GENETICS 2024 LECTURE 1.pdf

LECTURE ONE: GENETICS Objectives are: To list the branches of genetics. To define various terminologies that are used in the study of genetics. To state Mendel’s laws of I heritance. To explain factors that lead to a deviation from Mendel’s laws. To calculate the chi-squared tes...

LECTURE ONE: GENETICS Objectives are: To list the branches of genetics. To define various terminologies that are used in the study of genetics. To state Mendel’s laws of I heritance. To explain factors that lead to a deviation from Mendel’s laws. To calculate the chi-squared test.  INTRODUCTION TO GENETICS Genetics is the scientific study of heredity and variation in living organisms. Heredity is the process in which a parent passes certain genes onto their children.  Children inherit their biological parents’ genes that express specific traits, such as some physical characteristics, natural talents and genetic disorders The passing on of genetic instructions from generation to generation is called inheritance  DEFINITION OF TERMS Gene: It is the basic unit of inheritance for a given characteristic. A gene occupies a specific location on chromosome known as a locus. A gene usually consist of a pair alleles for each characteristic.  Alleles: are different forms of the same gene for a trait and occupy the same relative position on a pair of homologous chromosomes. One allele is of paternal origin and the other is of maternal origin. As such an allele is responsible for determining contrasting characteristics of the same gene.  Example a locus for the gene that codes for the height characteristic or trait in pea plants will have an allele that may code for a tall plant or for a short plant. Since a pea plant has two homologous chromosomes each carrying this gene, there will be two alleles. Genotype: It is the genetic composition of an organism with respect to alleles under consideration.  If both alleles coding for a particular characteristic (i.e height) are identical, then the organism is homozygous for that characteristic. (homo=same) e.g TT or tt. If the two alleles coding for a characteristic (i.e height) are different, then the organism is heterozygous. (hetero=different) e.g Tt.  e.g TT , Tt or tt can be genotype of a pea plants. Phenotype: This is the observable characteristics (appearance) of an organism resulting from the genotype. Often times it results from the interaction between the genotype and the environment.  Example of phenotype include colour or height of an organism. e.g. red flower, yellow maize kernel, tall sorghum plants. An allele may either be dominant or recessive. Dominant allele: An allele which influences the phenotype even in the presence of an alternative allele.  Dominant allele will mask or cover the effect of the recessive allele. A dominant allele is represented by a capital letter, for example, T for tall. Recessive allele: An allele which influence the phenotype ONLY in the presence of similar alternative allele.  Meaning a recessive allele can NOT expresses itself when the dominant allele is present. A recessive allele is represented by a small letter, for example, t for short. In genetic diagrams, dominant alleles are presented in capitals, recessive in small letters.  P (Parental)- Generation: They are true breeding parents whose genotype is homozygous. True breeding means that if you cross two individuals that are homozygous for the same characteristic all the resulting offsprings will show this same genotype and phenotype. The exception is if a mutation occurs.  Heterozygotes on the other hand are true breeders. F1 generation (first filial generation): The generation produced by crossing homozygous parental stocks. F2 generation: The generation produced when two F1 organisms are crossed (self pollinated offspring of F1 generation).  Character – This is a heritable feature that varies among individuals(hair colour, skin colour, height, body type etc). Trait - variations of characters (white or black colour, short or tall). True breeding- self pollination leads to offspring of the same variety (all white or all purple flowers).  Hybridization - the cross pollination of two true breeding varieties of plants. Monohybrid cross: This is a genetic cr oss in which one gene is being transmitted at a time from parents to offsprings through mating. e.g a monohybrid cross for flower colour in peas Dihybrid cross: This is a genetic cross in which two genes are being transmitted at once from parents to offspring through mating.  Test cross: This is a genetic crossing which an organism, with an unknown genotype is mated (crossed) a homozygous recessive organism to determine the unknown genotype.  Mendelian Genetics Gregor Mendel is regarded as the father of genetic. He is the first scientist to be credited with giving proper explanation regarding the way in which characters or traits are inherited. He was an Austrian monk who developed the particulate theory after performing a series of ingenious experiments in the 1860s.  He used garden peas in his experiments on to formulate laws which explained the manner of inheritance of characters.  MENDEL’S EXPERIMENTS Mendel chose pure-breeding pea plants to study the inheritance of several characteristics. A pure-breeding plant is obtained after many generations of self-pollination. They produce identical offspring and the offspring show the same traits as their parents.  In one of his experiment, Mendel chose two parent plants, one a pure-breeding tall plant and other a pure breeding short plant. He called this generation the parental generation or P generation.  He carried out cross-pollination on the two plants by transferring the pollen grains from the tall plant onto the stigma of the short plant. He collected the seed, planted them and found that all grew to become tall plants. The results of the parental cross appeared in the first generation called the first filial generation or F1 generation.  PUNNETT SQUARE A punnett square is another method that can be used to determine the possible outcome of a monohybrid cross and their expected frequencies. A punnett square can be set up as follows; 1. Set up a 2 by 2 Punnett square. 2. Write the alleles for parent 1 on one side of the Punnett square. 3. Write the alleles from parent 2 on the other Punnett square. 4. Cross the gametess and fill in the blank boxes.  Mendel’s conclusion (a) Inheritance depends on the transfer of heredity factors from parents to offspring. There is a heredity factor that determines a particular characteristic. (b) Each characteristic is controlled by a pair of factors. (c) The factors are passed from generation to generation unaltered.  (d) The factors may be dominant or recessive. Some factors are not expressed in every generation. (e)The factors segregate or separate during gamete formation so that each gamete contains only one of the factors for a given characteristic.  