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Genetics and inheritance

Genetics and inheritance. In your own words, explain the following terms. Chromosome Gene Features Gametes Zygote Diploid Allele Homozygous Heterozygous Selective breeding Genetic engineering Meiosis Mitosis Sexual reproduction Asexual reproduction Mutations Dominant Recessive.

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Genetics and inheritance

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  1. Genetics and inheritance

  2. In your own words, explain the following terms • Chromosome • Gene • Features • Gametes • Zygote • Diploid • Allele • Homozygous • Heterozygous • Selective breeding • Genetic engineering • Meiosis • Mitosis • Sexual reproduction • Asexual reproduction • Mutations • Dominant • Recessive

  3. chromosome The instructions that tell cells what we look like are carried in these. There are 23 pairs of them in a normal human cell.

  4. Genes These are the units which make up chromosomes. Responsible for inheritance of specific characteristics

  5. Cellular Functions of Human Genes

  6. FEATURES These are things like eye colour, skin colour and hair colour. They are controlled by genes.

  7. gametes Sperm and egg cells are both this type of cell. Contain half the amount of DNA of normal diploid cells

  8. zygote When a sperm and egg cell fuse together, they produce this.

  9. diploid We use this word to describe cells which contain the full complement of genetic material. In humans this would be 46 chromosomes (23 pairs)

  10. alleles The different versions of genes One of two to many alternative forms of the same gene (eg., round allele vs. wrinkled allele; yellow vs. green).Alleles have different DNA sequences that cause the different appearances we see.

  11. Homozygous Alleles of a given gene are identical (can be either dominant or recessive

  12. Heterozygous Alleles of a given gene are not identical

  13. selective breeding Where plants and animals with useful or desired traits are bred together to produce offspring with those desired traits

  14. Genetic engineering The altering of the character of an organism by inserting genes from another organism

  15. mitosis Division of a cell to produce 2 daughter cells which each has the same number and kind of chromosomes as the mother cell

  16. sexual reproduction Type of reproduction that involves fusion of gametes

  17. asexualreproduction Reproduction whereby individuals are produced from a single parent

  18. Mutation Random change in the genetic material of the cell

  19. dominant The allele that is expressed where an individual is heterozygous

  20. recessive The allele that is ‘hidden’ (not expressed) when an individual is heterozygous for a given gene

  21. Mendelian Genetics The laws of heridity Gregor Mendel (1822-1884): “Father of Genetics” Augustinian Monk at Brno Monastery in Austria (now Czech Republic) -> well trained in math, statistics, probability, physics, and interested in plants and heredity. Mountains with short, cool growing season meant pea (Pisum sativum) was an ideal crop plant. • Work lost in journals for 50 years! • Rediscovered in 1900s independently by 3 scientists • Recognized as landmark work!

  22. Garden Pea • Pisum sativum • Diploid • Differed in seed shape, seed color, flower color, pod shape, plant height, etc. • Each phenotype Mendel studied was controlled by a single gene.

  23. Terms • Wild-type is the phenotype that would normally be expected. • Mutant is the phenotype that deviates from the norm, is unexpected but heritable. • This definition does not imply that all mutants are bad; in fact, many beneficial mutations have been selected by plant breeders.

  24. Advantages of plants • Can make controlled hybrids. • Less costly and time consuming to maintain than animals. • Can store their seed for long periods of time. • One plant can produce tens to hundreds of progeny.

  25. Advantages of plants • Can make inbreds in many plant species without severe effects that are typically seen in animals. • Generation time is often much less than for animals. • Fast plants (Brassica sp.) • Arabidopsis

  26. Mendelian Genetics The laws of heridity • The Law of Segregation: • Genes exist in pairs and alleles segregate from each other during gamete formation, into equal numbers of gametes. Progeny obtain one determinant from each parent. • -> Alternative versions of genes account for variations in inherited characteristics (alleles) • -> For each characteristic, an organism inherits two alleles, one from each parent. (-> homozygote/heterozygote) • -> If the two alleles differ, then one, the allele that encodes the dominant trait, is fully expressed in the organism's appearance; the other, the allele encoding the recessive trait, has no noticeable effect on the organism's appearance (dominant trait -> phenotype) • -> The two alleles for each characteristic segregate during gamete production.

  27. The Principle of Segregation • Genes come in pairs and each cell has two copies. • Each pair of genes can be identical (homozygous) or different (heterozygous). • Each reproductive cell (gamete) contains only one copy of the gene.

  28. Mendel’s Principle of Segregation • In the formation of gametes, the paired hereditary determinants separate (segregate) in such a way that each gamete is equally likely to contain either member of the pair. • One male and one female gamete combine to generate a new individual with two copies of the gene.

  29. X Parental Lines Round Wrinkled All round F1 progeny Self-pollinate 3 Round : 1 Wrinkled Round 5474 Wrinkled 1850 Principle of Segregation(Mendel’s First Law)

  30. Important Observations • F1 progeny are heterozygous but express only one phenotype, the dominant one. • In the F2 generation plants with both phenotypes are observedsome plants have recovered the recessive phenotype. • In the F2 generation there are approximately three times as many of one phenotype as the other.

