vet-tech-therio-ch-7

• breeding between two related individuals
• reduces gene pool
• reduces heterozygosity, increases homozygosity because...
• ... an animal is more likely to receive the same allele from both mom and dad, so 2 of same gene
• increase in homozygosity reveals recessive genes because...
• ... recessive alleles in the genome are only epressed when they occur in the homozygous state
• inbreeding reveals the presence of recessive genes because they are expressed in the phenotype
• related animals more likely to have some alleles that are the same than non-related
• done when breeders are selecting for a particular characteristic and want to fix that characteristic in future populations
• the less the relationship, the less the chance of homozygosity at any locus

• increase in homozygosity fixes genes, animal is more likely to breed true because...
• ... when animal is homozygous it has 2 copies of the same allele to pass on at a particular locus
• a lot of the time this is bad
• as likely to fix "bad" recessive genes as good ones
• many recessive genes are "bad"
• "bad" carrier animal is less able to live or breed (ie less fit)
• unrelated individuals generally have different "bad" alleles so offspring are not homozygous

• the degree of relationship of the animals
• how many "bad" alleles the related animals have in their genome - don't know until breed
• whether the related animals have the same "bad" alleles

• to fix a desirable characteristic
• - 50/50 chance of getting a "bad" characteristic
• to detect whether one of the parents is the carrier of a recessive gene (eg deformity, illness)
• laboratory animals are deliberately inbred so that lines of animals are genetically the same
• - eg nude mice, immunocompromised mice

• describes the general reduction in vitality and viability of many inbred animals
• occurs because of increase in homozygosiy and expression of recessive genes that are deleterious
• most marked in fertility and progeny survival - inbred animals are often sterile
• main reason why inbreeding is considered a bad thing

the probability that the two genes present at a locus in that individual are identical by descent

• decrease in performance (inbreeding depression)
• - especially traits associated with viability and/or reproductive ability
• as level of inbreeding increases, so does incidence of:
• - embryonic mortality - spontaneous abortion
• - stillbirths
• - single-gene recessive disorders
• decrease in genetic variance

• form of inbreeding
• use of an individual more than once in a pedigree - eg show champion
• breeders attempting to make their animals as genetically similar to the champion ancestor as possible
• will also created inbreeding depression wth a reduction in fertility and viability

breeding of unrelated individuals

• opposite of inbreeding
• increases heterozygosity
• - unrelated individuals not likely to have same alleles, offspring will be heteroygous at many loci
• masking of recessive genes
• - when there is heterozygosity at a gene locus, no recessive genes are expressed
• reduces the likelihood of breeding true - due to heterozygosity at many loci
• increases fertility and viability of offspring
• - gives hybrid vigor or heterosis

• arises because of masking of recessive genes which depress viability in the homozygous state
• animals with hybrid vigor have a superior performance compared to their parents
• most obvious when two inbred (but different) lines of animals are mated
• each inbred line will show inbreeding depression because of homozygosity for different genes
• offspring will be heterozygous for all those genes
• - deleterious genes will be masked, offspring will show increased hybrid vigor