Principles of Inheritance and Variation — Revision Notes

⚡ 30-Second Revision

Inheritance: — Passing of traits.
Variation: — Differences among individuals.
Gene: — Unit of heredity.
Allele: — Alternative form of a gene.
Genotype: — Genetic makeup (e.g., TT, Tt).
Phenotype: — Observable trait (e.g., Tall, Dwarf).
Dominant: — Expressed in heterozygote (e.g., T).
Recessive: — Expressed only in homozygote (e.g., t).
Homozygous: — Identical alleles (TT, tt).
Heterozygous: — Different alleles (Tt).
Mendel's Laws:

- Dominance: One allele masks another. - Segregation: Alleles separate during gamete formation. - Independent Assortment: Genes on different chromosomes assort independently.

Monohybrid Cross F2 Ratios (complete dominance): — Phenotypic 3:1, Genotypic 1:2:1.
Incomplete Dominance/Co-dominance F2 Ratios: — Phenotypic 1:2:1, Genotypic 1:2:1.
Dihybrid Cross F2 Ratio (complete dominance): — Phenotypic 9:3:3:1.
Test Cross: — Unknown dominant phenotype x Homozygous recessive.
Non-Mendelian: — Incomplete dominance, Co-dominance, Multiple Alleles (ABO), Pleiotropy, Polygenic Inheritance.
Linkage: — Genes on same chromosome inherited together.
Recombination: — New gene combinations due to crossing over.
Genetic Disorders: — Mendelian (single gene) vs. Chromosomal (number/structure).

2-Minute Revision

Principles of Inheritance and Variation explain how traits are passed down and why individuals differ. Gregor Mendel's work with pea plants established the foundation, introducing genes and alleles. His three laws are crucial: Law of Dominance (one allele masks another), Law of Segregation (alleles separate during gamete formation), and Law of Independent Assortment (genes for different traits assort independently).

Monohybrid crosses (e.g., Aa x Aa) yield a 3:1 phenotypic ratio and 1:2:1 genotypic ratio in F2 under complete dominance. Dihybrid crosses (e.g., RrYy x RrYy) give a 9:3:3:1 phenotypic ratio. Deviations from Mendelism include incomplete dominance (intermediate phenotype, 1:2:1 F2 ratio), co-dominance (both alleles expressed, 1:2:1 F2 ratio, e.

g., ABO blood groups), multiple alleles, pleiotropy (one gene, multiple effects), and polygenic inheritance (multiple genes, one trait). The Chromosomal Theory of Inheritance posits that chromosomes carry genes.

Linkage describes genes on the same chromosome inherited together, while recombination generates new combinations. Sex determination varies (e.g., XY, ZW, XO). Genetic disorders are either Mendelian (single gene, e.

g., Sickle Cell Anemia) or chromosomal (chromosome number/structure, e.g., Down's Syndrome). Pedigree analysis helps trace inheritance patterns.

5-Minute Revision

Genetics, the study of inheritance and variation, began with Gregor Mendel's pea plant experiments. He defined genes (units of heredity) and their alternative forms, alleles. A genotype is the genetic makeup, while phenotype is the observable trait. Dominant alleles express themselves even in heterozygotes, masking recessive alleles. Homozygous individuals have identical alleles (e.g., TT, tt), while heterozygous individuals have different alleles (e.g., Tt).

Mendel's Laws:

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Law of Dominance: — In a heterozygote, one allele (dominant) masks the other (recessive). E.g., Tall (T) is dominant over dwarf (t), so Tt is tall.
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Law of Segregation: — Alleles for a trait separate during gamete formation, so each gamete receives only one allele. This explains the 3:1 phenotypic ratio in the F2 of a monohybrid cross (e.g., Tt x Tt -> 3 Tall : 1 Dwarf).
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Law of Independent Assortment: — Alleles for different genes assort independently during gamete formation, provided they are on different chromosomes. This leads to the 9:3:3:1 phenotypic ratio in the F2 of a dihybrid cross (e.g., RrYy x RrYy).

Test Cross: Crossing an individual with a dominant phenotype (unknown genotype) with a homozygous recessive individual to determine the unknown genotype. If all offspring are dominant, the unknown is homozygous dominant. If 1:1 dominant:recessive, the unknown is heterozygous.

Deviations from Mendelism:

Incomplete Dominance: — Heterozygote shows an intermediate phenotype (e.g., Red x White snapdragons -> Pink F1; F2 ratio 1 Red:2 Pink:1 White).
Co-dominance: — Both alleles express fully in heterozygote (e.g., ABO blood groups, $I^A I^B$ genotype -> AB blood type).
Multiple Alleles: — More than two alleles for a gene in a population (e.g., ABO blood groups have $I^A, I^B, i$).
Pleiotropy: — One gene affects multiple phenotypic traits (e.g., PKU).
Polygenic Inheritance: — Multiple genes contribute to a single trait (e.g., human skin color, height).

