8Non-mendelian's 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:
Image 1
  • •
    HbᴬHbᴬ → normal RBC
  • •
    HbˢHbˢ → sickle cell anaemia
  • •
    HbᴬHbˢ → carrier
Both normal Hb + sickle Hb produced in carrier
◉ ABO blood group:
Image 1
IᴬIᴮ
AB blood group; Iᴬ + Iᴮ both expressed
◉ Roan coat of cattle:
Image 1
Red hair allele + white hair allele
Mixed red + white hairs → roan coat
📖
Incomplete dominance
▢ Definition:
  • •
    Neither allele completely dominant
  • •
    Heterozygote → intermediate phenotype
  • •
    Both Genotype and Phenotype of F2 generation have 1:2:1 ratio. (CEE 2025)
▢ Plant Examples:
❖ Flower Colour:

Table 1: Incomplete dominance in plants

Plant
Cross
Genotype
Phenotype
F₂ Ratio
◉ Mirabilis jalapa / 4 o'clock plant:
Image 1
Red flower × White flower → Pink flower
  • •
    RR → Red
  • •
    rr → White
  • •
    Rr → Pink
Intermediate pink flower
1 Red : 2 Pink : 1 White
◉ Antirrhinum majus / Snapdragon:
Image 1
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:
Image 1
Black feather × White feather → Blue feather
  • •
    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
📖
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
Q1.
If a haemophilic man marries a normal women, the chance of their sons being haemophilic [BPKIHS 1997]
📅BPKIHS 1997
Q2.
A colour blind father marries a mother whose father was colour blind. The resulting progeny is: [BPKIHS 1998]
📅BPKIHS 1998
Q3.
Which of the following should be avoided while marriage: [IOM 1997]
📅IOM 1997
Q4.
If a colour blind woman marries a normal man, their children will be:
Q5.
A colorblind women marries with a normal man. The progeny will be: [IOM 1996]
📅IOM 1996
Q6.
Identical twins are produced by fertilization of: [BPKIHS 2000, 2001]
📅BPKIHS 2000, 2001
Q7.
The male chromosome of ant, bees & wasp is represented by [IE - 2002]
📅IE - 2002
Q8.
The disease caused by pleiotropic gene is: [IE -2002]
📅IE -2002
Q9.
The recessive gene that always produces its effect [IE 2006]
📅IE 2006
Q10.
The phenomenon in which a gene controls more than one trait is: [IE -2001]
📅IE -2001
Q11.
Sex - linked disease is [BPKIHS - 2016]
📅BPKIHS - 2016
Q12.
Haemophilia is caused due to deficiency of clotting factor? [BPKIHS 2014]
📅BPKIHS 2014
Q13.
ABO blood group is an example of [BPKIHS - 2013]
📅BPKIHS - 2013
Q14.
If the character of a gene is suppressed by the another non - allelic gene, then the suppressed gene, can be termed as [BPKIHS - 2011]
📅BPKIHS - 2011
Q15.
Barr bodies is present in [BPKIHS - 2010]
📅BPKIHS - 2010
Q16.
One of the examples of sex-linked trait is [MOE 2014]
📅MOE 2014
Q17.
Which one is a sex linked disease? [MOE 2012]
📅MOE 2012
Q18.
When a dihybrid ratio is obtained in a ratio of 1:4:6:4:1 instead of 9:3:3:1, it represents [MOE 2010]
📅MOE 2010