Agriculture Reference
In-Depth Information
MUTATIONS AND GENETIC DEFECTS
A mutated gene makes a fairly sudden change in how it exerts
itself in the next generation. For the most part, mutations are rare
— however, as with everything in nature, there are exceptions to
that rule. For example, the AIDS virus mutates fairly quickly, mak-
ing research on a cure challenging. Usually (thanks to television
and movies), we think that all mutations are bad, like the radiation-
mutated, three-headed, 2,000-pound killer-frog kind of thing. But
mutations don't have to be bad. Several of the breeds discussed
later in this chapter, such as the Booroola Merino and California
Variegated Mutant, are the result of a mutation that a farmer,
rancher, or researcher spotted in a fl ock and began purposefully
breeding for.
There are over 30 known or suspected genetic defects in sheep.
Many are lethal, while others are crippling or result in long-term
health and reproductive challenges. Some are more common in
particular breeds, while others seem to pop up from time to time
across different breeds. The good news is that with our quickly
expanding understanding of genomes, testing is becoming avail-
able for many of these defects. Quite a few breed organizations
are actively testing to map the genetic problems within their breed,
thus providing a tool for eliminating these costly problems through
controlled breeding selection. If you fi nd lambs born with genetic
disorders in your fl ock, talk to your veterinarian about having the
animals tested to identify the defect and either cull or control the
breeding of carrier animals.
Inheritance
So how do these genes actually get passed along? Well, in every cell of a body
— except the sex cells (eggs in the female and sperm cells in the male) —
there is a full complement of the genetic code that defi nes who and what that
animal is. This code is complex and consists of many gene pairs that are strung
together like two strands of string twisted around each other. In humans, for
example, there are more than 100,000 gene pairs that make up the code. Each
egg or sperm cell carries one-half of the parent's genetic string. Remember,
