Download Mendelian Genetics

Mendelian Genetics
Year 13 Biology
Revision – The Basics
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Monohybrid Cross
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Genetic cross that considers only one
characteristic.
Revision – The basics
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Back-cross or test cross
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By looking at the phenotype, you can’t tell if an organism’s
genotype is homozygous or heterozygous – do a test
cross by mating your unknown genotype with an organism
showing the recessive trait.
If any of the offspring show the recessive trait, the original
unknown parent was heterozygous.
If no recessive trait shown up in the offspring then, it was
homozygous.
Revision – The basics
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Dominance
1.
2.
Incomplete dominance – when one allele does
not completely dominate over the other e.g.
snapdragon flower. When a red flower plant is
crossed with a white flower plant, the
heterozygous plant is pink.
Codominance – same as above except the
heterozygote shows both traits.
Revision – The basics

Dominance
3.
Overdominance – e.g. if one homozygote is tall
and the other is short then the heterozygote will
be even taller than the tall homozygote. Can
lead to hybrid vigour.
Revision – The basics
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Lethal Genes
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Homozygous dominant is lethal (dies as an
embryo)
Instead of 1:2:1 ratio you get a 2:1 ratio
E.g. In mice yellow fur is dominant to grey. If two
yellow mice mate you get ¼ YY (died), ½ Yy
(yellow), ¼ yy (grey) – the homozygous dominant
is lethal.
Revision – the basics
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Multiple Alleles
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Three possible alleles for each gene.
E.g. Blood groups – A, B and O
A and B are codominant and they are both dominant
to O, which is recessive.
Blood Group
Genotype
Antigen on Cell
Serum agglutinin
A
IAIA or IAi
A
Anti-B
B
IBIB or IBi
B
Anti-A
AB
IAIB
A and B
None
O
ii
none
Anti-A and anti-B
Revision – the basics
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Dihybrid Cross
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Cross between organisms with two different
characteristics
Remember:
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Phenotype ratios for a dihybrid hetetozygous
cross 9:3:3:1
9/16 – the two dominant alleles
3/16 – one dominant + one recessive
3/16 – other dominant + other recessive
1/16 – the two recessives
Gene-Gene Interactions
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Situations where genes on two different
loci or on different chromosomes have an
affect on each other.
1. Collaboration
2. Complementary genes
3. Supplementary genes (epistasis)
Collaboration
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When one characteristic is
controlled by two or more
pairs of alleles.
Combinations of dominant
and recessive alleles
result in up to four different
phenotypes
E.g. comb shape in
chickens
9:3:3:1 phenotype ratio
Rose
Walnut
Pea
Single
Collaboration
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See page 132 biozone for the cross
9/16 P_R_ two dominants = Walnut
3/16 P_rr one dominant + one recessive = Pea
3/16 ppR_ other dominant +other recessive = Rose
1/16 pprr two recessives = Single
Complementary Genes
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Where one dominant allele can only be
expressed in the presence of another dominant
allele (of another gene pair)
E.g. white and purple flower colours – must
have both P and C alleles for purple.
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9:7 phenotype ratio
Only two “visible” phenotypes
See page 135 biozone for full cross
To make purple pigment, both P and C are needed i.e.
they are complementary
Supplementary Genes (epistasis)
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Where one particular allele (out of 4 alleles) must be present
for the full expression of another allele.
E.g. mouse fur colour
_ _ cc – albino
_ _ _ C – Brown
B_ C_ - black
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Allele C supplements allele B
9:3:4 phenotype ratio – only 3 “visible” phenotypes
See page 136 biozone for full cross.
Ratios you should know
Parents’ genotype
Type of inheritance
3:1
Aa X Aa
A dominant to a
1:2:1
Aa X Aa
Incomplete or
codominance
1:1
Aa X aa
Back or test-cross
2:1
Aa X Aa
Lethal genes (AA dies)
9:3:3:1
AaBb X AaBb
Heterozygous cross
1:1:1:1
AaBb X aabb
Back or test cross
9:3:4
AaCc X AaCc
Supplementary (epistasis)
9:7
AaCc X AaCc
Complementary
Offspring Ratio
Monohybrid crosses
Dihybrid Crosses