MENDELIAN GENETICS DR. A. TARAB DEPT. OF BIOCHEMISTRY HKMU

Transcription

1 MENDELIAN GENETICS DR. A. TARAB DEPT. OF BIOCHEMISTRY HKMU

2 Mendel s Studies of Characters Many of your characteristics or characters including the colour and shape of your eyes and the texture of your hair resemble those of your parents The passing of characters from parents to offspring is called heredity From the beginning of recorded history, humans have attempted to alter crop plants and domestic animals to give them traits that are more useful to us

3 * trait a genetically determined characteristic or condition. Traits may be physical, such as hair, colour or leaf shape, or they may be behavioral, such as nesting in birds and burrowing in rodents Before DNA and chromosomes were discovered, heredity was one of the greatest mysteries of science

4 Humans first applied genetics to the domestication of plants and animals between approximately 10,000 and 12,000 years ago This domestication led to the development of agriculture and fixed human settlements

5 Ancient peoples practiced genetic techniques in agriculture Assyrian bas-relief sculpture showing artificial pollination of date palms at the time of King Assurnasirpalli II, who reigned from B.C

6 Mendelian inheritance (or Mendelian genetics or Mendelism) is a scientific description of how hereditary characteristics are passed from parent organisms to their offspring; it underlines much of genetics This theoretical framework was initially derived from the work of Gregor Johann Mendel, a 19 th century Austrian priest/monk, published in 1865 and 1866

7 Gregor Mendel founder of modern genetics ( ) Studied at University of Vienna from Conducted breeding experiments from Presented his results at meetings of the Brno Natural Science Society in 1865 Published his results in 1866

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9 The laws of inheritance were derived by Mendel, conducting hybridization experiments in garden peas (Pisum sativum) Between 1856 and 1863, he cultivated and tested some 29,000 pea plants From these experiments, he deduced two generalizations which later became known as Mendel s Principles of Heredity or Mendelian inheritance

10 Garden peas

11 Pea plant (Pisum sativum)

12 Useful Features in Peas 1. Several characters of the garden pea exist in two clearly different forms For example, the flower colour is either purple or white there are no intermediate forms 2. The male and female reproductive parts are enclosed within the same flower You can allow self-fertilization or crosspollination

13 3. The garden pea is small, grows easily, matures quickly and produces many offspring Thus, results can be obtained quickly, and there are plenty of subjects to count

14 Traits Expressed as Simple Ratios Mendel s initial experiments were monohybrid crosses (involves one pair of contrasting traits) Mendel carried out the experiments in three steps: Step 1 allowed each variety of garden pea to self-pollinate for several generations. This ensured that each variety was true-breeding for particular character, i.e, all the offspring would display only one form of the character

15 These true breeding plants serve as the parental generation in Mendel s experiments The parental generation, or P generation, are the first two individuals that are crossed in a breeding experiment Step 2 then cross-pollinated two P generation plants that had contrasting traits, such as purple flowers and white flowers

16 Mendel called the offspring the first filial (from the Latin filialis meaning of a son or daughter ) generation or F 1 generation He then examined each F 1 plant and then recorded the number of F 1 plants expressing each trait Step 3 finally allowed F 1 generation to selfpollinate

17 He called the offspring of F 1 generation plants the second filial generation, or F 2 generation Again, each F 2 plant was characterized and counted

18 Mendel s Results When Mendel crossed purple flowers with white flowers, all of the offspring in F 1 generation had purple flowers In F 2 generation, 705 plants had purple flowers and 224 plants had white flowers a ration of 705 to 224, which is then rounded to 3:1 For each of the seven characters Mendel studied, he found the same 3:1 ratio of plants expressing the contrasting traits in the F 2 generation

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21 Mendel s conclusions, which were unappreciated for 45 years, laid the foundation of our modern understanding of heredity He died at the age of 61 on January 6 th, 1884, unrecognized for his contribution to genetics

22 Mendel s conclusions were largely ignored Although they were not completely unknown to biologists of the time, they were not seen as generally applicable The mechanisms by which characteristics are transmitted from one generation to the next remained a mystery until the late 1800s

23 Early Written Records The ancient Greeks gave careful consideration to human reproduction and heredity They believed in the concept of pangenesis, which proposed that specific particles, later called gemmules, carry information from the various parts of the body to the reproductive organs, from where they are passed to the embryo at the moment of conception Although incorrect, the concept of pangenesis was highly influential and persisted until the late 1800s

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25 The Rise of Modern Genetics Dutch spectacles makers began to put together simple microscopes in the late 1500s, enabling Robert Hooke ( ) to discover cells in 1665 Excessive enthusiasm for this new world of the very small gave rise to the idea of preformationism

26 According to this idea, inside the egg or sperm existed a tiny miniature adult, a homunculus, which simply enlarged during development Ovists argued that the homunculus resided in the egg, whereas spermists insisted that it was in the sperm

