Sheep - AgRIS

5. Genetics
5.1 Chromosome Profile
All animal are made up of microscopic ‗building blocks‘ called cells. The animal body
may contain many millions of cells of different sizes and shapes. They are all alike in that they
contain two major parts, the cytoplasm and the nucleus. The outer portion of the cell is the cell
membrane, which serves as a framework and maintains the shape of the cell. By proper
staining, cells can be seen to contain on oval-shaped body more or less near the center called
the nucleus. The nucleus is said to be the heart and brain of the cell, and it is this part which is
important in inheritance. The material between the nucleus and the cell membrane is called the
cytoplasm. Within the cytoplasm are various bodies that play important role in the function of
the cell but in farm animals, have little or nothing to do with the transmission of inheritance.
When the cell is properly prepared and stained, a number of thread-like bodies appear
within the nucleus. These microscopic threads are called chromosomes. The chromosomes are
present in pairs in the body cells. The members of each pair are very similar in appearance and
are called twins. In genetic terms are called these homologous chromosomes, (home comes
from the Greek work meaning equal or the same, and logous from the word meaning
proportion).
The different species of farm animals differ in the number of pairs of homologous
chromosomes swine possess 19 pairs, cattle and horses 30 pairs, and sheep 27 pairs. Within a
single species of farm animals, the number of pairs of chromosomes is constant; e.g., sheep
have 27 pairs of homologous chromosomes regardless of the breed. The same is true of the
other species of farm animals.
Each pair of homologous chromosomes is distinct from the other pairs. An individual
receives one member of each pair of homologous chromosomes from his father and the other
member from her mother. Each individual, in turn, will pass on to each of its offspring one
member of each pair of chromosomes it possesses. It is purely a chance, which one of members
of each pair the individual will receive from his parent and there is no way of predicting. Both
members of a pair are not passed to the offspring, but only one of each pair.
It is not known how many genes are there on any one of the 27 pairs of chromosomes in
sheep suppose are 10 and they do not interact, the 27 chromosowems in a sprerm and the 27 in
an ovum carry factors that determine 270 of the characteristics of the offspring of a ram and a
ewe. The actual number of genes is undoubtedly much greater. If the chromosomes from the
male and those from the female have within them genes for the same characteristics, the lamb
will be pure or homozygous or those chaacteristics, when the chromosomes unite to form body
cells with twice as many chromosmes as found in the reproductive cells. But, if one gene for
color, for example, on one chromosome is for white and the gene on the other chromosome
from the other parent is for black, then the lamb is r.ot pure for white color, although it may
appear white. In such cases the geneticist calls the lamb heterozygous for color. Exactly the
same thing may be true for all characteristics where it is possible to have one condition or
another or even where there may not be such absolute differences as there are between white
and black or horned and polled.
The factors for white and black, were received from the parents, one chromosome in the
cells of the lamb will carry the factor for white, and the other chromosomes will carry the factor
for black. In the subsequent formation of reproductive cells, at the point where the reduction of
the number of chromosmes occurs one cell will receive the chromosome bearing the factor for
white, and the other will receive the chromosme carrying the factor for black. Both, of course,
come from the same parent. Hence, it becomes immediately apparent that identical parentage
does not insure identical inheritance. If botlh chromosmes carried the same factor, then
inheritance would be identical for that character.
It is not easy to find two animals that are exactly alike. In the case of white and black, it
will become still more evident as other factors are added. Suppose a ram and a ewe, both
heterozygous (not pure for either white or black) for color, are mated. Each of them is capable
to produce two kinds of gametes (reproductive cells) with respect to color. One gamete would
carry the factor for white; and the would carry the factor for black. These two kinds of gametes
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