INHERITANCE
Gene: unit of inheritance located at a particular locus of a chromosome a specific DNA sequence
which codes for RNA/polypeptide
Allele: alternative form of a gene at a particular gene locus
Mendel’s First Law of Segregation (monohybrid)
During the formation of gametes, the paired alleles segregate randomly so that each gamete
receives one or the other with equal likelihood
Mendel’s Second Law of Independent Assortment (dihybrid)
The segregation of one pair of alleles is independent of the segregation of other pairs
Monohybrid inheritance
● Monohybrid cross: cross between two pure breeding organisms which differ in one
character
○ Reciprocal cross: not sex dependent
● Test cross: cross individual of dominant phenotype but unknown genotype with a
homozygous recessive individual
○ If homozygous dominant, all offspring will express dominant phenotype
○ If heterozygous, offspring will show phenotypic ratio of 1 dominant: 1 recessive
● Modifications of classical 3:1 ratio for F2 generation:
Incomplete dominance 1:2:1
● Neither of the two alleles is completely dominant to the other
● Heterozygote has intermediate phenotype
● E.g. flower colour in snapdragons (red-pink-white)
Codominance 1:2:1
● Both alleles are equally expressed in phenotype of heterozygote
● E.g. coat colour of shorthorn cattle (red-roan-white)
Multiple alleles No fixed ratio
● E.g. ABO blood group in humans
○ IA and IB are codominant to each other, i is recessive to both
Lethal genes 2:1
● Mutations which lead to a non-functional gene product can
sometimes be tolerated in the heterozygous state but not the
homozygous state
● E.g. Coat colour in mice (mice homozygous for dominant mutant
yellow allele Y die)
Dihybrid inheritance
● Dihybrid cross: cross involving two characters (controlled by two genes located on two
gene loci on two different chromosomes), in which parents possess different forms of
each character
● Test cross: cross individual that expresses two dominant traits but with unknown
genotype with an individual which is double homozygous recessive
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, Sex-linked inheritance
● Sex linkage: carrying of genes on the sex chromosomes (usually X chromosome)
● Mainly affects males (hemizygous for every sex-linked locus; possesses only one X
chromosome, hence whatever allele present on X chromosome in males will be directly
expressed in phenotype)
● E.g. Haemophilia, red-green colour-blindness, duchenne muscular dystrophy (all caused
by X-linked recessive alleles)
● Reciprocal cross: a pair of crosses in which traits of the two parents are reversed; to
discern if a trait is carried on a sex chromosome (non-identical results) or autosome
(identical results)
Pedigree analysis
● Autosomal recessive inheritance (e.g. albinism)
○ If both parents are affected, all children should be affected
○ When affected mates with unaffected, most children should be unaffected
○ Unaffected parents can produce affected individuals
○ Trait often skips generations
○ Both males and females affected with equal probability
● Autosomal dominant inheritance (e.g. Huntington’s disease)
○ Unaffected parents should not have affected children
○ Trait should not skip generations
○ If affected mates with unaffected and 50% of children are affected, affected is
heterozygous
○ Both males and females affected with equal probability
● X-linked recessive inheritance (e.g. red-green color blindness)
○ Most affected are males
○ Affected mothers always produce affected sons
○ Affected females are the result of a mating between affected fathers and
affected/carrier mothers
○ About half of sons of carrier females should be affected
○ If unaffected fathers, daughters will be unaffected but may be carriers
● X-linked dominant inheritance
○ Affected mothers produce affected sons (50% chance for carrier mothers)
○ Affected females are the offspring of affected mothers or fathers
○ All daughters, but none of the sons, of an affected father and unaffected mother
are affected
○ Trait does not skip generations
○ About half of children of affected heterozygous females are affected
Gene linkage
● Linked genes: genes that control different characters and are situated on the same
chromosome at different loci; do not show independent assortment
● Complete linkage: no crossing over occurs; all linked genes will pass into same gamete,
resulting in only parental gametes
● Incomplete linkage: genes are located some distance apart on same chromosome, can
be separated when crossing over occurs during prophase I of meiosis
