7.1 Inheritance
● Genotype: genetic constitution of an organism, the combination of alleles an
organism has for a specific trait
● Phenotype: expression of the genotype and its interaction with the environment
● Allele: version of a gene found at the same locus on homologous chromosomes.
May be dominant, recessive, or codominant
○ Dominant: an allele whose characteristics appear in the phenotype when
there is one copy
○ Recessive: an allele whose characteristics appear in the phenotype in the
absence of another allele
○ Codominant: both alleles are fully and equally expressed in the phenotype
● A diploid organism has two sets of chromosomes, so each gene and therefore allele
is present twice at a locus.
○ Homozygous: both alleles are the same, e.g. AA or aa
○ Heterozygous: different alleles, e.g. Aa or aA
Genetic diagrams:
● Monohybrid crosses: one gene, with two alleles, dominant and recessive
A a
A AA Aa
a Aa aa
○ If A dominant and a recessive, phenotype ratio 3:1
FR FW
FR FRFR FRFW
FW FRFW FWFW
○ If R and W are codominant, phenotype ratio (1:2:1) 1 red : 2 pink : 1 white
● Dihybrid crosses: inheritance of two different genes at the same time, where the
genes come from different homologous chromosome pairs that undergo
independent segregation during meiosis
SY Sy sY sy
SY SSYY SSYy SsYY SsYy
Sy SSYy SSyy SsYy Ssyy
sY SsYY SsYy ssYY ssYy
sy SsYy Ssyy ssYy ssyy
○ Phenotype ratio (3:3:3:1) 9 smooth yellow : 3 smooth green : 3 wrinkled
yellow : 1 wrinkled green
, ● Sex-linkage: X chromosome linked inheritance, XX female, XY male. Males express
whichever X allele they receive as the Y chromosome is too short so carries fewer
genes
XG Y
XG XGXG XGY
XB XGXB XBY
○ Phenotype ratio (1:1:1:1) 1 ginger female : 1 tortoiseshell female : 1 ginger
male : 1 black male
● Autosomal linkage: both genes are on the same chromosome, so are inherited as a
pair unless crossing over separates them
GN gn
GN GGNN GgNn
gn GgNn ggnn
○ Phenotype ratio (3:1) 3 grey body normal wings : 1 black body and vestigial
wings
○ Missing phenotypes proves autosomal linkage, and in the case those
missing phenotypes are present, crossing over must have occurred
● Multiple alleles: three or more different alleles
IA IO
IB IAIB IBIO
IO IAIO IOIO
○ Phenotype ratio for parents with the above gametes: 1 AB : 1A : 1B : 1O
○ A and B codominant, O recessive
○ A: IAIA or IAIO, B: IBIB or IBIO, AB: IAIB, O: IOIO
● Epistasis: one gene masks the expression of another gene
○ e.g. flower pigment is controlled by alleles Y and R, where Y codes for a
yellow pigment and R codes for an enzyme that turns the yellow pigment
orange. So if there is yy it won’t matter if there is the R allele as the flower
will be colourless.
YR Yr yR yr
YR YYRR YYRr YyRR YyRr
Yr YYRr YYrr YyRr Yyrr
yR YyRR YyRr yyRR yyRr
yr YyRr Yyrr yyRr yyrr
● Genotype: genetic constitution of an organism, the combination of alleles an
organism has for a specific trait
● Phenotype: expression of the genotype and its interaction with the environment
● Allele: version of a gene found at the same locus on homologous chromosomes.
May be dominant, recessive, or codominant
○ Dominant: an allele whose characteristics appear in the phenotype when
there is one copy
○ Recessive: an allele whose characteristics appear in the phenotype in the
absence of another allele
○ Codominant: both alleles are fully and equally expressed in the phenotype
● A diploid organism has two sets of chromosomes, so each gene and therefore allele
is present twice at a locus.
○ Homozygous: both alleles are the same, e.g. AA or aa
○ Heterozygous: different alleles, e.g. Aa or aA
Genetic diagrams:
● Monohybrid crosses: one gene, with two alleles, dominant and recessive
A a
A AA Aa
a Aa aa
○ If A dominant and a recessive, phenotype ratio 3:1
FR FW
FR FRFR FRFW
FW FRFW FWFW
○ If R and W are codominant, phenotype ratio (1:2:1) 1 red : 2 pink : 1 white
● Dihybrid crosses: inheritance of two different genes at the same time, where the
genes come from different homologous chromosome pairs that undergo
independent segregation during meiosis
SY Sy sY sy
SY SSYY SSYy SsYY SsYy
Sy SSYy SSyy SsYy Ssyy
sY SsYY SsYy ssYY ssYy
sy SsYy Ssyy ssYy ssyy
○ Phenotype ratio (3:3:3:1) 9 smooth yellow : 3 smooth green : 3 wrinkled
yellow : 1 wrinkled green
, ● Sex-linkage: X chromosome linked inheritance, XX female, XY male. Males express
whichever X allele they receive as the Y chromosome is too short so carries fewer
genes
XG Y
XG XGXG XGY
XB XGXB XBY
○ Phenotype ratio (1:1:1:1) 1 ginger female : 1 tortoiseshell female : 1 ginger
male : 1 black male
● Autosomal linkage: both genes are on the same chromosome, so are inherited as a
pair unless crossing over separates them
GN gn
GN GGNN GgNn
gn GgNn ggnn
○ Phenotype ratio (3:1) 3 grey body normal wings : 1 black body and vestigial
wings
○ Missing phenotypes proves autosomal linkage, and in the case those
missing phenotypes are present, crossing over must have occurred
● Multiple alleles: three or more different alleles
IA IO
IB IAIB IBIO
IO IAIO IOIO
○ Phenotype ratio for parents with the above gametes: 1 AB : 1A : 1B : 1O
○ A and B codominant, O recessive
○ A: IAIA or IAIO, B: IBIB or IBIO, AB: IAIB, O: IOIO
● Epistasis: one gene masks the expression of another gene
○ e.g. flower pigment is controlled by alleles Y and R, where Y codes for a
yellow pigment and R codes for an enzyme that turns the yellow pigment
orange. So if there is yy it won’t matter if there is the R allele as the flower
will be colourless.
YR Yr yR yr
YR YYRR YYRr YyRR YyRr
Yr YYRr YYrr YyRr Yyrr
yR YyRR YyRr yyRR yyRr
yr YyRr Yyrr yyRr yyrr
