Principles of Inheritance and Variation — BIOLOGY STD 12 Science — Question
Maharashtra BoardEnglish MediumSTD 12 ScienceBIOLOGYPrinciples of Inheritance and Variation3 Marks
Question
Explain the causes, inheritance pattern and symptoms of any two Mendelian genetic disorders.
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Answer
Haemophilia:
It is a sex-linked recessive disorder.
Patient continues to bleed even with a minor cut because of a defect in blood coagulation.
The gene for haemophilia is located on $X$ chromosome.
More males suffer from haemophilia than females because in males single gene for the defect is able to express as males have only one $X$ chromosome.
The defective alleles produce non-functional proteins which later form a non-functional cascade of proteins involved in blood clotting.
Females suffer from this disease only in homozygous condition, i.e., $Xc Xc$ .
Queen Victoria was a carrier of this disease and produced haemophilic offsprings.
Sickle–cell anaemia:
It is an autosome-linked recessive trait.
The disease is controlled by a single pair of allele $Hb^A$ and $Hb^S$ .
Only the homozygous individuals for $Hb^S$ , i.e., $Hb^S$ HbS show the diseased phenotype.
The heterozygous individuals are carriers $(Hb^AHb^S )$.
Due to point mutation, glutamic acid $(Glu)$ is replaced by valine (Val) at the sixth position of $\beta$-globin chain of haemoglobin molecule.
HbS behaves as normal haemoglobin except under oxygen stress where erythrocytes lose their circular shape and become sickle-shaped. As a result, the cells cannot pass through narrow capillaries. Blood capillaries are clogged and thus affect blood supply to different organs.
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