ABC of genetics

Genetics

Chromosomes: a threadlike strand of DNA that appear in a cell nucleus during cell division, (consisting of genes that are responsible for the transmission of hereditary characteristics)

Sex chromosome - (genetics) a chromosome that determines the sex of an individual; "mammals normally have two sex chromosomes"

Autosome: A chromosome not involved in sex determination.

Euchromatin is a lightly packed form of chromatin (DNA, RNA and protein) that is rich in gene concentration, and is often (but not always) under active transcription.

Heterochromatin is a tightly packed form of DNA

Barr body The condensed, inactive, single X-chromosome , appearing as a densely staining mass, that is found in the nuclei of somatic cells of female mammals. It is named after its discoverer, Murray Barr.

 The number of Barr bodies is thus one less than the number of X-chromosomes.

The human abnormalities called Kleinefelter's syndrome and Turner's syndrome both result from an unnatural presence or absence of a Barr body.

Kleinefelter's syndrome(47XXY genetically male but contain Barr body  and

 Turner's syndrome (45X0 genetically female but contain no Barr body .

Chromatin granule- the readily stainable substance of a cell nucleus consisting of DNA and RNA and various proteins; during mitotic division it condenses into chromosomes

Chromatid - one of two identical strands into which a chromosome splits during mitosis

Gene: a part of DNA molecule of a chromosome which directs the synthesis of a specific polypeptide chain.

Codon: a sequence of three adjacent nucleotides which codes for one amino acid

Haploid: the condition in which the cell contains one set of chromosomes (23). This is the chromosome number in a normal gamete

Homologous chromosomes: chromosomes which pair during meiosis & contain identical loci.

Locus: the site of a gene on a chromosome.

Alleles : Different forms of the same gene are called alleles,

Every trait (character :ex:  hair colour)   has two alleles one cone from mother and another come from father . Alleles are commonly represented by letters: A(black hair ) ,  a (brown hair ) 

Homozygous : If an organism has two copies of the same allele, for example AA (mother gene contain information about black hair and father gene contain information about black hair) or aa,( mother gene contain information about brown hair and father gene contain information about brown hair)  it is homozygous for that trait.

 Heterozygous:  If the organism has one copy of two different alleles, for example Aa(  mother gene contain information about black hair and father gene contain information about brown hair), it is heterozygous.

Dominant and Recessive: Alleles can be dominant or recessive.

A dominant allele takes precedence over a recessive allele.

Genotype:  the genetic constitution of an individual

Phenotype: the appearance of an individual which results from the interaction of the environment & the genotype.

Mendel’s law:  

Gregor Mendel describes the inheritance of traits passed from one generation to the next.

The mendel‘s law has three parts: 

1.      The Law of Dominance

Mendel's law of dominance states that one gene (dominant gene) is dominant or powerful can mask the expression of other gene (recessive gene).  
Single dominant gene can express the character
Two dose of recessive genes can express the character  

2.      The Law of Segregation

Mendel's second law, the law of segregation, states that two alleles of a gene segregated or separated during formation of gametes.  
So one copy of parent gene  only pass  to their offspring for a particular trait .
 Offspring gets second copy of gene from another parent. 
These  segregation or separation occur during meiosis.

3.      The Law of Independent Assortment

 During gamete formation alleles of genes located on different chromosomes are assorted or distributed independently. The transmission of one character does not influence the transmission of other character. 

Kyryotype: the number, size and shape of the chromosomes of an individual

Mutation: a change in genetic material, either of a single gene or in the number or structure of the chromosomes

Non-disjunction: the failure of two members of a homologous pair chromosome to separate during cell division so that both pass to the same daughter cell

DNA: deriboxynucleic acid that carries the genetic information in cells and some viruses, consisting of two long chains of nucleotides twisted into a double helix.  

Nucleotide: each of which consists of

1.      a pentose sugar

2.      a phosphate group.

3.      Four  nitrogenous bases: cytosine [C], guanine [G], adenine [A] or thymine [T],  

DNA replication: the process of copying the nucleotide sequence of the genome from one generation to the next.

Transcription: the process whereby genetic information is transmitted from the DNA in the chromosomes to messenger RNA.

Translation means the synthesis of protein from RNA

RNA: the nucleic acid which is found mainly in the nucleolus and ribosome

Composition of RNA : it is formed by single stand nucleotide . Each nucleotide contain pentose suger , phosphate, four nitrogenous bases :  (A), cytosine (C), guanine (G), or uracil  (U). Adenine

There are three types of RNA: 1. mRNA  2. tRNA  3.rRNA

Messenger RNA (mRNA): it transfers genetic information from the nucleus to the ribosomes in the cytoplasm and also acts as a template for the synthesis of polypeptides.

Transfer RNA: transfers activated amino acids from the cytoplasm to messenger RNA.

Ribosomal RNA :  (rRNA) is responsible for the translation of mRNA to protein.

itis the RNA component of the ribosome which is essential for protein synthesis in all living organisms. rRNA is the predominant RNA in most cells, composing around 80% of cellular RNA. Ribosomes are approximately 60% rRNA and 40% protein by weight.

Genome: the entire gene carried by a cell.