7Mendelian's law of inheritance

📚
G.J. MENDEL
▢ Basic Facts:

Table 1: Mendel: key facts

Point
Data
Birthplace
Austria; now Czech Republic
Experiment duration
1856–1864 = 8 years
Experimental material
Pisum sativum = garden pea; \(2n = 14\)
Unsuccessful experiment
Hawkweed / Hieracium
Reason for Hieracium failure
Cross-pollinated + parthenogenesis
Work presented at
Brunn Natural History Society
Paper title
Experiments in Plant Hybridization
Rediscovery
1900 → Hugo de Vries, Correns, Tschermak
Republication
Flora, 1901
▢ 7 Contrasting Characters of Pisum sativum:

Table 1: Mendel’s 7 pea traits

Trait / Character
Dominant
Recessive
Plant height / stem length
Tall
Dwarf / short
Pod colour
Green
Yellow
Pod shape
Inflated
Constricted
Cotyledon colour
Yellow
Green
Seed shape
Round
Wrinkled
Flower colour
Red
White
Flower position
Axial
Terminal
▢ Reasons for Choosing Pea Plant:
  • •
    Distinct contrasting characters
  • •
    Self-fertilization
  • •
    Hermaphrodite
  • •
    Short life span
  • •
    Hybridisation easy
▢ Reasons for Mendel’s Success:
  • •
    Single trait at a time
  • •
    Traits → no linkage observed
  • •
    Characters considered were effectively independently assorting
  • •
    Now proved by Blext → 7 traits on 4 chromosomes: chromosome 1, 4, 5, 7
  • •
    Greater gene distance → crossing over high → linkage not expressed
  • •
    Statistical analysis
📚
EXPERIMENTS OF MENDEL
📖
Monohybrid cross
Image 1
▢ Definition: Cross involving single pair of contrasting characters
▢ Example: Tall plant × dwarf plant → plant height
▢ Explains:
  • •
    Law of dominance
  • •
    Law of segregation
▢ F2 Results:

Table 1: Monohybrid cross

Parameter
Result
Phenotypic ratio
3 : 1
No. of phenotypes
2 → tall, dwarf
Genotypic ratio
1 : 2 : 1
No. of genotypes
3 → pure tall, hybrid tall, pure dwarf
▢ General Facts:
  • •
    Homozygous dominant in \(F_2\) = 1
  • •
    Homozygous recessive in \(F_2\) = 1
  • •
    Proportion of homozygous dominant/recessive in \(F_2\) = \(\frac{1}{4^n}\)
▢ Law of Dominance:
  • •
    Contrasting parents crossed → only one parental character expressed in heterozygous \(F_1\)
  • •
    Dominance not studied in haploids → algae, fungi, bryophytes
  • •
    Exceptions → incomplete dominance, co-dominance
▢ Law of Segregation:

Table 1: Law of segregation / Mendel’s first law

Synonym
Meaning
Law of splitting of hybrid
Dominant + recessive factors in \(F_1\) separate
Law of purity of gamete
Each gamete receives only one factor → \(T\) or \(t\)
Mendel’s first law
Alleles segregate during gamete formation
❖ Key Points:
  • •
    Dominant + recessive factors stay together in \(F_1\) but do not mix/blend
  • •
    Segregation occurs during anaphase-I of meiosis
  • •
    Evidence → 2 phenotypes in \(F_2\)
  • •
    Universal law
  • •
    Gametes never become hybrid
  • •
    Genes are particulate in nature
❖ MCQ Points:

Table 1: Segregation in Pisum sativum

Question focus
Answer
Stage
Gamete formation
Meiotic phase
Anaphase-I
Concept
Purity of gametes
📖
Dihybrid cross
Image 1
▢ Definition: Cross involving two pairs of contrasting characters
▢ Basic Idea: 2 alleles + 4 characters
▢ F2 Results:

