Science: Heredity: Blood Group, Heredity Disorders and Human Genome (For CBSE, ICSE, IAS, NET, NRA 2022)

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The Major Steps of DNA Duplication Are Simplified Below

  • The double stranded DNA molecule unwinds with the help of certain enzymes to expose two strands of DNA.
  • A DNA polymerase enzyme catalyzes the formation of a new daughter strand which can form a double helix with one strand of parental DNA molecule. So, two DNA molecules, each with a parental strand and a new strand get generated.

The two identical DNA molecules then become two chromatids which remain attached by a centromere.

Blood Group

  • There are four blood groups A, B, AB and O. Every human being has one blood group out of the four. The genes which control the inheritance of these blood groups are designated as IA, IB and i.
  • Apart from these blood groups, human beings may also belong to the groups designated as Rhesus positive (Rh +) or Rhesus negative. Most humans are Rh + . Some are Rhesus negative (Rh-) . The Rh + gene is dominant over Rh- gene.
Blood Group
Gene combinationBlood group
IA or IA iA
IB or IB iB

Sex Determination in Humans

  • The combination of sex chromosomes with autosomes determines whether the foetus will be a boy or a girl.
  • The foetus develops from the zygote which is formed by the fusion of the two gametes, the male gamete or sperm and the female gamete or egg. Gametes are haploid
  • Ova or eggs are of one kind only. These contain 22 autosomes and a single X chromosome. Sperms are of two kinds
    • having 22 autosomes and one X chromosome, or
    • having 22 autosomes and a Y chromosome (see figure 25.8) . When X bearing sperm fuses with the egg, a female child results with 44 autosome and two X chromosomes. If Y bearing sperm fuses with the egg, then a male child results with chromosomal constitution of 44 autosomes and one X and one Y chromosome

Heredity Disorders

Genetic disorder is a disease caused in whole or in part by a change in the DNA sequence away from the normal sequence. Genetic disorders can be caused by a mutation in one gene (monogenic disorder) , by mutations in multiple genes (multifactorial inheritance disorder) , by a combination of gene mutations and environmental factors, or by damage to chromosomes (changes in the number or structure of entire chromosomes, the structures that carry genes) . There are several genetic (hereditary) disorders. Three common hereditary disorders are Thalassemic, Haemophilia and Color blindness.


Patients suffering from this disorder are unable to manufacture hemoglobin, the pigment present in red blood corpuscles which carries oxygen to tissues. This is because the pair of genes controlling hemoglobin production are defective. Thalassemic (persons suffering from Thalassemic) require frequent blood transfusion in order to survive. The Thalassemic gene is present on an autosome.


Those persons suffering from haemophilia have either a defective gene or lack genes, which control production of substance responsible for blood clotting. In the

absence of such substance blood does not coagulate. Once bleeding starts, it does not clot easily.


Different kinds of color-blindness have been detected but in the most common form of the disorder, a person is unable to distinguish the blue color from green. Again, this is due to the presence of a defective gene or absence of the gene, responsible for color vision.

Human Genome

Human genome project (HGP) was an international scientific research project which got successfully completed in the year 2003 by sequencing the entire human genome of 3.3 billion base pairs. The HGP led to the growth of bioinformatics which is a vast field of the research. The successful sequencing of the human genome could solve the mystery of many disorder in humans and gave us a way to cope up with them.

Human Genome

Goals of the Human Genome Project

Goals of the human genome project include:

  • Optimization of the data analysis.
  • Sequencing the entire genome.
  • Identification of the complete human genome.
  • Creating genome sequence databases to store the data.
  • Taking care of the legal, ethical and social issues that the project may pose.

Methods of the Human Genome Project

In this project, two different and significant methods are typically used.

  • Expressed sequence tags wherein genes ere differentiated into the ones forming a part of the genome and the others which expressed RNAs.
  • Sequence Annotation wherein the entire genome was first sequenced, and the functional tags were assigned later.

Developed by: