For the aleurone to be colored, alleles C and R must be present. The genotypes C/C R/_ and C/c R/_ both result in a purple (Fig. 2) or red kernel. The homozygous recessive of either allele (c/c

or r/r) disrupts anthocyanin production and results in a colorless aleurone. As a result, a _/_ r/r or a c/c _/_ kernel will show the color of the endosperm. A red aleurone color is determined by another gene, Pr, which we will not be working with in this laboratory. Thus, for these experiments, the presence of the C/C R/_ or C/c R/_ genotypes will result in a purple kernel only. In addition to C and c, the C gene has a third possible allele, C1. The CI allele is dominant over both C and c. CI inhibits anthocyanin production, giving a colorless aleurone, regardless of the genotype at the R locus. If the aleurone is colorless, the kernel color will be that of the endosperm, either yellow or white (see Fig. 2). Normal corn endosperm color (yellow) occurs when the Y gene carries the allele Y. causes the production of carotenoid pigments in the endosperm (carotenoids are also responsible for the color of carrots and hot or sweet peppers). In the recessive condition (y/y) carotenoids are not produced and the endosperm is white. The Y alleles are masked by the presence of a colored aleurone. The Y gene is independent of the C and R genes. Kernel texture Normal corn endosperm is high in amylose starch, a large complex carbohydate. The gene Su in the homozygous recessive condition (su/su) produces endosperm that is high in sugars that are much simpler in their structure than starch. As corn dries, the sugary endosperm of sweet kernels lose water, and they wrinkle (see Fig. 3). Starchy kernels remain smooth.

Fig. 3. A corn ear showing starchy (smooth surface) and sweet (wrinkled) kernels. Note that the kernels are all yellow, meaning that the aleurone is colorless and the Y genotype for each kernel must be Y/_.

Recall that the scientific method includes development of complementary hypotheses, the null hypothesis, and the alternative hypothesis. In the scientific method, evidence is collected to determine whether the null hypothesis should be rejected or retained. If it is rejected, the alternative hypothesis is accepted as an explanation. PROCEDURES Your answers, including the following table, are (one week from today). Because this is a group assignment, only one copy of the assignment need be turned in for the group. See the course syllabus for more information about participating in group work. Part A For this part of the lab, you and your table partners should choose an ear with the number 6500 and an ear with the number 6540. You may assume that: 1) both of the ears represent the F2 generation from an F1 generation, produced in turn from true-breeding parents (P), and 2) the true-breeding parents (P) of the F1 are different phenotypes for the trait you observe. Ear #6500 is the F2 generation. The F1 generation was selfed to yield F2 ears. Two parental (P) corn breeds, one true-breeding purple and the other true-breeding yellow, were crossed to yield F1 progeny. Although purple color is known to be determined by more than one gene, you may assume that a single gene is responsible in this case. Ear #6540 is the F2 generation. The F1 generation was selfed to yield F2 ears. Two parental (P) corn breeds, one true-breeding starchy and the other true-breeding sweet, were crossed to yield F1 progeny. 1. Your objective is to evaluate whether the phenotypic patterns on these ears fits Mendel’s Principle of Segregation. Propose a null and alternative hypothesis that you can evaluate using these corn ears. Hint: In each hypothesis, be certain that you specify the ratio of phenotypes. Hint: You need a separate pair of hypotheses for each ear of corn. 2. Count the different types of kernels (purple and yellow in 6500; starchy and sweet in 6540) on each ear. Count all kernels on each ear. Do not remove the ears from the plastic wrapping or the label. To track your counting, consider inserting a tack or pin (provided) into each row as you count. Enter your observed totals in the table at the end of this handout. 3. Complete the appropriate portions of the table at the end of this handout. Use a Chi-Square test to evaluate your null hypothesis. (See your textbook for the Chi-Square Table of Probabilities.) 4. On the basis of your Chi-Square analysis, what conclusions should be made regarding your hypotheses? (Keep in mind that each corn breed should be evaluated separately.)

Part B For this part of the lab, you and your table partners should choose an ear with the number 6600. You may assume that: 1) the ears represent the F2 generation from an F1 generation, produced in turn from true-breeding parents (P); 2) the true-breeding parents (P) of the F1 are different phenotypes for the traits you observe; and 3) the F1 are double heterozygotes. Ear #6600 is the F2 generation. The F1 generation was selfed to yield F2 ears. Two parental (P) corn breeds, one true-breeding purple and starchy, and the other true-breeding yellow and sweet, were crossed to yield F1 progeny. Although purple color is known to be determined by more than one gene, you may assume that a single gene, R, is responsible for color. 5. Count the different types of kernels on each ear. For 6600, count four different types of kernels: purple and starchy, purple and sweet, yellow and starchy, and yellow and sweet. Count all kernels on each ear. Do not remove the ears from the plastic wrapping or the label. To track your counting, consider inserting a tack or pin (provided) into each row as you count. Enter your observed totals in the table at the end of this handout. 6. Your objective is to evaluate whether the phenotypic patterns on these ears fits Mendel’s Principles of Segregation and Independent Assortment. Propose a null and alternative hypothesis that you can evaluate using these corn ears. Hint: In each hypothesis, be certain that you specify the ratio of phenotypes. Hint: You need a separate pair of hypotheses for each ear of corn. 7. Use a Chi-Square test to evaluate your null hypothesis. Complete the appropriate portions of the Chi-Square table on the following page. (See your textbook for the Chi-Square Table of Probabilities.) 8. On the basis of your Chi-Square analysis, what conclusions should be made regarding your hypotheses? (Keep in mind that each corn breed should be evaluated separately.)

Table: Chi-Square Analyses of Phenotypes of Corn Ears Corn Breed 6500 Phenotypes

Observed Number (o)

Expected Number (e)

d (o – e)

d2

d2 / e

Total 2 = ___________

Degrees of freedom (df) = _________

________ > P > ________

Corn Breed 6540 Phenotypes

Observed Number (o)

Expected Number (e)

d (o – e)

d2

d2 / e

Total 2 = ___________

Degrees of freedom (df) = _________

________ > P > ________

Corn Breed 6600 Phenotypes

Observed Number (o)

Expected Number (e)

d (o – e)

d2

d2 / e

Total 2 = ___________

Degrees of freedom (df) = _________

________ > P > ________...