Table 1: Dihybrid cross

Parameter
Result
Phenotypic ratio
9 : 3 : 3 : 1
No. of phenotypes
4
Genotypic ratio
1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1
No. of genotypes
9
Non-recombinant : recombinant
5 : 3
Both traits dominant offspring
9
Genotypic ratio among both dominant traits
1 : 2 : 2 : 4
Genotype like parents
4
Parental offspring
\(\frac{10}{16}\)
Recombinant offspring
\(\frac{6}{16}\)
Homozygous offspring
\(\frac{4}{16}\)
▢ Law of Independent Assortment:
  • •
    Conclusion of dihybrid cross
  • •
    Two pairs of contrasting characters → assort independently
  • •
    Exception → linkage
  • •
    Mendelian recombinant → independent assortment
  • •
    Linkage recombinant → crossing over
  • •
    Mendelian principles not applicable to prokaryotes without sexual reproduction
📚
SPECIAL POINTS
▢ Gene Location + Result:

Table 1: Independent assortment vs linkage

Gene condition
Name
Result
Non-allelic genes on separate non-homologous chromosomes
Independently assorting genes
Independent assortment
Non-allelic genes on same chromosome
Linked genes
Linkage
Linked genes far apart
Crossing over frequency ≈ 50%
Independent assortment-like result
▢ Test Cross:
  • •
    Phenotypic ratio = genotypic ratio
  • •
    Number of classes = \(2^n\)
  • •
    Monohybrid test cross → 1 : 1
  • •
    Dihybrid test cross → 1 : 1 : 1 : 1
▢ Formulae:

Table 1: Hybridization formulae

Parameter
Formula / Rule
No. of offspring in \(F_2\)
\(4^n\)
Gamete type from homozygous genotype
Always 1
Example
\(AABBCC \rightarrow 1\); \(aabbccdd \rightarrow 1\)
Gamete type from heterozygous genotype
\(2^n\)
Example
\(AaBbCc \rightarrow 2^3 = 8\)
Example
\(AaBBCcddEe \rightarrow 2^3 = 8\)
Homozygous dominant type in hybrid
Always 1
Homozygous recessive type in hybrid
Always 1
Proportion of homozygous dominant/recessive gamete
\(\frac{1}{2^n}\)
Phenotypic ratio in \(F_2\)
\((3:1)^n\)
\(n\)
Type of cross / number of heterozygous gene pairs
📚
IMPORTANCE OF MENDELISM
  • •
    Animal breeds + plant varieties possible
  • •
    Eugenics → development of superior progeny
  • •
    Heterosis / hybrid vigour in plants + animals
📚
CROSSES
📖
Reciprocal cross
▢ Definition: Two crosses: dominant character from female parent + recessive from male parent; second cross vice-versa
▢ Result: Same result
▢ Proves: Sex has no effect on phenotype
▢ Exceptions:
  • •
    Sex-linked inheritance
  • •
    Cytoplasmic inheritance / uniparental inheritance / maternal inheritance
📖
Back cross
▢ Definition: Cross between \(F_1\) hybrid and any parent
▢ Examples:
  • •
    \(Tt \times TT\)
  • •
    \(Tt \times tt\)
📖
Test cross
▢ Definition: Back cross of hybrid with homozygous recessive parent
▢ Example: \(Tt \times tt\)
▢ Uses:
  • •
    Differentiate heterozygous from homozygous dominant
  • •
    Selfing can replace test cross to test purity of dominant individual
▢ Breeding Point:
  • •
    Animal breeders use back cross of hybrid with homozygous dominant → pure line
  • •
    Pure line = homozygous individuals of particular character
📖
Out cross
▢ Definition: Cross between \(F_1\) hybrid and dominant parent
▢ Example: \(Tt \times TT\)
📖
Mcq point

