31 Years NEET Previous Year Questions: Principles of Inheritance & Variation

So, the corresponding genotype will be AaBb.

Inheritance of skin colour in human is controlled by three genes, A B and C which is polygenic inheritance.

The two chains are antiparallel, one aligned in 5’→3’  direction the other in 3’ → 5’ direction.

Let GG  ⇒ homozygous yellow seed plant.
Gg ⇒ heterozygous green
gg ⇒ homozygous green According to the question : 

When the linked genes are situated quite close, the chances of crossing over are highly reduced. Due to this, large number of parental gametes are formed and only few recombinant gametes are formed. This results in higher number of parental types in F₂ generation as compared to recombinants.

Test cross, i.e. crossing of F₁-progeny to the recessive parent is used to find the genotype of the progeny.

The genotype AABbCC can produce different kinds of gametes based on the combinations of alleles. According to the principles of Mendelian inheritance, each heterozygous gene pair can produce two different types of gametes, while a homozygous pair produces only one type. Here, the gene pair Bb is heterozygous, so it can produce two different gametes (B or b), while AA and CC are homozygous, producing only one type of each (A and C, respectively). Thus, the total number of different gametes is calculated as 1 (A) * 2 (B or b) * 1 (C) = 2 types of gametes: ABC and AbC.

Phenotype is the appearance one organism shows while genotype is the gene complement it has from its ancestors. These genes only show their effect in phenotype but environment also plays an important role in this hence phenotype is a result of genotype and environmental interaction.

A cross of F₁ hybrid with its recessive homozygous parent is called the test cross. It is done to determine the genotype of a given plant. If the given plant has homozygous dominant traits then on test cross it gives all dominant trait plants but if it is heterozygous dominant than it gives dominant and recessive phenotypes in 1 : 1 ratio.

So the ratio of round seeded with yellow cotyledons : Round seeded green cotyledons : Wrinkled seeded yellow cotyledons : Wrinkle seeded green cotyledons in 9 : 3 : 3 : 1. So in the F₂ generation the round seeded character remains with yellow cotyledons & wrinkled seeds with yellow cotyledons.

In sickle cell anaemia RBCs become sickle shaped which are not supportive for the growth of malarial parasite Plasmodium so it provides immunity against malaria disease.

Sickle cell anaemia is a biochemical disorder in which shape of RBCs become sickle-shaped due to the defective haemoglobin. Haemoglobin becomes useless for oxygen transport.
Huntington Chorea in a disease in which atrophy of brain occurs resulting to respiratory irragulations, articulation of speech and irregular limbs movements take place. They both are genetic disease present in any person since birth hence congenital diseases.

Cri-du-chat syndrome in humans caused by the partial deletion of the short arm of chromosome no. 5. In this child has abnormalities in his/her facial expressions, mental retardation, speak like and they cry like that of cat, heart does not work properly.

Polygenic inheritance is the inheritance of traits which are dependent on the no. of genes such as the skin colour of human beings. eg. AABB is black AaBB in neither dark nor black. AaBb is again wheatish Aabb is light and aabb is white colour.

Colour blindness in a X-chromosome linked character. So they’ll be having all colour blind sons and carrier daughters.

This disease is due to an x - linked recessive mutation. Males suffer this disorder since they have only one X chromosome and hence express any trait on this chromosome.

Down's syndrome is caused by the presence of an extra chromosome number 21 and the off spring has 47 chromosomes.

Cystic fibrosis : It is a common disorder of caucasian race in which thick and more salty mucus blocks the respiratory tract. The homozygous recessive condition produces the defective protein which regulates chloride transport channel.
Cretinism : In this disorder the physical growth, mental growth and sexual growth in children is retarded. Such a dwarf and sterile child is called a cretin. It is due to hyposecretion of thyroid hormones.
Thalassemia : Due to defective production of α or β chains of haemoglobin, autosomal recessive.
Haemophilia : Sex linked disorder due to defective recessive gene.

Since the woman’s father was colour blind. She would be a carrier of the colour blindness gene. When she marries a colour blind man. Their progeny could be 

Glucose-6-P dehydrogenase is the first enzyme of glucose oxidation during Pentose Phosphate Pathway. RBC contain haemoglobin which combines with oxygen to form oxyhaemoglobin which gives its oxygen for oxidation of food. In haemolysis there is destruction of RBCs with release of haemoglobin into plasma resulting in juandice. So, now RBCs cannot provide oxygen for oxidation of food thereby causing deficiency of G-6-P dehydrogenase.

Polytene chromosomes were first reported by EG Balbiani in 1881. They are quite common in salivary glands of insects and are therefore popularly called salivary gland  chromosomes. Polytene chromosomes also occur in other organs of insects antipodal cells (of embryo sac), endosperm cells and suspensor cell of embryo. The chromosomes can reach length of 200 µm and contain 1000 to 16000 times DNA as compared to the ordinary somatic chromosomes. The giant chromosomes are formed by somatic pairing between homologous chromosomes, and repeated replication (endomitosis) of their chromatids.

The mathematical expression to calculate allele frequency can be given as p + q = 1. Where p represents the frequency of dominant allele and q represents the frequency of recessive allele. The bionomial expression of Hardy Weinberg’s law to calculate genotype frequency is p² + 2pq + q² = 1 where p² = homozygous dominant 2pq = heterozygous dominant  q² = homozygous recessive.
Frequency of heterozygous individuals = 2 × 0.6 × 0.4 = 0.48.

In order to find out the gamete or the genotype of an unknown individual, Scientists perform a test cross. In test cross the individual in question is crossed with the  homozygous recessive parent. Hence the answer is aabb.

A self-fertilizing trihybrid plant forms 8 different gametes and 64 different zygotes. The genotype of trihybrid plant is AaBbCc. Number of gametes formed = 2ⁿ = 2³ = 8 Here n = no. of heterozygosity = 3 in present case.
No. of zygotes formed = 8² = 64.

Change in genetic locus, changes the gene and in turn the amino acid it codes for. This alters the nature of protein synthesized which produces change in the organism. DNA replication is not affected neither the method of protein synthesis. Synthesis of  RNA over DNA template is called transcription.