Important points in heredity and evolution
Answers
Accumulation of Variations during Reproduction
The variations appear during reproduction based on whether organisms multiply Asexually or Sexually
Asexually
a) Variations are fewer
b) Occurs due to small inaccuracies in DNA copying. (Mutation)
Sexually
a) Variations are large
b) Occurs due to crossing over, separation of chromosomes, mutations
Importance of Variation
(i) Depending upon the nature of variations different individuals would have different kinds of advantage. Example, Bacteria that can withstand heat will survive better in a heat wave.
(ii) Main advantage of variation to species is that it increases the chances of its survival in a changing environment.
Mendel and His Work on Inheritance
Gregor Johann Mendel (1822 & 1884) : Started his experiments on plant breeding and hybridisation. He proposed the laws of inheritance in living organisms. Mendel was known as Father of Genetics.
Plant selected by Mendel : Pisum sativum (garden pea). Mendel used a number of contrasting characters for garden pea.
Medel’s Experimental Material : He chose Garden Pea (Pisum sativum) as his experiment material because of :
(i) Availability of detectable contrasting traits of several characters.
(ii) Short life span of the plant.
(iii) Normally allows self-fertilisation but cross-fertilisation can also be carried out.
(iv) Large no. of seeds produced.
Mendel’s Experiments : Mendel conducted a series of experiments in which he crossed the pollinated plants to study one character (at a time).
Monohybrid Cross
Cross between two pea plants with one pair of contrasting characters is called a monohybrid cross. Example : Cross between a tall and a dwarf plant (short).
Observations of Monohybrid Cross
(i) All F1 progeny were tall, no medium height plant. (Half way characteristic)
(ii) F2 progeny 1⁄4 were short, 3⁄4 were tall.
(iii) Phenotypic ratio F2 – 3 : 1 (3 tall : 1 short)
Genotypic ratio F2 – 1 : 2 : 1 – (TT : Tt : tt)
Conclusions
1. TT and Tt both are tall plants while tt is a short plant.
2. A single copy of T is enough to make the plant tall, while both copies have to be ‘t’ for the plant to be short.
3. Characters/traits like ‘T’ are called dominant trait (because it express itself) and ‘t’ are recessive trait (because it remains suppressed).
Dihybrid Cross
A cross between two plants having two pairs of contrasting characters is called dihybrid cross.
Phenotypic Ratio
Round, yellow : 9
Round, green : 3
Wrinkled, yellow : 3
Wrinkled, green : 1
Observations
(i) When RRyy was crossed with rrYY in F1 generation all were Rr Yy round and yellow seeds.
(ii) Self pollination of F1 plants gave parental phenotype and two mixtures (recombinants round yellow and wrinkled green) seeds plants in the ratio of 9 : 3 : 3 : 1.
Conclusions
1. Round and yellow seeds are Dominant characters.
2. Occurrence of new phenotype combinations show that genes for round and yellow seeds are inherited independently of each other.
How do these traits get expressed
Cellular DNA (Information source)
↓ For synthesis of
Proteins (Enzyme)
↓ Works efficiently
More Hormone
↓ produced
Tallness of plant
Therefore, genes control characteristics/traits.
Sex Determination
Factors responsible for Sex Determination are:
Environmental : In some animals, the temperature at which the fertilized eggs are kept decides the gender. E.g., in turtle
Genetic :In some animals like humans gender or individual is determined by a pair of chromosomes called sex chromosome.
XX – Female
XY – Male
Sex Chromosomes : In human beings, there are 23 pairs of chromosome. Out of these 22 chromosomes pairs are called autosomes and the last pair of chromosome that help in deciding gender of that individual is called sex chromosome.
XX – Female
XY – Male
Evolution
Evolution is the sequence of gradual changes which takes place in the primitive organisms, over millions of years, in which new species are produced.
Acquired and Inherited Traits
Acquired Traits
1. These are the traits which are developed in an individual due to special conditions.
2. They cannot be transferred to the progeny.
3. They cannot direct evolution. E.g., Low weight of starving beetles.
Inherited Traits
1. These are the traits which are passed from one generation to the next.
2. They get transferred to the progeny.
3. They are helpful in evolution. E.g., Colour of eyes and hair.