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FELASA Module 4 Genetics
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FELASA function ABD notes for module 4 concerning genetics
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FELASA Module 4: Genetics Mating, gestation and birth in rodents and rabbits
Rats and mice are fertile around seven weeks of age and can be mated after
this
You can mate one female to one male or let the male have several females
Females have an estrous cycle and can only be mated during estrous
Estrous lasts around 12 hours and happens every 4.5 days
The morning after mating there will be a vaginal plug in the female made of
sperm secretions from the male
This is used to check mating has occured
Continuous mating- where males are left with females all the time
Cycle mating- where you remove the male every time the rodents have
mated and let the females stay alone in a cage with their offspring
The stage of the estrous cycle can be checked with a vaginal swab
It is then smeared on a glass object and the glass is stained with Hemacolour (a
rapid cell staining kit)
The object is then looked at under a microscope
There will be very few cells if the mouse is in di-estrous (lasts 50-60 hours)
Pro-estrus will have some epithelial cells that have become keratinized (10-15
hours)
In estrous all epithelial cells will be keratinized (10-15 hours, usually at night)
In post-estrus there will be epithelial cells and small neutrophilic granulocytes
that have horse shoe shaped nuclei
Timed mating is used for when you need to know exactly when a mouse was mated
Mating is done within a restricted period
Vaginal smears are analysed to find females in estrous/ in pro-estrous in the
daytime
These females are then mated overnight and checked for vaginal plugs the next
morning
If done with vaginal smears the chance of a mating is only equal to the normal
gestation rate of the strain
Without analysis of vaginal smears the success rate of a timed mating is only
20%
Microscopic exams the day after mating can be used instead of checking for
vaginal plugs
In mice that have been mated you will see cells from post-estrous (epithelial and
small neutrophilic granulocytes) and long threads that are the tails of the sperm
When the fertilized egg divides it becomes a morula
When a cavity forms in the morula, it becomes a blastocyst
Gestation time for mice: 19-22 days
Gestation time for rats: 20-23 days
After three weeks a cold-blooded, nude, blind and deaf pup is born
After 9 days the ears and eyes will open, the pup gets fur and turns into a warm
blooded animal
17 days after birth the mouse looks more like a normal mouse
Mice and rats can be weaned at around 3 weeks of age
Other species of rodents and rabbits are bred slightly different
Guinea pigs have a gestation around 65 days (59-72 days)
This long gestation produces a highly developed pup which can survive without
the mother’s milk
, This isn’t recommended
Male guinea pigs mature at 3-4 months
Female guinea pigs mature at 2-3 months
The female cycle is approximately 15-17 days
Litter size is 3-4 pups
Normally bred in large pens with up to 50 females and one male
Offspring is weaned when it is large enough
Hamsters and gerbils are solitary animals so mating has to be set up according to
that
Gerbils mate for life so can be left in the cage together and have short gaps
between matings
Hamsters are set up in pairs but are separated between matings to avoid fights
Rabbits have a gestation period just over one month
Females mature at 6-7 months
Males mature between 4 and 8 months
Litter size is 5-10 pups
Females experience induced ovulation when they are mated
Male rabbits usually kill offspring to avoid competition which makes femals very
protective
Females are bought to the males cage for mating
Males ejaculate fast and faint afterwards
Breeding
In the early 19th and 20th century rodents used to come from pest control
authorities
Todays laboratory rodents derive from ones caught in the wild
1912- Wistar institute of Anatomy started selling “Wistar rats”
Albino strain that have spread to commercial breeders over the world
Major stock now used for research
Original came from a pet store
Outbreeding
As well as Wistar rats there were other colonies of outbred rats such as Sprague
Dawley
For mice there were albino colonies like Swiss Webster and NMRI
Colonies of guinea pigs, hamsters and gerbils also set up
Colonies from the first laboratory rodents are outbred
Outbreeding means they are maintained by random mating and there is a
certain gene pool in the colony
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