Transcript 14. Gene350 Animal Genetics 25 August 2009

Gene350 Animal Genetics
Lecture 14
25 August 2009
Last Time
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Examples of calculations for
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Mutation
Selection
Demonstrating changes in gene and genotypic frequencies
Today
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Single gene for animal breeding
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Coat colour
Carpet wool
Prolificacy in sheep
Polledness
Muscular hypertrophy in cattle and sheep
Dwarf poultry
Genes for sexing chickens
Pedigree checking
Coat colour
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Coat colour in mammmals is due to the presence in hair and wool of pigment
granules consisting of melanins in a protein framework
Melanins are formed by a series of metabolic pathways converting amino acid
tyrosine into either
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Eumelanins: - dark colour and often described as black but include brown and its
derivatives
Phaemelanins: - light colour. Contain sulphur. Often described as yellow but range
from yellow to red
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Melanin production occurs in organelles called melasomes
Melasomes are found in cells called melanocytes
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A melasome filled with pigment is secreted from the cell and is called a
pigment granule
Coat colour
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Genetic control of pigment formation in all mammals involves at least 6
autosomal loci
Each has multiple alleles
The loci influence production and distribution of pigment
Coat colour
Locus
Symbol Main Alleles
Effects and mode of action
Agouti
A
Ay, Aw, A, at, a, ae
controls regional distribution of
eumelanin and phaeomelanin over
the body and in individual hairs
Brown
B
Blt, B, b, bl
Affects concentration of eumelanins.
Locus encodes a tyrosinase-related
protein
Albino
C
C, cch, cb, cs, ca, c
Reduces intensity of pigmentation, first
phaeomelanin and then eumelanin,
none is left. Ecodes tyrosinase
Dilute
D
D, d, dl
Dilutes both eumelanin and
phaeomelanin by clumping pignemt
granules. Encodes a membranetransporter protein
Extension
E
Ed, E, ebr, e
Extends eumelanin or phaeomelanin
pigment in body as a whole.
membrane-transporter
P,p,ps
main effect on eumelasomes, with dark colours
much more diluted than light ones. Encodes
until
Encodes
protein
Pink-eyed
dilution
P
Coat colour
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Other loci are important in determining coat colour and pattern in certain
species
e.g. in cats autosomal Tabby locus has 3 alleles: Abyssinian tabbies (Ta),
striped tabbies (T), and blotched tabbies (tb)
X-linked orange locus gives rise to the tortoiseshell coat colour
Complications for coat colour
• Epistasis (interaction between loci):
– e.g complete masking of the tabby alleles at the tabby locus by the nonagouti allele
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Alleles at different loci can sometimes be used with more or less
equal validity to explain a certain coat colour
– e.g horse coat colour a source of great confusion
– 2 rules are valid.
– Chestnut Rule- mating chestnut to chestnut will produce a black, brown,
bay or grey;
– Grey Rule- a grey horse must have at least one grey parent.
Coat colour
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Trademark genes
Segregation of many coat-colour loci in various species
Breeders have developed a large number of different genotypes recognized as
distinct varieties or breeds
Some of these breeds were once different by alleles at only one or a few coatcolour loci
Diffrences in coat-colour can therefore become a trademark for a breed
For palomino horses, coat-colour trademark is due to heterozygosity at E
locus which means that it is impossible to obtain a population that breeds true
Carpet wool
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Drysdale breed is the basis for the carpet wool industry
Developed from the Romney breed (predominant sheep breed in New
Zealand)
Romney fleece consists of a majority of hair fibres which are
unpigmented and solid and a minority which have a hollow core (called
medulla) running down the centre of the fibre
Traditionally, selected against medulatted fibres
Extreme hairiness is due to autosomal incompletely dominant allele, Nd
Resulted from the mutation of normal, non-hairy allele, n
Fleeces from sheep that are homozygous for Nd have approximately
65% by weight more of medulated fibres- ideal for carpet manufacture
(virtually free of pigmentation and has good spinning qualities)
Heterozygotes (Ndn) have a level of medullation intermediate between
the 2 homozygotes
Nd Allele often refered to as the carpet-wool gene
Sheep vary considerably in the type of wool they
produce.
Fine wool from Merino
Carpet wool from a Karakul
One type of wool is not better than the other. They just have different
uses.
Fineness - fiber diameter
Long
Coarse
Medium
Crossbred
Fine
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Thicker
> 40 µ
Thinner
< 17µ
Grade refers to the relative diameter of the wool fibers (fineness).