Reason for choosing Garden peas (Pisum sativum) for his experiments 1. They are easy to cultivate and grow even small areas like pots. 2. They are easy to pollinate since they self pollinated. 3. They have a short life cycle hence several generation can be produced within a short time.  4. Produces large numbers of hybrid offsprings which are fertile. 5. They have several sharply defined variations i.e height, seed shape etc.  Reasons for Mendel successful experimentation: 1. Preliminary investigation was conducted to familiarize with the garden peas. 2. The experiments were carefully planned and focused only on one variable. 3.Accurate records were kept of all experiments and results obtained.  4. Significant data were obtained to have statistical significance. 5. Contrasting forms of the chosen characters showed dominance of the other  MENDEL’S LAWS OF INHERITANCE Mendel proposed three laws: Law of Dominance The Law of Segregation Law of independent assortment LAW OF DOMINANCE  This law states that in a heterozygous condition, the allele whose characters are expressed over the other allele is called the dominant allele and the characters of this dominant allele are called dominant characters.  The characters that appear in the F1 generation are called dominant characters. The recessive characters appear in the F2 generation.  It states the following – If one parent has two copies of allele X – the dominant allele, and the other parent has two copies of allele x – the recessive allele, in that case, the child inherits Xx genotype exhibiting the dominant phenotype. LAW OF SEGREGATION  This law states that when two traits come together in one hybrid pair, the two characters do not mix with each other and are independent of each other. Each gamete receives one of the two alleles during meiosis of the chromosome.  Mendel’s law of segregations supports the phenotypic ratio of 3:1 i.e. the homozygous dominant and heterozygous offsprings show dominant traits while the homozygous recessive shows the recessive trait. It proposes the following – During meiosis, two alleles are separated from each other.  Precisely, during the second stage of meiosis, two copies of each chromosome are isolated from each other which causes segregation or separation of the two distinct alleles from one another that are present on those chromosomes. For a particular gene, a parent may possess two distinct alleles, each located on the same locus. LAW OF INDEPENDENT ASSORTMENT This means that at the time of gamete formation, the two genes segregate independently of each other as well as of other traits. Law of independent assortment emphasizes that there are separate genes for separate traits and characters and they influence and sort themselves independently of the other genes.  This law also says that at the time of gamete and zygote formation, the genes are independently passed on from the parents to the offspring.  The Law of Independent Assortment proposes alleles for separate traits are passed independently of one another. That is, the biological selection of an allele for one trait has nothing to do with the selection of an allele for any other trait. Mendel found support for this law in his dihybrid cross experiments.  In his monohybrid crosses, an idealized 3:1 ratio between dominant and recessive phenotypes resulted. In dihybrid crosses, however, he found a 9:3:3:1 ratios. This shows that each of the two alleles is inherited independently from the other, with a 3:1 phenotypic ratio for each.  Dihybride crosses This is a cross between two individuals with two or more pairs of genes, each having two contrasting alleles. In one of Mendel’s Dihybrid cross he chose seed colour (yellow or green) and seed shape (round or wrinkled). Mendel crossed pure dominant yellow round seeds (YYRR) with plants having pure recessive green wrinkled seeds (yyrr).  From this cross heterozygous yellow round seed plants (YyRr) were obtained in F1. Thus yellow seed colour exhibited dominance over green and round seed shape was dominant over wrinkled seed shape.  The dihybrid cross shows that gametes are assorted independently because they end up a random mix of alleles rather than a predetermined set of either parents. This means that the inheritance of a dominant or recessive allele for one characteristic such as round or wrinkled seed has nothing to do with the inheritance of alleles for other characteristics such as yellow or green seed.  Therefore, an allele of one gene is equally likely to combine with any allele of the gene as they are passed from parents to offspring. The physical basis of independent assortment is the random orientation of each bivalent chromosome along the metaphase I plate with respect to other bivalents.  There are however, some exceptions to the law of independent assortment. This is the case where genes are located close to one another on a chromosome. This is called gene linkage as the genes that are tightly linked on chromosomes, are inherited together.  Another example is a situation where a phenotype is governed by more than one gene. Inheritance of such phenotype is polygenic inheritance. Examples of polygenic inheritance is seen in human height, skin colour, eye colour and weight.  TEST CROSS/BACK CROSS Genotype of an F1 organism, produced by the breeding of homozygous dominant and homozygous recessive parents, is heterozygous but shows the dominant phenotype. An organism displaying the recessive phenotype must have a genotype which is homozygous for the recessive allele.  In case of F2 organisms showing the dominant phenotype the genotype may be either homozygous or heterozygous. It may be of interest to a breeder to know the genotype and the only way in which it can be determined is to carry out a breeding exp. This involves the use of a technique known as Test cross.  By crossing an organism having an unknown genotype with a homozygous recessive organism it is possible to determine an known genotype within one breeding generation. Test crosses help to establish genotypes.  Summary of Mendel’s hypotheses 1-Each characteristic of an organism is controlled by a pair of alleles. 2-If an organism has two unlike alleles for a given characteristic, one may be expressed (dominant allele) while the other is suppressed. 3-During meiosis each pair of alleles separates (segregates) and each gamete receives one of each pair of alleles (principle of segregation).  4-During gamete formation in each sex, either one of a pair of alleles may enter the same gamete cell (combine randomly) with either one of another pair (Principle of independent assortment). 5-Each allele is transmitted from generation to generation as a discrete unchanging unit. 6- Each organism inherits one allele (for each characteristic) from each parent.