Samacheer Kalvi 12th Biology Chapter 14 Exercise Solutions

Codominance occurs when both alleles in a heterozygote are fully and equally expressed, producing both phenotypes simultaneously (e.g. I^A I^B = blood group AB, where both A and B antigens are present on RBCs). Incomplete dominance occurs when the heterozygote shows an intermediate phenotype between the two homozygous parents (e.g. red × white snapdragon = pink). In codominance, both traits appear; in incomplete dominance, a blended trait appears.

Erythroblastosis foetalis is a haemolytic disease of the newborn caused by Rh incompatibility. When an Rh− mother carries an Rh+ foetus (inherited from the father), her immune system produces anti-Rh antibodies. In subsequent Rh+ pregnancies, these antibodies cross the placenta and destroy the foetal RBCs, causing severe anaemia and jaundice. It can be prevented by administering anti-Rh immunoglobulin (RhoGAM) to the Rh− mother within 72 hours of delivery of an Rh+ baby.

ABO blood groups are controlled by the I gene with three alleles: I^A, I^B (codominant), and i (recessive). Cross: I^A i (blood group A) × I^B i (blood group B). Gametes: I^A, i from parent 1; I^B, i from parent 2. Offspring genotypes: I^A I^B (AB), I^A i (A), I^B i (B), ii (O). Phenotypic ratio: 1 AB : 1 A : 1 B : 1 O. This demonstrates both multiple allelism and codominance.

Pleiotropy is the phenomenon where a single gene influences multiple phenotypic traits. A classic example is sickle cell anaemia: the HbS gene affects RBC shape (sickling under low oxygen), causes anaemia, and in heterozygotes (HbA HbS) provides resistance to malaria. Another example is phenylketonuria (PKU), where the defective gene affects both brain development and skin pigmentation. Cystic fibrosis is also pleiotropic — the CFTR gene mutation affects the lungs, pancreas, liver, and sweat glands simultaneously.

X-linked recessive disorders are more common in males because males have only one X chromosome. If a male inherits the recessive allele on his X chromosome (X^h Y), he will express the disorder since there is no second X chromosome with a normal allele to mask it. Females have two X chromosomes, so they need two copies of the recessive allele (X^h X^h) to be affected. A female with one copy (X^H X^h) is a carrier and is usually unaffected.

Blood group A: I^A I^A (homozygous) or I^A i (heterozygous). Blood group B: I^B I^B (homozygous) or I^B i (heterozygous). Blood group AB: I^A I^B (heterozygous — codominant). Blood group O: ii (homozygous recessive).

A carrier female has one normal allele and one defective allele on her X chromosomes (X^H X^h). She does not express the disorder because the normal allele produces enough of the functional protein. An affected individual expresses the disorder — for males, this means having the defective allele on their single X chromosome (X^h Y); for females, it means having defective alleles on both X chromosomes (X^h X^h), which is very rare. Carriers can pass the defective allele to their children.

The point mutation at position 6 of the beta-globin gene substitutes glutamic acid with valine. This single amino acid change alters the structure of haemoglobin, producing HbS instead of HbA. Under low oxygen conditions, HbS molecules polymerise into long fibres, causing red blood cells to become sickle-shaped. These sickle cells block capillaries, reduce blood flow, and cause pain crises, organ damage, and anaemia. In heterozygotes (HbA HbS), this mutation also provides some protection against malaria — a classic example of pleiotropy.

ABO blood groups are controlled by a single gene I (isoagglutinin) located on chromosome 9. This gene has three alleles: I^A, I^B, and i. I^A and I^B are codominant to each other (both expressed when present together), while i is recessive to both. This is an example of multiple allelism because more than two alleles exist in the population, though each individual carries only two.

• Blood group A: Genotypes I^A I^A or I^A i. RBCs have antigen A; plasma has anti-B antibodies.
• Blood group B: Genotypes I^B I^B or I^B i. RBCs have antigen B; plasma has anti-A antibodies.
• Blood group AB: Genotype I^A I^B. RBCs have both A and B antigens; plasma has neither anti-A nor anti-B antibodies. This demonstrates codominance — both alleles are expressed simultaneously.
• Blood group O: Genotype ii. RBCs have no antigens; plasma has both anti-A and anti-B antibodies.

Codominance is demonstrated because I^A I^B individuals express both A and B antigens equally on their RBCs — neither allele is dominant or recessive. Multiple allelism is demonstrated because three alleles (I^A, I^B, i) exist for the same gene locus in the population.

Blood group AB can donate only to AB but can receive from all groups (universal recipient). Blood group O can donate to all groups (universal donor) but can receive only from O. This has critical implications for blood transfusion compatibility.

Sickle cell anaemia: Autosomal recessive disorder caused by a point mutation in the beta-globin gene on chromosome 11. The mutation substitutes glutamic acid with valine at the 6th position of the beta chain. This produces abnormal haemoglobin HbS, which polymerises under low oxygen conditions, causing RBCs to become sickle-shaped. Sickle cells block capillaries, causing pain crises, anaemia, and organ damage. Genotypes: HbA HbA (normal), HbA HbS (carrier — sickle cell trait, some protection against malaria), HbS HbS (disease — sickle cell anaemia). This is also an example of pleiotropy because the same gene affects RBC shape, causes anaemia, and provides malaria resistance in heterozygotes.

