Genetics

1. Genetics

2. GENETICS DEFINITION • The study of the means by which genetic information is stored, replicated, translated, mutated and transferred to future generations such that anatomical and physiological systems are integrated and parental characters appear in subsequent generations.

3. KEY ELEMENTS OF THE CELL • There are several structures and sub-cellular organelles in the cell, but the main ones of interest in genetics are the nucleus, ribosomes, nucleolus and centrioles.The nucleus contains the chromosomes, the ribosomes control RNA transcription, the nucleolus produces t-RNA and the centrioles serve as focal points for cell division.

4. DNA • DNA is an acronym for deoxyribonucleic acid. It is composed of a 5 carbon sugar, deoxyribose, phosphate bonds, and nitrogenous bases. These nitrogenous bases are adenine, thymine, cytosine and guanine. Chemically adenine can only bond with thymine and cytosine only with guanine. It is in the form of a double helix.

5. DNA continued • To visualize the structure of DNA, imagine a rubber ladder in space. The uprights of the ladder are composed of molecules of deoxyribose connected in single strands with phosphate bonds. The steps of the ladder are paired bases that join to each other and also the uprights of sugar and phosphate.

6. DNA CON’T. • The steps then would be a pair of bases composed of either adenine bonded with thymine or cytosine bonded with guanine. • If you were to grasp the top and bottom of this imaginary ladder and twist in opposite directions, you would form a twisted structure that would be in the shape of a double helix.

9. The code • Three of these bases in a row are called a codon or triplet. The sequence will code for a specific amino acid. Thus, as codons are put in sequence, the structure of a protein is coded. • If we put a codon that calls for the start of a gene in front of the above sequence and another at the end to stop it we have a gene.

10. Definitions • Locus-Point on a chromosome where a specific gene is always found. • Allele-Genes that are found at the same locus on homologous chromosome that affect the same trait in different ways. • Homologous-Chromosomes of the same shape, size and length that carry similar genes.

11. Diploid chromosomes • Each normal animal has an even number of total chromosomes. These are arranged in pairs of homologous chromosomes one of each pair coming from the sire and the dam. • Cattle=60, sheep=54, swine=38, goats=60, horses=64, donkey=62

12. Homologous chromosomes • Same length, size and shape that carry similar or the same genes at the same loci. • Sex chromosomes are not homologous. In mammals are X and Y; fowl are Z and W. Males XY, Female fowl are ZW.

13. Mendelian genetics • Each animal or plant has a maximum of two alleles. • Each gamete will receive one gene from a locus. • Thus the probability of a gamete receiving a particular gene depends upon the genotype of the animal or plant for that locus.

14. Probabilities • The probability of an event happening can never be less than zero nor more than 1.0. • Thus the sum of all the probabilities will add to 1.0. • E.g. with a legal coin a toss can either result in a head (1/2) or a tail (1/2). And (½ +½ = 1.0).

15. Segregation into gametes • Mendel’s first law indicates that genes are segregated into gametes at random based on the genotype of the individual. • Thus, an animal with a genotype Aa will form gametes with either A or a and it will do so at equal probabilities.

16. Example

17. Segregation and meiosis • Recall that homologous chromosomes arrive at the metaphase plate and line up randomly one on top of the other with no regard to whether the chromosome came from the sire or the dam. • Thus, each pair of homologous chromosomes act independent of every other pair.

18. Probability of two events • The probability of two or more independent events happening at the same time is the product of their independent probabilities. • Genes carried on nonhomologous chromosomes are independent events. • In calculating, we can find each loci probability and then multiply them to reach the final answer.

19. Two genetic loci • If an animal were to have the genotype AaBb and the two loci were on different pairs of homologous chromosomes, then the A locus could produce an A or an a gamete. • Likewise, the B locus could produce either a B or a b gamete.

20. Combining loci • Remember that a gamete contains only ONE of each gene from a locus. • Thus, in constructing the possible gametes from the previous example we could have an AB, an Ab, an aB or an ab.

21. Example

22. Probabilities • The probability of each of these gametes being formed is ¼. Pr (A) = ½ and Pr (B) = ½. ½ x ½ = ¼ • There are four possible gametes each with a probability of ¼, so they add to 4/4 or 1.0 and all the probabilities are accounted for. • “What is the probability of producing a gamete (abcDEf) from AaBbCcDdEEFf?

23. Answer • 1/32 • a=1/2, b=1/2, c=1/2, D=1/2, E=1.0, f=1/2. • ½ x ½ x ½ x ½ x 1.0 x ½ = 1/32 • Each is an independent event and so each probability is multiplied by all the others. • For practice, make up genotypes and calculate the probability of several gametic genotypes.

24. Recombining into Zygotes • Gametic production in one sex has no influence over gametic production in the other. Thus, these too are independent events. • If we want to know what the possibilities of certain genotypes resulting from known matings, we need only to calculate the probabilities of gametes and multiply.

