Chapter 4: Principles of Inheritance and Variation

Ans: Humans have an $X Y$ sex determination pattern. Female sex chromosomes are $X X$, while male sex chromosomes are $X Y$. Females can only generate gametes with $X$ chromosomes, but men can produce gametes with both $X$ and $Y$ chromosomes.

When a male $Y$ chromosome-containing gamete fertilizes with a female $-X$ chromosome-containing gamete, the result is a male fetus. When a male $X$ chromosome-containing gamete fertilizes with either a female or a male gamete, the fetus is a female.

Ans: Human blood groups are determined by three types of alleles: $I$, $I^{B}$ and $i$. While $I^{A}, I^{B}$ are codominant, $I$ is recessive. $I^{A}$ stands for blood group $A$, and $I^{B}$ stands for blood group $B$. The $I$ allele codes for the $O$ blood group.

There are two possible allele combinations for a person with blood group $A$. They are: $I^{A} I^{A}$ or $I^{A} i$. There are also two possible allele combinations for a person with blood group $B$. They are: $I^{B} I^{B}$ or $I^{B} i$. The blood group $A B$ is formed when both $I^{A}$ and $I^{B}$ are present, while the blood type $O$ is formed when both are absent. Because both the father and mother have dominant alleles but the child has a recessive phenotype, the parent's dominant $A$ and $B$ alleles must be present in the heterozygous situation. This is because if the alleles were homozygous in either parent, the child would have the dominant phenotype rather than the recessive phenotype. As a result, under the preceding scenario, the other offspring could have the $A, B$ or $A B$ blood group.

Ans: Codominance occurs when both opposing alleles of a gene pair are equally expressed. For example, in the human blood group, both $A$ and $B$ are equal prominent, and when both are present, the $A B$ blood group appears instead of either $A$ or $B$

1. Incomplete dominance

Ans: Incomplete dominance occurs when a dominant allele does not entirely cover the effects of a recessive allele, resulting in a blending of both alleles in the organism's

physical appearance. It's also termed as half dominance or semi-dominance. Take, for example, the flower color of a snapdragon plant. The red flower color (R) has an imperfect dominance over the white flower color (r). When both alleles (R) are present in the same plant, (R) is unable to completely repress (r), resulting in a pink flower phenotype.

Ans: Point mutation happens when a mutation arises in a single base pair of DNA. For example, in sickle cell anemia, glutamic acid is replaced by valine in the sixth position of the beta globin chain of hemoglobin, resulting in sickle-shaped RBCs rather than biconcave RBCs.

Ans: The chromosomal hypothesis of inheritance was presented by Sutton and Boveri in 1902. This states that chromosomes serve as carriers for gene inheritance.

Ans: 

1. Sickle Cell Anemia - When glutamic acid is replaced by valine in the sixth position of the beta globin chain of hemoglobin due to a point mutation, the biconcave shape of hemoglobin transforms into a sickle shape. This causes a reduction in blood oxygen delivery, as well as weakness and fatigue.

2. Phenylketonuria - Phenylketonuria is a condition in which a person lacks an enzyme that transforms amino acids and phenylalanine to tyrosine. The phenylalanine then builds up and is converted to phenyl pyruvic acid and other derivatives. Mental retardation is a symptom of this disease. They are eliminated in the urine due to inadequate renal absorption.