Fundamental Principles of Genetics
•Genetics – study of heredity
•Gene – determinant of heredity
•Chapter is outline of basic principles of genetics
Cell Theory of Inheritance
•Plants and animals composed of cells
•Plant cell – surrounded by cell wall
•Animal cell – surrounded by cell membrane
•Nucleus – location of chromosomes in cell
•Chromosomes – carrier of genes
Chromosomes
• Carriers of genetic material – genes
• Occur in pairs
• Homologous chromosomes – members of a
pair
• Number of chromosomes constant among
normal members of a species
Chromosome Number
• Diploid (2N) numbers
donkey 62
horse
mule
63
swine
sheep
54
cattle
human
46
mink
dog
78
cat
chicken 78
64
38
60
30
38
• Normal cells have diploid (2N) number
• Gametes (sperm and egg) have haploid (1N)
number
Chromosomes
• Two major types
–Sex chromosomes – one pair which influences
sex of organism
–Autosomes – all pairs other than sex
chromosomes
Sex Determination
• Mammals
–Sex chromosomes are:
• female
• male
XX
XY
homogametic
heterogametic
• Chickens and turkeys
–Sex chromosomes are:
• female
• male
ZW
ZZ
heterogametic
homogametic
Chromosomal Abnormalities
• Changes in number of chromosomes
–Aneuploidy – extra or missing chromosome
–Polyploidy – extra or missing sets of
chromosomes
• Usually lethal in animals
–Except aneuploidy of very small chromosomes
(eg Down Syndrome in humans is extra #21)
Chromosomal Abnormalities
• Changes in chromosome structure
–Deletion – piece of chromosome removed
–Duplication – piece of chromosome duplicated
–Translocation – exchange of material between
different chromosomes
–Inversion – segment of chromosome reversed
Chromosomal Abnormalities
• Changes in chromosome structure
–Cause loss of fertility
–Amount of loss dependent on type of change and
importance of the affected segment of the
chromosome
Cell Division
• Two types
–Mitosis – division of normal body cells
– Results in production of two identical diploid cells
–Meiosis – division of germinal cells to produce
gametes (sperm and egg)
-Results in production of four different haploid cells
Cell Division
• Mitosis
–Cell division of normal body cells
–Each daughter cell has same genetic complement
as original cell
Cell Division
• Mitosis
–Four phases
•
•
•
•
Prophase – Chromosomes become visible
Metaphase – Chromosomes line up across cell center
Anaphase – Sister chromatids separate
Telophase – Chromosomes condense and new nucleus
is formed
–Interphase – period between cell divisions
–Cytokinesis – the actual division of the cell
Mitosis
18. G2 Phase – Preparation
19. Prophase
20. Metaphase
21. Anaphase
22. Telophase
23. Cytokinesis
24. G1 Phase - Cell Growth
25. S Phase - Replication
Mitosis -- Meiosis
Cell Division
• Meiosis
–Division in germinal cells to produce sperm and
egg
–Each cell that results has 1N number of
chromosomes (half of number of chromosomes
in body cells
Cell Division
• Meiosis
–Two cycles of:
•
•
•
•
Prophase
Metaphase
Anaphase
Telophase
–Results in four cells with 1N number
Cell Division
• Meiosis
–Spermatogenesis – meiotic production of sperm
cells
–Oogenesis – meiotic production of egg cells
The Gene
• Composed of DNA – deoxyribonucleic acid
• Large double stranded polymer of units
called nucleotides
• Nucleotide
–Sugar – deoxyribose
–Phosphate group
–Nitrogeneous base
• adenine
guanine
• cytosine
thymine
DNA
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Transcription of RNA
• Bases in DNA code for transcription of
RNA
• RNA – ribonucleic acid
–Also nucleic acid
–Single stranded
–Uracil instead of thymine
–Ribose instead of deoxyribose
RNA
• Three primary types of RNA
• Messenger RNA
–Transfers code from DNA to protein synthesis
• Transfer RNA
–Carries amino acids to the ribosome
• Ribosomal RNA
–Forms part of the structure of the ribosome
Genetic Code
• Each amino acid coded by a three base
sequence (codon)
• Most amino acids have more than one codon
• One codon specifies beginning of a protein
• Three codons specify the end of a protein
