Genetics of animal breeding 9
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Genetics of animal breeding 9 Genetics of animal breeding 9 Presentation Transcript

  • Genetics of Animal Breeding Animal Science II Unit 9
  • Objectives Explain how genetics relates to improvement in livestock production Describe how cell division occurs Diagram and explain how animal characteristics are transmitted Diagram and explain sex determination, linkage, crossover and mutation
  • Additive and Non-Additive GeneEffects Two factors responsible for genetic variation in animals
  • Additive Gene Effects Many different genes involved in the expression of the trait Individual genes have little effect upon the trait Effects of each gene are cumulative with very little or no dominance between pairs of alleles Each member of the gene pair has equal opportunity to be expressed
  • Traits that Result from AdditiveGene Effects Most of the economically important traits  Carcass traits  Weight gain  Milk production All have moderate to high heritability Quantative Environment often influences expression Difficult to classify phenotypes into distinct categories because they usually follow continuous distribution Difficult to identify animals with superior genotypes
  • Non-Additive Gene Effect Control traits by determining how gene pairs act in different combinations with one another Observable Controlled by only one or a few pairs of genes Typically one gene pairs will be dominant if the animal is heterozygous for the trait being expressed. When combinations of gene pairs give good results the offspring will be better than either of its parents This called hybrid vigor or heterosis
  • Traits That Result From Non-AdditveGene Effects Qualitative Phenotype is easily identified Little environmental effect Genotype can be easily determined
  • Heritability Estimates Heritability: the proportion of the total variation (genetic and environmental) that is due to additive gene effects Heritability Estimate: expression of the likelihood of a trait being passed from the parent to the offspring Traits that are highly heritable show rapid improvement Traits with low heritability make take several generations of animals for desirable characteristics to become strong See Table 9-1,2,3 and 42-4 to see the heritability estimates for several species of livestock
  • Selecting Breeding Stock
  • Selecting Breeding Stock Computer programs and data bases developed by Universities available Breed associations provide information Breeding values and Expected Progeny Difference (EPD) help producers make fast genetic decisions Also 3 types of systems that producers can use to select breeding animals  Tandem  Independent Culling Levels  Selection Index
  • Tandem Traits are selected for one at a time and selection for the next trait does not begin until the desired level of performance is achieved with the first. Animals with one desirable trait but with other undesirable ones may be kept for breeding For the most profitable production, emphasis has to be placed on several traits when selecting breeding stock; Tandem selection does not do this! Simple to use but not recommended Least effective of the selection methods
  • Independent Culling Levels Establishes a performance level for each trait in the selection program. The animal must achieve that level to be kept for breeding stock. Selection for the breeding program is based on more than one trait Disadvantage to this type of selection is that superior performance in one trait cannot offset a trait that does not meet selection criteria Most effective when selecting for only a small number of traits Second most effective method of selection Most widely used
  • Selection Index Index of net merit is established that gives weight to traits based on the economic importance, heritability and genetic correlations that may exists between the traits Does not discriminate against a trait with only slightly substandard performance when it is offset by high performance in another trait Provides more rapid improvement in overall genetic improvement in the breeding group Extensive records are required to establish the index Is the most effective method of achieving improvement in genetic merit
  • The Practical Viewpoint Wise to use a combination of selection methods
  • The Cell and Cell Division Body is made up of millions of cells Cells are the most basic and the smallest part of the body that are capable of sustaining the processes of life Fig 9-1
  • The Parts of Cell Protoplasm- makes up most of the cell Nucleus- contains the chromosomes that contain the genes, it also controls the cells metabolism, growth and reproduction Cytoplasm- surrounds the nucleus and contains mitochondria, lysosmes, Golgi apparatuses, ribosomes Cell membrane- semipermeable, surrounds the nucleus and cytoplasm
  • Mitosis The division of cells in the animals body Allows animals (and us) to grow Replaced old cells that die
  • Chromosomes Occur in pairs in the nucleus of all body cells except the sperm and ovum Each parent contributes to one-half of the pair The number of pairs of chromosomes is called the diploid number The diploid number varies species to species but is constant for each species of animal
  • Common Livestock Diploid Number Cattle 30 Swine 19 Sheep 27 Goat 30 Horse 32 Donkey 31 Chicken 39 Rabbit 22
  • So What Happens During Mitosis? Chromosome pairs are duplicated in each daughter cell Figure 9-2 p. 196 shows a cell going through the 4 typical stages of cell division
  • What Causes Animals to Age Ability of cells to continue to divide is limited At the end of each chromosome in the nucleus there is specific repeating DNA sequence called a telomere Each time the cell divides some the of telomere is lost As the animal ages the telomere becomes shorter and eventually the cell stops dividing This causes the animal to eventually die of old age if it doesn’t die of some other cause first
  • Meiosis When cells divide by mitosis the daughter cells contain two of each type of chromosome, they are diploid Reproductive cells are called gametes The male gametes is the sperm, the female gamete is the egg When the sperm and egg unite they form a zygote If each gamete were diploid the zygote would have twice as many chromosomes as the parents, since that can not be there is a mechanisms that reduces the number of chromosomes in the gametes by one-half This specialized type of cell division is called meiosis.
