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