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Key Words
· Locus: gene length of DNA that occupy a specific
position on a chromosome
· Allele: different forms of genes
· Gene: a length of DNA that occurs at a locuz that codes
for a specific protein
· Homozygous: two identical alleles (AA or aa)
· Heterozygous: different alleles (Aa)
· Phenotype: observable characteristic or trait expressed
by genotype
· Genotype: makeup of alleles (AA or Aa or aa)
· Co-dominance: is when the heterozygous is an
intermediate between the 2 homozygous e.g. When the
feature will be a mixture of the two phenotypes…read more

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Monohybrid Inheritance
· One characteristic dominates above the other
· Bb Bb B Brown b blonde
Ratio 3:1 Chance 1 in 4
BB Bb Bb bb
Co-dominance
· Co-dominance: is when the heterozygous is an intermediate
between the 2 homozygous e.g. When the feature will be a mixture
of the two phenotypes
· Brown cow Red cow
H B HB H RH R
Homozygous
HB HR
Heterozygous
­ Both of these are expressed so it makes a browny-red cow…read more

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Sickle cell anaemia
Reduced volume
·Cant carry as much
oxygen less
haemoglobin
· Normal HbAHbA
· Sickle HbSHbS
· HbA HbS
­ (doesn't change the shape of the blood cell but still has the gene which
can protect the person from malaria).
­ There is more pressure for this phenotype so people can survive better.
­ Makes the cell more resistant to malaria parasite as they cant reproduce
in the cell
HbS HbS
Normal HbA HbAHbS HbAHbS
Sickle HbS HbSHbS HbSHbS…read more

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Multiple Alleles
A B
A A B B
A B
A antigens on B antigens on No antigens on
surface surface surface
Different combos
Allele Antigens
AA A antigens on surface
BB B antigens on surface Universal Acceptor
AB A + B antigens on surface
AO A antigens on surface
BO B antigens on surface Universal Donor
OO No antigens on surface…read more

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Sex-linked Diseases
· No gene on Y chromosome as it is smaller
· Male has 1 allele on X chromosome
· No allele pair in male
· So male only needs one recessive (from the X
chromosome) to get the disease
F1 F2
X nY X NX N XN XN Y
Y X NY XN X NX N X NY
Colour blind n
Normal N Xn X NX n Xn X NX n XnY
Ratio 1:1:1:1
Chance 50%…read more

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