Principles of Genetics - September 2012

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Presentation by Dr Charles Amadi, National Root Crops Research Centre, Umudike, Nigeria
Delivered at the B4FA Media Dialogue Workshop, Ibadan, Nigeria - September 2012
www.b4fa.org

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Principles of Genetics - September 2012

  1. 1. Lecture Presented By Amadi Charles National Root Crops Research Institute
  2. 2. INTRODUCTION • A cursory look at siblings immediately reveals similarities and differences amongst them and between them and their parents. • Understanding the basis of these similarities and differences and how they are transmitted from one generation to another is within the purview of genetics.
  3. 3. Genetics • Genetics is the science of heredity, dealing with resemblances and differences of related organisms resulting from the interaction of their genes and the environment (Online dictionary).
  4. 4. BASIC PRINCIPLES OF GENETICS
  5. 5. Gregor Mendel • Gregor Mendel, through a classical set of experiments was able to accurately describe the inheritance mechanism based on the assumptions of paired units and random transmission of the units from parents to offspring. • For this reason He is called the Father of Genetics Gregor Mendel (1822-1884) Source: Wikipedia
  6. 6. Some Basic Principles • Traits, or characteristics, are passed on from one generation of organisms to the next generation • The traits of an organism are controlled by genes • Organisms inherit genes in pairs, one gene from each parent • Some genes are dominant, whereas other genes are recessive • Dominant genes hide recessive genes when both are inherited by an organism
  7. 7. Mendelian Laws 1. The Law of Dominance 2. The Law of Segregation 3. The Law of Independent Assortment
  8. 8. The Law of Dominance • This law states that in a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. • Offspring that are hybrid for a trait will have only the dominant trait in the phenotype. • The trait whose appearance is suppressed in the hybrid is said to be recessive.
  9. 9. The Law of Segregation • This law states that during the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other. • Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring. • If we cross two tall hybrids with genotype Tt, we will get both tall and short plants in the ratio of 3 tall: 1 short plants.
  10. 10. The Law of Independent Assortment • Alleles for different traits are distributed to sex cells (& offspring) independently of one another.
  11. 11. Illustration of independent Assortment Gamete s RG Rg rG rg RG RRGG round RRGg round RrGG round RrGg round Rg RRGg round RRgg round RrGg round Rrgg round rG RrGG round RrGg round rrGG wrinkled rrGr wrinkle d rg RrGg round Rrgg round rrGg wrinkled rrgg wrinkle d • To illustrate independent assortment, let assume that the genotypes of our parents are • • RrGg x RrGg • where "R" = dominant allele for round seeds "r" = recessive allele for wrinkled seeds "G" = dominant allele for green pods "g" = recessive allele for yellow pods
  12. 12. Non Mendelian Inheritance • Not all genetic observations can be explained and predicted based on Mendelian genetics. • Other complex and distinct genetic phenomena may also occur eg – blood types, – skin colour, – height, – Lower colour – tuber yield etc.
  13. 13. Incomplete Dominance • In some allele combinations, dominance does not exist. Instead, the two characteristics blend to form a new character in the offspring. • For instance, snapdragon flowers display incomplete dominance in their color. • There are two alleles for flower color: one for white and one for red. But when one allele for red is present with one allele for white, the color of the snapdragons is pink.
  14. 14. Illustration of Incomplete dominance Gametes W (white) R (Red) RW (Pink) New characteristic
  15. 15. Codominance • With codominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype in which both of the parental traits appear together. • For example a cross between Red and White Parent will give rise to an offspring that is red and white spotted.
  16. 16. Illustration of Codominance • R = allele for red flowers W = allele for white flowers • red x white ---> red & white spotted RR x WW ---> 100% RW
  17. 17. Multiple Alleles • In certain cases, more than two alleles exist for a particular characteristic. • Even though an individual has only two alleles, additional alleles may be present in the population. • An example of multiple alleles occurs in blood type.
  18. 18. Human Blood Type • In humans, blood groups are determined by a single gene with three possible alleles: A, B, or O. • Red blood cells can contain two antigens, A and B. • The presence or absence of these antigens results in four blood types: A, B, AB, and O.
  19. 19. Polygenic inheritance • Polygenic characters are controlled by many genes at different locations on chromosomes. • There is a gradual variation in the character from one extreme to the other • An example of polygenic inheritance is human skin color. • A person with many genes for dark skin will have very dark skin color, and a person with multiple genes for light skin will have very light skin color.
  20. 20. Gene linkage • A chromosome has many thousands of genes. • It is common for a large number of genes to be inherited together if they are located on the same chromosome. • Genes that are inherited together are said to form a linkage group. • Gene linkage can show how close two or more genes are to one another on a chromosome. • The closer the genes are to each other, the higher the probability that they will be inherited together.
  21. 21. Sex linkage • There are 23 pairs of chromosomes in human cells. • One pair is the sex chromosomes. (The remaining 22 pairs of chromosomes are referred to as autosomes). • The sex chromosomes determine the sex of humans. • There are two types of sex chromosomes: the X chromosome and the Y chromosome. • Females have two X chromosomes; males have one X and one Y chromosome. • Typically, the female chromosome pattern is designated XX, while the male chromosome pattern is XY. • Thus, the genotype of the human male would be 44 XY, while the genotype of the human female would be 44 XX (where 44 represents the autosomes).
