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Basic Concepts in Genetics

Mendel and the Gene Idea. Basic Concepts in Genetics. The passing on of characteristics from parents to offspring Trait physical characteristic Genetics i s the study of heredity. What Is heredity. Patterns of Inheritance. Gregor Mendel

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Basic Concepts in Genetics

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  1. Mendel and the Gene Idea Basic Concepts in Genetics

  2. The passing on of characteristics from parents to offspring • Trait • physical characteristic • Genetics • is the study of heredity What Is heredity

  3. Patterns of Inheritance • Gregor Mendel • (1822-1884) is the called the “Father of Genetics” • Researched with garden peas • Developed the ideas that are the basis of genetics

  4. Mendel used peas… • Characters (inherited characteristic) are in two distinct forms (such as white and purple color) called traits. • Not many traits • Easy to keep track • The male and female gametes are enclosed within the same flower – He could control the fertilization process • Self-fertilization • Cross-pollination • The garden pea is small, grows easily, matures quickly and produces many offspring.

  5. Mendelian genetics • Mendel studied a number of characteristics in pea plants including: • Height - short or TALL • Seed color - green or YELLOW • Seed shape - wrinkled or ROUND • Seed coat color - white or GRAY • Pod shape - constricted or SMOOTH • Pod color - yellow or GREEN • Flower position - terminal or AXIAL

  6. questions • What did Mendel cross? • What are traits? • What are gametes? • What is fertilization? • What is heredity? • What is genetics?

  7. Gene – segments of DNA that code for the basic units of heredity and are transmitted from one generation to the next • Allele – genes that reside at the same locus on homologous chromosomes • Alleles are the result of mutation!!!!!! Chromosome Logical Structure

  8. Genotype • Gene(s) responsible for the trait • The alleles that are present on each homologous chromosome that code for the trait genotype

  9. Phenotype • Expression of the characteristic • The trait • The way we “look” • Red hair or Brown hair • The expression of the gene Phenotype

  10. The likelihood that a specific event will occur Probability = # of one kind of possible outcome total # of all possible outcomes a coin lands on “heads” 1 outcome Total possible outcomes = 2 heads or tails Possibility that the coin will land on heads = 1/2 Predicting Outcomes-probability

  11. At each locus (except for sex chromosomes) there are 2 genes. These constitute the individual’s genotype at the locus. The alleles that are present. • The expression of a genotype is termed a phenotype. For example, hair color, weight, or the presence or absence of a disease. Genotypes Phenotypes

  12. Mendel was a careful researcher • Carefully controlled experiments • Studied one trait at a time • Kept detailed data • Cross - combining gametes from parents with different traits • The offspring are called hybrids • offspring of parents with different traits • A monohybrid cross is one that looks at only one trait (let’s look at plant height – tall or short) • Cross fertilization • Pollen from one plant to fertilize another plant

  13. Step One: • Mendel allowed the peas to self-pollinatefor several generations. Mendel’s monohybrid crosses What Did Mendel Find?

  14. Step One: • Each variety was true-breeding for a particular character. • tall plants only produced tall plants • These plants served as the parental generation. • The P generationis the first two individuals that are crossed in a breeding experiment Mendel’s monohybrid crosses

  15. Step Two: • Mendelcross-pollinated two P generationplants with different traits • The offspring were the first filial generation or F1 generation • Mendel recorded the traits of the offspring Mendel’s monohybrid crosses What Did Mendel Find?

  16. Tall plant crossed with short plants produced all tall offspring • Purple flowers crossed with white flowers produced all purple offspring Mendel’s monohybrid crosses

  17. Step Three: • Finally, Mendel allowed the F1 generation to self-pollinate. • He called the offspring of the F1 generation, the second filial generation, or F2 generation • Again, Mendel recorded the traits of the offspring Mendel’s monohybrid crosses What Did Mendel Find?

  18. Mendel found that 3 out of 4 (¾) of the offspring were tall & 1 out of 4 (¼) were short • (the short plants reappeared!!!!!!) Mendel’s monohybrid crosses

  19. Mendel found the same 1:3 ratio (1 out of 4) in the other traits as well! Mendel’s monohybrid crosses

  20. He discovered different lawsand rules that explain factors affecting heredity Mendel’s results

  21. Before Mendel, people thought offspring were a blend of traits • Tall x short = medium • Mendel’s experiments did not support this theory • Mendel’s work led him to the understanding that traits are carried in pairs (one from each parent) Theory of Heredity

  22. Hypotheses 1 • For each inherited character, an individual has two copies of each gene • One on each chromosome • Alternative versions of genes account for variations in inherited characters. • Alleles are different versions of genes that impart the same characteristic. Mendel’s 4 hypotheses

  23. Hypotheses 2 • Offspring inherit 2 alleles, 1 from each parent for each characteristic (i.e. height, color, etc.) • Gametes (sperm or egg) carry only one allele as a result of pair separation during meiosis Mendel’s 4 hypotheses

  24. Hypotheses 3 • The Rule of Dominance • If the two alleles differ • one, the dominant allele, is fully expressed in the organism's appearance • the other, the recessive allele, has no noticeable effect on the organism's appearance. Mendel’s 4 hypotheses

