Background

Consider a cabbage. Why docs it look the way it does? Why does it have the shape of a ball? Why are some cab-bages red and others light green? What determines thai a cabbage seed bccomes a cabbage and not a rosebush? In a field of cabbage, why does each plant vary somewhat from the others even though the seeds were purchased from the same seed company? Why arc the cabbages grown in my garden never as big as those shown in catalogs? What factors influence the final appcarancc of any specific cabbage plant:

Consider your own family. What traits do you share with vour brothers and sisters? What traits arc differ-

w cnt? If you all had the same parents, why don't you all look exacth alike? Maybe you are an "identical" twin. Arc you really completely identical, or are there differences that your friends and parents have no problem recognizing?

The answers to these questions are fundamental to understanding the underlying mechanisms that influence the growth, development, survival, and reproduction of all living organisms. The answers also explain how organisms that seem so similar can be so different and also why some that seem so different arc actually very similar.

VVe know that an organism's appearance and growth pattern depend on both internal and external factors. Internal factors include the genetic coding stored in DNA, how the DNA is decoded to produce specific enzymes or proteins, and how these enzymes and other gene products affect metabolism. A structure, such as a leaf, is a product of many genes being activated and many metabolic steps occurring in specific sequences. The final product is influenced by external factors such as die amount of light, the available water, and the mineral nutrients in the soil. Laboratory Topic 5 explores some of the factors affccting photosynthesis, respiration, and transpiration. An organism's appearance, what we often call the phenotype, is a result of the interaction between the genetic potential of the individual and the physical and chemical influences of the environment.

This lab explores the variation existing within a group of economically important plants called the brassicas. Some members of this group still grow wild, some even as weeds. Other members have been bred and cultivated by humans for millennia as important foods for humans and livestock. Still another member of this group has been exploited as a research tool because it grows to maturity so fast. These plants are called rapid-cycling brassicas (Rbr) or Fast Plants. We will use several of these diverse yet related plants to explore which features are due to genes and which arc due to the environment.

This lab is divided into three exercises. Exercise A explores the diversity- of brassicas cultivated as food. In Exercise B, you will determine whether turnips and Chinese cabbage arc members of the same species by cross-pollinating the vegetables with a rapid-cycling brassica (Rbr, Fast Plant). In Exercise C, you will follow in the footsteps of generations of plant breed crs and select a trait to change in a population of Rbr plants. You will see if your population can evolve by human-guided selection, one of the basic mechanisms for crop improvement.

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