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The nucleus of a cell contains many X shaped chromosomes. They contain long
strands of DNA. A gene is a section of a molecule of DNA that has the genetic code for
making a particular protein.

DNA (deoxyribonucleic acid) is a very long molecule that contains 2 strands coiled…

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Genes which code for the same characteristic can be found at the same locus. We call
matching pairs alleles.

An allele can be dominant or recessive. We represent dominant genes with a capital
letter (e.g. B) and recessive genes with a lower case letter (e.g. b)

Heterozygous - having dissimilar…

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Monohybrid means that it is limited to 1 gene. However 1 gene = 2 alleles.

Example (1):
Huntingtons disease is an inherited disease which affects the brain and nervous
system. Symptoms do not arise until a person is between 35-45 years old. By this
time that person may have had…

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Sex is determined by sperm. Eggs contain X chromosomes ONLY. Sperm will contain
either an X or a Y.

Females are always XX.
Males are always XY.

Phenotype Male X Female
Genotype XY X XX
Gamete X
X and Y X and X

F1 Cross X X

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Mitosis occurs in all organisms and is a process of asexual reproduction in which the
cell divides in 2, producing a replica with an equal number of chromosomes (2
diploid daughter cells). This process makes all cells other than sex cells (somatic

Cell division by mitosis is used for…

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4 haploid daughter cells are produced and they are all genetically different. Meiosis
only occurs in the reproductive organs and the cells created are sex cells: female egg
cells or male sperm cells.


humans the diploid number of chromosomes is
46 and the haploid is 23.

Diploid cells are…

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The difference in the appearance of individuals in a species, is because those
individuals have different genotypes and they're surviving and living in different

Some genetic variation (discontinuous
variation) is entirely due to variations in
genes. An example of this is blood

Some genetic variation is due to…

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Natural Selection ­ when a plant or animal has an advantage over its competitor it is
more likely to breed and pass on the useful characteristic.

This characteristic may have come about by a rare beneficial mutation, which has
helped the animal/plant to compete and survive. If this is the…


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