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Genetics Survey of the Animal Industry Chapter 12 Chromosomes • Tissues composed of cells • Cells have outer membrane, cytoplasm, and nucleus • The nucleus contains the chromosomes • Body cells contain chromosomes in pairs Figure 12.1 The 30 pairs of chromosomes of a bull magnified several hundred times. Note the X and Y chromosomes. Courtesy of Texas A&M University. Cell division • Mitosis - each chromosome pair divides – results in two identical cells (Fig 12.2) • Meiosis - each gamete contains only one of the chromosome pairs – sex cells combine to create pairs – 1/2 pairs from male, 1/2 from female • Chromosome number table 12.1 Figure 12.2 Mitosis. Source: Colorado State University. Gamete production • Testicles and ovaries produce sex cells called gametes – called gametogenesis – testicles (in seminiferous tubules) - sperm - spermatogenesis – ovaries - eggs or ova - oogenesis • Gametes form through meiosis – chromosomes replicate and pair up synapsis • then called primary spermatocyte and primary oocyte Spermatogenesis • Two pairs of chromosomes synapsis • First maturation division chromosome pairs • Secondary maturation division only one of each chromosome • Spermatogenesis - lose of cytoplasm and development of tail Figure 12.3 Meiosis or reduction cell division in the testicle and ovary (example with two pairs of chromosomes). Source: Colorado State University. Oogenesis – Two pairs of chromosomes - synapsis – First maturation division - chromosome pairs • secondary oocyte and first polar body – Second maturation division - one of each chromosome • produces ovum and second polar bodies • first polar body may divide or die, all three die – Ovum or egg contains only one chromosome of the two chromosomes in a pair (like sperm) Figure 12.3 Meiosis or reduction cell division in the testicle and ovary (example with two pairs of chromosomes). Source: Colorado State University. Fertilization • Each gamete supplies one chromosome to the chromosome pair • Fertilized egg - zygote • Fertilization - union of the sperm and egg with establishment of paired chromosomes Figure 12.4 Combining of chromosomes through fertilization (two pairs of genes used for simplification of example). Source: Colorado State University. Haploid vs. Diploid • Diploid – has chromosomes in pairs – one chromosome from sire and one from dam • Haploid – has only one member of each chromosome pair – gametogenesis reduces the number of chromosomes by one half DNA • Chromosomes carry genes • Homologous - two that affect same heredity characteristics • Chromosomes composed of a protein sheath surrounding deoxyribonucleic acid (DNA) – deoxyribose sugar, phosphate, and four bases • Bases called nucleotides Figure 12.5 A simplified example showing a pair of chromosomes containing several pairs of genes. Source: Colorado State University. Nucleotides • Polymer - strand of many nucleotides • two wind around each other to form double helix that is the DNA molecule • Four bases (nucleotides) – adenine (A) and thymine (T) – guanine (G) and cytosine (C) A--T G--C Figure 12.6 DNA helix and structure of nucleotides. Nucleotides • During cell division strands of DNA pull apart and the corresponding nucleotides are replaced • Most genes code for proteins – amino acids coded in triplets – triplet is a sequence of three nucleotides – triplet called codon RNA • Ribonucleic acid (RNA) • types of RNA – transfer RNA (tRNA) – messenger RNA (mRNA) – ribosomal RNA (rRNA) Protein Synthesis • Transcription - mRNA reads one or few proteins from DNA • mRNA travels out of nucleus to ribosome • tRNA contain anticodon to codon on mRNA • tRNA brings amino acid specific to its anticodon to ribosome Figure 12.7 Protein synthesis in the cell. Protein Synthesis • Ribosome reads down the full length for the mRNA • Matches appropriate amino acid from tRNA • Peptide bonds between amino acids are formed • Protein leaves ribosome to go fulfill its role Genes • Genes are located on chromosomes • Since there are pairs of chromosomes also pairs of genes • Location of gene called locus • Genes on homologous