Heredity and Variation
Cells behave the way they do because of the ____________ they contain which are made of
proteins. Each protein in a cell is made according to the ___________ code that the cell
___________. The section of DNA that codes for an individual protein is known as a
___________ which are carried on long, thread-like structures called _______________.
Genes occur in ____________.
Cells may be divided into two types: diploid and ____________.
Diploid cells contain __________ complete sets of chromosomes – written as 2n.
___________ cells contain _____________ complete set of chromosomes – written as n.
Human body cells are ____________ while gametes are ____________. Human cells contain 23
different chromosomes. Human body cells contain two copies of these chromosomes totaling
_____ while gametes contain _________ copy totaling ______.
Homologous (partner) chromosomes are a pair of chromosomes with similar characteristics and
the same types of genes and are found in human body cells. Human body cells therefore have
_______ homologous chromosomes which is maintained when gametes fuse during
____________.
Cell Division
This occurs when the nucleus of a cell divides by one of TWO processes: mitosis and meiosis.
During mitosis each chromosome becomes ___________ and duplicates itself into two
____________ which remain connected via the ______________. The nuclear envelope then
disappears and the chromosomes align themselves along the center of the cell. Once there, the
________________ separate and move to opposite poles of the cell. The cell cytoplasm then
divides into two along the equator resulting in two identical cells that are identical to the parent
cell. The nuclear envelope then reforms.
During meiosis each chromosome becomes __________ and duplicates itself into two
______________ which remain connected via the ________________. The nuclear envelope
then disappears and the homologous chromosomes align themselves next to each other along the
____________ of the cell. Genetic information may then be exchanged between them – a
process known as ______________ ____________. The homologous chromosomes then
separate (each still made up of two ________________) and move to opposite ___________ of
the cell. The cytoplasm then ___________ ________ _______ along the equator resulting in
two non-identical cells. The chromosomes in each cell just formed now line up along the equator
again. Once there, the _________________ separate and move to opposite __________ of the
cell. The cell cytoplasm divides for both cells again resulting in ________ overall cells. These
cells contain _________ the number of chromosomes found in the parent cell and they are not
_____________ to it.
Mitosis involves _______________ division while meiosis involves ______________ divisions.
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Importance of Mitosis and Meiosis
Mitosis ensures that every cell produced have the _____________ number of chromosomes and
are genetically ________________. It is therefore used for growth, _____________ and
______________ of damaged cells.
Meiosis ensures that the cells produced have the ______________ number of chromosomes and
are genetically _________________. It is therefore used in the production of ______________.
Variation
Because meiosis produces genetically _________________ cells, variation is possible. Variation
refers to the phenotypic (physical features) differences between individuals of a species. These
differences may be due to genotype (the genetic make-up) or the environmental e.g. diet and
living conditions. Identical twins, for example, have the same _____________ but based on
exposure to their environment (eating habits, injury, even the formation of the finger prints) their
phenotype may differ.
Genetic (genotypic) variation may be continuous or _____________________.
Continuous variation is where there is a complete range of forms for a particular characteristic
within a population. Examples of features that show continuous variation include height, weight,
skin colour, hair colour and size.
Continuous variation usually occurs when a particular feature is controlled by a number of genes
– it is polygenic.
______________________ variation is where there are clearly defined differences for a
particular characteristic within a population. Examples of features that show
______________________ variation include ABO blood group, gender, fixed or free ear lobes
and rhesus blood group.
______________________ variation usually occurs when a particular feature is controlled just
one or two genes.
Causes of genetic variation include random fertilization, crossing over during meiosis and
mutation.
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