Mendelian's law of inheritance

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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:

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Distinct contrasting characters
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Self-fertilization
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Hermaphrodite
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Short life span
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Hybridisation easy

▢ Reasons for Mendel’s Success:

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Single trait at a time
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Traits → no linkage observed
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Characters considered were effectively independently assorting
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Now proved by Blext → 7 traits on 4 chromosomes: chromosome 1, 4, 5, 7
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Greater gene distance → crossing over high → linkage not expressed
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Statistical analysis

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EXPERIMENTS OF MENDEL

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Monohybrid cross

▢ Definition: Cross involving single pair of contrasting characters

▢ Example: Tall plant × dwarf plant → plant height

▢ Explains:

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Law of dominance
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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:

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Homozygous dominant in \(F_2\) = 1
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Homozygous recessive in \(F_2\) = 1
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▢ Law of Dominance:

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Dominance not studied in haploids → algae, fungi, bryophytes
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Exceptions → incomplete dominance, co-dominance

▢ Law of Segregation:

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

Synonym	Meaning
Law of splitting of hybrid
Law of purity of gamete
Mendel’s first law	Alleles segregate during gamete formation

❖ Key Points:

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Segregation occurs during anaphase-I of meiosis
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Evidence → 2 phenotypes in \(F_2\)
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Universal law
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Gametes never become hybrid
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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

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Dihybrid cross

▢ 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:

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Conclusion of dihybrid cross
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Two pairs of contrasting characters → assort independently
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Exception → linkage
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Mendelian recombinant → independent assortment
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Linkage recombinant → crossing over
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Mendelian principles not applicable to prokaryotes without sexual reproduction

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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:

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Phenotypic ratio = genotypic ratio
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Number of classes = \(2^n\)
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Monohybrid test cross → 1 : 1
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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
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

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IMPORTANCE OF MENDELISM

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Animal breeds + plant varieties possible
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Eugenics → development of superior progeny
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Heterosis / hybrid vigour in plants + animals

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CROSSES

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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:

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Sex-linked inheritance
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Cytoplasmic inheritance / uniparental inheritance / maternal inheritance

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Back cross

▢ Definition: Cross between \(F_1\) hybrid and any parent

▢ Examples:

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\(Tt \times TT\)
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\(Tt \times tt\)

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Test cross

▢ Definition: Back cross of hybrid with homozygous recessive parent

▢ Example: \(Tt \times tt\)

▢ Uses:

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Differentiate heterozygous from homozygous dominant
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Selfing can replace test cross to test purity of dominant individual

▢ Breeding Point:

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Animal breeders use back cross of hybrid with homozygous dominant → pure line
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Pure line = homozygous individuals of particular character

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Out cross

▢ Definition: Cross between \(F_1\) hybrid and dominant parent

▢ Example: \(Tt \times TT\)

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Mcq point

Table 1: Hybrid tall × dwarf pea plant

Cross	Answer
\(Tt \times tt\)	Back cross + test cross

7Mendelian's law of inheritance