Download Other Genetic Crosses

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DATE
PERIOD
Other Genetic Crosses
J. Levine and K. Miller; 2002
Not all genes show simple patterns of dominant and
recessive alleles. In most organisms, genetics is more
complicated, because the majority of genes have more
than two alleles. In addition, many important traits are
controlled by more than one gene. Some alleles are
neither dominant nor recessive, and many traits are
controlled by multiple alleles or multiple genes.
Incomplete Dominance
A cross between two four o’clock plants shows one of
these complications. The F1 generation produced by a
cross between red-flowered (R1R1) and white-flowered
(R2R2) plants consists of pink-colored flowers (R1R2).
Which allele is dominant in this case? Neither one.
Cases in which one allele is not completely dominant
over another are called incomplete dominance. In
incomplete dominance, the heterozygous phenotype is
somewhere in between the two homozygous phenotype.
2
R
R2
R1
R1 R2
R1 R2
R1
R1 R2
R1 R2
R1 R1= Red Flowers
R2 R2= White Flowers
R1 R2 = Pink Flowers
Sex-Linked Traits
Hemophilia, coat color in cats, color blindness are some
genes located on the X chromosome, thus referred to as
Sex-Linked Traits. While males have an X and Y sex
chromosome, females have two X’s. In cats, for example
the color for their coat is located on the X chromosome.
One X chromosome may have the allele for the color
orange, while the other X may have the allele for black.
So it’s an X-linked polygenic trait.
Since females have two X chromosomes, they could
haveboth of those colors. If you see a cat with 3 colors:
white, black, and orange, it’s almost certainly a female.
Calico cats that are male are rare and infertile (XXY).
Polygenic Traits
Many traits are produced by the interaction of several
genes. Traits controlled by two or more genes are said to
be polygenic traits, which means, “having many genes.”
Polygenic traits often show a wide range of phenotypes.
For example, the wide range of skin color in humans
comes about partly because more than four different
genes probably control this trait.
What are these examples of?
Codominance
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
A similar situation is codominance, in which both alleles
contribute to the phenotype of the organism. For
example, in cattle the allele for red hair (RR) is
codominant with the allele for white hair (WW). Cattle
with both alleles are called “roan,” or reddish brown and
white (RW), because their coats are a mixture of both
red and white hairs.
Q ui ck Ti m e ™ an d a
T IF F ( Un co m p re ss ed ) d ec om pr e ss or
a re ne ed ed t o s ee th i s pi c tu r e.
Qu i ck Ti m e ™ an d a
TI F F ( Un co m pr e ss ed ) d ec om p r ess or
ar e n ee de d t o s ee t h is pi ct u re .
Qu i ck Ti m e ™ an d a
TI F F ( Un co m pr e ss ed ) d ec om p r es so r
ar e n ee de d t o s ee t h is pi ct u re .
Multiple Alleles
Many genes have more than two alleles and are therefore
said to have multiple alleles. This does not mean that an
individual can have more than two alleles. It only means
that more than two possible exist in a population. One of
the best-known examples is coat color in rabbits. A
rabbit’s coat color is determined by a single gene that
has at least four different alleles. The four known alleles
display a pattern of simple dominance that can produce
four possible coat colors. Many other genes have
multiple alleles, including the human genes for blood
type.
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