  25. Hypotheses 4 • The two genes for each character segregate during gamete production. • The chromosome count is changed from the diploid number to the haploid number. • The genes are sorted into separate gametes, ensuring variation.  • This sorting process depends on genetic recombination Mendel’s 4 hypotheses

  26. Law of Random Segregation A parent randomly passes only one allele for each trait to each offspring mendel’s First law of heredity

  27. Law of Independent Assortment • Different gene pairs assort independently in gamete formation. • This Law is only true for genes on separate chromosomes! Mendel’s second law of heredity

  28. P GENERATION -True Breeding Parent Plants • All purple (PP) • All white (pp) • Gametes will be either P or p • FIRST FILIAL GENERATION F1 • F1 are all purple because of dominance (Pp) • SECOND FILIAL GENERATION F2 • F2 results in a mathematically predictable 3:1 ratio Law of independent assortment

  29. A dominant allele is always expressed A recessive allele is only visible when paired with another recessive allele. Patterns of inheritance

  30. Alleles – Patterns of inheritance • To test for patterns of inheritance, Mendel crossed two organisms with different traits • Each had two distinct forms • Each allele is represented by a letter • CAPITAL = dominant • lower = recessive • Homozygous- if the two alleles for a trait are the same (AA or aa) • Heterozygous - if the two alleles for a trait are different(Aa) • TT Tttt • Dominant Trait Recessive Trait

  31. Mendelian Inheritance • Mendelian inheritance patterns • Involve genes directly influencing traits • Obey Mendel’s laws • Law of segregation • Law of independent assortment • Include • Dominant / recessive relationships • Gene interactions • Phenotype-influencing roles of sex and environment • Most genes of eukaryotes follow a Mendelian inheritance pattern

  32. Predicting Inheritance • Mendel’s Principle of Dominance • Some genes (alleles) are dominant and others are recessive. • The phenotype (trait) of a dominant gene will be seen when it is paired with a recessive gene. • The phenotype of the recessive gene will be “hidden” (not exhibited)

  33. Predicting Inheritance • To determine the chances of inheriting a given trait, scientists use Punnett squares and symbols to represent the genes. • UPPERCASE letters are used to represent dominant genes. • lowercase letters are used to represent recessive genes.

  34. Predicting Inheritance • For example: • T = represents the gene for TALLin pea plants • t = represents the gene for short in pea plants • So: • TT & Tt both result in a TALL plant, because T is dominant over t. t is recessive. • tt will result in a short plant. • Remember there are two alleles for every trait!

  35. PUNNETT SQUARES • A diagram that predicts the outcome of a genetic cross • Considers all the possible combinations of gametes • 1ST DRAW A BIG SQUARE AND DIVIDE IT IN 4’S

  36. PUNNETT SQUARES The genes from one parent go here. The genes from the other parent go here.

  37. PUNNETT SQUARES

  38. PUNNETT SQUARES

  39. PUNNETT SQUARES

  40. PUNNETT SQUARES

  41. PUNNETT SQUARES

  42. PUNNETT SQUARES F1 generation

  43. Interpreting the Results • The genotype for all the offspring is Tt. • The genotype ratio is: • Tt– 4:4 = 100% heterozygous • The phenotype for all the offspring istall. • The phenotype ratio is: • tall – 4:4 = 100% tall

  44. PUNNETT SQUARES F2 generation

  45. Interpreting the Results • This time the ratios are different! Tt. • The genotype ratiois: • TT – 1:4 • Tt– 2:4 • tt – 1:4 • 1:2:1 • 2 – homozygous • 2 – heterozygous • The phenotype ratiois: • TT, Tt, Tt = 3 tall • tt = 1 short • 3:1 – tall : short

  46. PunnetT Squares • Simple dominance: one allele is completely dominant P=Aa x Aa A=red, a=clear A a A a F1 Genotype= AA: Aa: aa 1: 2: 1 F1 Phenotype= A (red): a (clear) 3: 1

  47. Laws of probability help explain genetic events • Genetic ratios are most properly expressed as probabilities: • 3/4 tall: 1/4 dwarf • The probability of each zygote having the genetic potential for becoming tall is ¾ • The probability of each zygote having the genetic potential for becoming small is ¼ • Probabilities range from • 0 (an event is certain NOT to happen) to • 1.0 (an event is certain to happen). • How do we calculate the probability of 2 or more events happening at the same time?

  48. Product law • For simultaneous outcomes (this AND that) • What is the chance that you will roll snake eyes with two dice? (1 and 1) • Chance of rolling 1 with first die = 1/6 • Chance of rolling 1 with second die = 1/6 • Chance of rolling two 1’s = 1/6 X 1/6 = 1/36

  49. Probabilities Multiplication Rule The probability that two independent events, A and B, are realized simultaneously is given by the product of their separate probabilities What fraction would we expect to be Round AND Green 3/4 x1/4 = 3/16

  50. Sum law • For outcomes that can occur more than one way (this OR that) • What is the chance that you will roll either a 1 or a 6 with one die? • Chance of rolling 1 = 1/6 • Chance of rolling 6 = 1/6 • Chance of rolling 1 or 6 = 1/6 + 1/6 = 2/6 = 1/3

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