Mendel’s Law And We The Humans: Mendel’s principles of classical genetics are fundamental laws. These are widely applicable to all organisms. Let us apply his law to human genetics.
17.7.1 Sickle – cell anemia
Sickle – cell anemia is a hereditary disease. The blood of affected individual has a reduced capacity of delivering oxygen to tissues. Hemoglobin is the oxygen – carrying protein in our red blood cells. It gives red blood cell their color. Red blood cells with normal hemoglobin are in the shape of a biconcave disc. They easily pass through capillaries and supply oxygen to tissues. Red blood cells with sickle – cell hemoglobin appear normal so long as oxygen is plentiful. As the oxygen level drops when blood exist tissues after supplying O2, this hemoglobin forms insoluble fibrous strands. These strands distort the shape of RBC into long thin sickles (Fig 17.25)
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. Hemozygotes 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
(a) 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.
(b)What is the probable risk of having a sickle – cell anemia child when a carrier woman marries a man suffering from sickle – cell anemia?
(c)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?
17.7.2 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?
Activity No. 6:Study of principle of inheritance (Law of independent assortment) through checkerboard.
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?