QS Study

Chromosome and Genes

Chromosomes are bundles of tightly coiled DNA (deoxyribonucleic acid) located within the nucleus of almost every cell in our body. Humans have 23 pairs of chromosomes. A gene is a length of DNA that codes for a specific protein. Chromosomes are structures within cells that contain a person’s genes.

From the moment of conception, a human cell is diploid, containing two copies of each of the 23 different chromosomes. Geneticists construct chromosome charts calls karyotypes that display the 23 chromosome pairs in size order. Pairs 1 through 22 are autosomes, chromosomes not involved in determining sex. The other two chromosomes, the X, and the Y determine sex and are called sex chromosomes.

Until recently, geneticists constructed karyotypes by separating white blood cells from a drop of blood, allowing them to splash down onto a microscope slide, and finding and photographing a view of a cell where the chromosomes are spread apart. The photo would be developed and enlarged, and images of individual chromosomes would be cut out and pasted in size order. Today, most laboratories use a computerized karyotype system that automatically selects a well-spread cell and aligns the chromosomes into a chart.

Since we have two copies of each chromosome, we also have two copies of each gene, each located at the same position on the chromosome. Sometimes the members of a gene pair are alike, their DNA sequences specifying the same amino acid sequence of the protein product. However, because a gene consists of hundreds of nucleotide building blocks, it may exist in several variant forms, called alleles. Thus, while homologous pairs of chromosomes are the same, the gene for gene, they may have different alleles of those genes. An individual who has two identical alleles of a gene is said to be homozygous for that gene. A person with two different alleles for a gene is said to be heterozygous for it. A gene may have a huge number of alleles, but an individual person can only have two alleles for a particular gene.

Dominant and Recessive Inheritance –

In heterozygotes, one allele determines the phenotype. Such an allele whose action masks that of another allele is termed dominant. The allele whose expression is masked is recessive.