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DNA Deoxyribonucleic acid What genes are made of Genes act by causing the production of certain enzymes Control all chemical reactions in the body Must occur in pairs Chromosomes in the nucleus of a particular cell contain the same genetic info as the chromosomes in every cell of the body Example: Tongue chromosomes are the same as shoulder chromosomes However, genes know their functions and know when to “turn on and off” Basic Genetics
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Basic Horse Genetics
• Gene• Basic unit of inheritance• Carried on the chromosomes in the gametes• Eggs from females• Sperm from males• Provide how animal will look• Alleles• Different forms of the same gene• Same location on the chromosome• Chromosomes• Varies species to species• Horses have 32 pairs, 64 individual
Basic Genetics
• DNA• Deoxyribonucleic acid• What genes are made of• Genes act by causing the production of
certain enzymes• Control all chemical reactions in the body• Must occur in pairs• Chromosomes in the nucleus of a particular
cell contain the same genetic info as the chromosomes in every cell of the body• Example: Tongue chromosomes are the same as
shoulder chromosomes• However, genes know their functions and
know when to “turn on and off”
Basic Genetics
• Genome• Complete set of instructions for making an organism• Contains “master blueprint” • Genes consist of tight coiled threads of DNA
and associated with protein molecules• Genes are arranged linearly along the
chromosomes• Nucleus contains 2 sets of chromosomes• One set from each parent• Each set has 31 single chromosomes (or autosomes)• And an X and Y sex chromosome• Female XX• Male XY
Gene Descriptions
• Made up of sugar and phosphate molecules • Rungs of the “ladder” are nitrogen-containing
chemicals called bases• Each strand is a linear arrangement of repeating
similar units called nucleotides• Composed of one sugar, one phosphate, and one nitrogen• 4 different bases are present• Adenine (A)• Thymine (T)• Cytosine (C)• Guanine (G)• The particular order is called the DNA sequence • Specifies exact instructions for a particular organism
Structure of DNA
• Each time a cell divides into 2 daughter cells a full genome is duplicated inside the nucleus
• Bases• A – T• C – G• Each daughter cell receives one old and one
new DNA strand
How the Code Works
• After fertilization (egg and sperm unite) a newborn foal will have a chromosome from stallion and from mare• Now in pairs again• Offspring will be genetically different because
the combination occurs randomly
Fundamentals of Inheritance
• Random Segregation• Random transfer of chromosomes and their genes to
form gametes• Causes major causes of genetic differences among
related individuals• Genetic Variation• Differences in genetic makeup
Randomness
Controlled by a single or few gene pairs• Phenotypes• Characteristics we can see
• Genotype• Identifying genes
• 3 Types of Gene Action:• Dominance• 2 different (heterozygous) alleles (genes) for the trait are present, the
dominant phenotype will be expressed• The phenotype of recessive alleles in only expressed in the homozygous
(same) condition• Codominance• 2 alleles share same phenotypic expression • Blend of both parents (ex. Black and white produce spots)
• Partial of incomplete dominance• 2 alleles influence phenotypic expression (ex. Red and white produce
pink)
Qualitative
• Controlled by hundreds or thousands of gene pairs located on different chromosomes
• Economically important traits
Quantitative Traits
Influence of Genetics and Environment on Some Traits
Trait Variation Due to Genetics
Variation Due to Environment
Height at Withers 45-50 50-55
Body Height 25-30 70-75Body Length 35-40 60-65Heartgirth 20-25 75-80
Running Speed 35-40 60-65Walking Speed 40-45 55-60
Movement 40-50 50-60Temperament 25-30 70-75Reproductive
Traits 10-15 85-90
• Phenotype = Genotype + Environmental Effects• In an ideal environment a horse presumed to have an
inferior genotype can exhibit a great phenotype• To reduce environmental effects, all horses
must be treated the same
Phenotypic & Genotypic Expression
• Most common coat colors in horses seem to be explained by the genetic effects of the alleles of at least 7 genes
• Determining the effects of these 7 genes is rather complex and relates to the dominant and recessive expression of each of the alleles of the 7 genes
Coat Color
• Defects in DNA can result in failure to form essential proteins of the formation of abnormal proteins• Can cause death or disease• Defects can be caused by abnormalities in a
single gene, the collective effect of a group of genes, or some chromosomal abnormality
Genetic Disease
Genetic Diseases of Horses Caused by a Single or Few Genes
Genetic Disease DescriptionFailure of immune system to form; horse dies of
infections; transmitted as recessive gene; mutation of a single gene
Defect in the movement of sodium and potassium in and out of muscles; causing intermittent attack
of muscle weakness, tremors and collapse; transmitted as autosomal dominant; involves on
geneFailure to produce blood clotting factor, bleeding into joints, development of hematomas (blood
clots); transmitted on X chromosomeLower jaw in shorter than upper jaw; incisor teeth
improperly aligned
Failure to form certain nerves in the intestinal tract; foals die of colic several days after birth
Animals tend to be small and weak at birth; show disorders of the reproductive system; mares are sterile; caused by failure of the X chromosome to
separate after duplication
Accumulation of fluid within compartments of the brain; resulting in crushing of brain
Hemolytic (destruction of red blood cells) disease of the newborn; caused by antibody in mare’s
colostrum; results in anemia and sometimes death; underlying cause in compatibility in blood types
Opening in the body wall at the navel (umbilical) or opening into the scrotum (inguinal) which allows
the intestines to drop through
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