Transcript Reproduction

Mating Systems
Chapter 14
Animal Breeders
• Two basic types
– purebred
– commercial
• Purebred - pure breeds for which
ancestry is recorded on a pedigree
• Commercial are usually crossbreds
Figure 14.1
Pedigree of a Rambouillet ewe (CSU #942695). Note that the sire and dam are listed
as part of the pedigree. Source: Colorado State University.
Breeding Knowledge
• Use to
– preserve genetic superiority
– take advantage of hybrid vigor
– improve herd genetics
• Two types of mating systems
– inbreeding
– outbreeding
Figure 14.2
Relationship of the mating system to the amount of heterozygosity or homozygosity.
Self fertilization is currently not an available mating system in animals.
Inbreeding
• Mating of related animals
– intensive inbreeding
– linebreeding
• Inbred offspring have an increased
homozygostiy
• Figure 14.3 brother-sister
• Figure 14.4 sire-daughter
Figure 14.3
Bracket pedigree and arrow pedigree showing animal A resulting from a full brothersister mating.
Figure 14.4
mating.
Bracket pedigree and arrow pedigree showing animal X resulting from a sire-daughter
Intensive Inbreeding
• Mating closely related animals for
several generations
• Inbreeding studies show
– Increased inbreeding is usually
detrimental to
• reproductive performance
• preweaning and postweaning growth
• More pg. 258-260
Figure 14.5
An arrow pedigree showing animal H resulting from three generations of sire-daughter
matings. Note that J is the sire of H and also the sire of K, the latter being the dam of H. Animal J
has resulted from two previous, successive generations of sire-daughter matings.
Linebreeding
• Mild form of inbreeding
• Used to maintain a high genetic
relationship to an outstanding
ancestor
• Mainly seedstock producers
Figure 14.8
Horse pedigree showing linebreeding.
Outbreeding
• Species cross
• Crossbreeding
• Outcrossing
• Grading up
Species Cross
• Can breed different species within
same genus
• Outside of the genus chromosome
number and genes are different
• Example – Jack x Stallion = ?
– Mare horse x Jack = Hinny
• Mare mules are usually sterile
Figure 14.9
Zoological classification that identifies the major species of farm animals. Adapted
from R. F. Plimpton and J. F. Stephens, Animal and Science for Man: Study Guide (Minneapolis:
Burgess, 1979).
Species Cross
• Zebu cattle with humped cattle
– some consider crossbreeding
• Bison and Cattle
– Cattalo or Beefalo
• Sheep and goats
– fertilization but embryos die
• Genetic engineering could make more
possible
Crossbreeding
• Main reasons for crossbreeding
– breed complementation
– heterosis (hybrid vigor)
• Breed complementation
– crossing so strengths and weaknesses
compliment each other
• Heterosis increase in productivity
above the average of breeds that are
crossed (Table 14.1)
Crossbreeding
• Amount of heterosis expressed is
related to the heritability of the trait
• Most commonly used in
– swine
– beef cattle
– sheep
• Very little in dairy cattle
Figure 14.11
Two-breed rotation cross. Females sired by breed A are mated to breed B sires, and
females sired by breed B are mated to breed A sires.
Figure 14.12
Three-breed rotation cross. Females sired by a specific breed are bred to the breed
of sire next in rotation.
Outcrossing
• Unrelated animals within the same
breed are mated
• Gene pairs are primarily heterozygous
• Some homozygous (between 10-20%)
due to being same breed
Grading up
•
•
•
•
•
•
Continuous use of purebred sires
1st generation 1/2 purebred 1/2 grade
2nd 3/4 PB
3rd 7/8 PB
4th 15/16 PB
Figure 14.4 page 269
Figure 14.14
Utilizing grading up to produce purebred offspring from a grade herd.
Forming new breeds
• Formed by crossing several breeds
• Sometimes called synthetic or
composite breeds
– Example - MARC I (Charolais, Brown
Swiss, Limousin, Hereford, and Angus)
• Also used in sheep, swine, and poultry
Summary (pg. 270)
• Genetic relationship estimates the
genes two animals have in common
• Mating systems are IDed by genetic
relationships
• Inbreeding increases homozygosity
• Crossbreeding increases heterosis