Inheritance and variation Introduction: There are two kinds of reproduction, sexual reproduction and asexual reproduction. Sexual reproduction necessitates the presence of two individuals male and female for production of sexual gametes.Sexual reproduction is a major source of genetic variation.

- Asexual reproduction comprises isolation of a body part either a spore cell or tissues and their growth into a new organism that fully resembles the original from which the above part has been isolated.

functions of sexual reproduction. Sexual reproduction allows genetic material to be passed from one generation to the next. Sexual reproduction allows genetic material from two individuals to be used in generating unique combinations of genes in the reproduction of new offspring.

The offspring may possess new characteristics that enhance their successful survival in a changing environment. to be used in generating unique combinations of genes e reproduction of new offspring.

characteristics of gametes in humans, as in all mammals Sperm and eggs are highly specialised haploid cells. Sperm are relatively small. They survive for a matter of only hours or a few days in the vagina.

Sperm are produced constantly and in enormous numbers (400 million in one ejaculation in man). There is a very high mortality rate so they need to be released in such high numbers. The sperm head contains the haploid nucleus. The head is tipped by a special body, the acrosome, which contains hydrolytic enzymes that help the sperm penetrate the egg.

Sperm contain large numbers of mitochondria behind the head. These provide ATP for movement of the tail, a flagellum. Sperm have to swim to the egg through the female reproductive tract. Eggs are relatively large. They have a large food reserve to nourish the fertilised egg, the zygote, in its first days

Eggs are produced in relatively low numbers in mammals. Primates normally produce single eggs because the survival rate of both the egg and the baby is very high. In addition, a single baby’s head (and brain) can achieve an optimum size with little competition in utero and the long period of dependency of the baby on the parent can more readily be satisfied with a single child

Eggs are non-motile as the sperm swim to meet them and the eggs are moved passively into position in the uterus by the ciliated cells of the oviduct.

-Asexual reproduction depends on mitotic division of cells, where cells resulting from this division receive a complete copy (2N) of original chromosomes.

--The nucleus of the plant or animal cell contains chromosomes. Each chromosome is formed of one molecule of the nucleic acid DNA which extends along the length of the chromosome in the form of a thread coiled around some basic proteins. The nucleic acid DNA carries the genetic information of the cell.

Chromosome is a double structure made of two identical chromatides,each chromatid contains one DNA molecule,appears in nucleus just before cell division, it carries genes..

-When the cell is about to divide, each chromosome condenses and become visible. Each chromosome appears as two threads. Each of the threads is called a chromatid and they are joined together at a point called centromere..

-The gene is a part of DNA, a sequence of nucleotide at a specific locus,which codes for just one polypeptide...

-The number of chromosomes remains constant within a certain species, yet the number varies from one species to another. For example in Man number of chromosomes in somatic cell is 46 chromosomes,in pea plant, the number of chromosomes is 14,in maize number of chromosomes is 20.The cells of the maize plant that have twenty(20)chromosomes are called somatic cells

and referred to as 2N because it contains diploid number of chromosomes and these cells are produced from mitotic division. On the other hand, the gametes (pollen grains, ovules or sperm)are produced from the gonads of higher plants or animals contain half the number of chromosomes and referred to as N because it is haploid. It contains one copy of the chromosomes.

These cells are produced from meiotic division and referred to as 2N because it contains diploid number of chromosomes and these cells are produced from mitotic division. On the other hand, the gametes (pollen grains, ovules or sperm)are produced from the gonads of higher plants or animals contain half the number of chromosomes and referred to as N because it is haploid. It contains one copy of the chromosomes.

These cells are produced from meiotic division

Variation is the difference between individual animals and plants and others of the same species even when they are offspring of the same parents. Variation can be due to environmental factors, or inherited factors. The following is a basic overview of variation and inheritance:

Environmental variation can be caused by diet, physical accidents, culture, climate, or lifestyle. Both environmental factors and genetic factors can influence variation as our genes decide what characteristics we inherit and our environment affects how these inherited characteristics develop. For example, an individual might inherit a predisposition to thinness, but a poor diet can result in the individual becoming overweight :

Although identical twins are genetically the same there will be differences in tastes, aptitude or personality according to the differences in their experience or environment. A good example of this would be identical twins adopted into two separate families. The difference in their experience and environment will lead to many variations between the two. Continuous and Discontinuous Variation:

Variation can be either continuous or discontinuous. Continuous variation is gradual or not so clear cut variation. Height is an example of continuous variation - individuals can have a complete range of heights, for example, 1.98, 1.99 or metres high. Other examples include weight, hand span, colour of skin, size of feet etc. Genes, environmental variation or both can cause continuous variation. :

Discontinuous variation is where individuals have to be within a distinct class or categories. These are cut and dried characteristics. You either have the characteristic or you don't. Genes cause discontinuous variation. For example, you are either one blood group or another - you cannot be in between. Another example is natural hair colour. The environment has little to no effect on this type of variation.. :

discontinous variation -Not affected by environment -Shows distinct phenotypes, no intermediates -controlled by one or few genes e.g, blood groups,gender, rolling of tongue

continuous variation - affected by environment -no distinct phenotypes, many intermediates -controlled by one or many genes e.G human height, intelligence

: 1- environmental variation (not inhrtited) Environment can affect the expression of a gene and induce it, so phenotypes

: 2-genteic variation (inherited) due to; a- Meiosis due to i-Random independent assortment of homologous chromosomes ii-Crossing over between chromatids of homologues chromosome

: b- Random mating c-Mutation a sudden,spontaneous change in the sequence of bases in a DNS molecule (a gene mutation )or in the structure or number of chromosomes in a cell (chromosome mutation

: Kinds of mutation 1- Gene mutation: chemical change in genes, can’t be detected by microscope 2- Chromosome mutation: changes in structure or number of chromosomes, can be detected by microscope.

: Mutagens : ‘’ factors that increase the frequency of mutations’’ 1- Chemicals ; such as nitrous acid, chemicals in tobacco smoke tar 2-Radiations;such as x- rays gamma rays, ultraviolet light,atomic radiation 3- Virus infection such as cancer cervix.

: What Causes Genetic Variation? The mixing of genetic material during sexual reproduction causes genetic variation. Inherited differences are due to genes we get from our parents. Genes control the characteristics we develop. The human body and other plant and animal species contain chromosomes, which carry the genes that control different characteristics

: On fertilisation a unique set of chromosomes is created – half from the mother and half from the father. The new individual is genetically unique.