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Chromosomes: Carriers Of Heredity


W.S. Sutton, in 1902, provided the evidence that gene is a part of chromosome. The mode of transmission of ‘factor’ from parent to offspring as conceived by Mendel and that of the transmission of chromosomes through gametes were strikingly similar. Both Mendelian factors or genes and chromosomes are present in pairs. Both segregate during meiotic cell division to form gametes in which they remain unpaired. After fertilisation, the paired feature is again restored in the zygote that develops into the offspring.

Chromosome Number


In all organisms, the number of chromosomes is fewer than the number of characteristic features, which are many. If ‘genes’ are responsible for characteristic features, they have to be certainly many more than the number of chromosomes.

For example, in human beings, the total number of chromosomes is 23 pairs but the total number of characters (genes) has been estimated to be between 30,000 and 40,000. Genes are, however, located on chromosomes at fixed positions.

  • The number of chromosomes within the nucleus is constant in all individuals of a given species. For example, there are 46 chromosomes in humans, 40 in house mouse, 8 in fruit fly (Drosophila melanogaster), 20 in corn (maize) and 48 in potato. As it represents the double dose of chromosomes received from two parents, this number is called the diploid number of chromosomes. The nucleus of a gamete contains half this number of chromosomes or the haploid number.
  • The chromosomes that bear genes for sexual characters are called sex chromosomes or allosomes which are designated as X and Y chromosomes while those that possess genes for the vegetative characters are called autosomes. The XX pair with similar patterns is found in females whereas the XY pair with dissimilar patterns is found in males.

Sex-linked Inheritance


Sex-linked inheritance is the appearance of a trait which is due to the presence of an allele exclusively either on the X chromosome or on the Y chromosome. Sex-linked inheritance was first discovered by T.H. Morgan (1910) in the fruit fly, Drosophila melanogaster.

  1. Characteristics of sex-linked inheritance
  • Defects that arise due to X-linked inheritance are more common in males than in females. The reason for this is that defects controlled by the recessive genes on the X chromosomes cannot be masked by the Y chromosome. But in the females, if they are heterozygous, the defective genes of one of the X chromosome may be masked by the normal genes of the other X chromosome. Such females are known as carriers.
  • The defective gene on the X chromosome of the mother is transmitted to the son and such defective genes on the X chromosome of the father are transmitted to the daughter. This phenomenon is called criss-cross inheritance.
  1. ‘X’-linked inheritance: Certain diseases caused due to heredity such as haemophilia and colour blindness are common in males than in females. Such defects are due to recessive genes, which occur on the ‘X’ chromosome.
  • Colour blindness is a genetic disorder in human beings. People who are colour blind are unable to distinguish red from green colour. This character is controlled by a recessive gene located on the chromosome.
  • Haemophilia is a genetic disorder in which the sufferers are at a risk of bleeding to death because the blood fails to clot in them.
  • The following cases explain the sex-linked inheritance of colour blindness in humans. It is also true for haemophilia.

Case 1:



XX°: Daughters heterozygous dominant, normal vision

XY: Normal sons

[None of the children are colour blind, but daughters are carriers of the defective allele for colour blindness.]


Case 2:



Case 3:



  1. ‘Y’-linked inheritance: The Y-chromosome-linked traits occur in males but not in females. For example, traits such as hypertrichosis of ears (hair growing out of ears), pattern baldness are found in men only. This is because the dominant genes of such traits are found on ‘Y’ chromosome which determines the male sex.

Genetic Disorders in Man


In females, there are 44 autosomes and two X chromosomes. In males, there are 44 autosomes, one X chromosome and one Y chromosome. The chromosomes of the human female are represented by the formula 44 + XX. In the same way, the formula for a male is 44 + XY. This is the diploid condition for both male and female.

