2024 Edexcel IGCSE Human Biology 模擬試題連答案詳解

(a) Normal blood clotting mechanism:

• When a blood vessel is damaged, platelets aggregate at the site and release clotting factors (thromboplastin).
• These factors trigger a cascade that converts the inactive plasma protein prothrombin into the active enzyme thrombin.
• Thrombin then catalyses the conversion of the soluble plasma protein fibrinogen into insoluble protein fibers called fibrin.
• Fibrin forms a dense mesh network across the wound that traps red blood cells and platelets, forming a solid clot to prevent further blood loss and pathogen entry.

(b) (i) Mode of inheritance:

• Recessive: Unaffected parents (such as 1 and 2, or 4 and 6) have an affected child (3 and 7 respectively). This indicates the allele can be masked in heterozygous parents.
• Sex-linked (X-linked): The condition only affects males in this pedigree, and unaffected father 6 has an affected son 7. This shows that the allele is carried on the X chromosome and the unaffected mother 4 is a carrier. Males only inherit one X chromosome (from their mother), so a single recessive allele on their X chromosome results in the disease.

(b) (ii) Genetic diagram:

• Individual 4 (Carrier female): \(X^H X^h\)
• Individual 6 (Unaffected male): \(X^H Y\)

The offspring genotypes are \(X^H X^H\), \(X^H X^h\), \(X^H Y\), and \(X^h Y\).

The probability of having an affected male child is 1 in 4 (or 25% or 0.25) of all births.

(c) Clotting time difference and management:

• Individual 3 has Hemophilia A, meaning they lack functional clotting factor VIII. Consequently, the chemical cascade that converts fibrinogen to insoluble fibrin is severely disrupted or absent, leading to a much slower clotting process (25 minutes vs 4 minutes in Individual 1).
• This presents a severe risk of prolonged internal or external bleeding from minor injuries.
• The condition is managed medically by regular intravenous infusions of recombinant/donor clotting factor VIII (replacement therapy).

(a) [Maximum 4 marks]

• MP1: Platelets release clotting factors / thromboplastin at the wound site;
• MP2: Prothrombin is converted to active enzyme thrombin;
• MP3: Thrombin catalyses conversion of soluble fibrinogen to insoluble fibrin;
• MP4: Fibrin forms a mesh that traps red blood cells (forming a clot);

(b) (i) [Maximum 3 marks]

• MP1: Recessive because unaffected parents have an affected child (e.g., 4 and 6 have 7) / trait skips a generation;
• MP2: Sex-linked / X-linked because only males are affected in the pedigree / unaffected father (6) has an affected son (7) meaning mother (4) must be a carrier;
• MP3: Explains mechanism: gene is on the X chromosome, so males (XY) expressing the single recessive allele are affected, while females (XX) need two copies or remain unaffected carriers;

(b) (ii) [Maximum 4 marks]

• MP1: Correct parental genotypes identified: \(X^H X^h\) and \(X^H Y\);
• MP2: Correct gametes shown in Punnett square;
• MP3: Correct offspring genotypes derived: \(X^H X^H\), \(X^H X^h\), \(X^H Y\), \(X^h Y\);
• MP4: Correct probability of an affected male child stated as 0.25 / 25% / 1 in 4 / 1/4 (Accept 50% of males if clearly qualified);

(c) [Maximum 3 marks]

• MP1: Individual 3 lacks factor VIII, meaning they cannot efficiently convert soluble fibrinogen to insoluble fibrin / cannot form a clot quickly;
• MP2: Leads to risk of internal bleeding (into joints/organs) or excessive blood loss from minor wounds;
• MP3: Managed by regular intravenous injections / infusions of factor VIII / clotting factor replacement therapy;