Science [2 Mark Questions]

Fertilization of the egg (22 + X) with a sperm [22 + X] will produce a female child [44 + XX] The fertilization of the egg (22 + X) with a sperm [22 + Y] will give rise to a male child [44 + XY].

Monohybrid cross:
Phenotypic ratio – 3 : 1
Genotypic ratio – 1 : 2 : 1

Dihybrid cross:
Phenotypic ratio – 9 : 3 : 3 : 1
Genotypic ratio – 1 : 2 : 2 : 1 : 4 : 1 : 2 : 2 : 1.

At the time of gamete formation in the F1 Hybrid genes character asserted independently resulted in the appearance of new combination of character in the F2 progene.

Based on the position of the centromere, the chromosomes are classified as follows:

• Telocentric: The centromere is at the proximal end. The chromosomes are rod – shaped.
• Acrocentric: The centromere is found at the end with a short arm and a long arm. They are rod-shaped chromosomes.
• Sub metacentric: The centromere is found near the centre of the – chromosome. Thus forming two unequal arms. They are J shaped or L shaped chromosomes.
• Metacentric: The centromere occurs in the centre of the chromosome and forms two equal arms. They are V – shaped chromosomes.

Each chromatid is made up of a spirally coiled thin structure called chromonema. The chromonema has a number of bead – like structures along its length which are called chromomeres.

When a pair of contrasting alleles are brought together in a heterozygote, the two members of the allelic pair remain together without mixing and when gametes are formed, the two separate out, so that only one enters each gamete.