Two animals: enough
to save a species?
In the movie ‘Noah’ and the biblical story of Noah’s ark,
two of each animal are brought aboard the ark. But could a
population be rebuilt realistically from just two animals?
BIOLOGY If an animal species is
reduced to only one male and one
female, such an extreme bottleneck
event would make future generations
highly inbred. Mating between brother
and sister or parents and offspring
could not be avoided, likely causing
poor fertility and viability. Inbreeding
is harmful because it reduces the
genetic variation of the animal group,
so harmful gene variants accumulate,
with potentially fatal consequences.
Most genes come in two variants,
of which one might be harmful. If the
other gene variant is functional, it
will normally outcompete the
harmful one, and the animal will
be heterozygous for the gene in
question. But when two hetero-
zygous animals mate, their genes are
mixed, with the harmful gene
variants ending up in the same
embryo in 25% of cases, making those
offspring homozygous for the gene in
question and so less viable. In a small
population in which inbreeding
cannot be avoided, the risk rises of
heterozygous animals mating.
Biologists are highly aware of the
consequences of inbreeding in zoos
and dog breeding, where new breeds
are produced based on very few
individuals. Nevertheless, some
dog breeds still struggle with
inbreeding symptoms. Dalmatians,
for example, are susceptible to kidney
stones because of a harmful gene
variant which, if it occurs in a double
version, releases excessive levels of
uric acid into their urine.If only two animals were used to
rebuild an entire species, as planned
on Noah’s Ark, it would probably
cause severe longterm gene diseases.
Gene variant gives some
Dalmatians kidney stones
A harmful gene variant can make Dalmatians
release too much uric acid, causing kidney
stones. Though the parents are not
themselves ill, they carry the harmful gene,
which will be expressed in 25% of puppies.ILM
/RI
TZAU (^) S
CAN
PIX
A
a
Healthy
male
Healthy
female
Healthypuppy Healthypuppy Healthypuppy Puppy produces too much
uric acid
Healthy gene for uric acid production
Harmful gene for
uric acid production
SPECIAL EFFECTS · “Noah”
Aa Aa
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