Biology Reference
In-Depth Information
(c) Show that the probability that the phenotypes are recessive is:
1/4
1/4
ΒΌ
1/16.
(d) How does this imply the same proportion of the phenotypes as the
Punnett square?
E
XERCISE
3-2
Suppose we now have three sites on three different chromosomes
occupied by A or a, B or b, and C or c, where upper case denotes the
dominant allele. If we mate two triply heterozygous parents, AaBbCc
AaBbCc:
(a) What is the probability that each site has at least one dominant
allele?
(b) What is the probability that the A and B sites are dominant and
the C site is recessive?
(c) What is the probability that the A site is dominant and the B and C
sites are recessive?
(d) What is the probability that all sites are recessive?
(e) List all of the possible phenotypes and the probability with which
each occurs.
Next, we want to investigate how the genetic make-up evolves. We
consider two mathematical models describing how the distribution of
alleles at a particular site comes to equilibrium.
III. HARDY-WEINBERG LAW OF GENETIC EQUILIBRIUM
It may appear that, over time, the genotypes of each
species should converge to a single, optimal genotype. The simplest
explanation for why this is inaccurate is that our view of ''optimal'' is
not nature's view. In fact, nature's view is that genetic diversity is
valuable. Genetic diversity allows some members of a species to survive
periods of catastrophic environmental change, and therefore promotes
the perpetuation of the species. We begin with some examples that
demonstrate how the lack of genetic variability can be disastrous.
In 1970, 15% of the U.S. corn (maize, Zea mays) crop was lost to infection
by the fungus Cochliobolus heterostrophus, the southern corn leaf blight.
This $1 billion loss occurred because a huge fraction of the U.S. corn crop
was planted in a single type of corn, Texas male-sterile cytoplasm
(or T-cytoplasm). This particular type was chosen because its seed was
