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Sex Limitation
Expression of certain characteristics limited to one sex
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Peacock tails
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This is due to gene interaction between the BRCA1
mutation and oestrogen which is
produced much more in women than men
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Autism
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Examples:
Tay-Sachs disease and hexosaminidase activity
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Codominance
Phenotypic effects of a gene’s allele are fully and
simultaneously expressed in the heterozygote
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Homozygotes for sickle cell haemoglobin suffer from the
disease, whilst heterozygotes have both
normal haemoglobin and sickle cell
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Can be analysed with electrophoresis of haemoglobin
to distinguish genotypes
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Examples:
Human eye colour variation
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Genotypes include: AA, AO, BB, BO, AB, OO
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Actually sub-varieties of alleles, so more than three
possible
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Dozens, perhaps more alleles leading to individuality
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Cystic Fibrosis caused by over 1000 different mutations
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Because of so many different mutations, can only tell
someone if they do carry a known
mutation, not if they don’t because it may
be a yet undiscovered rare mutation
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Examples:
Brachydactyly (dominant pattern of inheritance)
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If two Aa parents have offspring, 25% is likely to die
shortly after birth due to an AA genotype
causing severe skeletal damage
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Examples:
Effects of sickle cell substitution (change of one DNA
base and one amino acid)
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Gene Interaction
Multiple genes affect one characteristic
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Secretor phenotype used in forensic testing
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sese doesn’t produce these substances in fluids
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Deafness Case Study
Many different genes
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Example of pleiotropy and gene interaction
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g
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Deafness can be a pleiotropic effect - e
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as a side
effect of albinism
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In

humans, this means that female heterozygotes for
colour blindness have patches of normal and colour
blind cells in their retinas
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A
heterozygous female for DMD will likely possess some
of the characteristics of it, especially if the proportion of
affected X chromosome cells is higher than not because
as an X-linked disorder, some cells will have the
affected X and others won’t
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It gives a much higher
chance of homozygosity by relatedness
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Another example of inbreeding affecting health
is that it has been established that Tutankhamun had a
congenital foot defect - this is likely because his parents
were brother and sister
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There are about 20 known cases and
they are all clustered in the same area in Finland
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More distant inbreeding is exempli fied through
cystic fibrosis
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Intermediate dominance is when the heterozygote is an
intermediate between two homozygotes
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Hets for Tay
Sachs have an intermediate level of hexosaminidase
enzyme activity, whereas a heterozygote for the
recessive allele will have no activity and vice versa

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