domestic breeds, genes, genotypes and phenotypes

line breeding

Robert Bakewell (1725-1795)

selective breeding

gradual involvement of humans

animals have diversity due to environmental influence

• animals that display desired attributes are used in reproduction

Robert Bakewell

pioneer of livestock breeding natural selection, deliberate animal breeding

• great variety of breeds

origins of todays livestock breeds

natural selection and evolution

• descent with modification from a common ancestor

Charles Darwin- the origins of species

Galton’s conditions of domestication

They should be hardy

they should have an inborne liking to man

they should be comfort-loving

they should be found useful to the savages

they should breed freely

they should be easy to tend

Soay

Sheep

one of the more primitive breeds in the UK

representative of original breed

sheep breed development

Developed for living conditions

eg. mountains, hills, lowland

Hill Ewe- Scottish blackface

standardised for wool

with an average fibre diameter

dog breeds- rapid phenotypic evolution under domestication

animal genetics

study of inheritance in animals

Animal breeding

application of principles of animal genetics

the goal is improvement of animals

Breed

a group of domestic animals with a homogeneous appearance, behaviour,   and other characteristics that distinguish it from other animals of the same   species.

Selecting the best animals

no animal is best for all situations

systems approach

• knowledge of traits of importance

• how performance interacts with factors

• eg. physical environment

gene

The basic physical unit of heredity consisting of a DNA sequence and a specific location on a chromosome

locus

the specific location of a gene on a chromosome

Allele

an alternative form of a gene. One of two or more forms of the same gene

sheep karyotype

haploid number= 54

genotype

the combination of genes at a single locus or at a number of loci

homozygote

a one-locus genotype containing functionally identical alleles

heterozygote

a one-locus genotype containing functionally different alleles

genetic variation and mutations

1) chromosomal mutations

• deletion, translocation, duplication and inversions

2) gene mutations

• substitution, insertion, deletions. frameshifts

transfer of genetic information through cell division

mitosis- multiplication division

meiosis- re-shuffling of alleles, genetic variation

the benefits of sex

increase the rate of adaptive evolution by accelerating the speed at which beneficial mutations sweep through sexual, as apposed to asexual, populations

sexual reproduction

•Over time, genomes accrue mutations that have either a positive or a negative effect on an individual's fitness   (relative benefit or cost of each mutation is indicated by size)

•During sexual reproduction, chromosomes are shuffled by recombination, changing the mutations that are grouped together in offspring. This process enables individual mutations to be independently retained or removed by selection.

additive gene addition or co-dominance

type of gene action

performance of the heterozygous animals is exactly intermediate to the two homozygous types

complete dominance

gene action

a form of dominance in which the expression of heterozygote is identical to the expression of the homozygous dominant genotype

partial dominance

gene action

when the effect of one allele is partially, but not completely, masked by the presence of another allele

over dominance

gene action

when heterozygotes show more extreme performance than either homozygote

sex-linked genes

genes that are lactated on the sex chromosome, rather than the autosome

• e.g. Inverdale allele

sex limited

gene action

the trait is only shown in one sex

sex influenced genes

some sheep breeds, the male are horned and the females usually polled

genetic imprinting

gene action

epigenetic phenomenon by which certain genes are expressed in a parent-of-origin specific manner = mono-allelic expression.

• e.g callipyge genomic locus in sheep