In recent years, we find that a huge number of human disorders like chromosomal disorders, deformities and diseases  become hereditary. Reasons of some diseases are gene mutation and others are gross chromosomal abnormalities.  We can study these abnormal arrangement of chromosome by advanced genetical studies.

Various genetic disorders are classified into two broad categories

Mendelian disorders and Chromosomal disorders like Down’s syndrome.

Here I am trying to elaborate some important chromosomal disorders PKU or Down’s syndrome. The  reason of these disorders are absence or excess or abnormal arrangement of one or more chromosomes.

Down’s syndromeAutosomal aneuploidy in chromosomal disordersTrisomy of chromosome 21 in abnormal arrangement of chromosome in down’s syndromeMongolian eyelid fold; ever open mouth; protruding tongue; short neck; projecting lower lip in down’s ssyndrome.
Turner’s syndromeSex chromosomal aneuploidy.XO sex chromosomes.Sterile female underdeveloped breasts; short stature; neck with heavy muscles; loose skin and narrow hips.
AlkaptonuriaAutosomal gene mutation.Lack or inactivity of enzyme homogentisate oxidase. When the patient releases urine, it becomes black in  contact with air, cartilage becomes dark also.
Phenylketonuria (PKU)Autosomal gene mutation.Lack of enzyme phenylalanine hydroxilase.Extreme mental retardation.
AlbinismAutosomal gene mutation.Lack of enzyme tyrosinase in chromosomal disordersPatient lacks melanin pigment in skin. Hair, iris and skin becomes white.
Sickle-cell anemiaAutosomal gene mutation.Formation of hemoglobin in RBCs.Rupturing of red blood corpuscles; blocking of oxygen supply to tissues; jaundice.
Huntington’s diseaseAutosomal gene mutation (dominant)Production of an inhibitor of brain cell metabolism. Degradation of brain tissue after young age.
Cystic fibrosisAutosomal mutationFailure of chloride ion transport.Mucus clogging in lungs; abnormalities in liver and pancreas.
hemophiliaSex chromosomal gene mutation or abnormal arrangement of chromosomeLack of blood coagulantBlood does not clot; more common in males found in chromosomal disorders.
Red-green color blindnessSex chromosomal gene mutation.Lack of red and green color vision pigments in cone cells of retina.Patient can not differentiate  red and green color.
Muscular dystrophySex chromosomal gene mutation.Lack of protein dystrophin.Muscles degeneration; most sufferers are males.
Klinfelter’s syndromeSex chromosomal aneuploidy in chromosomal disorders.XXY sex chromosome. Male becomes impotent, having legs with more height, obese body with body hair. Mentally retarded.


Qs1: The linear arrangement of genes on the chromosomes is favored by which event properly?  How do you define this event? Where is it not happening?

 Ans1:  The phenomenon, that strongly favors the linear arrangement of genes on the chromosome which is an abnormal arrangement of chromosome, is linkage. Linkage is the tendency for alleles of different genes to pass together from one generation to the next. The genes located on different chromosomes cannot show any linkage.

Qs2: How many linkage group are present in an organism whose genomic chromosome number is n=8 in chromosomal disorder?

Ans2: The number of linkage groups in an organism corresponds to its haploid number of chromosomes. Therefore, the number of linkage groups in the given organism is 8.

Qs3: Explain why babies born to young women seldom show this abnormality in chromosomal disorders down’s syndrome?

Ans3: The incidence of non disjunction rises to the ovaries of aged women. That is why, women who are aged, more than year 40 years, they may have tendency to have Down’s syndrome. Women about 40 years age, if becomes pregnant, can have foetal testing for trisomy 21. One in every  600 children may have Down’s syndrome. Individuals having Down’s syndrome are likely to develop Leukemia and Alzheimer’s disease.

Qs4: What is familial Down’s syndrome?

Ans4:  This familial Down’s  syndrome may run in the family. It arises by translocation of a large segment of or whole of the third chromosome 21 to another chromosome which may term as abnormal arrangement of chromosome, usually 14. In such cases, the total number of chromosomes remains normal (46). But there is an extra chromosome 21 material. This is a rare type of Down’s syndrome. Extra chromosome small in size contains excess genetic information. These chromosomes interrupt many biological processes.


Qs5: Define super female.

Ans5: Super females have genotypes XXX, XXXX, XXXXX with total chromosome number 47, 48, or 49.  These females are having underdeveloped sexual characters and they are mentally retarded. If the number of X is increased then their characteristics become more dangerous.

 Qs6: Define super males.

Ans6: Super males have genotype XYY. They have unusual height, mental retardation and perhaps criminal bent of mind.  These people become very much aggressive in nature than normal people. They use to produce male hormone in excess quantity.


Qs7: What are the causes of PKU (phenylketonuria)?

Ans7: The homozygous recessive individual lacks the enzyme phenylalanine hydroxilase . They used to need to change one amino acid, phenylalanine, to another tyrosine. Phenylalanine accumulates in the tissues and some of it changes into phenyl pyruvic acid. The later is excreted in the urine. Accumulation of phenylalanine and metabolites damages the brain and causes the disease. Lack of the enzyme is due to the abnormal recessive gene. The heterozygous individual is normal, but carrier.

The affected baby is normal at birth but within a few weeks, phenylalanine level is plasma starts rising, and by the age of 6 month he develops severe mental retardation. It deficiency of phenylalanine hydroxylase  is detected in the newborn, we can easily prevent this with special  diet, low in phenylalanine.

Qs7: What is muscular dystrophy?

Ans7:  Deterioration of muscles at an early age we term as muscular dystrophy.  The mutated gene on X  chromosome fails to produce a protein called dystrophin. That protein is thought to relay the nerve’s signal to the calcium storage sacs in the muscle cell. As a result, calcium is not released. There is halt for muscular contraction at very first step. Further, the abnormal calcium levels trigger and release of an enzyme that destroys actin and myosin. The victims becomes invalid by the age of 10 and usually dies by the age of 20.

Qs8: State the special characters about red- green color blindness.

Ans8:  Female has two X chromosomes. For a female to be colorblind, it is necessary that each chromosome has a gene for color blindness. If only one X chromosome bears a gene for color blindness, its dominant homologous gene on the other X chromosome will check the expression of the  recessive gene. Hence, color blindness occurs more often in the males than in the female.


A normal woman and color blind man produce normal sons and carrier daughters. A normal man and a carrier woman produce 50% color blind sons and 50% carrier daughters. A criss-cross inheritance is noted in this case too. A color blind man and a carrier woman will produce color blind daughter.

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