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Genetics 101


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This presentation by Susan Schoenian is the first from a five-part webinar series on "Breeding Better Sheep & Goats." The topic of this presentation is "Genetics 101."

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Genetics 101

  1. 1. 2013 Winter Webinar Series: Breeding Better Sheep & Goats Genetics 101 SUSAN SCHOENIAN Sheep & Goat Specialist University of Maryland -
  2. 2. DNA, chromosomes, genes, and alleles
  3. 3. DNA (deoxyribonucleic acid)• The genetic material that controls how an animal looks and performs.• Exists as two long strands spiraled into a double helix.• Is organized into chromosomes.
  4. 4. Chromosomes• Found in pairs in the nucleus of every cell of the body.• There are two kinds of chromosomes. 1. Autosomes 2. Sex chromosomes
  5. 5. Species differ in the number of chromosomes they have. n=60 n=54 30 pairs 27 pairs
  6. 6. AUTOSOMES SEX CHROMOSOMES• Comprise all but one pair • Every animal has one pair of chromosomes. of sex chromosomes.• (Mostly) control features • There are two types of sex the same in male and chromosomes denoted by females. the letters X and Y. • Females have two of the same kind of sex chromosomes (XX). • Males have two distinct sex chromosomes (XY).
  7. 7. Sex determination • The sex of the offspring is determined by the sex chromosomes. • Males contribute either an X or Y chromosome, thus determine the sex of the offspring. • Diet may also affect sex ratio.
  8. 8. Genes • A unit of inheritance composed of a segment of DNA. • Also in pairs. • Encodes the amino acid sequence of a protein. • Physical location of gene on DNA molecule is called a locus (loci).
  9. 9. Alleles (usually denoted by letters) • An alternative form of a gene (one of the pair) that is located at a specific position of a specific chromosome. • Organisms have two alleles for each trait or the expression of a trait may be affected by multiple alleles.
  10. 10. HOMOZYGOUS HETEROZYGOUS• When the two alleles are • When the two alleles are the same. different.
  11. 11. RECESSIVE (lower case letter) • Allele that causes aDOMINANT (Capital letter) phenotypic trait that is only• One allele masks the seen in the homozygous expression of another allele genotype. at the same loci.• Overrides the traits of a A a recessive allele in a heterozygous pairing. AA A Aa a Aa aa
  12. 12. Dominant traits in sheep and goats • Hairy fleece • Red, tan hair sheep • White wool (most breeds) • White goats • Red Boers • Polled • Brown eyes • Wattles
  13. 13. Recessive traits in sheep and goats • Woolly fleece • Colored fleece (most breeds) • Horns • Blue eyes • Myotonia • Colored goats • Black Boers • Genetic defects – Spider lamb disease – Cryptorchidism – Entropion
  14. 14. Incomplete or partial dominance• When the dominant allele is only partially dominant.• Example: scurs
  15. 15. Sex-limited inheritance• Sex-linked - on X or Y chromosome Example: hemophilia• Sex-limited - all or none expressed by sex Example: milk production• Sex-influenced - genotype + sex determines phenotype Examples: horns in most sheep and beards in goats.
  16. 16. Epistasis• When the action of one gene depends upon another. Example: fleece color, skin color, coat color• Inheritance of fleece and coat color can be complicated and varies by breed.
  17. 17. Genetic linkages or co-inheritance • Traits that tend to be inherited together as a consequence of an association between their genes. Examples: polledness in goats and hermaphrodism (intersex in the homozygous female).
  18. 18. Genetic correlations and responses • Extent to which the genotypic values for one trait predict the genotypic values for the second trait. • Can be a positive or negative relationship. -1 ↔ +1 • Correlation can also be due to environmental influences.
  19. 19. Positive correlations (0-1) Birth weight Weaning weight Weaning weight Post weaning weight Post weaning weight Yearling weight Rib eye area Percent retail cuts Ovulation rate Litter size Fecal egg count FAMACHA© scoreFecal consistency score Dag score Frame size Carcass weight Scrotal circumference Semen productionUltrasound rib eye area Actual rib eye area
  20. 20. (-1 – 0) Negative correlations Birth weight Lambing ease Fleece weight Fiber diameter Staple length Fiber diameter Ovulation rate Embryo survival Fecal egg count Packed cell volume
  21. 21. No correlation (zero) Scrapie genotype Production traitsResistance to a disease Resistance to another disease Reproductive rate Growth and carcass traits Reproductive rate Wool traits Horn condition Productivity Color Productivity Genetic correlations tend to vary by breed and study.
  22. 22. Genetic defectsSimple recessive inheritance More complicated• Spider lamb syndrome • Jaw defects• Cryptorchidism • Entropion• Myotonia • Hernias• Hairy lamb syndrome • Teat defects• Polledness in goats • Structural defects • Fleece defects • Rectal and vaginal prolapse • Birth (congenital) defects
  23. 23. Inheritance• Each sperm and egg contains one chromosome from each pair of chromosomes of the parent.• Which chromosome of a pair ends up in a particular sperm or egg is determined purely by chance.
  24. 24. Inheritance • There are 134,217,728 (227) possible combinations of the 54 or 60 chromosomes in a sperm or egg produced by a male and female.? • This results in considerable variation in the progeny from the same mating.
  25. 25. Genotype vs. Phenotype
  26. 26. GENOTYPE (G) PHENOTYPE (P)• The genetic make-up of a • The observable or cell, organism, or animal. measurable traits of an• Determines hereditary organism: what we can see potential and limitations of or measure. the individual. • Determined by genetics, plus environmental influences (E). P=G+E
  27. 27. Environmental influences • Diet • Health • Weather • Age • Type of birth and rearing • Age of dam • Housing • Season of birth
  28. 28. Which buck is better?
  29. 29. Two kinds of traits1. Qualitative2. Quantitative (or polygenic)
  30. 30. QUANTITATIVEQUALITATIVE • Continuous in• Phenotype falls into expression. specific category. • Considerable variation• Usually affect by in phenotype. one or few genes. • Under the influence• Very little influence of many genes of the environment. • Much more environmental influence
  31. 31. Qualitative traits• Blood type• Eye color• Coat type• Fleece or coat color• Horns• Wattles• Beards• Inherited defects – Entropion – Spider lamb disease – Cryptorchidism – Myotonia
  32. 32. Quantitative traits • Reproductive rate • Growth rate • Milk production • Fiber production • Carcass characteristics • Disease resistance • Conformation • Wool shedding • Feed efficiency
  33. 33. Quantitative traits• Are usually the traits of greatest economic importance.
  34. 34. Methods of genetic improvement 1. Crossbreeding 2. Selection
  35. 35. Next webinar: Jan 29, 7 pm EST“Breeding systems” with Jeff Semler