  22. 22. Sex linked Characters • In humans, the Y chromosome is much shorter than the X chromosome. • Because of this shortened size, a number of sex-linked conditions occur. • When a gene occurs on an X chromosome, the other gene of the pair probably occurs on the other X chromosome. • Therefore, a female usually has two genes for a characteristic. • In contrast, when a gene occurs on an X chromosome in a male, there is usually no other gene present on the short Y chromosome. Therefore, in the male, whatever gene is present on the X chromosome will be expressed. • Examples of sex-linked conditions are Colour blindness and Hemophilia
  23. 23. The Cell • The cell is the basic structural and functional unit of all known living organisms. • It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. • Most plant and animal cells are between 1 and 100 µm and therefore are visible only under the microscope.
  24. 24. Diagram of Plant Cell Diagram of Onion Cells. Source Wikipedia Cell(Biology) Diagram of a plant cell. Source: Wikipedia Cell (Biology)
  25. 25. Chromosome • A chromosome is a long, stringy aggregate of gene that carries heredity information (DNA). • A chromosome has many thousands of genes; there are an estimated 100,000 genes in the human genome. • Inheritance involves the transfer of chromosomes from parent to offspring through meiosis and sexual reproduction.
  26. 26. Structure of Chromosome Source: National Institutes of Health
  27. 27. Gene • Genes are segments of DNA located on chromosomes. Traits are passed from parents to offspring through gene. Genes contain the codes for the production of specific proteins.
  28. 28. Allele • An Allele is one of two or more alternative forms of a gene at corresponding sites (loci) on homologous chromosomes, which determine alternative characters in inheritance
  29. 29. Deoxy Ribose Nucleic Acid (DNA) • Deoxy Ribose Nucleic acid (DNA) is the genetic material in most of the organisms. • DNA is mainly found in the chromosomes in the nucleus. • It consists of smaller molecules called nucleotides. • Each nucleotide consists of a sugar, phosphate group and a nitrogenous base.
  30. 30. Genotype Genotype TT = homozygous(pure) Tt = heterozygous(hybrid) tt = homozygous(pure) The genes present in an organism make up the genotype. That is the genetic makeup of an organism.
  31. 31. Phenotype • The manifested characteristic or the physical appearance of an organism. • Examples of phenotypes are – blue eyes – brown fur – striped fruit – yellow flowers
  32. 32. Hazel EyeGreen eye Steel Gray EyeElectric blue eye Source Wikipedia from link www.obsidianbookshelf.com Human Eye Colour
  33. 33. Flower colours Source: Wikipedia
  34. 34. Sexual Reproduction • The production of new living organisms by combining genetic information from two individuals of different types (sexes). • In higher plants this usually involves: – Pollination (Transfer of pollen grains from the anther to the stigma of flower of a plant of the same type) – Fertilization (Mixing of the male and female gamete) – Embryogenesis (embryo formation).
  35. 35. Diagram of a Flower Source: http://askabiologist.asu.edu/mendel-garden
  36. 36. Gametes • Gametes are the reproductive or sex cells produced in the sex organs of a plant or animal. • The male sex cell (Male gamete) is known as sperm • The female sex cell is known as egg
  37. 37. Symbols for Gametes Symbol for female gamete Symbol for male gamete
  38. 38. Quantitative Genetics: Basics • They physical appearance of an individual, the phenotypic value (P) is the combined result – its genetic makeup, the genotype (G) and – the effects of the environment (E): P = G + E
  39. 39. Genotypic Variation Additive effects Dominance effects
  40. 40. • Additive variation represents the cumulative effect of individual loci, therefore the overall mean is equal to the summed contribution of these loci. • Dominance variation represents interaction between alleles. If a trait is controlled by a dominant allele, then both homozygous and heterozygous individuals will display the same phenotypic value.
  41. 41. Environmental Effect Effect due to environment Effects due to interaction between genes and environment
  42. 42. • Interaction (I) between different genes can modify the observed phenotypes. • This is called epistasis, or non- allelic interaction, distinguishing it from dominance. P = A + D + E + I
  43. 43. • The total phenotypic variation (V) of a population is the sum of the variation in additive (A), dominance(D), gene-interaction (I), environmental (E)and gene- environment interaction (GE) effects: VP = VA + VD + VI + VE + VGE
  44. 44. Why is this important? • Being able to estimate how the total variance is partitioned between genetic and environmental effects is important to quantitative geneticists trying to improve a given trait. • If the proportion of variation is mostly due to genetic effects (heritable), then selecting for individuals that possess the desired genetic value is a worthwhile investment. • If however, the genetic variance is low (and therefore the environmental variation has more impact on phenotype), then a more strategic approach would be to optimize environmental conditions.
  45. 45. Introduction • A search of the internet for '' genetics games or simulations" using google search engine scored 25.6 million hits in 33 seconds. • This underscores the emerging tendency to simulate the outcomes of genetic studies using computer technology. • Many reasons have been adduced for the use of computer applications or programs to simulate the results of genetic studies.
  46. 46. Why use of computer applications or programs to simulate the results of genetic studies • Many generations of genetic research can be carried out more quickly than with live organisms. • It eliminates the drudgery associated with carrying out field experiments • Organisms do not need to be created or destroyed • Simulators allow the application of class lessons to real world situations. • Complete crossing programs that are impossible in live organisms can be carried out rapidly at almost no cost.
  47. 47. Genetic Simulation Programs • There are many computer simulation and animation programs available for genetic studies. Some of these include – “Hands On Genetics” programs – Drosophilab, – Classical genetics simulator, – EasyPop, – ModelMage, – PABSIM, etc.
  48. 48. Excercise • We will practice simulation with Drosophilab a free program. • www.drosophilab.com

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