Introduction The black rhinoceros (Diceros bicornis) is a critically endangered species that only remains in parts of southern Africa. There are many environmental factors involved in its decline, but perhaps the biggest threat to its survival is human predation. Poaching for rhino horn has reduced the black rhino population by 96% since the 1970 s, when there were over 100,000 rhinos. Rhino horn is composed of compressed keratin, the same material as our hair and fingernails. Rhino horns are believed to possess strong medicinal value in traditional Asian cultures, and are sold illegally throughout the world. Among the fragmented populations of black rhinoceros, there have appeared two traits that enhance the survivability of individuals of the species. (That doesn t improve the survivability of the species, unless human predation is stopped!) One of these traits is an extra-long prehensile lip. This lip allows the animal to grab hold of tough, thorny vegetation (like acacia) so that it can more easily feed on the food source. This means that the animal can spend less time in actual feeding and is more able to be alert for predators or potential mates. The second trait is extra-long ears. The external ear of any animal is like a sound collector. The larger the collector, the more sensitive it is to sounds. Thus, the larger ear enables the animal to hear predators approaching from a greater distance. This improves the chance that this animal will survive to reproduce (and have offspring that also have longer ears.) Unfortunately, humans hunting with highpowered rifles from a distance won t be heard in time for the animal to escape! These two traits are expected to be uniformly distributed among the surviving members of Diceros bicornis. The question for you to answer is: are these traits dominant, co-dominant, or recessive? To do so, you will need the rhinoceros population data on page 4, your knowledge of genetic dominance, and the ability to perform dihybrid test crosses with Punnett Squares. Procedure Refer to the descriptions of the traits on pages 2 and 3 of this handout. Data for the surviving numbers of Diceros bicornis and the numbers of individuals with traits for extra-long prehensile tongues and extra-long ears are summarized on page 4. From the data, make a claim for the level of dominance for each trait. Then, test your claim by choosing letters for each trait and make a dihybrid cross of the two traits. Your claim will determine what the genotype of the two parents should be. After obtaining the results of your test cross, justify your claim by an analysis of the data to see if it fits your claim. If it does not, start again. When you have determined the correct genotypes, prepare a presentation of your finding to the class to defend your claim. Your presentation will include a title page with the names of the members of your group, the course name, and the date. On a second slide, present your hypothesis and the method by which you tested your hypothesis. On a third slide, show the data from your test cross. On the final slide provide your conclusions with the evidence obtained from the original population statistics and your data. Blank templates for Punnett Squares with a dihybrid cross are provided on page 4. You may use these and paste them into your notebooks, or create your own templates; either in your notebook or in a spreadsheet. Be sure to keep track of your progress, including your step-by-step procedure in your notebook. The notebook is your proof that the evidence you collected is your own! 1
The prehensile lip is used for browsing from very tough vegetation. This extra-long version makes for more efficient feeding. This is a normal length prehensile lip. This animal can feed from tough vegetation, too. However, it takes longer for it to grab the tough vegetation and chew it up than it does for animals with the extra-long version. 2
Black rhino ears detect the approach of predators. Since they are choice food for lions, tigers, and hyenas, the ears are necessary for survival. These are normalsized ears. This baby is the first black rhino born in captivity at the St. Louis Zoo in 20 years. Notice its extra-long ears. These ears give the rhino early-warning capacity for approaching predators. (Today, the most deadly predator is the Human.) 3
The latest data on the population of black rhinos in South Africa is 5,055 individuals. 1 Some scientists sorted through the data and came up with the following information about the appearance of extra-long prehensile lips and extra-long ears: Individuals with normal ears and normal prehensile lips: ~ 2,600 Individuals with normal ears and extra-long prehensile lips: ~ 1000 Individuals with extra-long ears and normal prehensile lips: ~ 900 Individuals with extra-long ears and extra-long prehensile lips: ~ 300 Punnett Squares Templates Mother Father Mother Father 1 Rhino population figures, Save the Rhino, http://www.savetherhino.org/rhino_info/rhino_population_figures, accessed 8/13/14. 4
Punnett Squares Solution Key Mother PpEe PE Pe pe pe Father PpEe PE PPEE PPEe PpEE PpEe Pe PPEe PPee PpEe Ppee pe PpEE PpEe ppee ppee pe PpEe Ppee ppee ppee Both Homozygous, dominant: 1/16 Both Homozygous, recessive: 1/16 Heterozygous, both dominant: 8/16 Long prehensile lips only: 3/16 Long ears only: 3/16 5