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The Genetics of
Color
Genes, found on an animal’s chromosomes,
control specific functions of
individuals. Cattle have about 200,000
pairs of genes on 30 pairs of
chromosomes, found within the nucleus of
each cell in the body. Every function,
including the color of an animal, is
controlled by specific genes on specific
chromosomes.
Sex cells in
bulls (sperm) and cows (eggs) are made
up a little differently from other cells
in the body. Each sex cell contains only
one of the two genes required for
offspring’s functions. When the sperm
and egg unite, the resulting embryo
receives 30 chromosomes from each parent
to make up the 30 pairs of chromosomes
the offspring requires. In this way, the
new individual always has half its genes
from each parent.
We can
predict the genes of the offspring from
the genes of the parents. A simply
inherited trait (like color) is often
easy to predict.
The
Color Game
Gelbvieh cattle have a basic color of
either red or black. In cattle, the gene
for black color is dominant to the gene
for red color. Individuals with two
black genes are called homozygous black
and will be black. Similarly,
individuals with two red genes are
called homozygous red and will be red.
However, since the black gene is
dominant over the red, cattle with one
gene for black and one gene for red are
called heterozygous and will be black.
When a
bull is called double-black, it simply
means that both of his parents are
black. As you can see from Table 1,
offspring from two black parents may or
may not be black. If both parents are
proven to be heterozygous, the chance is
three in four that their offspring will
be black. But the chance is one in four
that their offspring will be red. The
only way to guarantee a black calf is to
mate two black parents with at least one
being proven homozygous black.
Remember,
genes for color have no influence on
production or reproduction traits.
TABLE 1
PROBABILITY OF THE
COLOR PRODUCED BY DIFFERENT GENE
COMBINATIONS
(represents chances of color with each
calf born, B=black gene, b=red gene )
|
A
homozygous red bull
(bb)
mated to a:
-
heterozygous
female (Bb)
results in 2Bb,
2bb (50% chance
heterozygous
black, 50%
chance
homozygous red)
-
homozygous red
female (bb)
results in 4bb
(100% chance of
homozygous red)
-
homozygous black
female (BB)
results in 4Bb
(100% chance
heterozygous
black)
A
heterozygous bull
(Bb)
mated to a:
-
heterozygous
female (Bb)
results in 1BB,
2Bb, 1bb (25%
chance
homozygous blk.
50% chance
heterozygous blk,
25% chance
homozygous red)
-
homozygous red
female (bb)
results in 2Bb,
2bb (50% chance
heterozygous
black, 50%
chance
homozygous red)
-
homozygous black
female (BB)
results in 2BB,
2Bb (50% chance
homozygous
black, 50%
chance
heterozygous
black)
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In
Search of the Gene
A
black bull can be tested through matings
to determine if he is heterozygous or
homozygous. For example, 10 black calves
from 10 red cows means that there is a
99.9 percent probability that the sire
is homozygous black. 20 black calves
from 20 known heterozygous, black calves
give a 99.7 percent probability that the
sire is homozygous black (see table 2).
Table 2
PROBABILITY OF A BULL
BEING HOMOZYGOUS BLACK
|
Number of
Matings
|
Red Cows (bb)
|
Known
Heterozygous
Black Cows (Bb)
|
| |
% |
% |
|
1 |
50 |
25 |
|
2 |
75 |
44 |
|
3 |
87 |
58 |
|
4 |
94 |
68 |
|
5 |
97 |
76 |
|
6 |
98 |
82 |
|
7 |
99 |
87 |
|
8 |
99.6 |
90 |
|
9 |
99.8 |
92.5 |
|
10 |
99.9 |
94.4 |
|
15 |
--- |
98.7 |
|
20 |
--- |
99.7 |
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Dilution Gene
The dilution gene causes black color to
dilute to gray (in a range from dark to
light) and red color to dilute to gold.
