Chapter Seven Heredity and Breeding. If viewing this in PowerPoint, use the icon to run the show (bottom left of screen). Mac users go to “Slide Show.

Chapter Seven

Heredity and Breeding

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ALAT Presentations Study Tips

Breeding and Husbandry

Genetics deals with heredity.

why offspring resemble or differ from their parents

Similarities and differences:

due to heredity

caused by environmental factors or mutations

Successful breeding of animals depends on:

an understanding of basic genetics

breeding system that suits the needs of a particular

protocol.

Genes & Chromosomes

Physical characteristics of organisms are passed from one generation to next by genes.an inherited characteristic such as fur or eye color,

number of toes or the length of the intestine

Genes are on chromosomes & made of DNA. Chromosomes = paired structures in cell nuclei.

Number of pairs differs for various species.Normal members of a species have = number of pairs.

Only ova & sperm chromosomes are not paired.When the ovum is fertilized by the sperm, the zygote

forms paired chromosomes from the unpaired sets.Half of each parent’s genes passed to next generation.

Expression & Mutation

Expression is result of

interaction of paired genes. Genes can be either dominant or recessive. Recessive brown fur gene & dominant black fur =

expression of dominant black gene (black fur). Gene may mutate when passed parent to offspring. Mutations may occur spontaneously or by chemical

& physical environmental factors.Can be either harmful or beneficial.

Such animals have potential use as a model to help understand diseases or phenomena.

Reproduction

1st step in sexual reproduction is production of sperm & eggs.

Fertilization location/manner depend on species. In most mammals fertilization occurs within ’s

reproductive tract. In other types of animals, it occurs outside the body.

Development site also depends on species. In most mammals, zygote becomes implanted & grows

in the uterus. In birds and reptiles, embryonic development usually

occurs outside body, inside an egg.

(Image) Female Internal Reproductive Organs

Gestation & Estrus

Time required for a zygote to develop into a fetus & be born = gestation period.

Natural end of gestation period called parturition. Males produce sperm continually & are ready to

mate almost any time after reaching maturity. Females mate only during specific times. During estrous cycles, female’s eggs undergo

changes which prepare them for fertilization.During estrus the female allows mating.Ovulation usually occurs at or near this time.Cells in vaginal wall change in phases of cycle.

In some species samples can be collected from the vagina & examined to determine phase of estrous cycle.

(Image) Embryonic Growth Phases

Breeding Schemes

Breeding schemes determine how similar or different parents are from offspring.Scheme depends on animal & requirements.

Inbreeding is used to produce animals with minimal genetic variation.share characteristics & are identified as specific strain

Strains are inbred after >20 generations. Rodents are the most frequently inbred animals.

C57BL, DBA/2, C3H, and BALB/c

A mating to an unrelated animal or a different strain contaminates the inbred line.Escaped animal should not be returned to a cage.

Breeding Schemes II Outbreeding is a scheme of breeding in which

only unrelated animals of the same stock are mated.Frequently used in rodent colony management.Results in maximum genetic difference among

animals.Usually more vigorous animals & larger litter size.CF1, ICR, and Swiss

Other breeding schemes usually involve mating for desirable characteristics.

Animals share a common ancestor = line breeding. Cross breeding = mating animals of different breeds.

Mating Systems

After determining genetic type required for a project & selecting mating system appropriate to species being used, more than one mating system may be appropriate. depends on management considerations

Monogamous mating = 1 & 1 paired together for the duration of their breeding life.Simplifies record keeping & lends itself well to

maintaining inbred or outbred colonies.

Gerbil is bred most successfully with monogamous pairs.

Harem Mating

Polygamous mating system = 1 with > 2 s results in large # of young from least # of breederseconomical method of production difficult to keep accurate records

s often share nursing responsibilities, causes uncertainty of which female gave birth to offspring.

for colonies when not critical to know which is the mother

Cage must be large enough for adults and young until offspring are weaned.