  31. Mendel’s Results

  32. 3 : 1 Ratio • The 3 : 1 ratio is the key to interpreting Mendel’s data and the foundation for the the principle of segregation.

  33. X Parental Lines Round Wrinkled All round F1 progeny Self-pollinate 3 Round : 1 Wrinkled Round 5474 Wrinkled 1850 Round vs. Wrinkled

  34. A Molecular View Parents F1 F2 Progeny WW ww Ww ¼WW ¼Ww ¼wW ¼ww 1: 2 : 1 Genotype = 3: 1 Phenotype

  35. One Example of Mendel’s Work Dwarf Tall x Phenotype P dd Genotype DD Homozygous Dominant Homozygous Recessive All Tall Clearly Tall is Inherited… What happened to Dwarf? F1 Dd • Tall is dominant to Dwarf • Use D/d rather than T/t for symbolic logic Heterozygous F1 x F1 = F2 possible gametes Punnett Square: D d 3/4 Tall 1/4 Dwarf -> Phenotype: 3:1 F2 D Tall DD Tall Dd possible gametes Dwarf is not missing…just masked as “recessive” in a diploid state d Tall Dd Dwarf dd

  36. Dihybrid crosses reveal Mendel’s law of independent assortment • A dihybrid is an individual that is heterozygous at two genes • Mendel designed experiments to determine if two genes segregate independently of one another in dihybrids • First constructed true-breeding lines for both traits, crossed them to produce dihybrid offspring, and examined the F2 for parental or recombinant types (new combinations not present in the parents).

  37. Mendel and two genes Round Yellow Wrinkled Green x All F1 Round, Yellow Wrinkled Yellow 101 Wrinkled Green 32 Round Yellow 315 Round Green 108

  38. Dihybrid cross produces a predictable ratio of phenotypes genotype phenotype number phenotypic ratio • Parent Y_R_ 315 9/16 • Recombinant yyR_ 108 3/16 • Recombinant Y_rr 101 3/16 • Parent yyrr 32 1/16 Ratio of yellow (dominant) to green (recessive)=3:1 (12:4) Ratio of round (dominant) to wrinkled (recessive)=3:1 (12:4)

  39. Ratio for a cross with 2 genes • Crosses with two genes are called dihybrid. • Dihybrid crosses have genetic ratios of 9:3:3:1.

  40. Mendel and two genes Wrinkled Yellow 101 Wrinkled Green 32 Round Yellow 315 Round Green 108 Yellow = 416 Green = 140 Round = 423 Wrinkled = 133 Each gene has a 3 : 1 ratio.

  41. Summary of Mendel's work • Inheritance is particulate - not blending • There are two copies of each trait in a germ cell • Gametes contain one copy of the trait • Alleles (different forms of the trait) segregate randomly • Alleles are dominant or recessive - thus the difference between genotype and phenotype • Different traits assort independently

  42. Rules of Probability Independent events - probability of two events occurring together What is the probability that both A and B will occur? Solution = determine probability of each and multiply them together. Mutually exclusive events - probability of one or another event occurring. What is the probability of A or B occurring? Solution = determine the probability of each and add them together.

  43. Mendelian Genetics The laws of heridity 2. The Law of Independent Assortment Members of one pair of genes (alleles) segregate independently of members of other pairs. -> The emergence of one trait will not affect the emergence of another. -> mixing one trait always resulted in a 3:1 ratio between dominant and recessive phenotypes -> mixing two traits (dihybrid cross) showed 9:3:3:1 ratios -> only true for genes that are not linked to each other 3:1 9:3:3:1

  44. Linked Genes • Genes found on same chromosome will be inherited together • do not exhibit independent assortment

  45. Mendelian Genetics Problems with doing human genetics:-> Can’t make controlled crosses!-> Long generation time-> Small number of offspring per crossSo, human genetics uses different methods!! The laws of heridity

  46. Mendelian Genetics Major method used in human genetics is -> pedigree analysis(method for determining the pattern of inheritance of any trait)Pedigrees give information on:-> Dominance or recessiveness of alleles-> Risks (probabilities) of having affected offspring The laws of heridity

  47. Mendelian Genetics Standard symbols used in pedigrees: The laws of heridity carrier ”inbreeding”

  48. Modes of Heredity Most dominant traits of clinical significance are rareSo, most matings that produce affected individuals are of the form:Aa x aa Autosomal Dominant -> Affected person can be heterozygote (Aa) or homozygote (AA) -> Every affected person must have at least 1 affected parent -> expected that 50% are affected /50% are uneffected -> No skipping of generations -> Both males and females are affected and capable of transmitting the trait -> No alternation of sexes: we see father to son, father to daughter, mother to son, and mother to daughter

  49. Autosomal dominant disorders • both homozygotes and heterozygotes are affected • usually heterozygotes (inherited from one parent) • both males and females are affected • transmission from one generation to the other • 50% of children are affected

  50. Modes of Heredity Autosomal Dominant Examples: Tuberous sclerosis (tumor-like growth in multiple organs, clinical manifestations include epilepsy, learning difficulties, behavioral problems, and skin lesions) and many other cancer causing mutations such as retinoblastoma Brachydactyly

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