Dealing with Recessive Genetic Defects
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Transcript Dealing with Recessive Genetic Defects
Genetic Defects:
Current Status and
Breeding Management
Jon Beever
Brown Bagger Series
October 14, 2009
Most genetic defects are going to have
recessive patterns of inheritance
◦ not problematic if present at a low allele
frequencies
◦ commercial cross-breeding programs have less
risk
Recognition of genetic defects typically
occurs after it is “too late”
◦ allele frequency is sufficiently high to cause
consistent frequency of affected calves
◦ threat proportional to population size
background
New genomic technologies insure rapid
solutions to emerging problems
◦ short- to mid-term time frame for the
identification of causative genes/mutations
development of DNA-based tests
◦ assembly of sufficient material = short-term success
◦ high accuracy
◦ cost effective
◦ breeding decisions assisted by molecular tools
◦ potential for elimination of deleterious mutation without
loss of valuable germplasm
solution
Idiopathic Epilepsy (IE)
Arthrogryposis Multiplex (AM)
Hypotrichosis (HY)
Neuropathic Hydrocephalus (NH)
Osteopetrosis (OS)
Fawn Calf Syndrome (FCS)
genetic defects
Generalized “seizure” disorder
◦ neurologic
◦ Parkinson’s-like “locking up” syndrome
Origin in Hereford cattle
◦ Putative proband born circa 1982
DNA-based test released in January 2008
◦ more than 18,000 cattle tested to date
◦ relatively low frequency – <2%
◦ has been observed in “baldie” based
commercial operations
Idiopathic Epilepsy (IE)
◦ arthrogryposis
◦ scoliosis/kyphosis
◦ muscular hypoplasia
AM phenotype
Research initiated in September 2008
◦ DNA test released December 15, 2008
Relatively high allele frequency
◦ ~8% in AI sires – slightly higher in cow herd
Rapid implementation
◦ ~80,000 registered animals tested
Long-term policies in place
◦ ability to secure high merit genetics
◦ eventual reduction in frequency or elimination
current status
Commonly referred to as “marble bone”
disease
◦ late term abortion, small body size
240 to 275 days
◦ brachygnathia (parrot mouth)
may be accompanied by other skull malformations
◦ brittle, dense bones
no marrow cavities (solid bones)
reported in both Angus, Red Angus and
Hereford – present prior to 1970
Osteopetrosis (OS)
Red Angus diagnostic developed
collaboration between USDA MARC & BARC, UNL,
UW and UI
announcement of confirmed carriers by RAAA on
March 17, 2009
Low/moderate frequency
◦ probably between 1.5 to 3%
Currently not recommended for use in
breeds other than Red Angus
◦ continued investigation into Black Angus
mutation
Invariably lethal
Generalized absence of central nervous
system tissue
◦ high estimated embryonic and fetal losses
◦ pronounced hydrocephalus
◦ arthrogryposis and muscular hypoplasia
DNA-based test released in
Spring 2009
Also relatively high
frequency
◦ ~10%
courtesy of David Steffen, UNL
Neuropathic Hydrocephalus (NH)
partial absence of hair at birth
predominantly a Polled Hereford issue
◦ stems from early 60s proband
diagnostic developed and currently being
deployed
◦ low/moderate frequency
Hypotrichosis (HY)
Semi-lethal
◦ joint laxity/contractures
connective tissue
Recessive inheritance
◦ confirmed by WGA/
homozygosity analysis
◦ 18 calves – 1.5 Mb interval
Gene identified
◦ preliminary test shows low frequency
◦ DNA-test available soon
Fawn Calf Syndrome (FCS)
Two major components to accuracy
◦ scientific basis and testing process/execution
Tests are based on specific mutations
associated with each genetic defect
◦ tests do not use “linked” or “associated”
changes in the DNA
Testing process starts at sample collection
and ends at reporting
how accurate are the tests?
Expense vs. outcome
◦ low cost – no affected calves born
sires only – no affected calves born to genetically
“free” sires
◦ moderate cost – on the road to elimination
sires, herd matriarchs and annual replacement
heifers
◦ highest cost – complete management
all animals in the herd
◦ does not imply elimination, only management
breeding management
female parent gametes
male parent gametes
A
a
A
A
AA
AA
25%
25%
Aa
Aa
25%
25%
A mating using at
least one free (AA)
parent
Free parent can
only produce A
gametes
No affected
offspring produced
50:50
recessive inheritance
Are there other defect-free animals with
equal genetic value?
Is it worth the $$/opportunity cost?
Is your management good enough?
What is the purpose of retaining carriers?
How important is it to eliminate defects
from the population?
should I use carrier animals?
Differs based on place in production
system
◦ Seedstock
highest management
◦ Commercial with replacement
commitment to manage female base
◦ Commercial terminal
little or no risk
implementation
education
the psychology of breeders toward genetic
defects
industry wide standard reporting
processes – reimplementation of “old”
protocols
central location(s) for establishing
collections for DNA analysis
future directions
genetic defect research should be viewed
as “preventative” investment
solutions can be very rapid
must have a proactive and positive
attitude toward defect surveillance and
reporting
summary