In
instances where an organism displays a dominant characteristic but its genetic
makeup remains uncertain—whether it's homozygous or heterozygous—a test cross
is employed. This cross involves pairing the organism with an individual that
has a phenotypically recessive trait. In the resulting F1 generation, a 1:1
segregation ratio of dominant to recessive traits signifies that the unknown
characteristic is heterozygous in nature.
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| Test Cross |
Illustrative
Example: Consider the case of plant height,
which can be TT (tall) or Tt (tall):
a)
TT x tt = Tt, Tt, Tt, Tt (all tall) indicating a homozygous genotype.
b)
Tt x tt = Tt, Tt, tt, tt (2 tall: 2 short, 1:1) suggesting a heterozygous dominant
genotype.
Mendel's
Concept Supported: Mendel's idea
of allele segregation during gamete formation was substantiated through the
test cross. This cross entails breeding an organism of unknown genetic
composition with one exhibiting a recessive phenotype, ultimately unveiling the
genotype of the mysterious organism.
Significance: The test cross validates Mendel's hypothesis of allele
segregation. A monohybrid test cross is used to discern whether an individual
with a dominant phenotype is homozygous dominant or heterozygous for a specific
trait.
Problem
Scenario 1: White vs. Black Sheep
Considering
sheep coat color, where white is dominant over black, the genotype of a white
sheep can be deduced through crossbreeding. A white sheep mated with a black
(homozygous) sheep will yield either all white or a 2:2 segregation of white
and black offspring.
Solution: White x Black
a)
WW x ww = Ww (All white)
b)
Ww x ww = WW, Ww, ww, ww (2 white: 2 black)
Problem
Scenario 2: Fruit Flies' Wing Length
When
a long-winged (L) fruit fly is crossed with a short-winged (I) counterpart,
resulting in a 1:1 offspring ratio, the genotypes of all flies involved can be
ascertained.
Solution: Long-winged fly x Short-winged fly
Offspring: Ll, Ll, ll, ll (Long-winged: 50%, Short-winged: 50%, 2:2 or 1:1
ratio)
