Chapter 5 Principles of Inheritance and Variation Solutions
Question - 11 : - How is sex determined in human beings?
Answer - 11 : -
The chromosomal mechanism of determination of sex in human beings is of the genotype XX-XY. The nucleus of each cell consists of 23 pairs of chromosomes or 46 chromosomes out of which 22 pairs are autosomes and the last pair, the 23rd pair is the sex chromosome. Females are homomorphic, i.e., they possess two same sex chromosomes XX whereas males are heteromorphic, they possess two different sex chromosomes XY. Females are homogametic, producing only one type of egg (22+X) whereas males are heterogametic producing two types of sperms – (22+X) & (22+Y).
In the process of fertilization, if the sperm containing X chromosome fertilizes the egg, then the developing offspring would be a female (XX) and if the sperm containing Y chromosome fertilizes the egg, then the developing offspring would be a male (XY). Therefore, the sex ratio that is produced in the progeny is 1:1. Heterogamesis is the chromosomal mechanism of sex determination which can be male heterogamety or female heterogamety.
Question - 12 : - A child has blood group O. If the father has blood group A and mother blood group B, work out the genotypes of the parents and the possible genotypes of the other offsprings.
Answer - 12 : - A set of three alleles – IA, IB and i control the blood group characteristics in humans. Where alleles IA and IB are equally dominant and allele i is recessive to the other alleles. The table below depicts the genotypes and blood group.
| Individuals with Genotype | Blood group they would possess |
| IA, IA, IA, i | A |
| IB,IB, IB, i | B |
| IA, IB | AB |
| ii | O |
Thus, if father has blood group A and mother has blood group B, then the possible genotype of the parents will be as follows:
A cross between heterozygous parents willproduce progenies with AB blood group (IA IB) and Ogroup (ii)
Explain the following terms with example
(a) Co-dominance (b) Incomplete dominance
Answer - 13 : -
1. Co-dominance – These are the alleles that are able to express themselves independently when found together in a heterozygote. They are termed as co-dominant alleles.
2. Incomplete dominance – it is the phenomenon of inheritance where none of the contrasting characters in a pair are dominant. In the F1 hybrid, the expression of the trait is a fine mixture of the expression of the two factors. For instance, the Mirabilis jalapa produces pink flowers when a hybrid cross between a red and white flower is carried out.
Question - 14 : - What is point mutation? Give one example.
Answer - 14 : -
The abrupt change in the gene structure due to a change in the single-base pair of DNA due to inversion and substitution without any alteration in the reading of succeeding bases. For example – sickle cell anemia.
A change in the shape of the RBCs from biconcave discs to elongated sickle-shaped structures is caused due to substitution of a single nitrogen base guanine to adenine at the sixth codon of the β-globin chain of the haemoglobin molecule. A typical RBC is biconcave, resembling flexible disc-like that facilitates the cells to pass through large and small blood vessels to convey oxygen. Whereas sickle-shaped RBCs are elongated, non-flexible and rigid in structure which causes the cells to stick to the walls of the blood vessels leading to a blockage slowing or stopping the blood flow, hence sickle cell anemia.
Question - 15 : - Who had proposed the chromosomal theory of the inheritance?
Answer - 15 : - In 1902, the chromosomal theory of inheritance was independently put forward by Theodore Boveri and Walter Sutton.
Question - 16 : - Mention any two autosomal genetic disorders with their symptoms.
Answer - 16 : -
These disorders are caused as a result of a defect in the gene found on the autosomes. Some autosomal genetic disorders are – Down’s syndrome, sickle cell anemia, Patau syndrome, Phenylketonuria.
Down’s syndrome
Symptoms:
• Flat hands, short neck
• Broad forehead
• Partially open mouth, furrowed tongue
• Mongolian type eyelid fold and stubby fingers
• Stunted psychomotor, physical and mental development
• Heart deformities and deformity of other organs
• Underdeveloped genitalia and gonads
Sickle cell anemia
Symptoms:
• Shape of RBCs change from elongated to sickle-shaped(curved) under the influence of low oxygen tension
• These sickle-shaped RBCs are more rapidly destructed than the normal ones causing anaemia