Copyright © 2019 tonyndr
Gene: unit of inheritance located at a particular locus of a chromosome a specific DNA sequence
which codes for RNA/polypeptide
Allele: alternative form of a gene at a particular gene locus
Mendel’s First Law of Segregation (monohybrid)
During the formation of gametes, the paired alleles segregate randomly so that each gamete
receives one or the other with equal likelihood
Mendel’s Second Law of Independent Assortment (dihybrid)
The segregation of one pair of alleles is independent of the segregation of other pairs
Monohybrid inheritance
● Monohybrid cross: cross between two pure breeding organisms which differ in one
character
○ Reciprocal cross: not sex dependent
● Test cross: cross individual of dominant phenotype but unknown genotype with a
homozygous recessive individual
○ If homozygous dominant, all offspring will express dominant phenotype
○ If heterozygous, offspring will show phenotypic ratio of 1 dominant: 1 recessive
● Modifications of classical 3:1 ratio for F2 generation:
Incomplete dominance 1:2:1
● Neither of the two alleles is completely dominant to the other
● Heterozygote has intermediate phenotype
● E.g. flower colour in snapdragons (red-pink-white)
Codominance 1:2:1
● Both alleles are equally expressed in phenotype of heterozygote
● E.g. coat colour of shorthorn cattle (red-roan-white)
Multiple alleles No fixed ratio
● E.g. ABO blood group in humans
○ IA and IB are codominant to each other, i is recessive to both
Lethal genes 2:1
● Mutations which lead to a non-functional gene product can
sometimes be tolerated in the heterozygous state but not the
homozygous state
● E.g. Coat colour in mice (mice homozygous for dominant mutant
yellow allele Y die)
Dihybrid inheritance
● Dihybrid cross: cross involving two characters (controlled by two genes located on two
gene loci on two different chromosomes), in which parents possess different forms of
each character
● Test cross: cross individual that expresses two dominant traits but with unknown
genotype with an individual which is double homozygous recessive
Copyright © 2019 tonyndr
, Sex-linked inheritance
● Sex linkage: carrying of genes on the sex chromosomes (usually X chromosome)
● Mainly affects males (hemizygous for every sex-linked locus; possesses only one X
chromosome, hence whatever allele present on X chromosome in males will be directly
expressed in phenotype)
● E.g. Haemophilia, red-green colour-blindness, duchenne muscular dystrophy (all caused
by X-linked recessive alleles)
● Reciprocal cross: a pair of crosses in which traits of the two parents are reversed; to
discern if a trait is carried on a sex chromosome (non-identical results) or autosome
(identical results)
Pedigree analysis
● Autosomal recessive inheritance (e.g. albinism)
○ If both parents are affected, all children should be affected
○ When affected mates with unaffected, most children should be unaffected
○ Unaffected parents can produce affected individuals
○ Trait often skips generations
○ Both males and females affected with equal probability
● Autosomal dominant inheritance (e.g. Huntington’s disease)
○ Unaffected parents should not have affected children
○ Trait should not skip generations
○ If affected mates with unaffected and 50% of children are affected, affected is
heterozygous
○ Both males and females affected with equal probability
● X-linked recessive inheritance (e.g. red-green color blindness)
○ Most affected are males
○ Affected mothers always produce affected sons
○ Affected females are the result of a mating between affected fathers and
affected/carrier mothers
○ About half of sons of carrier females should be affected
○ If unaffected fathers, daughters will be unaffected but may be carriers
● X-linked dominant inheritance
○ Affected mothers produce affected sons (50% chance for carrier mothers)
○ Affected females are the offspring of affected mothers or fathers
○ All daughters, but none of the sons, of an affected father and unaffected mother
are affected
○ Trait does not skip generations
○ About half of children of affected heterozygous females are affected
Gene linkage
● Linked genes: genes that control different characters and are situated on the same
chromosome at different loci; do not show independent assortment
● Complete linkage: no crossing over occurs; all linked genes will pass into same gamete,
resulting in only parental gametes
● Incomplete linkage: genes are located some distance apart on same chromosome, can
be separated when crossing over occurs during prophase I of meiosis
Copyright © 2019 tonyndr
