Genetics

             Genetics is study of science of genes, heredity and variation in living organisms. Heredity is the cause of similarity between individuals and variation is the cause of differences between individuals. Genetics deals with transmission of characters from parents to offspring and the molecular structure and function of genes. The study of genes is done at three levels.

Transmission Genetics

Population Genetics    

Molecular Genetics 

TRANSMISSION GENETICS          

The two most fundamental questions in biology are – what is the nature of genetic material and how is it transmitted from parents to offspring? The fact that living things inherit traits from their parents has been used in prehistoric times to improve crop plants and animals through selective breeding. However modern genetics only began with the work of Gregor Mendel. His breeding experiments in pea plants revealed that hereditary characteristics are determined by discrete factors (genes) that are transmitted to the next generation.

THE LAWS OF INHERTANCE

          The study of science of genetics begins with the work of Gregor Mendel. He was a clergyman and a teacher. He performed his own experiment on garden pea. He restricted his attention to the single character and kept pedigree records for each plant. Mendel studied the nature of inheritance in plants and presented his work in 1885 in the paper “Experiments on Plant Hybridisation “. The importance of his work did not get noticed until 1890s, after his death. The advantages of garden pea (Pisum sativum) as experimental system were:

The characters were readily observable.

Plants were easy to grow and self fertilise before the flowers opened

Mendel worked with 14 pure breeding varieties and selected seven pairs of characters and propounded the following laws:

The law of dominance: Mendel stated that the hereditary characters are determined by pair of factors (genes) in an individual. In a monohy--brid cross all the plants in F₁ generation showed only one character   but in F₂ the other character appears. The character in F₁ generation is dominant and the one which expresses in F₂ generation is recessive e.g., the cross between two homozygous parents (P), one with two dominant alleles for tall plant (TT) and the other with two recessive alleles for dwarf plant (tt). The phenotype of the offspring in F₂ generation is dominant and the one which expresses in F₂ generation is recessive e.g., the cross between two homozygous parents (P), one with two dominant alleles for tall plant (TT) and the other with two recessive alleles for dwarf plant (tt).  The phenotype of the offspring in F₁ generation is tall plant but the genotype is Tt.  The F₁ heterozygote produces four kinds of gametes. When this plant self-pollinates, the T and t egg and sperm cells combine randomly to form ¼TT (tall), 2/4 Tt (tall) and ¼ tt (dwarf) offspring. The F2 generation has 1:2:1 genotypic ratio and 3:1 phenotypic radio.

Luminous bodies

Luminous bodies: Those objects which emit light by themselves are called luminous bodies.

e.g. - sun, stars, electric bulb etc.

Non- luminous bodies: those objects which do not emit light by themselves but are visible by the light falling on them emitted by self luminous bodies are called non-luminous bodies.

A material can be classified as:

(i)                  Transparent: the substances which allow most of the incident light to pass through them are called transparent e.g. glass, water.

(ii)                Translucent: The substances which allow a part of incident light to pass through them are called translucent bodies e.g. oiled paper.

(iii)               Opaque: The substances which do not allow the incident light to pass through them are called opaque bodies. E.g. mirror, metal, wood etc.

Reflection of light: Light moving in one medium when falls at the surface of another medium, part of light returns back to the same medium. This phenomenon of returning back of light in the first medium at the interface of two media is known as reflection of light.

Laws of reflection

(i)                  The incident ray, reflected ray and normal to the reflecting surface at the incident point all lie in the same plane.

(ii)                The angle of reflection is equal to the angle of incidence.

Reflection from plane mirror

(i)                  The image is virtual, laterally inverted.

(ii)                The size of image is equal to that of object.

(iii)               The distance of image from the mirror is equal to distance of object from the mirror.

(iv)              If an object moves towards (or away from) a plane mirror with speed v, relative to the object the image moves towards (or away) with a speed 2v.

(v)                If a plane mirror is rotated by an angle θ, keeping the incident ray fixed, the reflected ray is rotated by an angle 2θ.

(vi)              To see his full image in a plane mirror, a person requires a mirror of at least half of his height.

(vii)             If two plane mirrors are inclined to each other at an angle θ the number of images (n) of a point object formed are determined as follows:

(a)    If 360  /θ  is even integer, then n= 360 /θ = 1

(b)   If  360/ θ is odd integer,

Then n=360/ θ -1 for the object is symmetrically placed and

N = 360 /θ for the objects is not symmetrically placed.

(c)    If 360 /θ are a fraction then n is equal to integral part.