What Is The Relationship Between Genes And Chromosomes
Chromosomal Theory of Inheritance
- It was proposed by Sutton and Boveri independently in 1902.
- In 1933 T. H. Morgan confirmed the findings of Sutton and Boveri.
- He was the first to associate a specific gene with a specific chromosome.
- It states that genes are located at specific loci on the chromosomes and it is the chromosomes which segregate and assort independently during meiosis and recombine at the time of fertilization in the zygote.
Salient Features
- Gametes constitute bridge between one generation and the next generation.
- Both the sperm and egg contribute equally in the heredity of the offspring.
- The sperm provides only nuclear part to the zygote.
- There is fusion of the sperm and egg nuclei during fertilization.
- Nucleus contains chromosomes. Therefore, chromosomes must carry the hereditary traits.
- Every chromosome or chromosome pair has a definite role in the development of an individual. Loss of a complete or part of the chromosome produces structural and functional deficiency in the organism.
- Like the hereditary traits the chromosomes retain their number, structure and individuality throughout the life of an organism and from generation to generation.
- Both chromosomes as well as genes occur in pairs in the somatic or diploid cells.
- A gamete contains only one chromosome of a type and only one of the two alleles of a character.
- The paired condition of both chromosomes as well as genes is restored during fertilization.
- Genetic homogeneity and heterogeneity, dominance and recessiveness can be suggested by chromosomal type and behaviour.
- Homologous chromosomes synapse during meiosis and then segregate independently into different cells.
- In many organisms, sex of an individual is determined by specific chromosomes called sex chromosomes.
Parallelism between Genes and Chromosomes
>>Both genes and chromosomes are transferred from generation to generation without any change.
>>In diploid cells, chromosomes occur in homologous pairs.
- Genes also occur in allelic pairs.
- One member of a pair is obtained from maternal parent and the other from the paternal parent.
>>Prior to cell division both chromosome and an allele of a gene get replicated.
- During mitosis the replicated chromosome and the replicated allele split which pass into the two daughter cells.
- The process of replication and distribution maintains similarity in the genetic composition of the cells of a multicellular organism.
>Both segregate during gamete formation (meiosis) so that a gamete receives only one chromosome and one allele of each pair.
>> Fusion of two haploid gametes restores the diploid chromosome number and allelic pairs in the offspring.
>>Both chromosomes and alleles follow law of segregation.
>>Both genes and chromosomes show law of independent assortment.
What Is The Relationship Between Genes And Chromosomes
| Chromosomes | Genes |
| Occur in pairs | Occur in pairs |
| Segregate at the time of gamete formation such that only one of each pair is transmitted to a gamete | Segregate at gamete formation and only one of each pair is transmitted to a pair is transmitted to a gamete |
| Independent pairs segregate independently of each other | One pair segregates independently of another pair |
Human Genetic Disorders
Chromosomal disorders
>>These are caused due to absence or excess or abnormal arrangement of one or more chromosomes.
>>These are heritable and pedigree analysis of a family does not help in tracing the pattern of inheritance of such chromosomal disorders.
Autosomal Abnormalities
Down’s Syndrome (Mongolian Idiocy, Mongolism, 21 trisomy
>> The disorder was first reported in 1866 by Langdon Down.
>> It is an autosomal aneuploidy, caused by the presence of an extra chromosome number 21.
>> Both the chromosomes of the pair 21 pass into a single egg due to nondisjunction during oogenesis.
>>Thus the egg possesses 24 chromosomes instead of 23 and offspring has 47 chromosomes (45 + XY in male, 45 + XX in female) instead of 46.
Symptoms
>>Rounded face, broad forehead
>>Permanently open mouth with protruding tongue
>>Projecting lower lip and short neck
>> Flat hands and stubby (small) fingers
>> Many “loops” on fingertips
>>Broad palm with characteristic palmar crease, which runs all the way across the palm and mongolian type eyelid fold
(epicanthus).
> >Gonads and genitalia are undeveloped.
Down’s Syndrome (Mongolian Idiocy, Mongolism, 21-trisomy
>>It affects one in 750 babies at birth but over half of foetuses suffering from this syndrome abort spontaneously (miscarriage).
