What is recombination? Discuss the applications of recombination from the point of view of genetic engineering.
Recombination refers to the generation of new combination of genes which is different from the parental types. It is produced due to crossing over that occurs during meiosis prior to gamete formation.
Applications of Recombination
(i) It is a means of introducing new combinations of genes and hence new traits.
(ii) It increases variability which is useful for natural selection and under changed environment.
(iii) Since, the frequency of crossing over depends upon the distance between the two genes, the phenomenon is used for preparing linkage chromosome maps.
(iv) It has proved that genes lie in a linear fashion in the chromosome.
(v) Breeders have to select small or large population for obtaining the required cross-overs. For obtaining cross-overs between closely linked genes, a very large population is required.
(vi) Useful recombinations produced by crossing over are picked up by breeders to produce useful new varieties of crop plants and animals. Green revolution has been achieved in India due to this selective picking up of useful recombinations. Operation flood or white revolution is also being carried out on the similar lines.
What is artificial selection? Do you think it affects the process of natural selection? How?
Artificial selection (or selective breeding) describes intentional breeding for certain traits or combination of traits by humans, for exploiting the variations existing among species. It is of three types-mass selection, pure-line selection and clonal selection.
Yes, it affects the process of natural selection. Natural selection selects for/or against traits based on their effect on the fitness of the organism. In artificial selection, traits are selected based on human preference for improving traits.
The process of natural selection leads to evolutionary change in the expression of the trait in the population, whereas the artificial selection, though being the same process, involves the traits preferred by humans for its own benefit. It is a much faster process than the natural selection but it may impose threat on diversity in long run making it unfit to the environment.
With the help of an example differntiate between incomplete dominance and co-dominance.
Incomplete dominance is a phenomenon where two contrasting alleles are present together but neither of the alleles is dominant over other and the phenotype formed is intermediate of the two alleles.
e.g., the kind of inheritance in the dog flower (Snapdragon or Antirrhinum species) in which the intermediate trait is expressed in $\mathrm{F}_1$-generation.
Codominance is a phenomenon in which when two contrasting alleles are present together and both of the alleles express themselves.
e.g., AB blood group in humans where both the alleles are expressed to produce RBC surface antigens $A$ and $B$.$$ \text { (i) Coss showing incomplete dominance } $$
$$ \text { (ii) Blood group showing co-dominance } $$
| Genotype | Surface Antigen | $$ \text { Blood Group } $$ |
|---|---|---|
| $\mathrm{I}^{\mathrm{A} i}$ (dominance) | A | A |
| $\left.\mathrm{I}^{\mathrm{A}}\right|^{\mathrm{A}}$ | A | A |
| $\mathrm{I}^{\mathrm{B}} \mathrm{i}$ (dominance) | B | B |
| $\mathrm{I}^{\mathrm{B}} \mathrm{I}^{\mathrm{B}}$ | B | B |
| $\mathrm{I}^{\mathrm{A}} \mathrm{I}^{\mathrm{B}}$ (co-dominance) | AB | AB |
| ii | - | O |
It is said, that the harmful alleles get eliminated from population over a period of time, yet sickle-cell anaemia is persisting in human population. Why?
Sickle-cell anaemia is an autosomal recessive disease caused by haemoglobins an oxygen carrying protein in blood cells.
Despite the disease's lethal symptoms, it protects the carrier from malaria. Its allele are most common in the people of African descent (about 7\% people of African descent carry an allele) and some other are as where malaria in prevalent.
It provides the vital protection from malaria. Individuals with HbAS heterozygotes tend to survive better than individuals with HbSS (homozygotes) as they are not exposed to the same severity of risk.
In a plant tallness is dominant over dwarfness and red flower is dominant over white. Starting with the parents work out a dihybrid cross. What is standard dihybrid ratio? Do you think the values would deviate if the two genes in question are interacting with each other?
The standard dihybrid ratio is $9: 3: 3: 1$. Yes, the values will show deviation if the two genes in the above case are interacting with each other. When the genes are linked, they do not assort independently but remain together in the gametes and the offsprings, give a dihybrid ratio of $3: 1$ and show a test cross ratio of $1: 1$ instead of $1: 1: 1: 1$.