27 Another early notion of heredity was blending inheritance, which proposed that the offspring are a blend, or mixture, of parental traits This idea suggested that the genetic material itself blends, much as blue and yellow pigments blend to make green paint However, we realize today that individual genes do not blend

28 Preformationism The homunculus: A myth Well into the 19 th century, many prominent microscopists believed they saw a fully formed, miniature fetus crouched within the head of a sperm

29 Preformationism Preformationism a popular idea of inheritance in the seventeenth and eighteenth centuries Homunculus inside a sperm

30 Lamarckism (or Lamarckian inheritance) is the idea that an organism can pass on characteristics that it acquired during its life time to its offspring (also known as heritability of acquired characteristics or soft inheritance) It is named after the French biologist Jean- Baptiste Lamarck ( )

31 Lamarck did not originate the idea of soft inheritance, which proposes that individual efforts during the lifetime of the organisms were the main mechanism driving species to adaptation, as they supposedly would acquire adaptive changes and pass them on to offspring

32 Jean-Baptiste Lamarck

33 When Charles Darwin published his theory of evolution by natural selection in On the origin of Species, he continued to give credence to what he called use and disuse inheritance, but rejected other aspects of Lamarck s theories

34 Charles Darwin

35 Later, Mendelian genetics supplanted the notion of inheritance of acquired traits, eventually leading to the development of the modern evolutionary synthesis, and the general abandonment of the Lamarckian theory of evolution in biology

36 Twentieth Century Genetics The year 1900 was a water shed in the history of genetics In 1900, Mendel s work was rediscovered by three European scientists Its most vigorous promoter was William Bateson who coined the term genetics and allele to describe many of its tenets Walter Sutton proposed in 1902 that genes are located on chromosomes

37 Thomas Hunt Morgan and his assistants later integrated the theoretical model of Mendel with the chromosome theory of inheritance, in which the chromosomes of cells were thought to hold the actual hereditary material, and create what is now known as classic genetics, which was extremely successful and cemented Mendel s place in history

38 Geneticists began to use bacteria and viruses in the 1940s At about this same time, evidence accumulated that DNA was the repository of genetic information James Watson and Francis Crick described the three dimensional structure of DNA in 1953, ushering in the era of molecular genetics

39 James Watson and Francis Crick

40 DNA - Structure

41 Mendel suggested that genes come in alternative versions that account for the variations seen in inherited characteristics The gene dictating seed colour, for example, exists in two flavours ; one that directs the production of yellow peas and one that produces green Such alternative versions of a gene are today called alleles

42 A large contribution to Mendel s success can be traced to his decision to start his crosses only with plants he demonstrated were true-breeding He also only measured absolute (binary) characteristics, such as colour, shape, and position of the offspring, rather than quantitative characteristics He expressed his results numerically and subjected them to statistical analysis His method of data analysis and his large sample size gave credibility to his data

43 He also had the foresight to follow several successive generations (F 2,F 3 ) of his pea plants and record their variations Finally he performed test crosses (backcrossing descendents of the initial hybridization to the initial true breeding lines) to reveal the presence and proportion of recessive characters Without his hard work and careful attention to procedure and detail, Mendel s work could not have had the impact it made on the world of genetics

44 Mendel discovered that when crossing white flower and purple flower plants, the result is not a blend Rather than being the mix of the two, the offspring was purple flowered He then conceived the idea of heredity units, which he called factors, one of which is a recessive characteristic and the other dominant *dominant an allele that is expressed in organism s phenotype; recessive an allele that is hidden

45 Mendel said that factors, later called genes, normally occur in pairs in ordinary body cells, yet segregate during the formation of sex cells Each member of the pair becomes part of the separate sex cell The dominant gene, such as the purple flower in Mendel s plants, will hide the recessive gene, the white flower

46 After Mendel self-fertilized the F 1 generation and obtained the 3:1 ratio, he correctly theorized that genes can be paired in three different ways for each trait: AA, aa and Aa The capital A represents the dominant factor and lower case a represents the recessive.

47 Mendel stated that each individual has two factors for each trait, one from each parent The two factors may or may not contain the same information If the two factors are identical, the individual is called homozygous for the trait same allele (BB or bb) If the two factors have different information, the individual is called heterozygous different alleles (Bb) The alternative forms of a factor are called allele

48 The genotype of an individual is made up of the many allele it possesses An individual s physical appearance, or phenotypes, is determined by its alleles as well as by its environment An individual possesses two alleles for each trait, one allele is given by the female parent and the other by the male parent * genotype the entire set of genes

49 They are passed on when an individual matures and produces gametes; egg and sperm When gametes form, the paired alleles separate randomly so that each gamete receives a copy of one of the two alleles In heterozygous individuals the only allele that is expressed is the dominant The recessive allele is present but its expression is hidden

50 Mendel summarized his findings into two laws; the Law of Segregation and the Law of Independent Assortment Law of Segregation (The first Law ) The two alleles for each trait separate (segregate) during gamete formation, then unite at random, one from each parent, at fertilization

