Principles of Inheritance and Variation

Key Concepts

Introduction to Genetics

Genetics: The branch of biology dealing with inheritance and variation.
Inheritance: The process by which characters are passed from parent to progeny.
Variation: The degree by which progeny differ from their parents.

Mendel’s Laws of Inheritance

Gregor Mendel conducted hybridization experiments on garden peas (Pisum sativum) for seven years (1856-1863).

True-breeding Line: A line that shows stable trait inheritance and expression for several generations through continuous self-pollination.
Monohybrid Cross: A cross involving one pair of contrasting traits (e.g., Tall vs. Dwarf).
- F1 Generation: All offspring show the dominant trait.
- F2 Generation: Phenotypic ratio is 3:1 (Dominant:Recessive); Genotypic ratio is 1:2:1 (TT:Tt:tt).

Test Cross A cross between an individual with a dominant phenotype (but unknown genotype) and a homozygous recessive parent. It is used to determine the genotype of the dominant individual.

Mendel’s Principles:
1. Law of Dominance: Characters are controlled by discrete units called factors (genes). In a dissimilar pair, one factor dominates the other.
2. Law of Segregation: Alleles do not show blending; they segregate during gamete formation so that each gamete receives only one allele.
3. Law of Independent Assortment: In a dihybrid cross, the segregation of one pair of characters is independent of the other pair. Phenotypic ratio: 9:3:3:1.

Exceptions to Mendelian Principles

Incomplete Dominance: The F1 phenotype is an intermediate between the two parents (e.g., Pink flowers in Snapdragon - Antirrhinum sp.).
Co-dominance: Both alleles express themselves equally in the F1 generation (e.g., ABO blood grouping in humans).
Multiple Alleles: More than two alleles govern the same character (e.g., Three alleles IA, IB, and i for ABO blood groups).

Chromosomal Theory of Inheritance

Proposed by Walter Sutton and Theodore Boveri (1902).

Argued that the pairing and separation of a pair of chromosomes would lead to the segregation of a pair of factors (genes) they carried.
Experimental verification was provided by T.H. Morgan using Drosophila melanogaster (fruit fly).

Linkage and Recombination

Linkage: Physical association of genes on a chromosome. Tightly linked genes show very low recombination.
Recombination: Generation of non-parental gene combinations due to crossing over.
Genetic Mapping: Alfred Sturtevant used recombination frequencies to map the positions of genes on a chromosome.

Polygenic Inheritance and Pleiotropy

Polygenic Inheritance: Traits controlled by three or more genes (e.g., human skin colour, height). The effect is additive.
Pleiotropy: A single gene exhibits multiple phenotypic expressions (e.g., Phenylketonuria).

Sex Determination

Male Heterogamety: Males produce two types of gametes.
- XO Type: e.g., Grasshopper (Males have one X, Females have XX).
- XY Type: e.g., Humans, Drosophila (Males have XY, Females have XX).
Female Heterogamety: Females produce two types of gametes.
- ZW Type: e.g., Birds (Females have ZW, Males have ZZ).
Haplodiploid System: e.g., Honey bees. Females are diploid (32 chromosomes), males (drones) are haploid (16 chromosomes) and develop via parthenogenesis.

Haplodiploid System (Honey Bees) Males (drones) develop from unfertilized eggs (parthenogenesis) and are haploid (n=16). Females (queens/workers) develop from fertilized eggs and are diploid (2n=32).

Mutation

Alteration of DNA sequences leading to changes in genotype and phenotype.

Point Mutation: Change in a single base pair (e.g., Sickle-cell anaemia).
Frame-shift Mutation: Deletions or insertions of base pairs.

Genetic Disorders

Categorized into Mendelian and Chromosomal disorders.

Mendelian Disorders: Determined by alteration in a single gene.
- Colour Blindness: Sex-linked recessive.
- Haemophilia: Sex-linked recessive (blood clotting protein affected).
- Sickle-cell Anaemia: Autosomal recessive. Substitution of Glutamic acid by Valine at the 6th position of the beta-globin chain.

Sickle-cell Anaemia: Molecular Basis It is caused by a point mutation in the $\beta$ -globin gene, where GAG is converted to GUG, leading to the substitution of Glutamic acid by Valine at the 6th position.

Phenylketonuria: Autosomal recessive inborn error of metabolism.
Thalassemia: Autosomal recessive blood disease (reduced synthesis of globin chains).
Chromosomal Disorders: Caused by absence, excess, or abnormal arrangement of chromosomes.
- Aneuploidy: Gain or loss of a chromosome (e.g., Trisomy, Monosomy).
- Down’s Syndrome: Trisomy of chromosome 21.
- Klinefelter’s Syndrome: Extra X chromosome in males (47, XXY).
- Turner’s Syndrome: Absence of one X chromosome in females (45, XO).

Principles of Inheritance and Variation

Key Concepts

Introduction to Genetics

Genetics: The branch of biology dealing with inheritance and variation.
Inheritance: The process by which characters are passed from parent to progeny.
Variation: The degree by which progeny differ from their parents.