Table 1: Hybrid tall × dwarf pea plant

Cross
Answer
\(Tt \times tt\)
Back cross + test cross
Q1.
If two pea plants with red flowers of unknown genotype are crossed and 75% red, 25% white flowers are produced, parents are
📅CEE MBBS 2025
Q2.
Number of gametes that can be obtained from AaBBCc:
📅CEE MBBS 2024B
Q3.
How many types of gametes are possible from \\(AaBbDdEE\\)?
📅CEE MBBS 2023
Q4.
Which cross gives a 1:1 genotypic ratio?
📅CEE MBBS 2023
Q5.
A cross in which the sources of gametes are reversed is:
📅CEE MBBS 2022
Q6.
Which trait of garden pea studied by Mendel was recessive?
📅CEE MBBS 2022
Q7.
A dwarf pea plant treated with gibberellic acid grows as tall as a homozygous tall plant. If this gibberellic-acid-treated plant is crossed with a tall pea plant, the phenotypic ratio in next generation is likely to be:
📅CEE MBBS 2021
Q8.
How many different types of gametes will be produced by a plant having genotype AaBBCc?
📅CEE BDS 2024
Q9.
A plant with AaBb genotype is self pollinated, the probability of getting AABB genotype will be:
📅CEE BDS 2024
Q10.
A homozygous plant having round (R) & yellow (Y) seed is crossed with the homozygous plant having green (r) & wrinkled (y) seed; ratio of yellow & wrinkled in F₂ generation will be:
📅CEE BDS 2023
Q11.
If there is cross between hybrid tall and dwarf parents, it is:
📅CEE BDS 2021
Q12.
Which one is recessive trait of Pea plant on Mendel's experiment?
📅CEE Nursing 2024
Q13.
Test cross is:
📅CEE Nursing 2024
Q14.
The segregation ratio in F_2 generation for Mendel dihybrid cross is: [MOE 2008, 2055, I.E. 2004, IOM 1998]
📅MOE 2008, 2055, I.E. 2004, IOM 1998
Q15.
A cross between F1 hybrid and the recessive parent is: [MOE-2060, IOM 1998]
📅MOE-2060, IOM 1998
Q16.
Hybrids between 'red flowered' and 'white flowered' strains were crossed back to the pure 'red flowered' strain. The progeny of this cross had: [MOE-2058, 2056, 2054]
📅MOE-2058, 2056, 2054
Q17.
The genetic cross between pure red and pure white genes produce: [MOE-2055]
📅MOE-2055
Q18.
Red and white flowers are crossed, offsprings are all red and this red flower is crossed with white flowers then F2 generation will be: [MOE]
📅MOE
Q19.
In the garden pea experiment, Mendel used: [BPKIHS 1998]
📅BPKIHS 1998
Q20.
If a tall plant Pisum sativum with red flower crossed with dwarf plant with white flower. How many plants are tall with white flower in F2 generation? [IOM 2002]
📅IOM 2002
Q21.
Mendel was successful in formulating the law of inheritance whereas his predecessors were not because: [IOM 1996]
📅IOM 1996
Q22.
When an albino plant is crossed with a normal plant, the resulting offspring is normal because: [BPKIHS 2001]
📅BPKIHS 2001
Q23.
Phenotype refers to: [BPKIHS 2002]
📅BPKIHS 2002
Q24.
Which is not true about phenylketonuria? [BPKIHS 2002]
📅BPKIHS 2002
Q25.
A man with brown eyes and dark hair (whose father was blue eyed) marries a women with blue eyes and dark curly hair (whose mother was red straight haired). They have four children what is the genotype of children: [BPKIHS 2004]
📅BPKIHS 2004
Q26.
Which of the following terms represents the genes of contrasting expression of the same genetic trait? [BPKIHS 2007]
📅BPKIHS 2007
Q27.
The probability of obtaining AaBbCc offsprings in the cross of AABbCc and aaBBcc is: [IE - 2001]
📅IE - 2001
Q28.
In general independent assortment of chromosomes in cell with n pairs of chromosomes yields how many different kinds of gametes? [IE - 2002]
📅IE - 2002
Q29.
In mendels experiment, he selected a pair of alleles for the tall and dwarf plants and a pair alleles for the round and wrinkled seeds. How many kinds of eggs the dwarf plant? [IE - 2002)
📅IE - 2002)
Q30.
Which of the following crosses would produce genotype ratio of 1:2:1 in F2? [IE - 2003]
📅IE - 2003
Q31.
Dihybrid cross is related to: [IE - 2008]
📅IE - 2008
Q32.
Cross done to study a single character between 2 hybrids is called [BPKIHS - 2013]
📅BPKIHS - 2013
Q33.
When two hybrid tall plants are crossed, then % of dwarf plant is: [BPKIHS - 2013]
📅BPKIHS - 2013
Q34.
Mendelian factors is segregated during [BPKIHS - 2011]
📅BPKIHS - 2011
Q35.
Test cross is cross between [MOE 2014]
📅MOE 2014
Q36.
What is the phenotypic proportion of dihybrid cross [MOE 2068]
📅MOE 2068
Q37.
Pair of contrasting character is known as [MOE 201)
📅MOE 201)