Carpet wool
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The numbers of NdNd and Ndn sheep were increased rapidly giving rise
to the Drysdale breed
Other carpet wool genes include the Nt gene which is completely
dominant to n, and is seen in the Tukidale breed
A mutation at another loci has also given rise to the Elliotdale carpetwool breed from the Tasmanian Romneys
Prolificacy in sheep
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Booroola gene, fec (B), is aa autosomal gene which is a major cause of
the prolificacy seen in Booroola strain of Australian Merino
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Considerable publicity in 1993
Linkage analysis identified 2 markers that are linked to it
Using strong chromosomal homologies between sheep, cattle and
humans shown that gene is located on chromosome 6
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Another prolificacy gene discovered in sheep in New Zealand
Inverdale fecundity gene, Fec X(I) is X-linked
Unfortunately, homozygotes for FecX(I) have non-functional ovaries
Booroola merino
Polledness
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Generally, presence or absence of horns is due to action of 2 alleles at an
autosomal locus
Polled (P) is dominant to horned (p)
Many breeds are horned and therefore homozygous for p
Practical problems of carcase damage and difficult handling associated
with horned animals
Breeders now prefer animals to be polled
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Difficulty is in achieving complete homozygosity because recessive allele
remains hidden in heterozygotes
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Problem of polled gene in goats illustrates pleiotropy
In mammals, XX individuals develop into normal females
In goats, XX individuals that are horned (pp) or heterozygous for polled
(Pp) are normal females
All XX goats that are homozygous for the polled allele (PP) are intersexes
A proportion of XY goats that are homozygous for the polled allele (PP) are
sterile.
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Muscular hypertrophy in
cattle and sheep
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Double muscling is the term for this trait
Mutation of gene encoding for myostatin, a protein that counteracts
muscle growth
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Advantages include a substantially higher percentage of muscle and
lower percentage of fat, and increased food conversion efficiency
In cattle, associated with increased dystocia hence limited popularity
Allele has reached high frequency in the Belgian Blue- financial rewards
sufficient to justify veterinary assistance in avoiding dystocia
Trait also observed in sheep
Linkage analyses identified linked markers on sheep chromosome 18
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Belgian blue
Dwarf poultry
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Several different loci in domestic chicken give rise to birds with a
markedly lower mature body size
Most dwarfing genes seriously affect viability or hatchability therefore
there is no commercial interest
There is a Z-linked dwarfing gene that is of commercial value
Encodes the chicken growth hormone receptor (GHR) and the dw allele
at this locus produces a defective transcript that is only ¾ the length of
the normal transcript
In at least one strain of broilers due to a deletion in GHR gene
Prctically, dw allele can be regarded as recessive
Dwarf males are ZdwZdw and dwarf females are ZdwW
Broiler breeder lines that homozygous for zw gene have been developed
dw lines are used as a source of female parents which are mated to
males from a normal broiler strain
dw broiler-breeder lines more economical than normal broiler lines
because of reduced feed consumption and a smaller space requirement
Genes for sexing chickens
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Poultry breeders continually need to separate day-old chicks into males
and females
Sexing of day-old chicks is notoriously difficult
Conventoinal method is vent sexing
Requires skilled operators and therefore expensive
Cheaper alternative is to exploit genetically-determined differences
between sexes
Determined easily by unskilled workers
Most common difference arises from segregation of alleles at 2 Z-linked
loci i.e the feathering locus and feather-colour locus
Slow feathering allele (Ks) is dominant to rapid feathering (K+)
Silver (S) is dominant to gold (s)
Pedigree checking
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Breeding animals bought on strength of a pedigree
Incorrect pedigrees occur due to mistakes
Breed societies conduct checks on pedigrees to verify
Done by checking compatibility of pedigrees with Mendelian inheritance
at one or more identifiable loci
Traditionally involved examination of inheritance patterns at coat
colour and blood group loci
These loci do not provide a powerful test
Powerful tests provided by DNA fingeprinting using microsatellites, SNP
etc
DNA samples used come from blood and other tissues such as skin and
hair samples
These paternity tests also used in wild animals e.g birds, lions,
identifying the source of biological material e.g tusks from poached
elephants, assessing the level of inbreeding and relationship within
populations of free-ranging animals, assesing evolutionary relationship
(genetic distance) between populations