Genetic Terminology

Chromosome Terminology:
Before understanding the genetic terminology, it is necessary to know in brief about chromosome, DNA, and RNA present in each cell. Each human cell contains 46 chromosomes, which are arranged in pairs and numbered from 1 through 22. Besides them, there are 2 sex chromosomes, X and Y in male and X and X in females. The DNA of the cells is contained in its chromosomes. The 22 pairs, which are non-sex chromosomes, are known as autosomes.

Karyotype:
Karyotype of an individual is transcribed by placing the total number of chromosomes, then placing a comma without space and then placing the sex chromosomes. Upper cased letters are always used for sex chromosomes.

For example:

• 46, XX: normal female karyotype
• 46, XY normal male karyotype
• 46, XXX: abnormal female karyotype with 1 extra X chromosome.
• 45, XO: abnormal karyotype with missing X chromosome.

Chromosomes are divided in to groups from A through G as given below.

Chromosome Group

1-3 A
4,5 B
6-12, X C
13-15 D
16,18 E
19,20 F
21,22, Y G

Beasides these, every chromosome has a short arm, which is designated by lower-cased p, and a long arm, which is designated by a lower-cased q. They are transcribed with no space between chromosome and its arm.

For example:
14q (l4th chromosome, long arm)
9p (9th chromosome, short arm)
Xp (X chromosome, short arm)

Bands and regions: Each arm (long, short) is further divided into 4 parts or rather in to 4 regions. These regions are further divided into bands, which can be further subdivided. The region and bands are transcribed immediately following arm without any space. If there is still a subdivision, it is placed following a decimal point immediately after band designation.

For example:

• 17p.1 (17th chromosome, short arm, region 1)
• 21q22 (21st chromosome, long arm, region 2, band 2)
• 18p22.21 (18th chromosome, short arm, region 2, band 2, subdivision 21)

Translocation:
For example: t(l4p;20q): translocation from the short arm of the 14th chromosome to long arm of the 20th chromosome.

In some cases, bands translocated are dictated. In such a case, the bands are placed in parenthesis preceding the parenthesis for chromosome number followed by t.

For example: t(2;4)(l3q;15p): translocation from band 2 of long arm of 13th chromosome to band 4 of short arm of 15th chromosome.

Trisomy:
Transcribe the word trisomy lower cased followed by a space and then followed by chromosome number of the group of chromosome.

For example:
trisomy C means extra chromosome in group C
trisomy 21 means extra chromosome with chromosome 21

Genes:

DNA:
Branched DNA: bDNA
Complementary DNA: cDNA
Double-stranded DNA: dsDNA
Mitochondrial DNA: mtDNA
Nuclear DNA: nDNA
Ribosomal DNA: rDNA
Single-stranded DNA: ssDNA

These are transcribed as dictated with no space or comma in the abbreviations.

RNA:
Complementary RNA: cRNA
Heterogeneous nuclear RNA: hnRNA
Messenger RNA: mRNA
Mitochondrial RNA: mtRNA
Nuclear RNA: nRNA
Ribosomal RNA: rRNA
Transfer RNA: tRNA

Oncogenes:
Prefix 'v' or 'c' are used to indicate the location of oncogene (v for viral) and (c for chromosomal or cellular counterpart). Lower cased c or v is to be used followed by hyphen followed by the name of oncogene. For example: c-abl, v-mos, v-myb

Normal genes that have been disrupted by a virus are called as oncogenes. Their replication is the basis for certain types of cancer. They are expressed as 3-letter terms, which are derived from the names of virus they are associated with. For example: Abl, myb, myc RNA is composed of bases cytosine, adenine, guanine, and uracil. Some abbreviations commonly used with RNA are given below: The genetic code of cell is contained in DNA. The DNA is composed of bases, which are thymine, cytosine, adenine, and guanine. Some common abbreviations used with DNA are given below: Genes are functional unit of heredity, which occupy a special place or locus on the chromosome. A gene contains DNA (deoxyribose nucleic acid) and RNA (ribonucleic acid). The condition of an extra chromosome in any of the autosomes or either of sex chromosomes is referred to as trisomy. When a segment, which is normally found in a specific arm of a particular chromosome, is found in a different location, it is known as transcription . Translocation is transcribed by a lower cased "t" followed by parenthesis (enclosed brackets) without a space. The parenthesis contains the number of the chromosome and the arm where it was actually found first, followed by a semicolon, followed by the number and arm of chromosome where it has been translocated. No space is used in between any of the entities. Karyotype indicates the number of chromosomes plus the sex chromosomes present in an individual.