Separate housed & are together for breeding.Reduces # of animals needed, permits accurate record

keeping, but labor costs are high.Used when s kill the young or & are aggressive.

Other Breeding Concerns

Consideration should be given to selection of good

breeding stock.

Animals should be healthy, young &

nonaggressive.

Females should exhibit good mothering

characteristics & adequate milk production.

The health of breeding animals must

be monitored and sign of illness or

disease should be reported immediately.

Maintenance of Breeding Animals

Rodents & rabbits may desert, kill or cannibalize their young if disturbed.Delay cage changes for a few days following birth.Minimal handling of newborn animals is a general rule.

Wean ~ 21 days of age in rats and mice. Rodents build nests if provided soft paper,

shredded wood fiber or cotton. Rabbits require a nest box & pull hair to line nest. 12–14 hours of light is best for

rodent breeding colonies. The longer light period helps

establish consistent estrous cycles.

Record Keeping

The following are examples of information that

could be kept for a breeding colony:Breed, strain and type of animal

Parentage or ancestry

Animal identification number

Sex

Date mated

Date of birth and number of offspring

Date weaned and number of offspring

Sex of young

Veterinary information

Animal Identification

Identify individual animals. critical for determining parentage and lineage

Mixing animals in a cage or placing an animal in a

different cage without proper identification can be

devastating to a research project.

Cage cards are used to

identify each animal or

group of animals. Card contains information about

history, genetic background,

IACUC #, type of experiment & contact.

Short-Term ID

Clipping or shaving various

locations or patterns Nontoxic, waterproof dyes or markers Markings, colors, sex, hair or breed Collars for cats, dogs and nonhuman primates

Must be routinely checked as animal grows.Loose-fitting collar could get snagged on cage.

Dogs and cats purchased from commercial suppliers have a USDA number on record.USDA tag may be left on collared animals.

Collars may cause skin irritation & strangulation & interfere with neck bandages & other apparatus.

Permanent Identification Methods Implantable microchip transponders transmit ID.

Subcutaneous chip implanted Recording device scans chip and reads #

Ear notching = holes at various positionsNotches & holes = a numbering code.Easy to read and produces little trauma to animal.Used in pigs & rodents, not hamsters & guinea pigs.Holes or notches close over a period of time.Sanitize punch tool between animals.

Toe clipping must have strong justification. removal of 1st bone of certain toes = a # code>time, >difficult, >stress than ear markingAnesthesia is recommended.

Ear & Wing Tags Small metal clips are stamped with individual #’s.

Using special pliers, they are applied near the

base of the ear or to the wing.

Procedure is quick and causes minimal pain.

Used on rodents, rabbits, sheep & birds.

As with ear notching, the ID can be lost.

In birds a numbered leg band is an alternative to

a wing tag.

Tattooing

2 types of tattooing devices: Hand-operated instrument clamps onto ear so pointed tips

pierce skin and ink dyes the underlying tissue. Electrical pen-like tool has a reciprocating multi-point

needle which marks the skin.

Ink is massaged into skin perforations. Clean & disinfect instruments between animals. Ear tattoo ID in rabbits

Avoid damage to medial artery or marginal ear vein.Use black ink on lighter colored breeds.

Green ink on dark-colored breedsTattoo may spread as animal grows, making the tattoo

unreadable and tattoo may have to be redone.

Tattooing Continued

Ears - g. pigs, cats, dogs, monkeys & ungulates Dogs - ears, skin of flanks or oral cavity

Dogs anesthetized and tattoo area is clipped, washed & dried.

Nonhuman primates - chest or inner thigh Even individually housed animals should be

tattooed & a cage card posted w/ all other info. Neonatal rats & mice may be permanently ID’d

by injecting tattoo ink subcutaneously

into ears, tail, hocks or toes, using

a series or pattern of dots.

Additional Reading

Nicholas, F.W. Veterinary Genetics. Iowa State University Press, Ames, IA. 1987.