Genetics

• Concepts
• Decisions
• Formulas
• Phenotype
• Improvement
• Selection
• Crossbreeding
• Other topics

Chromosomes

• Paired chromosomes in each cell of the body
• Sheep have 27 pairs of chromosomes (humans have 23)
• Each chromosome is made up of DNA that contains genes
• The DNA in each member of a pair of chromosomes is not exactly the same; thus the two genes for a trait (alleles) are not necessarily the same
• The physical location of a gene on a chromosome is called a locus

Transfer of genetic information

• Sperm and eggs contain a single copy of each chromosome
• Because there are 27 pairs, there are 2²⁷ = 134,217,728 possible genetically different offspring from a ram or a ewe

• Structural proteins (muscle)
• Regulatory proteins (enzymes, hormones)
• Both genes (the two alleles on each chromosome) contribute to a structure or function
• Homozygous: the two alleles are the same
• Heterozygous: the two alleles are different (dominance, codominance, recessive)
• Epistasis: expression of allele at one locus influences expression of another allele at another locus (gene A influences expression of gene B)
• Spider syndrome: severe bone deformities shown at birth or within 30 days of birth

Only ss animals have the spider syndrome phenotype. The defective gene that causes spider syndrome has been identified by Dr. Noelle Cockett at Utah State University. (Example of dominance and recessive alleles.)

• Horn growth: controlled by three alleles of the same gene: P (polled), p' (horned in both sexes), p (horned in rams and polled in ewes)
• Fleece color: controlled by 16 loci (genes)

Inbreeding and crossbreeding

• Inbreeding: parents are more closely related than the average of the population. (More likely for deleterious recessive alleles that cause abnormalities to be present on both chromosomes.)
• Crossbreeding: parents are less closely related than the average of the population. Results in heterosis: superiority of the crossbred compared to the average of the parents (not superior to the best parent). Caused by fewer individuals with homozygous recessive alleles.

Qualitative trait: controlled by only a few genes; traits like the examples above.

Quantitative trait: controlled by many alleles of several genes (several different sequences of the same gene and/or many different genes)

• Usually of more economic importance
• Fleece weight, growth traits (mature size), reproductive traits

Booroola Merino fecundity allele (Fec^B)

• Example of a quantitative trait affected by a major gene. Ewes with one copy of this allele add about 1.5 eggs to each ovulation. Ewes with two copies add about 3 eggs to each ovulation.