First and Second Laws of Inheritance
* Gregor Johann Mendal carried out several hybridisation experiments with truebreeding pea lines which had sharply contrasting characters.
* A monohybrid cross is the study of inheritance of one pair of contrasting characters.
* The law of Dominance states that a monohybrid, pure line cross between parents with contrasting traits, the dominant form appears in the F1 generation and both forms appear in a ration of 3:1 in the generation.
* The law of segregation states that alleles do not show any blending but segregate from each other during gamete formation into different gametes.
* Punnett Square is a graphical representation that helps calculate the probability of all possible genotypes of the progeny in a genetic cross.
* A test cross is a cross between individuals of the F1 generation and a homozygous recessive parent to determine the genotype of an individual.
Incomplete Dominance
Mendel’s laws were not universal in occurrence.
* The phenomenon where a dominant allele does not completely mask the phenotypic expression of the recessive allele in a heterozygote, resulting on a blending of both dominant and recessive traits in the F1 and F2 heterozygotes, is called incomplete dominance.
* In incomplete dominance, both the genotypic and phenotypic ratios are 1:2:1.
* Incomplete dominance causes a distortion of the normal phenotypic ratio.
Dihybrid Cross
* A cross between plants that differ in two traits or characters is known as dihybrid cross.
* The law of Independent Assortment states that when two pairs of traits or character are united in a hybridisation experiment, the segregation of one pair of characters.
* In a dihybrid cross, the genes assort independently.
Chromosomal Theory of Inheritance
* Mendel’s work did not receive due recognition until the beginning of the 20th century.
* Theodore Boveri and Walter Sutton were two noted scientist who used the movement of Chromosome Theory.
* Both Sutton and Boveri were of the opinion that pairing pairing and separation of a pair of chromosomes would result in the segregation of a pair of genes or factors carried by them.
* Sutton combined the hypothesis of chromosomal segregation and Mendelian principles and called it the Chromosome Theory of Inheritance.
Sex Determination
* Sex- determining chromosomes are called sex chromosomes while the rest of the chromosomes are called autosomes.
* Of the 23 pairs of chromosomes in human beings, just one pair is the sex chromosomes while the remaining 22 pairs are autosomes.
* The very first indication that sex chromosomes were different from autosomes came from experiments conducted by German biologist Hermann Henking in 1891.
* The four important types of sex- determination systems are the XX-XY types, XX-XO type, ZW-ZZ type and the ZO_ZZ types.
* A system of sex determination where the genetic make-up of the sperm determines the sex of the offspring is known as male heterogamety.
* A system of sex determination where the genetic make-up of the ovum determines the sex of the offspring is known as female heterogamety.
Introduction to Genetics
* Inheritance is the process of passing character from parents to offspring.
* Variation is the degree by which an offspring differs from its parents.
* Advances in the field of genetics were made by Gregor Johann Mendel in the mid-19th century.
* Gregor Mendal proposed several laws of inheritance by conducting extensive research and hybridisation experiments on garden peas for about seven years.
* Hybridisation is the process of crossing two individuals differing in at least one character, resulting in a hybrid individual.