  • What Happens During Meiosis? Chromosome pairs are divided so that each gamete has one of each type of chromosome The gamete cell has a haploid number of chromosomes The zygote that results from the union of the gametes has a diploid number of chromosomes
  • Fertilization Takes place when a sperm cell from a male reaches the egg cell of a female The two haploid cells (the sperm and the egg) unite and form one complete cell or zygote Zygote is diploid, it has a full set of chromosome pairs This results in many different combinations of traits in offspring
  • Transmission ofCharacteristics
  • Genes Pass heritable characteristics from one animal to another Located on the chromosomes Composed of DNA Occur in pairs just like the chromosome Gene pairs that are identical are homozygous and they control the trait in the same way If the gene pairs code for different expression of the same trait they are heterozygous and the genes are called alleles  For example one gene may code for black and another for red. The same trait is being affected but the alleles are coding for different effects Genotype is the combination of genes that an individual poses
  • Genes Provide the code for the synthesis of enzymes and other proteins that control the chemical reactions in the body These reactions determine the physical characteristics The physical appearance of an animal, insofar as its appearance is determined by its genotype, is referred to as its phenotype Environmental conditions can also influence physical characteristics  For example; the genotype of a beef animal for rate of gain determines a range for that characteristic in which it will fall but the ration the animal receives will determine where it actually falls in that range.
  • Genes Some traits controlled by a singe pair Most traits however are controlled by many pairs  Carcass traits, growth rate, feed efficiency are all controlled by many gene pairs
  • Coding Genetic Information Read p. 199-200 stop at Dominant and Recessive Genes
  • Dominant and Recessive Genes In a heterozygous pair the dominant gene hides the effect of its allele The hidden allele is called a recessive gene When working problems involving genetic inheritance the dominant gene is usually written as a capital letter and the recessive gene is written as a lowercase letter For example the polled condition in cattle is said to be dominant so it would be written as Pp
  • Example Dominant & RecessiveTraits Black is dominant to red in cattle White face is dominant to color face in cattle Black is dominant to brown in horses Color is dominant to albinism Rose comb is dominant to single comb (chicken) Pea comb in chickens is dominant to single comb Barred feather pattern in chickens is dominant to nonbarred feather—the dominant gene is also sex-linked Normal size in cattle is dominant to “snorter” dwarfism
  • Homozygous Gene Pairs Homozygous gene pair carries two genes for a trait  For example a polled cow might carry a gene pair PP or a horned cow must carry the gene pair pp  For a cow to have horns she must carry two recessive genes
  • Heterozygous Gene Pairs Carry two different genes (alleles) For example a polled cow may carry the gene pair Pp
  • Six Basic Crosses Homozygous x Homozygous (PP x PP) (Both Dominant) Heterozygous x Heterozygous (Pp x Pp) Homozygous x Heterozygous (PP x Pp) Homozygous (dominant) x Homozygous (recessive) (PPxpp) Heterozygous x Homozygous (recessive) (Pp x pp) Homozygous (recessive) x Homozygous (recessive) (pp x pp)
  • Predicting Results Punnett Square Male gametes on top Female gametes on the left Male Gametes side P P Female Gametes P PP PP P PP PP
  • Multiple Gene Pairs When you have more than 1 gene combination you must account for all the possible combinations For example you are crossing a polled black bull (PpBb) and a polled black cow (PpBb) both are heterozygous for polledness and color
  • Multiple Gene Pairs MALE PB Pb pB pb PB PPBB PPBb PpBB PpBb FEMALE Pb PPBb PPbb PpBb Ppbb pB PpBB PpBb ppBB ppBb pb PpBb Ppbb ppBb ppBb
