Agriculture Reference
In-Depth Information
Geneticists use letters to represent all possible alleles on each gene pair.
A capital letter is used for the dominant allele and a small letter for the reces-
sive allele. Let's look at a simplifi ed example. For domestic sheep, white is a
dominant color and black is recessive. We'll use the capital letter
W
for the
white allele and the small letter
w
for the black allele. In this example, both
the ram and the ewe are white, and both have
one dominant allele (represented by
W
) and
one recessive allele (
w
). On average, these two
animals would produce three white lambs out
of every four lambs they have as offspring.
Remember, this is a very simple example;
the fi nal color of the
W
fl eece over the whole
body is actually controlled by as many as
16 gene pairs working together, depending
on the breed of sheep. This is why, in colored
fl ocks, there are many shades of black, gray, and
brown. However, in dark sheep, age can also
play a role in the color of fl eece, as brown may
turn to tan or black may gradually turn gray,
just as hair turns gray with age in humans.
W
w
W
WW
Ww
w
Ww
ww
Color inheritance probability
of lambs born to two white-
colored sheep with a recessive
black allele (
Ww
). On average,
there is a 75% probability their
lambs will be white.
(W = white
allele; w = black allele)
Heritability
Some traits, such as eye color, are fully the result of genetics, but most traits
are infl uenced by both genetics and environment. A particular breed may
have a high incidence of multiple births, but if the ewes aren't fed adequately,
their conception rate won't be as high despite their genetic potential. Or a
breed may be known for producing really fi ne fl eece, but if an individual ani-
mal has been sick, its fl eece may be of poor quality. Since many traits are at
least partially heritable, they can be taken into account when you're making
breeding program decisions.
As a rule, sheep are considered to be seasonal breeders, meaning they are
able to breed only during the late fall and early winter because their estrus
cycles are controlled by hours of daylight. But some breeds are able to breed
out of season. Lambs born out of season can command good prices, and this
trait also may be desirable for shepherds interested in accelerated lambing.
This refers to a program in which ewes give birth more than once per year on
average, but accelerated lambing requires exceptional skill and management
and does not always provide “accelerated” profi ts.
