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

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• -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

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• -The gene is a part of DNA, a sequence of nucleotide at a specific

locus ,which codes for just one polypeptide.

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• -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 1.995 metres high. Other examples include weight, hand span, colour of skin, size of feet etc. Genes,

environmental variation or both can cause continuous variation.

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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.

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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

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1- environmental variation (not inhrtited)

Environment can affect the expression of a gene and induce it, so phenotypes

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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

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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

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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.

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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

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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

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

individual is genetically unique.