Unit 2: Genetics and Evolution - Chapter 1: Principles of Inheritance and Variation

2.1 Principles of Inheritance and Variation

Heredity and Variation

Heredity: The process by which characters (traits) are passed on from parent to progeny. It is the basis of inheritance.
Variation: The degree by which progeny differ from their parents. Variations arise due to crossing over, mutation, recombination, and environmental influences.

Gregor Johann Mendel (1822-1884), known as the "Father of Genetics," conducted hybridization experiments on garden peas (Pisum sativum) for seven years (1856-1863) and proposed the laws of inheritance.

Monohybrid Cross: A cross involving a single pair of contrasting characters.
- Law of Dominance: States that in a heterozygous individual, only one allele (the dominant allele) expresses itself, while the other (recessive allele) remains unexpressed. The characters are controlled by discrete units called factors (now known as genes).
- Law of Segregation (Law of Purity of Gametes): States that the two alleles of a gene separate (segregate) from each other during gamete formation, such that each gamete receives only one allele. These alleles do not blend.
Dihybrid Cross: A cross involving two pairs of contrasting characters.
- Law of Independent Assortment: States that when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters. This law applies to genes located on different chromosomes or far apart on the same chromosome.
Back Cross: A cross between an F1 hybrid and either of its parents.
Test Cross: A cross between an F1 hybrid and its recessive parent. It is used to determine the genotype of an individual showing the dominant phenotype (whether it is homozygous dominant or heterozygous).
Punnett Square: A graphical representation to calculate the probability of all possible genotypes and phenotypes of offspring in a genetic cross. Developed by Reginald C. Punnett.

Terms: Gene, Allele, Homo/Heterozygous, Dominant/Recessive, Genotype/Phenotype

Gene: A unit of inheritance; a segment of DNA that codes for a particular trait.
Allele: Alternative forms of a gene (e.g., T for tall, t for dwarf).
Homozygous: An individual having identical alleles for a particular trait (e.g., TT or tt).
Heterozygous: An individual having two different alleles for a particular trait (e.g., Tt).
Dominant Allele: An allele that expresses its phenotype even in the presence of a recessive allele.
Recessive Allele: An allele that expresses its phenotype only when present in a homozygous state.
Genotype: The genetic constitution of an individual (e.g., TT, Tt, tt).
Phenotype: The observable characteristics of an individual, determined by its genotype and environment (e.g., Tall, Dwarf).

Deviations from Mendelian Inheritance

Incomplete Dominance:
- Neither allele is completely dominant over the other, resulting in a blended phenotype in the heterozygote.
- Example: Flower color in Snapdragon (Antirrhinum majus) or Four O'clock plant (Mirabilis jalapa).
  - Red (RR) x White (rr) → Pink (Rr) in F1 generation.
  - F2 phenotypic and genotypic ratio is 1:2:1 (Red:Pink:White).
Codominance:
- Both alleles express themselves fully in the heterozygote.
- Example: ABO blood group system in humans.
  - Controlled by gene I, which has three alleles: I^A, I^B, and i.
  - I^A and I^B are codominant, while i is recessive to both.
  - Genotype I^A I^B results in AB blood group, where both A and B antigens are expressed.
Multiple Alleles:
- More than two alleles exist for a single gene in a population.
- Example: ABO blood group system (I^A, I^B, i).
Polygenic Inheritance:
- A trait controlled by three or more genes (multiple genes).
- The phenotype reflects the additive effect of multiple genes.
- Example: Human skin color, human height.
Pleiotropy:
- A single gene affects multiple phenotypic traits.
- Example:
  - Phenylketonuria (PKU): A single gene mutation leads to mental retardation, reduction in hair and skin pigmentation.
  - Starch synthesis in pea seeds: A single gene controls both seed shape (round/wrinkled) and starch grain size.

Pedigree Analysis

Pedigree Analysis: The study of inheritance patterns of genetic traits in humans (or other organisms) by analyzing the family history over several generations.
Symbols: Standardized symbols are used to represent individuals, their sex, affected status, and relationships.
Interpretation: Used to determine the mode of inheritance (autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive) of a genetic disorder.