51 More precisely, the law states that when any individual produces gametes, the copies of the gene separate so that each gamete receives only one copy (allele) In meiosis, the paternal and maternal chromosomes get separated and the alleles with the trait of the character are segregated into two different gametes

52 Law of Independent Assortment (the Second Law ) states that separate genes for separate traits are passed independently of one another from parents to offspring More precisely, the law states that alleles of different genes assort independently of one another during gamete formation

53 That is, if a gene on chromosome 1 has two alleles, a and b, and a gene on chromosome 2 has two alleles, c and d, the combinations a and c, a and d, b and c, and b and d, are all equally likely There is no preference for a to be with either c or d

54 Since chromosome 1 and 2 line up on the metaphase plate independently at the first meiotic division, with equal chance of the maternal or paternal homolog going to one pole for each chromosome, these combinations have an equal chance of occuring Thus, alleles of genes that lie on different chromosomes assort independently of one another These two laws, the law of segregation and the law of independent assortment, are the basis of Mendelian inheritance

55 While Mendel s experiments with mixing one trait always resulted in a 3:1 ratio (Fig. 1) between dominant and recessive phenotypes, his experiments with mixing two traits (dihybrid cross) showed 9:3:3:1 ratios (Fig. 2) But the 9:3:3:1 table shows that each of the two genes are independently inherited of each other, so that there is no relation, between the cats colour and tail length * dihybrid cross when two traits/genes are under consideration

56 Fig. 1 - Monohybrid Cross Dominant and recessive phenotypes (1) Parental generation (2) F 1 generation (3) F 2 generation Dominant (red) and recessive (white) look alike in F 1 generation and show 3:1 ratio in F 2 generation

57 Fig. 2 Dihybrid Cross Two traits/genes are under consideration The phenotype ratio of the offspring is 9:3:3:1

58 Fig. 2 The phenotypes of two independent traits show a 9:3:3:1 ratio in the F 2 generation In this example, coat colour is indicated by B (brown, dominant) or b (white) while tail length is indicated by S (short, dominant) or s (long) When parents are homozygous for each trait (SSbb and ssbb), their children in F 1 generation are heterozygous at both loci and only show the dominant phenotypes

59 Fig. 2 If the children mate with each other, in the F 2 generation all combination of coat colour and tail length occur: 9 are brown/short (purple boxes), 3 are white/short (pink boxes), 3 are brown/long (blue boxes) and 1 is white/long (green box)

60 Table showing how the genes exchange according to segregation or independent assortment during meiosis.

61 .and how this translates into Mendel s laws

62 The reasons for these laws is found in the nature of the cell nucleus. It is made up of several chromosomes carrying the genetic traits In a normal cell, each of these chromosomes has two parts, the chromatids A reproductive cell, which is created in meiosis, usually contains only one of those chromatids of each chromosome

63 By merging two of these cells (usually one male and one female), the full set is restored and the genes are mixed The resulting cell becomes a new embryo The fact that this new life has half the genes of each parent (23 from mother, 23 from father for the total of 46 in the case of humans) is one reason for the Mendelian laws

64 The second most important reason is the varying dominance of different genes, causing some traits to appear unevenly instead of averaging out (whereby dominant doesn t mean more likely to reproduce recessive genes can become the most common, too) There are several advantages of this method (sexual reproduction) over reproduction without genetic exchange

65 Instead of nearly identical copies of an organism, a broad range of offspring develops, allowing more different abilities and evolutionary strategies Sexual reproduction can help a species survive in an unpredictably variable environment If two parents produce many offspring with a wide variety of gene combinations, the chance that at least one of their progeny will have the combination of features necessary for survival is increased

66 Sexual reproduction might also speed the elimination of deleterious genes from a population: by matting with only the fittest males, females select for good genes and allow bad genes to be lost from the population more efficiently than they would otherwise be

67 Punnett Squares A Punnett square is a diagram that predicts the outcome of a genetic cross by considering all possible combinations of gametes in the cross Named for its inventor, Reginald Punnet, the simplest Punnett square consists of four boxes inside a square The possible gametes that one parent can produce are written along the top of the square The possible gametes that the other parent can produce are written along the left side of the square

68 Each box inside the square is filled in with two letters obtaining by combining the allele along the top of the box with the allele along the side of the box. The letters in the boxes indicate the possible genotypes of the offspring Punnett squares can be used to predict the outcome of a monohybrid cross, where 100% of the offspring are expected to be heterozygous

69 Monohybrid Cross - The genotypic ratio of the offspring is 1:2:1 while the phenotypic ratio is 3:1 Heterozygous

70 Dihybrid Cross - Heterozygous The genotypic ratio of the offspring between spherical dented yellow seeded plants and spherical dented green seeded plant is 1:2:1:2:4:2:1:2:1 and the phenotypic ratio is 9:3:3:1