Download probability and statistics

Zool 3300
Genetics
Laboratory Exercise
PROBABILITY AND STATISTICS
The large ears and sweet kernels of today's maize (corn) plants are the product of about 8000 years
of genetic experimentation (Galinat 1992). The earliest plant breeders, native American Indians,
transformed the ancestor of maize, teosinte, from a wild grass with inch-long ears and about thirty
kernels into plants with ears containing about 1000 kernels. No other plant breeders –with or
without modern knowledge and science– have been able to produce such a great change (Wallace
and Brown 1988). In the early 1900s, American geneticists systematically applied Mendel's
principles to maize to produce numerous highly-productive strains; many of these are used today.
Not only has genetics been important to maize, but maize has been important to genetics. Because
of its economic value, maize has long been the subject of intensive genetic investigations. Many of
the basic principles of transmission genetics were a result of investigations of maize. In addition,
maize has been and continues to be important to many discoveries of molecular genetics. In 1984,
Barbara McClintock [1902-1992] was awarded the Nobel Prize for Medicine/Physiology for her
discovery of transposable genetic elements in maize. Today, these DNA sequences are known to be
present in all organisms and may be responsible for the overall organization of eukaryotic genomes
(Pennisi 1998). In addition, several human diseases are caused by transposable element-induced
mutations.
This experiment investigates the familiar principles of transmission genetics using the results of
various crosses between different strains of maize. You will obtain ears of corn and score each for
phenotype of the kernels. The traits include those that affect color, shape, and texture. By observing
the F2 phenotypic ratios, you should be able to infer the parental and F1 phenotypes and genotypes,
and to infer the genotypes of the F2 ears.
REFERENCES
Galinat, W.C. 1992. Corn, Columbus and Culture. Perspectives Biol. Med. 36:1-12.
Pennisi, E. How the genome readies itself for evolution. Science 281:1131-1134.
Wallace, H.A. and W.L. Brown. 1988. Corn and its Early Fathers. Iowa State University Press,
Ames Iowa.
EXPERIMENTAL OUTLINE
There are four groups of corn ears, A, B, C, and D. Working in teams of 4-6 students, you should
obtain a single ear of corn from each group. Thus, your team will have four ears total to study. Each
kernel represents an F2 individual that resulted from a particular cross. Below is a key of what to
look for in each.
Group A
Monohybrid cross. Among the traits to score are purple/yellow; OR red/white; OR
starchy/sweet.
Group B
Dihybrid cross. Kernels may be purple/yellow, starchy/sweet.
Group C
Dihybrid cross. Kernels may be purple/yellow, starchy/sweet; OR
red/white; OR purple/red/white; OR yellow/purple.
Group D
Dihybrid cross. Kernels may be purple/yellow, starchy/sweet;
Note: Some of the F2 may result from crosses between heterozygous F1 individuals, some may be
the result of a testcross, some may show epistasis. All of the details are for you to work out.
• Examine each ear and determine the range of phenotypes (possibilities listed above).
• Count the numbers of kernels belonging to each phenotypic class. To make sure that your count
reflects the true phenotypes, a large sample size is best. It is suggested that you count 100-200
kernels from each ear.
• Determine the observed phenotypic ratios and make a hypothesis based on these observations. Use
your knowledge of genetics to identify possibilities (e.g., 9:3:3:1 [and modifications, reflecting
epistasis]; 3:1; 1:2:1; 1:1:1:1, etc.).
• Write down the inferred genotypes of the F2 ears (match genotype with phenotype). Then write
down genotypes of P1 and F1 generations.
• Using the chi-square formula, determine if observed ratio agrees with your expected ratio.