Genetics and Plant Breeding

A.Polygenes – Inheritance is controlled by two or more genes in which the dominant alleles have cumulative effect, with each dominant allele expressing a part of functional polypeptide and full trait is shown when all the dominant alleles are present, is called Polygenic or Quantitative inheritance.

B.Linked genes – The tendency of some genes to inherit together is known as linkage, and the genes are linked genes.

C. Pseduoalleles - Genes are present together side by side and they produce related phenotypes are called pseudoalleles. These are distinguished from true alleles through rare crossing over.

D. Hypostatic gene – A gene whose effect are masked is by another gene is called Hypostatic gene. In the F₂ generation 12:3:1 ratio.

In a dihybrid cross between two heterozygotes, AaBb x AaBb, if we get a 3:1 ratio among offspring, the reason would be Linked genes.

So, the correct option is ‘Linked genes’.

A.Dominance - When two contrasting alleles for a character come together in an organism, only one is expressed and another one is not expressed. Expressed character is called Dominant character and the character which is not expressed is called Recessive character. It is the Mendel’s first principle.

B.Purity of gametes/segregation - In the living organisms, two alleles controlling each character maintains identity in the organism, but during the formation of gametes or spores, by meiosis move apart due to separation of the homologous chromosomes which bear them, so that each gametes or spore receives only one allele of each character on random basis. It is called Law of segregation. It is the Mendel’s second principle.

C.Linkage – The physical association of genes in a chromosome is called Linkgage. It is not the Mendel’s principle.

D.Indepedent assortment - When a cross is made between two individuals different from each other in two or more characters, then the inheritance of one character is independence of the inheritance of another character.

So, the correct option is ‘Linkage’.

• Linkage refers to the presence of two different genes on the same chromosome. Two genes that occur on the same chromosome are said to be linked, and those that occur very close together are tightly linked.
•  Crossing over is the exchange of chromosome segments between non-sister chromatids during the production of gametes.
• When the expression of one gene depends on the presence or absence of another gene in an individual, it is known as gene interaction.
• The pattern of inheritance for a qualitative trait is typically monogenetic, which means that the trait is only influenced by a single gene. Inherited diseases caused by single mutations are good examples of qualitative traits.

To find gene sequence from crossover percentages, identify genes with lowest recombination (closest) and arrange to satisfy all distances. J-M (20%) and M-L (15%) suggest J-M-L. L-K is 15%, so J-M-L-K. M-N is 50% (far apart), and J-N is 20%, meaning N is close to J. The sequence J-M-L-K-N satisfies: J-M (20%), M-L (15%), L-K (15%), M-N (50% via J), and J-N is close. Option C matches.

All three statements are correct: linkage prevents independent assortment, distance determines linkage strength, and strength is inversely (not directly) proportional to distance. However, in many contexts, 'proportional' is used loosely; given the options, A is the intended answer.

The methods used to identify the locus of a gene and the distances between genes is called gene mapping. It indicates the % of crossing over occurs between the linked genes. The essence of all genome mapping is to place a collection of molecular markers onto their respective positions on the genome. Recombination between linked genes can be used to map their distance apart on the chromosome. The distance between the two linked genes is measured by centimorgans or map units.

So, the correct option is ‘ratio of crossing over between them’.

The number of linkage groups is four in number which is less than humans. Fruit-fly Drosophila melanogaster has four linkage groups (4 pairs of chromosomes), human beings 23 linkage groups (23 pairs of chromosomes) So, the correct option is 'False'.

The chromosome no. 4 of pea plant carries genes for flower position, pod form and plant height. Out of these three genes, the two genes namely the gene for pod form and the one for plant height are closely placed on the chromosome that they show linkage. However, the gene for flower position is located far apart from the other two genes and do not show linkage. Hence, option A is correct.