Qualitative
Traits:
The
traits showing the simplest type of
inheritance are called as qualitative traits. These involve
only one pair of genes. Evident examples of such traits in dairy
cattle are hair colour, horned versus
polled, some inherited abnormalities and blood antigens. Coat
colour is still important from the standpoint of breed requirements.
A pair of genes is responsible for hair colour in cattle. A Milking
Shorthorn having two genes for red (RR) is actually red in colour,
while an animal having two genes for white (rr) is white in colour.
A Milking Shorthorn that has one gene for red (R) and one for white
(r) is neither red nor white but roan (Rr), which is a mixture of
red and white. Thus, red and white matings in Milking Shorthorn
cattle usually produce roan offspring. Likewise, white and white
matings generally produce white offspring, even though white Milking
Shorthorn is seldom pure because the face bristles, eyelashes, and
ears usually carry red hairs. Roans, having one gene for red and one
for white on the paired chromosomes, never breed true and when mated
together produce calves in the
proportion of one red, two roans, and one white. The most certain
way to produce roan Milking Shorthorns, is to
mate red cows with a white bull,
or vice versa; this produces all roan calves. If a roan animal is
bred to a red one, one-half of the offspring will be red,
whereas the other half will be roan. Likewise, when a roan animal is
bred to a white one, approximately an equal number of roan and white
calves will be produced.
Quantitative Traits: |
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Quantitative traits
are also termed as multiple gene inheritance. Relatively a few
traits of economic importance in farm animals are inherited in as
simple a manner as the coat colour or the polled condition. Most
traits of economic importance, such as milk yield and composition,
conformation, feed efficiency, disease resistance, are controlled by
multiple genes. Because such traits may be expressed in all possible
gradations, from high to low performance, they are known as
quantitative traits. Estimates of the number of pairs of genes
affecting each economically important characteristic vary greatly,
but the majority of the geneticists agree that ten or more pairs of
genes are involved for most such traits. In addition to being
influenced by many pairs of genes, quantitative traits differ from
qualitative traits because they are frequently strongly influenced
by the environment.
Factors
Governing
Heritability of Traits in Dairy Animals:
Thee
phenotypic expression of a trait, such as milk production,
depends on two factors: Inheritance (the genetic potential for
expression of a particular trait) and environment (the opportunity
to express the inherited trait). Heritability is 100% when the
expression of the trait varies solely because of inheritance. A
trait that varies solely because of environment has a heritability
zero.
Variation in the
expression of most traits is neither totally environmental nor
completely hereditary. The heritability of some important dairy
animal traits is given in Tables below. Traits with higher
heritabilities allow more rapid genetic progress. In addition,
traits with high heritabilities increase the value of the animal's
own phenotype as an estimator of genotype. Even with the most highly
heritable production traits, however, at least 50% of the population
variation is attributable to environment or heredity-environment
interactions. Although some environmental variables are not easily
managed (weather or climate, season of calving), many are directly
management influenced (feeding, housing, reproductive handling,
health care). Most traits of economic importance are primarily
controlled by a combination of both management skills and genetics.
The following
example shows how heritability can be computed: assume that a herd
averages 10,000 litres of milk on a mature basis. A sire used in
that herd is capable of transmitting inheritance for 12,000 litres
of milk production. He is mated to select animals in the herd with
production records averaging 12,000 litres of milk in a normal
lactation of 305 days. Because heritability is 30%, we expect only
3/10th of the apparent superiority of the parents to be expressed in
the offspring. The selected parents averaged 2,000 litres of milk
higher than the herd. Three-tenths of the 2,000 equals 600 litres of
milk. Thus, the offspring are expected to average 10,600 litres of
milk in this herd when given the same opportunity (environmental
effects) as the parents.
Heritabilities of Various Production and Type Traits of Cattle:
Traits |
Heritability (h2) |
Traits |
Heritability (h2) |
Milk
yield |
0.3 |
Fat
yield |
0.25 |
Fat
percentage |
0.5 |
Protein percentage |
0.50 |
Reproduction |
0.07 |
Milking rate |
0.3 |
Stature |
0.42 |
Feet
and legs score |
0.17 |
Strength |
0.31 |
Fore
attachment |
0.29 |
Body
depth |
0.37 |
Rear
udder height |
0.28 |
Dairy
form |
0.29 |
Rear
udder width |
0.23 |
Rump
angle |
0.33 |
Udder
cleft |
0.24 |
Thurl
width |
0.26 |
Udder
depth |
0.28 |
Rear
legs-side view |
0.21 |
Front
teat placement |
0.26 |
Rear
legs-rear view |
0.11 |
Teat
length |
0.26 |
Foot
angle |
0.15 |
Final
score |
0.29 |
Genetic control (heritability, h2) for various economic
traits in Nili-Ravi buffaloes:
Traits |
h2
(%) |
305-day milk yield (kg) |
20 |
Lactation length (days) |
10 |
Dry
period (days) |
3 |
Age
at first calving (days) |
10 |
Calving interval (days) |
5 |
Service period (days) |
5 |
Service per conception (number) |
3 |
Gestation period (days) |
3 |
Lifetime milk yield (kg) |
15 |
Herd
life (days) |
5 |
Productive life (days) |
20 |
Breeding efficiency (%) |
3 |
Genetic control (heritability, h2) for various economic traits in
Sahiwal cattle
Traits |
h2
(%) |
305-day milk yield (kg) |
15 |
Lactation length (days) |
12 |
Dry
period (day) |
10
|
Age
at first calving (days) |
12 |
Calving interval (days) |
10 |
Service period (days) |
15 |
Gestation period (days) |
5 |
Lifetime milk yield (kg) |
15 |
Herd
life (days) |
2 |
Productive life (days) |
2 |
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