Download Black Rhino Dihybrid Cross

Name
Black Rhinoceros Genetics
Class:
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
Name
Black Rhinoceros Genetics
Class:
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
Name
Black Rhinoceros Genetics
Class:
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
Name
Black Rhinoceros Genetics
Class:
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
Name
Black Rhinoceros Genetics
Class:
Punnett Squares Solution Key
Mother PpEe
Father PpEe
PE
Pe
pE
pe
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:
5
3/16
Pe
PPEe
PPee
PpEe
Ppee
pE
PpEE
PpEe
ppEE
ppEe
pe
PpEe
Ppee
ppEe
ppee