Chapter Seven Heredity and Breeding ALAT Presentations Study Tips If viewing this in PowerPoint, use the icon to run the show (bottom left of screen). Mac users go to “Slide Show > View Show” in menu bar Click on the Audio icon: when it appears on the left of the slide to hear the narration. From “File > Print” in the menu bar, choose “notes pages”, “slides 3 per page” or “outline view” for taking notes as you listen and watch the presentation. Start your own notebook with a 3 ring binder, for later study! 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 breeders economical 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 positions Notches & 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 marking Anesthesia 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 breeds Tattoo 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.