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Inheritance and Genetics
DNA and cell division
DNA GENES AND CHORMOSOMES
DNA; DNA molecules are large and complex
...

Each person’s DNA is unique
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DNA can be cut up and
separated, forming a sort of 'bar code' that is different from one person to the next
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It is the unit of heredity, and may be copied and
passed on to the next generation
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These are long threads of DNA, each made up of many
genes
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Each strand of DNA is made of chemicals called bases
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There are four different bases in DNA:



thymine, T
adenine, A




guanine, G
cytosine, C

There are chemical cross-links between the two strands in DNA, formed by pairs of bases
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The chromosomes in a pair carry the same genes in the
same places
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Different versions of the same gene are called alleles
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For any gene, a person may have the same two alleles or two different
ones
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For example, the allele
for brown eyes is dominant
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Two copies will still give you
brown eyes
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For example, the
allele for blue eyes is recessive
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MITOSIS
In mammals, body cells are diploid
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The diagram shows how it works
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MEIOSIS
Meiosis is the type of cell division that produces gametes
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Human gametes are haploid –
so their nucleus only contains a single set of 23 unpaired chromosomes
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Unlike mitosis,
meiosis is a reduction division – the chromosome number is halved from
diploid (46 chromosomes in 23 pairs in humans) to haploid (23 chromosomes
in humans)
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Gametes are
produced by meiosis
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It contains just one of the
chromosomes from each pair, and half the genetic information of a body cell (which is diploid)
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One of these pairs controls the inheritance of
biological gender - whether offspring are male or female:



Males, X and Y
Females XX

CODOMINANCE
Some alleles are both expressed in the same phenotype, a situation called codominance
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For example, feather colour
in hens may be white, black or speckled (it has both white feathers and black feathers)
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BLOOD GROUPS
The gene controlling human ABO blood groups has three alleles, not just two:



IA and IB are not dominant over one another
both are dominant over IO

The table shows the possible genotypes (alleles present) and phenotypes (blood group)

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