Mendel’s Laws of Inheritance

Gregor Mendel conducted hybridization experiments on garden peas (Pisum sativum) for seven years (1856-1863).

True-breeding Line: A line that shows stable trait inheritance and expression for several generations through continuous self-pollination.
Monohybrid Cross: A cross involving one pair of contrasting traits (e.g., Tall vs. Dwarf).
- F1 Generation: All offspring show the dominant trait.
- F2 Generation: Phenotypic ratio is 3:1 (Dominant:Recessive); Genotypic ratio is 1:2:1 (TT:Tt:tt).

Test Cross A cross between an individual with a dominant phenotype (but unknown genotype) and a homozygous recessive parent. It is used to determine the genotype of the dominant individual.

Mendel’s Principles:
1. Law of Dominance: Characters are controlled by discrete units called factors (genes). In a dissimilar pair, one factor dominates the other.
2. Law of Segregation: Alleles do not show blending; they segregate during gamete formation so that each gamete receives only one allele.
3. Law of Independent Assortment: In a dihybrid cross, the segregation of one pair of characters is independent of the other pair. Phenotypic ratio: 9:3:3:1.

Exceptions to Mendelian Principles

Incomplete Dominance: The F1 phenotype is an intermediate between the two parents (e.g., Pink flowers in Snapdragon - Antirrhinum sp.).
Co-dominance: Both alleles express themselves equally in the F1 generation (e.g., ABO blood grouping in humans).
Multiple Alleles: More than two alleles govern the same character (e.g., Three alleles IA, IB, and i for ABO blood groups).

Chromosomal Theory of Inheritance

Proposed by Walter Sutton and Theodore Boveri (1902).

Argued that the pairing and separation of a pair of chromosomes would lead to the segregation of a pair of factors (genes) they carried.
Experimental verification was provided by T.H. Morgan using Drosophila melanogaster (fruit fly).

Linkage and Recombination

Linkage: Physical association of genes on a chromosome. Tightly linked genes show very low recombination.
Recombination: Generation of non-parental gene combinations due to crossing over.
Genetic Mapping: Alfred Sturtevant used recombination frequencies to map the positions of genes on a chromosome.

Polygenic Inheritance and Pleiotropy

Polygenic Inheritance: Traits controlled by three or more genes (e.g., human skin colour, height). The effect is additive.
Pleiotropy: A single gene exhibits multiple phenotypic expressions (e.g., Phenylketonuria).

Sex Determination

Male Heterogamety: Males produce two types of gametes.
- XO Type: e.g., Grasshopper (Males have one X, Females have XX).
- XY Type: e.g., Humans, Drosophila (Males have XY, Females have XX).
Female Heterogamety: Females produce two types of gametes.
- ZW Type: e.g., Birds (Females have ZW, Males have ZZ).
Haplodiploid System: e.g., Honey bees. Females are diploid (32 chromosomes), males (drones) are haploid (16 chromosomes) and develop via parthenogenesis.

Mutation

Alteration of DNA sequences leading to changes in genotype and phenotype.

Point Mutation: Change in a single base pair (e.g., Sickle-cell anaemia).
Frame-shift Mutation: Deletions or insertions of base pairs.

Genetic Disorders

Categorized into Mendelian and Chromosomal disorders.

Mendelian Disorders: Determined by alteration in a single gene.
- Colour Blindness: Sex-linked recessive.
- Haemophilia: Sex-linked recessive (blood clotting protein affected).
- Sickle-cell Anaemia: Autosomal recessive. Substitution of Glutamic acid by Valine at the 6th position of the beta-globin chain.

Phenylketonuria: Autosomal recessive inborn error of metabolism.
Thalassemia: Autosomal recessive blood disease (reduced synthesis of globin chains).
Chromosomal Disorders: Caused by absence, excess, or abnormal arrangement of chromosomes.
- Aneuploidy: Gain or loss of a chromosome (e.g., Trisomy, Monosomy).
- Down’s Syndrome: Trisomy of chromosome 21.
- Klinefelter’s Syndrome: Extra X chromosome in males (47, XXY).
- Turner’s Syndrome: Absence of one X chromosome in females (45, XO).

Principles of Inheritance and Variation

Principles of Inheritance and Variation

Key Concepts

Introduction to Genetics

Mendel’s Laws of Inheritance

Exceptions to Mendelian Principles

Chromosomal Theory of Inheritance

Linkage and Recombination

Polygenic Inheritance and Pleiotropy

Sex Determination

Mutation

Genetic Disorders

On this page

Principles of Inheritance and Variation

Principles of Inheritance and Variation

Key Concepts

Introduction to Genetics

Mendel’s Laws of Inheritance

Exceptions to Mendelian Principles

Chromosomal Theory of Inheritance

Linkage and Recombination

Polygenic Inheritance and Pleiotropy

Sex Determination

Mutation

Genetic Disorders

On this page