  • When there is meiosis to produce gametes, all the eggs of a female have a 22 + X composition, which is the haploid condition. However, in the male, two types of sperms are produced: one bears the 22 + X composition and the other, 22 + Y. In other words, for 100 sperms, 50 have Y chromosomes and 50 have X chromosomes. Males produce two types of gametes, and females produce one type of gamete.
  • When it comes to the sperm fertilising the egg, any one of the two types can do it. If a Y-bearing sperm fertilises the egg, the zygote has a 44 + XY composition and the resulting embryo grows to be a male. When an X-bearing sperm fertilises the egg, the resulting zygote has a 44+XX composition. This embryo develops into a female.
  • But anomolies in the chromosomes lead to several disorders in human beings. Aneuploidy is a condition which involves the absence of 1 or 2 chromosomes (hypoploidy) or the presence of 1 or 2 extra chromosomes (hyperploidy) in a normal set.
    Aneuploidy is further classified into the following types:
  1. Monosomy: One chromosome is absent (2n - 1 ⇒ 46 - 1 = 45).
  2. Trisomy: One extra chromosome is present (2n + 1 ⇒ 46 + 1 = 47).
  3. Nullisomy: Both chromosomes of a pair are absent (2n - 2 ⇒ 46 - 2 = 44).
  4. Tetrasomy: Two extra chromosomes are present (2n + 2 ⇒ 46 + 2 = 48).
  • Aneuploidy is caused due to non-disjunction. It is a meiotic error that occurs due to the failure of a pair of homologous chromosomes to segregate. As a result, one daughter cell receives both the chromosomes of a pair and the other without a chromosome of that pair. Non-disjunction may occur either in allosomes or in autosomes. It results in various disorders.
  • Down’s syndrome or 21st trisomy was identified by Longdon Down, in 1866, and thus it is named after him. It was earlier known as mongolism or mongoloid idiocy. The karyotype of this anamoly shows 47 chromosomes (46 + 1 = 47, which may be 44 + 1 + XX or 44 + 1 + XY) with one extra chromosome in the 21st pair of autosomes.
  • Klinefelter’s syndrome was reported by Harry Klinefelter. It is an allosomal aneuploidy produced due to the presence of two ‘X’ chromosomes and one ‘Y’ chromosome along with the autosomes. The resulting individual will have 47 chromosomes with a genotype of 44A + XXY. Such an individual will be a sterile male.
  • Turner’s syndrome (XO complex) was reported by Henry Turner. It is an allosomal aneuploidy produced due to the presence of only one ‘X’ chromosome along with the autosomes and the other sex chromosome is absent. Therefore, they are monosomic for allosomes. The resulting individual will have 45 chromosomes with a genotype of 44A + XO. Such an individual will be a sterile female.
  • Cri-du-chat syndrome refers to the cry of the baby syndrome which sounds like that of a cat. It is an autosomal disorder caused due to a deletion of the short arm of 5th chromosome. It is an example of partial monosomy.

Gene Disorders


The diseases caused by defective genes are called genetic diseases or inborn errors. This concept of inborn errors was first explained by William Bateson in 1902.

Sickle cell anemia

  • Sickle cell anemia is inherited through a recessive gene in a homozygous condition.
  • It is a metabolic disorder characterised by decreased oxygen transport in the blood.
  • This is due to the presence of a defective S-haemoglobin (Hbs) in place of the normal haemoglobin (Hb).
  • The S-haemoglobin differs from the normal haemoglobin in having an amino acid, valine in place of glutamic acid.
  • RBCs with S-haemoglobin are elongated and sickle-shaped with pointed edges instead of the normal biconcave shape.
  • When sickle cell RBCs pass through narrow capillaries, the lining of the blood vessel may be damaged leading to loss of blood in the body.
  • Sickle cell RBCs have a shorter life span and reduced oxygen-carrying capacity. As a result, there is a decrease in the oxygen supply to vital organs such as heart, brain, liver and kidney. This may lead to various disorders.
  • Sickle cell anemia is generally fatal.

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