CHROMOSOMAL MUTATION ALBINA SUBBA DEPARTMENT OF ZOOLOGY ST

CHROMOSOMAL MUTATION ALBINA SUBBA DEPARTMENT OF ZOOLOGY ST. JOSEPH’S COLLEGE

DEFINITION: The changes in the genome involving chromosome parts, whole chromosomes, or whole chromosome sets are called chromosome aberrations or chromosome mutations �

TYPES: � A. STRUCTURAL CHANGES IN CHROMOSOME 1. DELETION OR DEFICIENCY 2. DUPLICATION 3. INVERSION 4. TRANSLOCATION

� 1. 2. B. CHANGES IN NUMBER OF CHROMOSOMES ANEUPLOIDY

DELETION or DEFICIENCY Deletion is a type of chromosomal aberration where a segment of the chromosome is lost. Deletion is of two types: i. Terminal deletion – In this type of deletion, the terminal segment is lost. ii. Intercalary deletion – Here, an intermediate segment of chromosome or portion of chromosome is lost. �

# In human babies, deletion of a segment of chromosome number 5 causes a disease called, “cri-du-chat ” syndrome where the baby has a cat-like cry, is mentally retarded with small head.

CRI-DU-CHAT

“CRI-DU-CHAT’’ Syndrome q q q Also known as chromosome 5 p deletion syndrome, 5 p- syndrome or Lejeune’s syndrome. It was first described by Jerome Lejeune in 1963. It is a rare genetic disorder due to the partial deletion of chromosome 5. It is a French term (cat cry or call of the cat) referring to the characteristic cat-like cry of the affected children.

CAUSES v v v It is caused due to the partial deletion of the short arm of chromosome number 5, also called “ 5 p monosomy” or “partial monosomy. ” Multiple genes are missing as a result of this deletion and loss of each genes may contribute to the symptoms of this disorder. One of the deleted gene known to be involved is h. TERT ( telomerase reverse transcriptase).

� � � SYMPTOMS The syndrome gets its name from the characteristic cry of affected infants, which is similar to that of a meowing kitten, due to problems with the larynx and nervous system. Low birth weight and poor growth. Unusual facial features which may change over time. Excessive drooling. Microcephaly (small head) Micrognathia (small jaws)

Wide eyes, skin tags in front of eyes. � Single palmar creases. � Cardiac defect. � Mental retardation � Partially webbed fingers or toes. � Slow or incomplete development of basic skills. Infertility is not associated with Cri-du-chat. � Single palmar crease

STATISTICS � � � Approximately, 90% of this syndrome results from a randomly occuring deletion. The remaining 10% occurs purely by inheritance. It affects an estimated 1 in 20, 000 to 50, 000 newborns. It occurs in all races and in both genders, although there is a slight female predominance by a 4: 3 ratio. 30%of the infants have heart defects.

DIAGNOSIS AND MANAGEMENT � � The diagnosis is based on the distinctive cry and accompanying physical problems. Genetic counseling and genetic testing offered to families with individuals who have this syndrome. Children maybe treated by speech, physical and occupational therapists. Heart abnormalities often require surgical correction.

DUPLICATION Duplication is a chromosomal aberration where a segment is repeated or added. Hence, a set of genes is present in double doses. In Drosophila, the bar eye is due to duplication. Duplication produces position effect. A phenotypic effect produced by change in position of a gene or a group of genes is called position effect.

TYPES OF DUPLICATION I. II. TANDEM DUPLICATION: The additional chromosome segment is located just after the normal segment, the gene sequence being the same. REVERSE TANDEM DUPLICATION: The additional chromosome segment is located just after the normal segment, the gene sequence of the additional segment is inverted.

iii. DISPLACED DUPLICATION: The additional segment is located in the same chromosome but away from the normal segment. iv. TRANSLOCATION DUPLICATION: Here the additional segment is located in a non homologous chromosome.

INVERSION Inversion is a chromosomal aberration where a segment of chromosome breaks and reunites in the reverse order. In inversion, there is no loss or gain in the genetic material but simply a rearrangement of the gene sequence. v It brings about position effect.

TYPES OF INVERSION 1. PARACENTRIC INVERSION – In a paracentric inversion, the centromere is located outside the inverted segment. (Centromere is not involved)

PERICENTRIC INVERSION – In a pericentric inversion, the centromere is located within the inverted segment. (Centromere involved) Paracentric inversions do not alter the morphology of affected chromosomes, while pericentric inversions may alter the centromere position. 1.

TRANSLOCATION The shifting or transfer of a chromosome segment to a non-homologous one is called translocation. There is no loss or addition of genes but only a rearrangement i. e. , changes in the sequence and position of gene. It maybe of three types; Simple translocation, Shift or Intercalary translocation and Reciprocal translocation.

TYPES OF TRANSLOCATION 1. 2. 3. SIMPLE TRANSLOCATION – They involve a single break in a chromosome. The broken piece gets attached to one end of a non-homologous chromosome. SHIFT TRANSLOCATION – The broken segment of one chromosome gets inserted interstitially in a non-homologous chromosome. RECIPROCAL TRANSLOCATION- A segment from one chromosome is exchanged with a segment from another non homologous one, so that two translocation chromosomes are simultaneously achieved.

END.