Interactions Between Genes
• Epistasis
–Two or more gene pairs in which one gene pair
influences expression of another gene pair
• Horned, polled, scurred
–Two gene pairs (horned vs polled) (smooth vs
scurred)
–If horned – scurred vs smooth does not matter
–If polled – scurred vs smooth can express
Genes and Embryological Development
• After union of sperm and egg
–Cell division occurs
–Early development - all cells are alike
–Subsequent development – cells differentiate
–Embryological development is genetically
controlled
Genes and Embryological Development
• Lethal genes
–Genes which stop development
–Many must be received from both parents to
cause death
–Cause of high percentage of embryonic death
Biotechnology
• Biotechnology
–All technologies that pertain to molecular
manipulation of living material
–Very difficult word to characterize
• Genetic engineering
–New methods for modifying the animal genome
Transgenesis
• Transgenesis
–Movement of genes from one species into
another
–First use – development of mouse with extra
genes for growth hormone
Genetic Engineering in Plants
• Round-up-ready crops
–Several crop species have been engineered to be
resistant to the herbicide Round-up
• Bt crops
–Corn and cotton have genes from a microbe that
causes them to be resistant to corn borer and boll
weevil
• Flavr-Savr tomato
–Gene altered to lengthen shelf life
Cloning
• Clone – genetic identical
–Identical twins are clones
–Cloning by embryo splitting has been available
for several years
• Cloning from an adult DNA donor not
possible until “Dolly”
Cloning
• Dolly
–Developed at Roslin Institute
–Sheep cloned from cells from adult ewe
mammary gland
–Cells had to be “started over” to remove results
of cell differentiation
Cloning
• Potential uses for cloning in animals
–Limited importance in routine livestock
improvement
–Combine with transgenesis to produce animals
with unique genetic makeup
• transgenesis to create first copy of animal with
specific gene inserted
• cloning to make multiple copies of that animal
Marker Assisted Selection
• Current selection
–Based on actual traits measured in animals
• Marker assisted selection
–Based on identification of genetic markers that
are associated with performance traits
–Can be applied as soon as appropriate tissue
(blood, skin etc) can be obtained
–Shortens time to obtain information for choosing
superior parents
Mutation
• Mutation – change in the base sequence
–May result in change in amino acid sequence in
protein
–May result in change in the phenotype
–Change is usually detrimental
–Only source of new genetic material
Phenotypic Expression of Genes
• Phenotype = Genotype + Environment
• Gene action varies among different genes
• Dominance relationships vary widely
Phenotypic Expression of Genes
• Dominant and recessive
• Color in Angus
Genotype
BB
Bb
bb
Phenotype
Black
Black
Red
–Black is dominant to red
–Red is recessive
–BB or bb – homozygous
–Bb
- heterozygous
Phenotypic Expression of Genes
• Lack of dominance
• Color in Shorthorns
Genotype Phenotype
BB
Red
Bb
Roan
bb
White
• Both red and white are expressed in
heterozygote
Sex-linked Inheritance
• Some genes on the sex chromosomes
• Inheritance pattern affected because males
have only one X chromosome
• Hemophilia in humans
•Deficiency in ability for blood to clot
Sex-influenced Inheritance
• Inheritance that is affected by sex of
individual
• eg. scurs - dominant in males, recessive in
females
• eg. horns in sheep – dominant in males,
recessive in females
Sex-limited Inheritance
• Some traits express in only one sex
• Female
–Milk production, egg production, age at first
estrus
• Male
–Scrotal circumference
Genome Project
• Human Genome project
–Initiated by National Institutes of Health and
Department of Energy
–Large multi-year, multi-location project to map
the human genome
• Other genome projects
–Cattle, sheep, swine, horses, dogs, cats, turkeys,
chickens, mice and many other species