25. Aa X Aa • The probability of the first genotype producing an A or an a gamete is ½ each. • Likewise for the second genotype. • If an A sperm fertilized an A ovum, then the result would be an AA zygote. The probability of this occurring is ½ x ½ = ¼. • The same would be true for an a sperm fertilizing an a ovum.

26. The other result • If an A sperm fertilized an a ovum the result would be an Aa zygote (also ¼). • But, if an a sperm fertilized an A ovum the result would be aA. (also ¼) • The two genotypes are equivalent, it making no difference which sex contributed what gene to the heterozygote. • Thus, we add the two probabilities to = ½.

27. The result • From any heterozygous mating, we will get 1 homozygote, 2 heterozygotes and 1 homozygote (opposite). • This is the classic 1:2:1 ratio. • We can construct a chart known as a Punnet Square to illustrate this.

28. Punnet Square

29. Two loci • If we add another independent heterozygous locus, we can still treat each as an independent event. • Thus, gametes at each locus are created in equal numbers. • AaBb would yield the four gametes referred to earlier. If we construct a chart for their recombination we would get:

30. AaBb x AaBb

31. Same genotypes

32. Same genotypes

33. Same genotypes

34. Same genotypes

35. Same genotypes

36. Different genotypes

37. Phenotypic expression of genes • Non-additive gene action-shows some type of dominance • Complete or total dominance • Incomplete or partial dominance • Co-dominance • Over-dominance • Epistasis

38. Complete dominance • One allele when in the homozygous or heterozygous form will always be expressed in the phenotype. Totally covers the expression of its recessive allele. • Common type first explored by Mendel. • Aa x Aa yields: AA, 2 Aa, aa • Note: heterozygotes never breed true.

39. Incomplete dominance • One allele only partially covers the phenotypic expression of its recessive allele. • Results in a blending type inheritance. Recessive allele ‘bleeds’ into the phenotype. • Example: red flower crossed with white flower yields a pink flower. • Yields three distinct phenotypes rather than only two such as complete dominance results in.

40. Co-dominance • Neither allele will cover or dominate. Both will be expressed completely in the phenotype. • Red flower crossed with white flower would yield a red and white flower. Also the type of gene action in roan shorthorns and AB blood groups in humans. • Three phenotypes will be expressed.

41. Over dominance • Interaction between genes that are alleles such that the heterozygote is superior in performance to either homozygote. • Is a major factor in the phenomenon known as heterosis or hybrid vigor. • Graph heterosis on blackboard.

42. Heterosis • An increase in performance in crossbred or out bred animals over their parents. • Average of the F1 – the average of the P1 divided by the average of the P1 times 100 gives the percent heterosis. Note: the average of the progeny only has to exceed the average of the parents, it does not have to be superior to the best parent.

43. Epistasis • Interaction between non-allelic genes such that there is created a new phenotype. • Very common gene action in coat color inheritance in animals. The dilution gene in horses affects the b locus to dilute to palomino or cremelo. • The e locus in labs will result in a yellow lab if the e locus is ee and the b locus is either BB, Bb or bb. Difference is in the nose color.

44. Additive gene action • Shows no dominance. Instead has residual genes and contributing genes. • A residual gene is expressed in the phenotype as a particular level of performance. A1, B1 are examples. • A contributing gene only adds on to the performance of the residual, doesn’t cover. A2, B2 are examples.

45. Additive continued • A1A1 X A2A2 yields all A1A2 which would be intermediate in phenotype to the homozygotes. • Another attribute of additive genes are that they are affected by environmental influences whereas non-additive genes are affected very little by environment. • This makes it difficult to tell the genotypes from the phenotypes because a contributing gene homozygote in a poor environment might be confused with a heterozygote in a good environ.

46. Additive con’t. • This type of gene action shows itself most often in what are termed ‘economically important traits’. These include average daily gain, feed efficiency, carcass traits, etc. • Many genes and environment affect them, males are heavier and more efficient than females and the measures are continuous.

47. Additive con’t. • Other characteristics of additive genes: • Involve many gene pairs-polygenic • Shows no heterosis • Shows sex effects • Transgressive variation • Medium to high heritability estimates • Phenotypes are not distinct but instead follow a continuous variable format.

48. Non-additive comparison • Little environmental influence. • Few pairs of genes involved. • Sex effects are few. • Heterosis and inbreeding depression expressed. • No transgressive variation. • Zero to low heritability estimate. • Distinct phenotypes.

49. Facts of gene expression • Some traits affected by only non-additive or additive gene action. • Some traits are affected by both types of gene action. • Select differently depending on type. • Additive: identify the best and breed best to best. • Non-additive: easiest to outbreed or crossbreed.

50. Polygenetic traits • Most traits of economic importance are affected by polygenes. This means that several loci of genes all affect the same trait in some fashion. Our example of coat color inheritance in horses is an example of polygenes. • Additive traits are affected by many pairs of polygenes each affecting the trait in a small way.