9Patterns of Inheritance

📚
ALLELIC-GENE INTERACTION
📖
Co-dominance
▢ Definition: Both alleles express equally in heterozygous condition
▢ Examples:

Table 1: Co-dominance examples

Example
Genotype / Alleles
Phenotype / Expression
Sickle cell anaemia
◉ Genotypes:
  • •
    HbᴬHbᴬ → normal RBC
  • •
    HbˢHbˢ → sickle cell anaemia
  • •
    HbᴬHbˢ → carrier
Both normal Hb + sickle Hb produced in carrier
ABO blood group
IᴬIᴮ
AB blood group; Iᴬ + Iᴮ both expressed
Roan coat of cattle
Red hair allele + white hair allele
Mixed red + white hairs → roan coat
📖
Incomplete dominance
▢ Definition:
  • •
    Neither allele completely dominant
  • •
    Heterozygote → intermediate phenotype
▢ Plant Examples:
❖ Flower Colour:

Table 1: Incomplete dominance in plants

Plant
Cross
Genotype
Phenotype
F₂ Ratio
Mirabilis jalapa / 4 o'clock plant
Red flower × White flower → Pink flower
◉ Genotypes:
  • •
    RR → Red
  • •
    rr → White
  • •
    Rr → Pink
Intermediate pink flower
1 Red : 2 Pink : 1 White
Antirrhinum majus / Snapdragon
Red flower × White flower → Pink flower
Rr → Pink
Intermediate flower colour
1 : 2 : 1
▢ Animal Examples:

Table 1: Incomplete dominance in animals

Animal
Cross
Genotype
Phenotype
F₂ Ratio
Andalusian fowl
Black feather × White feather → Blue feather
◉ Genotypes:
  • •
    BB → Black
  • •
    bb → White
  • •
    Bb → Blue
Intermediate blue feather
1 Black : 2 Blue : 1 White
📖
Lethal genes of human
▢ Definition: Genes causing death or severe survival disadvantage in certain genotypes
▢ Examples:

Table 1: Human lethal gene examples

Condition
Key Point
Congenital ichthyosis
Genetic skin disorder → dry, thick, scaly skin
Epilepsy
Abnormal genes → disturbed brain electrical activity
Huntington's chorea
Progressive brain cell degeneration → abnormal movements + mental changes
Retinoblastoma
Genetic tumour of retina; usually children
Sickle cell anaemia
HbˢHbˢ → severe disease
Tay-Sachs syndrome
Lipid accumulation in nerve cells → progressive nervous system damage
📚
NON-ALLELIC GENE INTERACTION
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Pleiotropy / pleiotropism
▢ Definition: One gene → many characters
▢ Examples:

Table 1: Pleiotropy examples

Example
One gene effect
Sickle cell anaemia
◉ Effects:
  • •
    Sickle-shaped RBC
  • •
    Resistance from malaria
Coat colour of mice
◉ Effects:
  • •
    Coat colour
  • •
    Lethal effect
📖
Multiple alleles
▢ Definition:
  • •
    More than 2 alleles of a gene present in population
  • •
    One individual carries only 2 alleles
▢ Examples:

Table 1: Multiple allele examples

Example
Alleles / Note
ABO blood group
◉ Alleles:
  • •
    Controlled by 3 alleles → Iᴬ, Iᴮ, i
  • •
    Iᴬ + Iᴮ → co-dominant
  • •
    Iᴬ and Iᴮ dominant over i
Eye colour of Drosophila
Multiple allelic series
Coat colour of rabbit
Multiple allelic series
Self-incompatibility / self-sterility in Nicotiana tabacum
Multiple alleles
📖
Polygene / polygenic inheritance
▢ Definition: One character controlled by many genes
▢ Core Rule: More dominant alleles → greater expression of trait
▢ Plant Examples:

Table 1: Polygenic inheritance in plants

Trait
Expression
Kernel colour of wheat
More dominant alleles → darker red; fewer dominant alleles → lighter colour
Number + size of flowers
Quantitative variation
Number + size of fruits
Quantitative variation
Number + size of seeds
Quantitative variation
▢ Animal Examples:

Table 1: Polygenic inheritance in animals

Trait
Expression
Skin colour
More melanin-producing alleles → darker skin
Height
Continuous variation
IQ
Continuous variation