A separate set of genes determines if
the color of an animal is diluted or not
diluted. Since the gene for dilution is
dominant to the gene for non-dilution,
an animal that has one gene for dilution
and one for non-dilution will have a
diluted color. An animal with two genes
for dilution will also be diluted (see
table 3). The terms "black" and "dark
red" indicate that the animal has two
genes for non-dilution. Use caution when
representing your cattle.
Another
factor, called the modifier gene, may
also influence color. For example, the
shade of red in Hereford cattle varies
from dark to light red. This variation
is presumably the effect of the modifier
gene, and does not involve the dilution
gene. These modifiers, while still not
fully understood, are probably at work
in Gelbvieh cattle as well, especially
in those upgraded from Hereford cattle.
Therefore, the assumption that gold and
light red animals carry the dilution
gene may be incorrect. Lighter color in
these animals could be a result of a
modifier gene. The modifier gene does
not seem to have any effect on black; in
other words, it doesn't cause gray
animals.
The best
way to test a bull for the dilution gene
is to mate him to black cows.
Table 3
COMBINATIONS OF COLOR AND
DILUTION GENES
|
B=black gene |
b=red gene |
|
D=dilution gene |
d=non-dilution
gene |
| |
|
|
Genotype (gene
combination) |
Color of Animal |
|
BBDD (black,
diluted) |
gray |
|
BBDd (black,
diluted) |
gray |
|
BBdd (black,
non-diluted) |
black |
|
BbDD (black,
diluted) |
gray |
|
BbDd (black,
diluted) |
gray |
|
Bbdd (black,
non-diluted) |
black |
|
bbDD (red,
diluted) |
light yellow to
light red |
|
bbDd (red,
diluted) |
light to medium
red |
|
bbdd (red,
non-diluted) |
medium to dark
red |
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Color
Q & A
Q:
What is the difference between
double-black and homozygous black?
A: Double-black simply
means that the animal's sire and dam are
both black. The parents could be either
heterozygous or homozygous black to show
black color. Assuming that the bull does
not have a dilution gene, a heterozygous
black bull will always sire some black
and some red calves. After a black bull
has sired red calves, he is proved to be
heterozygous black. At that point,
calling him double-black is meaningless.
His parents may both still be black, but
his calves will be both black and red.
Q:
Where do gold calves come from?
A: Gold calves are the
product of a set of dilution genes
acting on the genes for red color. Red
color dilutes to gold, just like black
color dilutes to gray.
Q:
If an animal is really black, does it
carry the dilution gene?A:
No. Any animal with genes for black
color that is also carrying the dilution
gene will be gray.
Q:
Can a "dark red" bull carry the dilution
gene?
A: The marketing term
"dark red" usually means that the animal
carries two non-dilution genes. The term
"black" usually means the same
thing-that the animal carries two
non-dilution genes. However, because
much misunderstanding exists about these
terms, it is always best to question
when you purchase, rather than after you
have gold or gray calves on the ground.
Q:
Is it possible for an animal to be 87
percent (or any other percent)
homozygous?
A: No. An animal either
carries two of the same genes for color
(homozygous) or two different genes for
color (heterozygous). You may hear
someone say there is an 87 percent
chance that the bull is homozygous,
meaning that the bull has three progeny
on the ground out of red dams, and all
of them are black. With no red calves
out of three, there is an 87 percent
chance that the bull is homozygous
black. Once a bull sires a red calf, he
is proved to be heterozygous.
Q:
If a bull sires gray or gold calves,
does that prove he is carrying the
dilution gene?
A: No. Since the
dilution gene is dominant, the progeny
could have received the dilution gene
from the dam side of its pedigree.
Additionally, due to the effects of the
modifier gene believed to be at work in
Hereford cattle or cattle upgraded from
Here-ford, there is often no way to tell
by looking if a red animal carries the
dilution gene. The best way to test a
bull for the dilution gene is to mate
him to black (non-dilution) cows. If
gray or gold calves result, the dilution
gene is present. |
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