Mendel’s principles of classical genetics are fundamental laws. Even though Mendel’s laws were first tested in pea plants and fruit bees, the evidence has grown rapidly that they apply to all organisms. As mutations provided the means to understanding the genetics of fruit bees, the pedigree of families affected by the disease provided the first example of Mendelian inheritance in humans. These are widely applicable to all organisms. Let us apply his law to human genetics.
Sickle Cell Anemia
Sickle cell anemia is a hereditary disease. The blood of the infected person is less likely to carry oxygen to the tissues. Hemoglobin is oxygen. Which holds protein in our red blood cells. It gives red blood cells their color. The normal hemoglobin red blood cells are in the form of a beacon cue disk. They pass through the capillaries easily and provide oxygen to the tissues.
As long as oxygen is high, red blood cell capillaries – cell hemoglobin appears as usual. When blood oxygen is present after the supply of O2, the oxygen level decreases, so this hemoglobin forms the pool of non-dissolving fibers which is called fibrous strands. These strands distort the shape of the RBC in long thin sickles.
Distorted cells cannot pass through narrow capillaries. So capillaries become clogged. The tissues are starved for oxygen. Sickle cells may rupture and cause anemia.
A hereditary disease is a body malfunction caused by a gene. It cannot be cured unless the malfunctioning gene is repaired or replaced by a normal one.
Hemoglobin is a large protein. It has four chains of amino acids; two identical alpha (α) chains and two identical beta (β) chains. The alpha chains of normal and sickle cell hemoglobin are alike, but their chains differ by a single amino acid. The amino acid glutamic acid at 6th position is replaced by valine in sickle cell chain. The chain of normal hemoglobin is encoded by a gene called human globin gene (HbA) Fig. (17.26)
Anemia is a condition of blood with lack of red corpuscles.
The DNA triplet of HbA gene encoding for glutamic acid is CTT. A point mutation changes just the middle base of the triplet from T to A. as it becomes CAT a new allele HbS emerges. CAT encodes for valine. So, the allele for normal hemoglobin is HbA and allele for sickle – cell hemoglobin is HbS. These are present on chromosome No: 11 (Fig 17.27)
Individuals homozygous for normal hemoglobin are HbA / HbA. They do not have the disease. Homozygote for sickle cell allele HbS / HbS have the disease. Heterozygotes HbA / HbS are called sickle – cell carriers. The carriers show no symptoms of the disease under normal circumstances. Only 1 % of their RBC becomes sickle shaped.
Fig 17.27: Human chromosome 11 with linked gene of sickle – cell anemia and albinism.
It is an autosomal recessive trait (Fig 17.28). If a normal woman (HbA / HbA) marries a sickle cell anemia man (HbS / HbS), all their children will be carrier (HbA / HbS).
Activity No. 5: Study of principle of inheritance (Law of segregation) through checkerboard
- Work out with a punnett square the risk of a sickle – cell anemic childbirth in a family of a normal man married to a carrier woman for sickle – cell anemia.
- What is the probable risk of having a sickle – cell anemia child when a carrier woman marries a man suffering from sickle – cell anemia?
- Earlobe is a normal human trait. Free earlobe is the dominant phenotype determined by allele ‘E’. Attached earlobe is its recessive phenotype produced by allele ‘e’ in homozygous condition. What type of earlobes do you and your family members have? Can you trace transmission of this trait in your family?
Diabetes Mellitus
It is a hereditary disease caused by a recessive allele‘d’ in homozygous condition. Diabetics are unable to use glucose in their body metabolism. They pass glucose in their urine. Normal individuals have dominant allele ‘D’. Their urine is without glucose.
Can you guess why the chances of diabetes in children increase when both parents are diabetics, than when one parent is normal but he other diabetic?
If a man suffering from diabetes but homozygous normal for hemoglobin marries a woman homozygous normal for sugar metabolism but suffering from sickle – cell anemia, what are the chances in their children of being diabetic and sickle – cell anemic at a time?