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Edexcel Biology GCSE Topic 3 – Genetics
Sexual Reproduction
Both the mother and father produce gametes
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These cells are all haploid (half the number of chromosomes compared to a
normal cell)
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This creates a zygote
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The zygote
then divides by mitosis to develop an embryo
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Meiosis
Gametes divide by meiosis
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Steps of meiosis:
1) The cell duplicates all the genetic information before duplicating
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3) The pairs are pulled apart, creating cells that only have one copy of each
chromosome
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Asexual reproduction
Asexual reproduction only involves 1 parent, so all o spring are genetically identical to
the parent
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The advantages of asexual reproduction are that the reproductive cycle is much faster
than sexual reproduction, allowing for fast population growth
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This allows fast colonisation of an area
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Sexual reproduction
Sexual reproduction requires 2 organisms, allowing for genetic variation
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The disadvantages, however, include the need to find a mate, consuming time and
energy
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It is the chemical that forms all genetic material
in a cell
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DNA is found in the nucleus of eukaryotic cells (cells with a nucleus) in
structures called chromosomes
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DNA is a polymer, a molecule built from a chain of smaller
molecules (monomers)
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The complete set of DNA in a gene is called a genome
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There are four di erent nucleotides, each
with a di erent base
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e
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Each base on a strand is linked to the
complimentary base on the other strand
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They are joined by hydrogen bonds
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Proteins are made of
chains of molecules called amino acids
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Each amino acid chain folds in a specific way, which is why enzymes have a
specific shape for their active site
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They are
joined together to create the protein that the gene coded for
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Mutations are rare and random changes to an organism’s DNA
sequence that can be inherited
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Protein synthesis has two steps, transcription and translation
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RNA is similar to DNA, but is
shorter and single stranded
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RNA

Polymerase (an enzyme) binds to the non-coding region before the start of the targeted
gene, causing the strands to unzip
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Once created,
the mRNA moves out of the nucleus and joins with a ribosome in the cytoplasm
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When the mRNA is bound, the protein can be
created
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The order in which the amino acids are brought to the ribosome matches the
order of the triplets in mRNA, which are known as codons
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This complimentary feature ensures that the amino acids are assembled in
the right order
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Mutations can also happen in the non-coding regions of DNA, changing the phenotype
of the organism
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If there is a mutation, the RNA polymerase will struggle to bind to the gene,
hindering the ability to create a certain protein
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These alleles changes the protein that
the gene codes for, and therefore changes the phenotype
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Alleles can either
be dominant or recessive
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If an allele is recessive, they need two
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If an organism
has two di erent alleles, they are heterozygous for that trait
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g
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g
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If you are female, you
have X and X
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Sex-linked disorders are usually present
on the X chromosome and are usually recessive
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This means that if a male has the
recessive allele, they will show the recessive trait, making them more likely to have the
trait than females
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There are three alleles for blood
type: IA, IB and IO
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IO is the recessive allele

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