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Topic 1: Introduction

Course: Advanced Animal Breeding MS program in Animal Production Faculty of Graduate Studies An-najah National University Instructor: Dr. Jihad Abdallah. Topic 1: Introduction. Some definitions.

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Topic 1: Introduction

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  1. Course: Advanced Animal BreedingMS program in Animal ProductionFaculty of Graduate StudiesAn-najah National UniversityInstructor: Dr. Jihad Abdallah Topic 1: Introduction

  2. Some definitions • Gene: the basic physical unit of heredity consisting of a DNA sequence at a specific location on a chromosome. • Locus: the specific location of a gene on a chromosome. • Allele: an alternative form (copy) of the same gene. • Biallelic (or diallelic) gene: If a gene has only two alleles. • Multiallelic gene: If the gene has more than two alleles.

  3. Genotype: the combination of alleles at a single locus or at a number of loci. • Homozygote (homozygous genotype): the alleles carried by the individual at a locus are the same. • Homozygosity: the probability of an individual to be homozygous at a given locus (frequency of homozygotes in the population) • Heterozygote (heterozygous genotype): the alleles carried by the individual at a locus are different. • Heterozygosity: the probability of an individual to be heterozygous at a given locus (frequency of heterozygotes in the population)

  4. Haplotype: a specific combination of alleles at different loci on the same chromosome. • Population: a group of interbreeding individuals living in time and space. It is usually a subdivision of a species. • Phenotype: the visible state of the individual.

  5. Types of Gene Action

  6. 1. Additive Gene Action (no dominance) • Each allele has a specific value (average effect) that it contributes to the final phenotype (independent gene effects) • The expression of the heterozygote is exactly midway between the expressions of the homozygous genotypes. J`J` JJ` JJ midpoint Phenotypic expression No allele is dominant over the other

  7. 2. Dominance • An interaction between alleles at the same locus such that in heterozygotes one allele has more effect than the other. The allele with the greater effect is dominant over the other allele (called recessive).

  8. Complete dominance: a form of dominance in which the expression of the heterozygote is the same (identical) to the expression of the homozygous dominant genotype. JJ` midpoint J`J` JJ J is dominant over J`

  9. Examples: - Mendel’s Pea plants (the tall allele, T, is completely dominant over the dwarf allele, t) TT  tall Tt  tall tt  dwarf • Presence and absence of horns in cattle (the polled allele is dominant over the horned allele): PP  polled Pp  polled pp  horned

  10. Coat color in cattle (the black color allele is dominant over the red color allele) - BB  black - Bb  black - bb  red • Spider syndrome in sheep: - SS  normal lambs - Ss  normal lambs - ss  crooked legs (have the disease phenotype) The s allele is lethal recessive.

  11. Partial dominance: a form of dominance in which the expression of the heterozygote is intermediate to the expression of the homozygous genotypes but more closely resembles the expression of the homozygous dominant genotype. midpoint J`J` JJ` JJ J is dominant over J` JJ J`J` JJ` midpoint J` is dominant over J

  12. Overdominance: a form of dominance in which the expression of the heterozygote is outside the range defined by the expressions of the homozygous genotypes and most closely resembles the expression of the homozygous dominant genotype. midpoint J`J` JJ JJ` J is dominant over J` JJ` JJ J`J` midpoint J` is dominant over J

  13. 3. Epistasis • An interaction among genes at different loci such that the expression of genes at one locus depends on the alleles present at one ore more other loci. • Example: Labrador dogs have three colors (black, chocolate and yellow) determined by genes at two loci: B locus (black color locus) and E locus (extension of pigmentation locus) • B_E_  black (BBEE, BBEe, BbEE, BbEe) • bbE_  chocolate (bbEE, bbEe) • _ _ ee  yellow (BBee, Bbee, bbee) - Only yellow dogs breed true (if two yellow dogs are mated, they produce only yellow dogs)

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