chromosomes – homozygous - correspond in controlling traits – heterozygous - differ in controlling traits Genes • Genes at same loci in homologous chromosomes are called alleles • When a gene at one loci overpowers the gene on the corresponding loci on the homologous chromosome it is dominant – Is not masked by recessive trait • Represented by – Dominant - capital letter – Recessive - lower case X and Y chromosomes • Not all of X chromosome corresponds with Y • Y is much shorter in length than X • Determines sex of animal – XX - female – XY - male • Females can only supply a X chromosome • Males supply X or Y X and Y Chromosome • In most livestock animals males supply X or Y which determines sex of offspring • In all bird species including poultry the female determines the sex of offspring – males supply X – females supply X or Y Types of Mating • Homozygous-dominant x homozygous dominant (BB x BB) • Homozygous-dominant x heterozygous (BB x Bb) • Homozygous-dominant x homozygous recessive (BB x bb) Figure 12.8 Mating of homozygous-dominant (BB) homozygous-dominant (BB). Figure 12.9 Mating of homozygous-dominant (BB) heterozygote (Bb). Figure 12.10 Mating of homozygous-dominant (BB) homozygous-recessive (bb). Types of mating • Heterozygous x heterozygous (Bb x Bb) • Heterozygous x Homozygousrecessive (Bb x bb) • Homozygous-recessive x homozygous recessive (bb x bb) Figure 12.11 Mating of heterozygote (Bb) heterozygote (Bb). Figure 12.12 Mating of heterozygote (Bb) homozygous-recessive (bb). Figure 12.13 Mating of homozygous-recessive (bb) homozygous- recessive (bb). Multiple Gene Pairs • • • • B = Black, b = red P = polled, p = horned Heterozygous for both traits BbPp Homozygous for both traits – dominant BBPP or – recessive bbpp Gene Interactions • Allelic Interactions - two unlike genes occupy corresponding loci – complete dominance – lack of dominance • Lack of dominance and additive gene action – D 0.1 lb per day, d 0.05 lb per day = • DD 0.2, Dd 0.15, dd 0.1 lb per day Figure 12.14 Bar graphs illustrating: (A) complete dominance; (B) lack of dominance; (C) overdominance. Heterozygous • Some heterozygous individuals are superior to either of the homozygote's – higher rate of gain – higher milk production – etc. • Referred to as heterosis • Also called hybrid vigor Epistatic • Gene may influence many other genes in their expression • Example horses – B black, b chestnut – BB & Bb = black, bb = chestnut • white gene masks all other genes – _ _ W_ = white, _ _ ww = black or chestnut Genes and Environment • Genotype + Environment = Phenotype • Example – Genetically similar animals fed different levels of nutrition – Are they going to perform different? • Environment effects at differing levels • Important to remember environment Biotechnology • Genetic Engineering – first implemented at turn of century • selection and hybridization – now have many other tools • embryo splitting, embryo transfer, etc • Most recently technology to manipulate genes Figure 12.15 A fertilized swine egg photographed at the moment it is microinjected with new genetic material. The vacuum in the large pipette at the bottom anchors the cell while a mixture containing the genetic material is forced through the smaller pipette into one of the egg’s pronuclei. Courtesy of R. E. Hammer and R. L. Brinster, University of Pennsylvania School of Veterinary Medicine. Gene splicing • Enzymes used to cut DNA at specific places • Cut DNA is then placed into a new cell • DNA displays its trait in the new cell – produce hormones – produce drugs – etc. Figure 12.16 Somatotropin production for use in cows and pigs. Biotech’s Future • Expectations – ID of genome for humans and animals http://www.genome.washington.edu/uwgc/ http://www.informatics.jax.org/ – increase productivity – disease resistance and treatment • gene therapy • Concerns – public acceptance Figure 12.17 The normal-appearing boar is a transgenic pig. He received a growth gene (from both mouse and cattle origin) by the process illustrated in Figure 12.15. Courtesy of R. E. Hammer and R. L. Brinster, University of Pennsylvania School of Veterinary Medicine.