HEREDITYAND VARIATION By :Reynaldo Thomas

What Are Genes?  Each cell in the human body contains approximately 25,000 to 35,000 genes.  Your genes has the information which determines your features or characteristics passed to you (inherited) from parents.  For example, if both of your parents have blue eyes, it is possible you might inherit blue eyes from them.

Where are Genes Found?  Are very small and can not be seen with the naked eyes.  Genes are found along threadlike structures called chromosomes.

What are Chromosomes  Chromosomes are found in the nucleus of cells.  Each chromosomes contains hundred of genes  Chromosomes comes in pairs.  Humans have 46 individual chromosomes or 23 pairs in each normal body cells.  This is called the diploid number.

 Each Gametes has 23 chromosomes and is called the haploid number.  Hence when the sex cells fuse during fertilization each nucleus gives 23 chromosomes.

 Chromosomes come in various striping patterns and lengths.  Chromosomes are lined up by their sizes and similar striping pattern.  First twenty two pairs of Chromosomes are called autosomes.

 The last pair of chromosomes are known as the sex chromosomes, X and Y.  This pair of chromosomes will determine if a child is boy or girl.  Male have one X and one Y chromosomes.  While female have two X chromosomes.

Homologous Chromosomes  These are matching chromosomes.  The chromosomes in the picture have been arranged in order of size.  One chromosome from the father and the other from the mother.  Homologous chromosomes are similar but not identical.  Homologous chromosomes Contain the same genes in the same places.

DNA  Chromosomes and genes are made up of DNA.  Individual chromosome contains a single long DNA molecule.  DNA molecule look like a Twisted ladder.

Mitosis  Life starts as a single cell.  The cell then divides over and over until all body cells are produce.  As a result of mitosis each new cell receives 46 chromosomes.

 Same as the original cell.  The new cells are genetically identical (clone).  The two news cells are called daughter cells.  Mitosis is a types of cell division.  Use to make new cells for growth and repairs.

Stages of Mitosis One  The cell first makes an exact copy of each chromosome.  The two copies are called chromatids.  The chromatids are held firmly together by centromere.

Stage Two  Chromosomes become shorter and fatter.

Stage 3  The chromosomes line up in the middle of the cell.

Stage 4  The two chromatids will break apart and pulled by spindles to opposite ends of the cell.

Stage 5  Chromatids are now separated.

Stage 6  Finally the two cells will form.

Making of Gametes (Sex Cells)  For sexual reproduction two gametes are fuse together to make a zygote.  This new zygote must have two full sets of chromosomes (46 in total).  Hence each gamete must have only one set of chromosomes.  When fused together, the zygote Will have two sets.

Meiosis  Sex cells are made in the testes and ovaries.  In meiosis each new cell contain only one cell contain only one set of chromosomes. set of chromosomes.  Meiosis cut the number of chromosomes by half (46 to 23).

Stages of Meiosis Stage 1  Cell will first make a copy of each chromosome, as in mitosis.

Stage 2  Homologous chromosomes in the cell will pair up with each other.

Stage 3  The chromosomes line up at the center of the cell in pairs.

Stage 4  The chromosomes are pulled apart, to opposite sides of the cell.

Stage 5  The cell now divide into two new cells with each having 23 chromosomes ( Haploid number).

Stage 6  Each of the two new cells will now divide again, just as they would in mitosis.  The final result will be four new cells, each having 23 chromosomes.

Variation  You and your classmates belong to the same species, as you all humans.  However you are not the same.  This is an example of variation between you.

Types of Variation Discontinuous Variation  These are clear cut variations.  Each individual falls into a particular category.  Example, the person blood group can be A,B,AB, or O.  Each person has one of these four blood group.  For discontinuous variation there are no in- between categories.

 The person either has the characteristic or not.  Characteristics normally qualitative and cannot be measured.  Discontinuous variation are cause by genes only and cannot be change by your environment.

Continuous  This variation is not clear- cut.  Each individual lies somewhere on a continuum.  Height is an example of continuous variation.  You can be any where between the lowest and the highest extremes.  Normally quantitative and can be measured.  Cause by both genes and environment.