Haemophilia: X-linked recessive disorder. Haemophilia A is caused by deficiency of clotting factor VIII; Haemophilia B by deficiency of factor IX. The gene is located on the X chromosome. Males are predominantly affected (X^h Y). Females can be carriers (X^H X^h) but are rarely affected (X^h X^h). Cross: carrier female (X^H X^h) × normal male (X^H Y) → 25% normal female, 25% carrier female, 25% normal male, 25% haemophiliac male. Queen Victoria of England was a famous carrier of haemophilia, which spread through royal families across Europe — this is called the "royal disease."

Colour blindness: X-linked recessive disorder affecting the ability to distinguish colours (most commonly red-green). Caused by mutations in opsin genes on the X chromosome. Inheritance pattern is similar to haemophilia — males are more frequently affected. A colour-blind male (X^c Y) cannot pass the trait to his sons but transmits the allele to all daughters (who become carriers). Carrier females (X^C X^c) have a 50% chance of passing it to sons. The most common type is red-green colour blindness, where individuals cannot distinguish between red and green colours.

The Rh (Rhesus) factor is an antigen present on the surface of RBCs in most people (Rh+). Individuals who lack this antigen are Rh−. The Rh gene is dominant: Rh+ (DD or Dd) and Rh− (dd). Erythroblastosis foetalis (Haemolytic Disease of the Newborn) occurs when an Rh− mother carries an Rh+ foetus.

Mechanism: During the first pregnancy with an Rh+ foetus, some foetal RBCs enter the maternal circulation at delivery. The Rh− mother's immune system recognises Rh antigen as foreign and produces anti-Rh antibodies (IgG). In a subsequent Rh+ pregnancy, these antibodies cross the placenta and attack the foetal RBCs, causing haemolysis, severe anaemia, jaundice, and in extreme cases, stillbirth.

Prevention: Anti-Rh immunoglobulin (RhoGAM) is administered to the Rh− mother within 72 hours of delivery of an Rh+ baby. This destroys any Rh+ foetal cells in the mother's circulation before her immune system can mount a response, preventing antibody formation. It must be given after every Rh+ pregnancy, miscarriage, or abortion. Without RhoGAM, the risk of erythroblastosis foetalis increases with each subsequent Rh+ pregnancy.

Important note: The first Rh+ pregnancy is usually unaffected because the mother has not yet been sensitised. Erythroblastosis foetalis typically manifests from the second Rh+ pregnancy onward, when pre-existing anti-Rh antibodies are already present in the maternal blood.

• Codominance: Both alleles in a heterozygote are fully and equally expressed. The heterozygote shows both phenotypes simultaneously, not a blend. Example: ABO blood group — I^A I^B genotype produces both A and B antigens on RBCs (blood group AB). Neither allele is dominant or recessive; both contribute to the phenotype independently. The F2 ratio for a cross involving codominance differs from complete dominance because the heterozygote is distinguishable from both homozygotes.

• Incomplete dominance: The heterozygote shows an intermediate phenotype between the two homozygous parents. Neither allele is completely dominant. Example: Snapdragon flower colour — RR (red) × rr (white) → Rr (pink) in F1. The F2 ratio is 1:2:1 (red:pink:white), which is the same as the genotypic ratio, because the heterozygote can be distinguished from both homozygotes. This differs from complete dominance, where the F2 phenotypic ratio is 3:1.

• Pleiotropy: A single gene affects multiple, seemingly unrelated phenotypic traits. This occurs because the gene product (protein) is involved in multiple metabolic pathways or affects different tissues. Example: Sickle cell gene (HbS) — affects RBC shape (sickling), causes anaemia, and in heterozygotes provides resistance to malaria. PKU gene — affects brain development (intellectual disability) and skin pigmentation (fair skin).

Key differences: Codominance and incomplete dominance both involve heterozygotes, but codominance shows both traits while incomplete dominance shows a blend. Pleiotropy is about one gene affecting multiple traits, not about heterozygote expression. Codominance and incomplete dominance are about how alleles interact within a single gene, while pleiotropy is about how a single gene affects multiple phenotypic outcomes.

Sex-linked inheritance refers to the inheritance of genes located on the X or Y chromosome. Haemophilia is an X-linked recessive disorder caused by mutations in genes on the X chromosome — factor VIII gene (Haemophilia A) or factor IX gene (Haemophilia B). Since the gene is on the X chromosome, its inheritance pattern differs between males and females.

Inheritance pattern — Cross: Carrier female (X^H X^h) × Normal male (X^H Y)

Gametes: X^H, X^h from mother; X^H, Y from father

Offspring:

• X^H X^H — Normal female (25%)
• X^H X^h — Carrier female (25%)
• X^H Y — Normal male (25%)
• X^h Y — Haemophiliac male (25%)

Key observations: 50% of sons are haemophiliac. 50% of daughters are carriers. No daughter is affected in this cross.

Why affected females are extremely rare: A female has two X chromosomes. To be affected, she needs two copies of the recessive allele (X^h X^h). This can only happen if her father is haemophiliac (X^h Y) AND her mother is either a carrier (X^H X^h) or affected (X^h X^h). Since haemophiliac males often did not survive to reproductive age before modern treatment, the combination was exceedingly rare. Even today, the probability of this genotype is very low. In contrast, a male needs only one copy of the recessive allele (X^h Y) because he has only one X chromosome, making the disorder far more common in males.