Principles of Inheritance and Variation

Sex determination, mutation and genetic disorders
The sex of an organism is determined in very early stage of its development, whether it will be a male, female or a hermaphrodite.Sex determination is a phenomenon by which the sex of an individual can be defined.
In insects, the females have two X-chromosomes while the male has only one X-chromosome.
The females are designated as XX and the males as XO. ZZ/ZW Methodis mainly observed in birds.They produce two types of ova, one with autosomes and a Z-chromosome and the other with autosomes and a W-chromosome. Males produce only one type of sperms, having autosomes and one Z-chromosome. The diploid males have autosome plus two Z-chromosomes.

In XX/XY Method, the male and female sexes have a set of chromosomes which are common, and another set which are different. The chromosomes which are different in two sexes are called sex chromosomes. The remaining set of chromosome is called autosome. In case of the XY method, males produce two types of gametes, one with an X-chromosome and another with the Y-chromosome.

In humans, a normal female has 22 pairs of autosomes and a pair of sex chromosomes (XX). A male has 22 pairs of autosomes and a pair of sex chromosome as XY.

Mutation is defined as change in the genetic material. The factors or agents that can bring about mutation are known as mutagens. Gene mutation is the inheritable change that occurs in the DNA of an organism.

The study of human genetics by the use of pedigree charts is called pedigree analysis. Mendelian disorders are caused due to alteration or mutation in a single gene.Some of the common Mendelian disorders are Haemophilia, Sickle cell anaemia, Colour blindness, Phenylketonuria, Thalassaemia etc.

Some of the common chromosomal disorders are Down’s syndrome, Klinefelter’s syndrome, Turner’s syndrome etc.

Mendel’s Laws of Inheritance
The branch of biology which deals with principles of inheritanceand its practices is called genetics
In the mid-nineteenth century, Gregor Mendel was the first to study this phenomenon systematically. He conducted hybridisation experiments on garden peas for seven years (1856-1863) and proposed the laws of inheritance in living organisms.

He proposed that the ‘factors’ or genes regulate the characters and are found in pairs known as alleles. Some characters are dominant and are expressed when factors are in heterozygous condition. This law is called Law of Dominance.
Characters are segregated during the formation of gametes. This law is called Law of segregation.
When Mendel studied theinheritance of two characters together, it was found that the factorsindependently assort and combine in all permutations and combinations. This law is called Law of Independent Assortment.
The genetic makeup of a character is called genotype while the physicalexpression of the characters is called phenotype.
 The Mendel’s laws wereextended in the form of ‘Chromosomal Theory of Inheritance’.

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