BIOLOGY OF HEREDITARY AND VARIATION (GENETICS)
TERM: 2nd Term
BIOLOGY OF HEREDITARY AND VARIATION (GENETICS)
DEFINITIONS OF GENETIC TERMS
Genetics is the scientific study of heredity and variation in all living things. The word genetics was coined by Dilliam Bateson (1906).
Hereditary/Inheritance is the transmission and expression of characters or traits from parents to offspring. It accounts for the usual resemblance between a parent and its offspring.
Variation isthe differences which exist between parents and offspring and among the offspring.
Genes are hereditary or basic units of inheritance located in chromosomes and responsible for the transmission of characters from parents to offspring. The hereditary units were named genes by Johannsen in 1909.
Chromosomes are rod or thread like bodies found in the nucleus of a cell which house the genes.
Characters are inheritable features possessed by an organism e.g height, complexion, colour etc.
Gamete is a mature sex cell which takes part in sexual reproduction. If is of two types male and female gamete e.g pollen grains and ovules in plants, sperms and ova in man. Gamete is usually haploid.
Zygote is a single cell formed as a result of the union of male and female gamete. It is diploid.
Allelomorphs (alleles) are pairs of genes on a particular position of chromosomes. They control contrasting characters.
Locus is the site or location of a gene in the chromosome.
Genotype is the sum total of the genes or the genetic make up of an individual inherited from both parents. It includes both dominant and recessive traits.
Phenotype is the sum total of all observable features of an organism. It includes the physical, physiological and behavioural traits e.g. height, skin colour etc.
Dominant trait is expressed in an offspring when two individuals with contrasting characters are crossed. Dominant trait is controlled by dominant gene e.gTt
Recessive trait is masked, with no effect in the presence of a dominant character. This trait is controlled by a recessive gene
Homozygous is when an individual has two similar genes for the same character i.e. the individual has two identical alleles at a locus as a pair of chromosomes e.g. TT, tt.
Heterozygous is when an individual has different or contrasting alleles located on the locus as a pair of chromosomes e.g. Tt.
Filial generation is made up of offspring of parents. The generations are represented as F1, F2, F3, e.t.c one giving rise to the other.
Hybrid is an offspring from a cross between parents that are genetically different but of the same specie.
Hybridization is the crossing of plants with contrasting characters. This can be monohybridization (i.e. two pure traits crossed) or dihybridization (two pairs of contrasting characters crossed).
Haploid is when an organism has one set of chromosomes in the gamete (23 in number). It is represented by small letter n.
Diploid is when an organism has two sets of chromosomes in the body cell (46 in number). It is represented by 2n.
Mutation is the change in the genetic makeup of an organism resulting in a new inheritable characteristic.
HEREDITARY VARIATION AND CAUSES
Hereditary variation is the differences among individuals which can be passed from the parents to their offspring (progenies).
NOTE: No two offspring inherit exactly the same set of characteristics from parents except in identical twins. Hereditary variation arises because of
Transmittable characters in animals
These include: body stature, shape or size of the head, nose and ear, colour of skin, hair colour, eye colour, intelligence, height, and characteristic voice of speech, blood group, baldness, tongue rolling, sickle cell anaemia, haemophilia, colour blindness, finger prints and ability to taste phenylthiocarbamide (PTC)
Transmittable characters in plants
These include height, weight or shape of plants, its fruit, leaf, fruit taste, food content, colour of leaf or flower, fruit or seed, resistance to environmental factors like disease, pest and wind, leaf texture, life span etc.
How characters get transmitted
Only characters controlled by genes are transmissible. Diploid organisms produce gametes by meiosis in their reproductive organs, therefore the male gamete (sperm cell) and the female gamete (ovum) are haploid organisms containing one set of chromosomes in their cell (one copy of each gene from homologous pair.
When fertilization takes place during sexual reproduction, the gametes (spermatozoon and ovum) fuses together to form a zygote.