Chromosomal Theory of Inheritance

Proposed by Walter Sutton and Theodor Boveri in 1902.
States that Mendelian factors (genes) are located on chromosomes.
Key points:
- Chromosomes, like genes, occur in pairs.
- Homologous chromosomes separate during meiosis, and each gamete receives one chromosome from each pair.
- The pairing and separation of a pair of chromosomes would lead to the segregation of a pair of factors they carry.

Sex Determination

The mechanism by which the sex of an individual is determined.
Humans (XY type):
- Males are heterogametic (XY), producing two types of sperm (X and Y).
- Females are homogametic (XX), producing only one type of egg (X).
- Sex of the offspring is determined by the sperm that fertilizes the egg.
Birds (ZW type):
- Females are heterogametic (ZW), producing two types of eggs (Z and W).
- Males are homogametic (ZZ), producing only one type of sperm (Z).
- Sex of the offspring is determined by the egg.
Honeybee (Haplodiploidy):
- Males (drones) develop from unfertilized eggs and are haploid (n).
- Females (queens and workers) develop from fertilized eggs and are diploid (2n).
- Sex is determined by the number of sets of chromosomes.
Grasshopper (XO type):
- Males are XO (have only one X chromosome).
- Females are XX.
- Sex of the offspring is determined by the sperm (X-carrying or O-carrying).

Linkage and Crossing Over

Linkage: The phenomenon where genes located on the same chromosome tend to be inherited together and do not assort independently.
- Discovered by T.H. Morgan in Drosophila melanogaster.
- Linked genes are located close together on the same chromosome.
Crossing Over: The exchange of genetic material between homologous chromosomes during meiosis (prophase I).
- Leads to recombination (new combinations of genes).
- The frequency of crossing over between two linked genes is directly proportional to the distance between them on the chromosome. This is used for genetic mapping.

Mutation

Mutation: A sudden heritable change in the DNA sequence.
Spontaneous Mutations: Occur naturally due to errors during DNA replication or repair.
Induced Mutations: Caused by mutagens (physical, chemical, or biological agents).
Point Mutations: Changes in a single base pair of DNA.
- Transition: Replacement of a purine by a purine (A↔G) or a pyrimidine by a pyrimidine (C↔T).
- Transversion: Replacement of a purine by a pyrimidine or vice versa (A↔C, A↔T, G↔C, G↔T).
- Frameshift Mutations: Insertions or deletions of base pairs that alter the reading frame of the genetic code, leading to a completely different protein sequence.

Sex-linked Inheritance

Inheritance of genes located on the sex chromosomes (X or Y).
In Drosophila: Morgan studied sex-linked inheritance of eye color.
In Humans:
- Haemophilia (Bleeder's Disease): X-linked recessive disorder. Affected individuals lack a clotting factor, leading to prolonged bleeding.
- Colour Blindness: X-linked recessive disorder. Affected individuals cannot distinguish between certain colors (most commonly red and green).

Mendelian Disorders

Genetic disorders caused by alteration or mutation in a single gene.
Thalassemia: Autosomal recessive blood disorder characterized by reduced synthesis of globin chains of hemoglobin, leading to anemia.
Sickle-cell Anaemia: Autosomal recessive blood disorder. A point mutation in the beta-globin gene leads to the substitution of glutamic acid by valine, causing red blood cells to become sickle-shaped under low oxygen tension.
Phenylketonuria (PKU): Autosomal recessive metabolic disorder. Deficiency of the enzyme phenylalanine hydroxylase, leading to accumulation of phenylalanine and its derivatives, causing mental retardation.

Chromosomal Disorders

Caused by absence or excess or abnormal arrangement of one or more chromosomes.
Down’s Syndrome (Trisomy 21): Caused by the presence of an extra copy of chromosome 21 (47, XX or XY, +21). Symptoms include mental retardation, short stature, characteristic facial features.
Turner’s Syndrome (XO): Caused by the absence of one X chromosome in females (45, XO). Symptoms include sterile females, rudimentary ovaries, lack of secondary sexual characteristics.
Klinefelter’s Syndrome (XXY): Caused by the presence of an extra X chromosome in males (47, XXY). Symptoms include overall masculine development but with feminine features (e.g., gynecomastia), sterile males.

Principles of Inheritance