  • Incomplete Dominance Occurs when the alleles at a gene locus are only partially expressed Usually produces a phenotype in the offspring that is intermediate between the phenotypes that either allele would express
  • Codominance Occurs when neither allele in a R R heterozygous condition dominanates the other and W RW RW both are fully expressed Example W RW RW  Roan color in Shorthorn Cattle R W R RR RW W RW WW
  • Sex-Limited Genes The phenotypic expression of some genes is determined by the presence or absence of one of the sex hormones Limited to one sex Example: Plumage patterns in male and female chickens  Males neck and tail feathers are long, pointed and curving
  • Sex-Influenced Genes Some traits are expressed in one sex and recessive in the other In humans male pattern baldness is an example In animals horns in sheep and color spotting in cattle  Horns are dominant in male sheep and recessive in females
  • Sex Determination: Mammals Sex of the offspring is determined at X Y fertilization Female mammals have two sex chromosomes in addition to the regular chromosomes.  They are shown as XX X XX XY Male mammals have only one sex chromosome, the other chromosome of the pair is shown as Y  Thus the male is XY X XX XY Sex of offspring is determined by the male
  • Sex Determination: Birds X Female determines the sex of Z Z the offspring Male carries two sex chromosomes Female carries one Z ZZ ZZ After meiosis all the sperm cells carry a Z chromosome and only one-half of the egg W ZW ZW cells carry a Z, the other half carry a W
  • Sex Linked Characteristics Genes are only carried on sex b b Z Z chromosomes Example is barred color in chickens Barred is dominant to black ZB ZB Z b ZB Z b Result of crossing a barred female ZB W with a black male b b Z Z W Z bW Zb W
  • Linkage Tendency for certain traits to stay together in the offspring The closer the genes are located together on a chromosome the more likely they are to stay together
  • Crossover May result in the predictions of mating not always happening During one stage of meiosis the chromosomes line up very close together. Sometimes the chromosomes cross over one another and split This forms new chromosomes with different combinations of genes The farther apart two genes are on a chromosomes the more likely they are end up in new combination
  • Mutation Generally genes are not changed from parent to offspring However, sometimes something happens that causes genes to change When a new trait is shown which did not exist in either parent is called mutation Radiation will cause genes to mutate Some mutations are beneficial, some harmful and other are of no importance Very few mutations occur and are not depended on for animal improvement Polled Hereford cattle are thought to be the result of a genetic mutation
  • Summary Livestock improvement is the result of using the principles of genetics Gregor Mendel is considered the father of genetics The amount of difference between parents and offspring is caused by genetics and the environment Heritability estimates are used to show how much of a difference in some traits might come from genetics Animals grow by cell division Ordinary cell division is called mitosis During mitosis each new cell is exactly like the old cell Reproductive cells are called gametes Gametes divide by meiosis Male gamete is the sperm Female gamete is the egg
  • Summary Fertilization occurs when the sperm cell penetrates the egg and the chromosome pairs are formed again when fertilization takes place Genes control an animals traits Some genes are dominant and some are recessive Animals may carry two dominant or two recessive genes for a trait. They are called homozygous pairs Animals may also carry a dominant and recessive gene pair. They are called heterozygous pairs Sex of mammals is determined by the male Sex of birds is determined by the female Some characteristics are sex linked and are located on the sex chromosome Crossover occurs when chromosomes exchange genes Genes are sometimes changed by mutation and they are of little value in improving livestock