mendelian laws of segregation, inheritance and genetics and its application. In order to understand the Mendelian Laws and its usefulness in plant breeding, it is important to know some technical words which we shall come across in crop improvement.
Definition of Some mendelian laws of segregation, Genetic Terms
(i) Genes: Genes are hereditary units or basic units of inheritance. They are located in chromosomes and responsible for the transmission characters from parents to offspring
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(ii) Chromosomes: Chromosomes am rod or thread-like bodies found n the nucleus of a cell. The chromosomes house or contain the genes.
(iii) Characters or traits: These are the inheritable attributes or features possessed by an organism, e.g. colour, seed size, plant height, plants.
(iv) Gamete: Gamete is a matured sex cell which takes part in sexual reproduction. There are two types of male gamete or spermatozoon (animals) and pollen grains (in plants) and female gamete, egg or ovum (in animals) and ovules (in plants). Gametes are usually haploid.
(v) Zygote: Zygote is a single cell formed as a result of the union of a male gamete with a female gamete. Gamete is usually diploid.
(vi) Allelomorphs: Allelomorphs are pairs of genes on the position of a chromosome (i.e. locus) that control contrasting characters. A pair allelornorphs are called allelic pa while each member of the pair is t allele of the other.
(vii) Phenotype: Phenotype is the sum total of all observable features of an organism, i.e. the physical, physiological and behavioural traits, e.g. height, weight and skin colour.
(viii) Genotype: This term is used to describe those traits or sum total of the genes inherited from both parents. In other words, the genotype of an individual is his genetic make-up or constitution. Genotype includes both the dominant and the recessive traits that form the genetic make-up of an individual.
(i) Dominant character: This is a trait or character that is expressed in an offspring when two individuals with contrasting characters or traits are crossed. Dominant genes on the other hand are genes which control dominant characters. For instance, in a very tall plant, there may be the gene for shortness but the gene has no influence on the gene for tallness.
(ii) Recessive character: This is the character or strait from one parent which is masked or does not produce the effect in the presence of dominant character. From the illustration above, shortness is the recessive character while tallness is the dominant character. Recessive genes on the other hand are genes which control recessive characters.
(iii) Homozygous: An individual is said to be homozygous if it has two similar genes for the same character, i.e., it has two identical alleles at the same position on a pair of chromosomes; the pair of genes controlling a given pair of contrasting characteristics are identical, e.g. (TT) for tallness or (tt) for shortness and this is the critical aspect of mendelian laws of segregation
(xi) Heterozygous: An individual is said to be heterozygous if the two members of a pair of genes controlling a pair of contrasting characters are different, i.e., it has two different or contrasting alleles located on the same position on a pair of chromosomes, e.g. (Tt) for tallness or a plant with Regenetic composition has a heterozygous red flower plant.
(xii) Filial generation: The offspring of parents make up the filial generation. The first, second and third generations of offspring are known as the first, second and third filial generation respectively, and are denoted by the symbols F1, F2 and F3 respectively, too. F1 generation gives rise to F2 generation.
(xiv) Hybrid: Hybrid is an offspring from a cross between parents that are genetically different but of the same species.
(xv) Hybridization: Hybridization is the crossing of plants with contrasting characters. Mono hybridization involves the crossing of two pure traits while Di hybridization involves the crossing of plants with two pairs of contrasting characters.
(xvi) Locus: Locus is the site for location of a gene in a chromosome.
(xvii) Haploid: Haploid is when an organism has one set of chromosomes in the gamete. Gametes at certain stages in the life cycle of plants are haploid. It is represented by small letter n.
(i) Diploid: Diploid is when an organism has two sets of chromosomes in the body cell. The bodies of animals and plants are diploids. Diploid number is double the haploid number of chromosomes and is represented by 2n.
(ii) Mutation: Mutation is a change in the genetic make-up of an organism resulting in a new characteristic that is inheritable.
(iii) Back cross: Back cross is the crossing of an organism with the homozygous recessive organisms from the original parental generation.
(xxi) Test cross: Test cross is the crossing of an organism with the homozygous recessive organism. Back cross and test cross are used to determine the genotype of organisms showing dominant phenotype.
(xxii) Test cross: Test cross is the crossing of an organism with the homozygous recessive organism. Back cross and test cross are used to determine the genotype of organisms showing dominant phenotype
MENDEL’S WORK IN GENETICS
Gregor Mendel (1822 – 1884) was a monk in an Augustinian monastery in Brunn, Austria. He often regarded as the father of genetics because his work formed the foundation for scientific study of heredity and variation. MENDELIAN LAWS OF SEGREGATION, INHERITANCE AND GENETICS
MENDEL’S EXPERIMENTS
Gregor Mendel carried out several experiments on how hereditary characters are transmitted from generation to generation. He worked with garden pea (Pisum sativum). His major aim was to find out the pattern of inheritance of different characteristics on the pea plant.
METHODS USED GREGOR MENDEL IN HIS EXPERIMENT
Gregor Mendel used two major methods in conducting his experiments. These methods were grouped into monohybrid inheritance and dihybrid inheritance. MENDELIAN LAWS OF SEGREGATION, INHERITANCE AND GENETICS
REASONS FOR MENDEL’S CHOICE OF PEA PLANT
Gregor Mendel decided to use the pea plant for his experiment because of the following reasons: (1) peas are usually self-pollinating and he could pollinate them by himself.
