Genetics and Animal Welfare - Genetics and the Behavior of Domestic Animals - page 449

weaker. Harper et al. (1988) found that the “bobbler” defect in turkeys is not
linked to feather color. The “bobbler” defect involves abnormalities in the
inner ear and balancing difficulties. However, Silversides and Smyth (1986)
found a relationship between lighter feather color and congenital tremor. The
affected birds had abnormal myelin in the cerebellum of their brain and a
reduction in melanin pigment in the eyes and feathers. A possible explana-
tion for less of an effect in poultry might be due to the fact that baby chicks
of the white poultry breeds are born with yellow down and adult white poul-
try have yellow-pigmented legs. The mutation that makes some of the feath-
ers white in bronze colored Leghorns is associated with weakness and the
homozygous mutant does not survive ( Somes, 1979 ). Photos accompanying
this paper show that the partially white mutants also have very pale (almost
white) feet and beaks as adults. Albinism and depigmentation in most mam-
mals causes abnormalities in the rods in the retina that are used for night
vision. However, albino birds do not have retinal abnormalities ( Jeffery,
1997 ). Unlike most mammals, birds have a cone (color-vision)-dominant
retina and the development of the cones is unaffected. A few albino
mammals, such as the squirrel, have relatively normal retina development
because cones predominate ( Jeffery, 1997 ).
More recently, Oskina et al. (2009) studied the involvement of glucocor-
ticoid hormones in the appearance of white spots during embroygenesis.
A delay in the migration and development of melanoblasts (pigment cells) in
the embryos of fully pigmented gray rats was caused either by restraint stress
(emotional) or by administering dexamethasone in drinking water.
Dexamethasone is a potent synthetic member of the glucocorticoid class of
steroid drugs. At days 12

14 of gestation, pregnant females were assigned
to a treatment plan that consisted of being restrained in a small plastic cage
for 45 minutes three times a day, or in the second treatment dexamethasone
was given to the pregnant rats in drinking water. In both treatment groups,
three times more offspring were born with large areas of depigmented skin
and hair on the ventral side of the body compared to the untreated control
groups. It was also demonstrated that in adult offspring of mothers treated
with dexamethasone, the response of the hypothalmo adrenal axis

to emotional stress was lower. Prenatal emotional stress in fully pigmented
mother rats caused a rise in glucocorticoid (stress hormone) which is associ-
ated with digmented areas of skin and hair of the offspring. There is also a
down-regulation of the HPA-axis leading to a reduced response to stress in
adult offspring. The same effects were shown by artificial administration
dexamethasone ( Oskina et al., 2009 ).

pituitary

FLUCTUATING ASYMMETRY AND GENETIC PROBLEMS
Another possible warning sign of both genetic problems and detrimental
effects of the environment on development is greater fluctuating asymmetry