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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.