Download chapter 3-5: Incomplete Dominance

CHAPTER
THREE
chapter 3 - 5 : Incomplete D o m i n a n c e
In Mendel's experiments with pea plants, the dominant allele
masked the expression of the recessive allele; the heterozygous
offspring had phenotypes that were dictated by the dominant
allele. We saw, for instance, that yellow dominates over green in
seed color, and that round dominates over wrinkled in seed
shape. In heterozygous individuals, the dominant round and yellow alleles are expressed if they are present, and the recessive
wrinkled or green alleles are hidden.
Sometimes two alleles are both expressed so that the phenotype of the hybrid is intermediate between the two phenotypes of
the parent individuals. This pattern of inheritance is called incomplete dominance. It causes an apparent blending of the
phenotypes, but it is not a true blending because the alleles for
the trait have not been altered.
In this plate, we will study the phenomenon of incomplete dominance in flowers called snapdragons.
Begin your work by focusing on the first portion of the
plate, and read the paragraphs below.
Snapdragons have two true-breeding strains. The red snapdragon (A) bears red flowers and the while snapdragon (B)
bears white flowers. You should color the stem (C) green. We will
start by assuming that the genotype of the red snapdragon is RR,
and that the genotype of the white snapdragon is WW.
One example of incomplete dominance can be seen when
white snapdragons are crossed with red ones. All the members
of the resulting F, generation are pink snapdragons (D); the heterozygous individuals have phenotypes that are intermediate
between the phenotypes of the two parent plants. Keep in mind
that the alleles for flower color have not changed even though the
phenotype is blended. The genotype of the offspring is RW, so
this heterozygous plant contains a gene from each of the parent
strains.
When two pink snapdragons are crossed, you can predict the
phenofype pattern in the F2 generation if you know Mendel's law
of segregation. We cross two hybrid pink snapdragons (D), both
members of the F1 generation. The snapdragon on the left produces sperm cells by meiosis and we will put these gametes (E)
vertically on the left. (They are enclosed in a bracket.) One is a
gamete with the red allele (F) and the other is a gamete with the
white allele (G). At the top of the Punnett square are the egg cells
that were produced by the second snapdragon, and again there is
one gamete with the red allele (F) and one with the white allele (G).
Now we are ready to do the crosses. The first individual that
results from this cross will have the genotype RR, and will be a red
snapdragon (A). Two individuals of the cross will have the genotypes RW, just as their parents did in the F, generation. They are
pink snapdragons (D). One individual in the F2 generation will be
a white snapdragon (B), and will have the genotype WW.
The results of this cross show that the individual alleles were
not changed, despite the fact that neither of them was expressed
strongly in the F, generation. We saw a ratio of genotypes of
1:2:1, which translates into 1RR:2RW:1WW. Interestingly, the
ratio of phenotypes is also 1:2:1, that is, 1 red:2 pink:! white.
Thus, the genotype and phenotype ratios are identical. (Recall
that in the monohybrid cross, the genotype ratio in the F2 generation was 1:2:1, while the phenotype ratio was 3:1.)
One example of incomplete dominance in humans can be
seen in'Tay-Sachs disease. An individual with two recessive alleles exhibits the symptoms of this genetic disorder. However, a
heterozygous individual shows slight symptoms, indicating that
the presence of the dominant allele does not completely overshadow the recessive allele.
As we have noted above, the alleles are not changed
in the offspring in cases of incomplete dominance.
This will become apparent when we interbreed the
hybrids of the F, generation. Continue your reading
below, and focus on the lower portion of the plate.
O
O
O
O
O
O
O
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Incomplete Dominance
Red Snapdragon
A
W h i t e Snapdragon
B
Stem
C
Pink Snapdragon
D
Gametes
E
Gamete with Red A l l e l e
F
Gamete with White All ele .......... G
o
o
Generation
fD
ct
O
O
DJ
::s
o
CD
B
Generation
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