Each zygote is a diploid organism having two set of chromosomes. Hence two copies of each gene (a copy donated by each gamete)
Male parent (2n) Female parent (2n)
Spermatozoon (n) X ovum (n)
Hence, characters determined by genes are transmitted from parents to offspring through gamete
MENDEL’S WORK IN GENETICS
In the study of genetics, Gregor Mendel (1866) worked with the garden pea
(Pisumsativum) because of three unique properties present in it. These are
The Mendel’s methods of studying genetics are two:
Mendel crossed two different plants which differ in one pair of contrasting characters e.g. tall and short plants. The procedures he followed are as follows:
Parents’ genotypes: TT x tt
(Tall plant) (Short plant)
Gametes: T Tt t
TtTtTtTt (All tall)
Self pollinating: Tt x Tt
T t T t
Genotypic ratio (GR) => 1: 2: 1
Phenotype ratio (PR) => 3: 1
This experiment resulted into Mendel’s first law of inheritance which is based on the principle of complete dominance.
Mendel’s first law of inheritance otherwise known as the law of segregation of genes states that paired alleles segregate from each other when the homologous chromosomes on which they reside separate during meiosis. Each gamete receives one of the two alleles. The actual segregation occurs in F2 generation.
Gregor Mendel crossed plants which differ in two pairs of contrasting characteristics e.g seed shape (round or wrinkled seeds) and seed colour (yellow or green seeds). He crossed plants having round and yellow seeds with these having wrinkled and green seeds. The F1 seeds were having round and yellow seeds. Self pollinating F1 plants produced the F2 plants of four (4) types
This experiment resulted into Mendel’s second law of inheritance
Mendel’s second law of inheritance otherwise known as law of independent assortment of genes state that alleles of genes on different chromosomes assort independently during meiosis
Parents’ genotypes: RYRY X ryry
(Round yellow seed) (Wrinkled green seed)
Gametes: RY RYry ry
F1: RYryRYryRYryRYry (all round yellow)
RYry x RYry
RY rY Ry ry
Round and yellow seeds: (1), (2), (3), (4), (5), (7), (9), (10), (13) : 9
Wrinkled and yellow seeds: (6), (8), (14) : 3
Round and green seeds: (11), (12), (15) : 3
Wrinkled and green seeds: (16) : 1
DETERMINATION OF THE GENOTYPE OF A DOMINANT PHENOTYPE
A dominant phenotype has the genotypic patterns RR, Rr. The genotype is determined using test cross or back cross
Test cross is the crossing of an organisms with an homologous recessive organism
Back cross is the crossing of an organism with an homologous recessive organism from the original parental generation
PRINCIPLE OF INCOMPLETE DOMINANCE
This deal with the ability of two contrasting alleles to interact and produce a heterozygous phenotype that is different from the two homologous phenotypes: Examples of organisms exhibiting incomplete dominance include: Mirabilisjalapa,4 o’clock plant, Audlausian fowl. This principle opposes Mendel’s principle of complete dominance.
Parent genotypes: RR X rr
Phenotypes: (red flower) (white flower
Gametes: R Rrr
RrRr Rr Rr (All pink flowers)
Self pollinating: Rr x Rr
RR RrRr rr
Red Pink White
PR = GR = 1: 2: 1
CO – DOMINANCE
In co-dominance both alleles in the heterozygous individuals are fully expressed. The effect of one is not modified by the presence of the other. Therefore, three distinct phenotypes are produced e.g Inheritance of human “ABO” blood group
Phenotype (Blood group)
Allele 1A implies the addition of antigen A to the cell surfaces of red blood cells resulting in a person with group A blood. Likewise Allele 1B implies the addition of antigen B to the cell surfaces of red blood cells resulting in a person with Group B.
In a heterozygous individual, (1A 1B) both antigens A and B are added to the cell surfaces of red blood cells. So the individual has blood group AB.
NOTE: 1A and 1B are co-dominants while 1Ois recessive.
Genes that have more than two alleles in the population are said to have multiple alleles e.g. the human ‘ABO’ blood grouping
Sex determination in human beings
Each body cell of human beings has 23 pairs (46) of chromosomes, 22 of which are autosomes and a pair is sex chromosome. In male the two sex chromosomes in each body cell are X and Y chromosomes, therefore, each male gamete carries either X or Y chromosome. In the female, all egg cells of the body contain two X chromosomes. Therefore all egg cells contain one X chromosomes each. At fertilization, the combinations of an egg with a sperm carrying either X or Y chromosome occurs by chance. The formation of a male or female offspring has equal chances as shown below parents:
XY x XX
Gametes: X Y X X
XX XX XY XY 50: 50
GR = PR = 1: 1
(1) 2XX Girls
(2) 2XY Boys.
College Biology, chapter 18, page 409 – 428.
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