(1) They have a very short life span because they are annual plants
(2) The pea plant was known to have several unique characteristics which exist in contrasting pairs such as in mendelian laws of segregation
(i) Some seeds were round while others were wrinkled
(ii) Some plants were tall while others were short
(iii) Some weed were yellow while others were green
(iv) Some flowers were axial while others were terminal
(v) Some pods were green while some were yellow
(vi) Some flowers were white while some were red
(vii) Some pods were smooth while some were constricted
MENDELIAN LAWS OF SEGREGATION, INHERITANCE AND GENETICS
METHOD USED BY MENDEL IN HIS EXPERIMENT
Gregor Mendel used two major methods in conducting his experiments. These methods were grouped into monohybrid inheritance and dihybrid inheritance.
MONOHYBRID INHERITANCE
Mendel used artificial method to cross two different plants at a time, which differed in one pair of contrasting characters, e.g tall and short plants. This procedure was called a monohybrid inheritance and it was an example of complete dominance.
He carried out the experiment in the following order.
(i) He planted tall plants for several generations and discovered that the plants produced were all tall plants. In the same way, he planted short plants for several generations and discovered that the plants produced were all short.
(ii) He proceeded to plant tall plants and short plants. By the time the flowers were produced, he collected the pollen grains of the tall plants tagged the male and pollinated the stigma of the short plant tagged the female. He also collected the pollen grains of the short plant and place them on the stigma-plant of the tall plant.
Mendel then covered the artificially pollinated flowers with small paper bags to prevent the chance of natural pollination by insects
.MENDELIAN LAWS OF SEGREGATION, INHERITANCE AND GENETICS
(iii) Mendel once again picked the seeds formed after the cross. When he planted the seeds, the plants obtained were all tall plants. These he referred to as the first filial generation or F.
(iv) Mendel then crossed the F, plants, collected their seeds and sowed them. The plants he got from these were tall and short plants in a ratio of 3:1 respectively. He then called this stage the second filial generation or F.
Mendel’s First Law of Inheritance
Law of segregation of genes/
MENDELIAN LAWS OF SEGREGATION, INHERITANCE AND GENETICS
This first law is also called the
law of segregation of genes
. The law states that genes are responsible for the development the individual and that they are independently transmitted from one generation to another without undergoing alteration. law of segregation of genes
From Mendel’s first law of segregation of genes, the actual segregation occurs in the F, generation. The phenotypic and genotypic ratios in F, generation can be summarised as follows:
(i) Phenotypic ratio = 3:1 (i.e., 3 tall and 1 short)
(ii) Genotypic ratio = 1:2:1 (i.e., 1 TT, 2Tt, Itt)
Note: Letters are used to represent the genotypes of the traits. In the case of complete dominance, the capital letter form of the first letter of the dominant trait is used to denote the dominant gene. The small letter form of it is used to represent the recessive gene.
mendelian laws of segregation, inheritance probables
Since tallness in the plant is dominant over shortness,
(i) T represents gene for tallness.
(ii) TT represents genotype of the pure breeding tall plants. Such a plant is described as homozygous for tallness.
(iii) t represents gene for shortness.
(iv) tt represents genotype of the pure breeding short plant, homo-zygous for shortness.
(v) A cross between two organisms is shown by a multiplication sign x.
(vi) Each gamete is represented by only one encircled letter, i.e. (T) or (t) depending on the trait being discussed. This is in compliance with Mendel’s law of segregation of germinal units.
(vii) A heterozygous individual is represented by one dominant gene and one recessive gene, i.e; Tt. Such individuals are called carriers of a trait. MENDELIAN LAWS OF SEGREGATION, INHERITANCE AND GENETICS
Dihybrid Inheritance
Gregor Mendel also carried out several experiments in which he crossed plants which differed in two pairs of contrasting characteristics such as seed shape (round and wrinkled seeds) and seed colour (yellow and green seeds). Mendel therefore called the whole set up as dihybrid inheritance because two pairs of contrasting characters are involved in mendelian laws of segregation
When Mendel crossed plants which had round and yellow seeds with those which had wrinkled and green seeds, all the F plants produced round and yellow seeds. However, when the F plants were self-pollinated-pollination, the F2 plants were of four types.
(i) Plant that produced round and yellow seeds
(ii) Wrinkled and yellow seeds
(iii) round and green
(iv) Wrinkled and green
All these were in the ratio of approximately 9:3:3:1
MENDELIAN LAWS OF SEGREGATION, INHERITANCE AND GENETICS
Mendel then concluded that this could result if the contrasting characteristics of round and wrinkled seeds and the contrasting characteristics of yellow and green seeds were inherited independent of each other.
The outcome of this experiment led to Mendel’s second law of inheritance.
Mendel’s Second Law of Inheritance
law of independent assortment of genes
This second law is also called the law of independent assortment of genes.
Mendel’s second law of independent assortment of genes states that each character behaves as a separate unit and is inherited independently of any other character. law of independent assortment of genes
Mendel’s work can be represented by letters and their explanations as below:
Parents round yellow x wrinkled green
The four phenotypes which appear in the ratio 9:3:3:1 are as follows:
(1) 9 round yellow r1m 2, 3, 4, 5, 7, 9, 10, 13
(2) 3 round green r6, 8, 14
(3) 3 wrinkled yellow r 11, 12, 15
(4) 1 wrinkled green r 16
The genotypes which include 4 homozygous and 5 heterozygous conditions are:
(1) 1 is hormozygous for both round an yellow (1)
(2) 1 is homozygous for both round and green (6)
(3) 1 is homozygous for both wrinkled and green (16)
(4) 1 is homozygous for both wrinkled and yellow (11)
(5) 2 are homozygous for round an heterozygous for yellow (2, 5)
(6) 2 are homozygous for round and heterozygous for yellow (2, 5)
(7) 2 are heterozygous for round and hormozygous for green (8, 14)
(8) 2 are homozygous for wrinkled and heterozygous for yellow(12, 15)
MENDELIAN LAWS OF SEGREGATION, INHERITANCE AND GENETICS
