Animal Breeding Basics - genes

colouredsheepNaming genes
  • We use letters for this and there is an international convention to avoid confusion.  Some genes are called after the person discovering them.
  • The capital letter is used for the dominant allele and the lower case letter for the recessive.
  • Example:
  • P = polled allele in cattle        p = horned allele
  • W = white wool in sheep        w = black wool colour
  • When we describe the animal, as it has a ‘diploid’ or double state of the allele, two letters are used.
  • Example:
  • PP = the polled cow               pp = horned cow
  • WW = white sheep                ww = black sheep
Homozygous and heterozygous
  • A homozygote is an individual that has identical alleles at a given locus – eg PP or pp.
  • A heterozygote has non-identical alleles at a given locus – eg Pp.
  • When you mate a polled bull (PP) to a polled cow (PP) – the offspring are all polled (PP).
  • When you mate a horned bull (pp) to a horned cow (pp) – the offspring are all horned (pp).
  • When you mate a polled bull (PP) to a horned cow (pp) the offspring are all polled (Pp).  The dominant big P allele stops the small p allele from being expressed. 
  • Note these are heterozygous as they have non-identical alleles and consequently won’t breed true.
  • If you cross a Pp bull with a Pp cow, you get a range of types:
    • 1 homozygous polled (PP)
    • 2 heterozygous polled (Pp)
    • 1 homozygous horned (pp)
  • Mendel discovered this 1:2:1 ratio (where there are one pair of alleles) in his early experiments.
  • If you complicate things by crossing two pairs of alleles, you get a different ratio.
  • Example: Cross a black (BB) polled (PP) Angus bull with a red (bb) horned (pp) Hereford cow.  The results are:
    • 9 black polled (containing B and P)
    • 3 red polled (containing bb and P)
    • 3 black horned (containing B and pp)
    • 1 red horned (containing bb and pp)
    • This is Mendel’s 9:3:3:1 ratio.
  • So things can become quite confusing if you keep on adding extra pairs of alleles. For example:
No. of alleles No. of gametes No. of genotypes
1 2 3
2 4 9
3 8 27
n 2 to the power n 3 to the power n
  • If n = 20, the possible number of genotypes is more than a thousand million.
  • A gamete is a reproductive cell- a sperm or egg.
  • Coat color and pattern is a good example of many alleles that have to be taken into account in predicting the results in matings.  It can be very complex.
Phenotype and genotype
  • Phenotype is what the animal looks like – so the PP and Pp animals look the same (polled).
  • Genotype is their genetic makeup – so PP and Pp are very different genetically.
  • In breeding, if you want to differentiate between these types, you do it by back crossing to a known genotype, eg using PP or pp, and predicting that the answer should be.  Then you check the predictions with the observed to decide the answer.
Lethal genes
  • Some genes kill the carrier or cause severe physical defects.
  • Examples:
    • Dropsical (bulldog) calves in cattle.
    • Imperforate anus in calves, sheep and pigs.
    • Hydrocephalus in cattle, sheep and pigs.
    • Cleft palate (general).
    • Nakedness in poultry.
    • Hairlessness (general)
    • Amputated limbs (general)
  • These genes can have dominant or recessive alleles and it’s important to know which it is.  Example:
    • For a dominant lethal allele D
    • DD dies; Dd dies; dd lives
    • For a recessive lethal allele ‘l’
    • LL lives; Ll lives; ll dies.
  • The concern for breeders is to know which animals are carriers of these lethal genes, as you may buy or sell them in all innocence with unfortunate or legal implications.
Linked genes
  • Some genes that lie on the same chromosome appear to be transmitted as a group.  This is linkage and the genes are said to be linked.
  • So the independent assortment of genes discovered by Mendel does not always happen.
  • It’s only of concern when good and bad genes are linked.  For example in the Drysdale sheep where we want the hairy fleece for carpets but it’s linked to horns in both sexes, which make shearing and handling more difficult.
Sex linkage
  • This is where certain genes are on the sex chromosomes.
  • Where the gene is carried on the larger X chromosome, both sexes will show it.
  • Where the gene is carried on the smaller Y chromosome, only the male (XY) will show the effect and it will be passed on from father to son.
  • It’s important to know if the sex-linked gene is dominant or recessive.
  • If the allele is dominant, every affected offspring will have an affected parent, ie it is seen in every generation. An example is severe rickets in humans.
  • If the allele is recessive, the gene appears to skip generations.  An example is red-green colour blindness in humans.
  • Sex linkage was used in poultry to sex day-old chicks instead of using vent inspection.  A dominant ‘silver’ allele and a recessive “gold” allele were used, so the males were one colour and the females another.
Mutations
  • A mutation is a change in a gene, and they give rise to new alleles at a particular locus.
  • It’s the way we get new genetic variation and can form new breeds, seen best in dogs.  The Bedlington terrier is a woolly version of a whippet, and miniature cattle and horses have come for mutations.
  • Mutation rates for a gene are usually very low eg. one in a million.
  • Mutations can be hastened by radiation, certain chemical treatment, abnormally high or low temperatures or by other genes carried by the organism.
Summary of Mendel’s genetics
  • Mendelian genetics is still the basis of animal breeding.
  • It explains how simple traits, controlled by few genes are inherited
  • In breeding animals, you’ll find them most useful in :
    • Predicting what color the offspring from a cross will be
    • Whether there is a likelihood of some genetic recessive arising from a mating.
    • But that’s about it!
  • A great deal of dog, cat and bird breeding is mainly about these kinds of traits.
  • In breeding farm animals, these traits are not our main concern.
  • When we want to improve the productivity of a group of animals, by changing their weight, fertility or milk production, then we need to understand population genetics.
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