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MENDELIAN LAWS
Gregor Mendel (1882 – 1884) was an Austrian monk who carried out simple experiments on heredity for nine years (1856 – 1865) using the common garden pea. He published his research findings “Experiments on Hybridization” in the journal of Natural History in Austria in 1866/67.
He formulated two principles of inheritance;
(i) First Mendelian law
The law of segregation; states that “ the two factors segregate or separate from one another unaltered and unblended as they pass from one generation to the next”. The pair of factors (genes) segregate during the formation of gametes (meiosis). Only one passes into a single gamete. During fertilization genes pair up in new ways.
(ii) Second Mendelian law
The law of Independent assortment; states that “two pairs of factors in the same cross assort or separate independently of each other”. In other words, a member of a pair of genes can combine separately with any other member of another pair. It acts randomly and it is thus inherited.
Mendel arrived at this law from his findings on the experiment he carried out with two pairs of contrasting characters (dihybridization). For example, in the crossing of round yellow seeds with wrinkled green seeds, the F1 generation showed the dominant character of round and yellow (RRYY), but the F2 generation showed a proportion that was the square of 3: 1.
The result was 16 combinations with four phenotypes in the ratio 9:3:3:1
- Nine round yellow
- Three round yellow
- Three wrinkled green
- One wrinkled green.
There were nine genotypes which include four homozygous and five heterozygous conditions.
SUB-TOPIC 2: CHROMOSOMES; THE BASIS OF HEREDITY
Chromosomes are located in the nucleus of living cells of plants and animals.
All plants and animals possess two types of cells;
- somatic or vegetative cells and
- gametes or sex cells
- Somatic cells are the body cells which are not concerned with reproduction. They carry the diploid (2n) set of chromosomes. They are obtained as a result of fertilization, i.e. the contribution of a set of chromosomes from a male and a female individual. Nuclear division in somatic cells is referred to as mitosis.
- Sex cells (gametes) are cells which are found in the reproductive organs of male and female organisms. The gametes (e.g. sperm, Ovum, pollen grain) are haploid i.e. they carry half the number of chromosomes of somatic cells. Nuclear division which results in their formation is referred to as meiosis (reduction division).
Each organism has a specific number of chromosomes in its somatic cells i.e. the number of chromosomes in a species is constant. Every human has 23 pairs (46) of chromosomes in each somatic cell, while each gamete has half i.e. 23 chromosomes. Drosophila (fruit fly) has 4 pairs and tomato plant has 10 pairs of chromosomes each.
Structure of Chromosomes
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