Chromosomal Theory of Inheritance: Genes are located on chromosomes, and chromosome behavior during meiosis explains Mendelian inheritance.

Linkage and Recombination: Genes on the same chromosome are linked and tend to be inherited together. Recombination (crossing over) breaks linkage, creating new allele combinations. Recombination frequency is proportional to gene distance.

Sex Determination: Mechanisms vary (e.g., XY in humans, ZW in birds, XO in grasshoppers, haplo-diploidy in bees).

Genetic Disorders:

Mendelian Disorders: — Single gene mutations (e.g., Sickle Cell Anemia - autosomal recessive, Hemophilia - X-linked recessive).
Chromosomal Disorders: — Abnormalities in chromosome number (aneuploidy like Down's Syndrome - Trisomy 21, Klinefelter's - XXY, Turner's - XO) or structure.

Pedigree Analysis: Uses family trees to trace inheritance patterns of traits/disorders, helping determine if a trait is autosomal/sex-linked and dominant/recessive.

Prelims Revision Notes

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Mendel's Laws:

* Law of Dominance: In a heterozygote, the dominant allele masks the recessive. E.g., Tall (T) over dwarf (t). * Law of Segregation (Purity of Gametes): Alleles separate during gamete formation; each gamete gets one allele. Explains 3:1 phenotypic ratio in F2 of monohybrid cross (e.g., Tt x Tt). * Law of Independent Assortment: Genes on different chromosomes assort independently. Explains 9:3:3:1 phenotypic ratio in F2 of dihybrid cross (e.g., RrYy x RrYy).

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Key Ratios:

* Monohybrid F2 (complete dominance): Phenotypic 3:1, Genotypic 1:2:1. * Incomplete Dominance/Co-dominance F2: Phenotypic 1:2:1, Genotypic 1:2:1. * Dihybrid F2 (complete dominance): Phenotypic 9:3:3:1.

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Test Cross: — Cross with homozygous recessive to determine unknown dominant genotype. If all dominant offspring, unknown is homozygous dominant. If 1:1 dominant:recessive, unknown is heterozygous.
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Non-Mendelian Inheritance:

* Incomplete Dominance: Intermediate phenotype in heterozygote (e.g., Mirabilis jalapa flower color). * Co-dominance: Both alleles fully expressed in heterozygote (e.g., ABO blood groups $I^A I^B$ -> AB blood type).

* Multiple Alleles: More than two alleles for a gene in a population (e.g., ABO blood groups $I^A, I^B, i$). * Pleiotropy: Single gene affects multiple traits (e.g., Phenylketonuria, Sickle Cell Anemia).

* Polygenic Inheritance: Multiple genes contribute to one trait, showing continuous variation (e.g., human skin color, height).

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Chromosomal Theory of Inheritance: — Sutton and Boveri proposed chromosomes as carriers of genetic material. Parallel behavior of genes and chromosomes during meiosis.
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Linkage: — Genes on the same chromosome tend to be inherited together. Stronger linkage for genes closer on the chromosome.
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Recombination: — Exchange of genetic material between homologous chromosomes during meiosis (crossing over), leading to new gene combinations. Recombination frequency is a measure of distance between linked genes.
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Sex Determination:

* XY type: Humans, Drosophila (XX female, XY male). * ZW type: Birds, some reptiles (ZZ male, ZW female). * XO type: Grasshoppers (XX female, XO male). * Haplo-diploidy: Honeybees (diploid female, haploid male).

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Mutation: — Sudden heritable change in genetic material.

* Point Mutation: Change in single base pair (e.g., Sickle Cell Anemia). * Chromosomal Aberrations: Changes in chromosome structure or number.

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Genetic Disorders:

* Mendelian Disorders (single gene): * Autosomal Recessive: Sickle Cell Anemia, PKU, Cystic Fibrosis, Thalassemia. * Autosomal Dominant: Myotonic Dystrophy, Huntington's disease. * X-linked Recessive: Hemophilia, Color Blindness. * Chromosomal Disorders (chromosome number/structure): * Aneuploidy: Down's Syndrome (Trisomy 21), Klinefelter's Syndrome (XXY), Turner's Syndrome (XO).

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Pedigree Analysis: — Use symbols to trace traits/disorders through generations. Helps determine mode of inheritance (autosomal dominant/recessive, X-linked dominant/recessive). Look for key patterns: affected parents having unaffected children (recessive), affected females having all affected sons (X-linked dominant).

Vyyuha Quick Recall

My Laws Dictate Segregation Independently:

Mendel's Laws
Dominance
Segregation
Independent Assortment