>>The nondisjunction is more in females.
>>The frequency of this disorder increases with mother’s age.
Edward’s Syndrome (18-Trisomy)
>> It was described by Edward in 1960.
>> This syndrome is due to an extra chromosome number 18.
>>It occurs more often in females than in males.
>> The frequency of this abnormality is about 1 per 8000 live births.
Symptoms
>> The affected person keeps the fingers tightly clenched against the palm of the hand.
>> Other symptoms are small jaws, deformed ears, small mouth, nose and fingers, small sternum and pelvis.
>> The patient is mentally retarded and dies within 6 months after birth.
Patau’s Syndrome (13-Trisomy)
>>It was described by Patau in 1960.
>> This syndrome is due to an extra chromosome number 13.
>> It occurs in about 1 in 20,000 live births.
>> The average lifespan of the affected person is about 4 months.
Symptoms
>>Small head and abnormalities of the face, eyes and forebrain
>>Cleft lip and palate
>>Low set deformed ears small chin
>>Clenched hands
Cri-du-chat (Cat Cry) Syndrome
>>The affected newborn cries like mewing of a cat.
>>Hence it is named Cri du chat (Cat Cry).
>>This condition is due to a deletion of half part in the short arm of the chromosome number 5.
Symptoms
>>>Small head with widely spaced eyes
>>>Moon like face
>>>Cry like kitten with receding chin
>>>Congenital heart disease.
Myelogenous Leukemia
>> It is caused by deletion of some portion of long arm of chromosome 22 and its addition to chromosome 9 (reciprocal translocation).
>>The deficient 22nd chromosome whose a little segment from long arm is deleted, is called Philadelphia chromosome, as it was first reported in the city of Philadelphia in 1959.
Myelogenous Leukemia
>>It is a cancer of white blood cells characterised by increased and unregulated growth of myeloid cells in the bone marrow and accumulation of these cells in the blood.
>>There is excess production of granular leucocytes, and hence called chronic granulytic leukemia or chronic myeloid leukemia (CML).
Turner’s Syndrome
>>It is due to monosomy (2n – 1).
>>It is formed by the union of an allosome free egg (22+0) and a normal X sperm or a normal egg and an allosome free sperm (22+0).
>>The individual has 2n = 45 chromosomes (44 +XO) instead of 46.
>> One in every 3000 female births is a victim.
>> 44 +YO combination (or male Turner’s syndrome) is unviable so that it does not occur in nature.
Symptoms
>>Sterile females with rudimentary ovaries filled with connective tissue
>>Short stature (less than 5 feet)
>>Undeveloped breasts, small uterus
>>Webbed neck and abnormal intelligence
>>Cardiovascular abnormalities and hearing impairment.
Klinefelter’s Syndrome
>> It is due to trisomy of sex (X) chromosome.
>>It is formed by the union of an abnormal XX egg and a normal Y sperm or normal X egg and abnormal XY sperm.
> >The individual has 47 chromosomes (44+XXY).
Symptoms
>> Sterile males with undeveloped testes
>> Mental retardation
>> Female like sparse body hair
>> Long limbs
>> Feminine pitched voice and enlarged breasts (gynaecomastia).
Superfemales ( Poly X female syndrome)
>> Such individuals have 47 (44 + XXX), 48 (44 + XXXX) or 49 (44 + XXXXX) chromosomes.
>> These females are characterised by abnormal sexual development and mental retardation.
>>The number of barr bodies is one less than total number of X chromosomes.
>>The frequency at birth of 44 + XXX is 1 in 1500.
>>The symptoms are more severe with the increase in number of X chromosomes.
>>Such females are taller with commonly normal fertility.
Supermales (Poly Y male syndrome )
>> It is also known as Jacob’s syndrome.
>> Such individuals have 47 (44 + XYY) chromosomes.
>>Its frequency is 1 in 1000.
>>Such males are taller with IQ 80-120.
>>They are normal in sexual function, fertility and genitalia.
Symptoms
>>Abnormal height
>>Overproduction of male sex hormones
>>More aggressive than normal males
>>Antisocial and criminal bent of mind (criminal syndrome or Jacob’s syndrome).
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