Genetics
Introduction
Genetics is the branch of biology that studies heredity, variation and the transmission of biological information from one generation to the next. The basic unit of heredity is the gene, a segment of DNA that contains instructions for building and maintaining an organism. Genes are arranged on thread-like structures called chromosomes, which are found in the nucleus of eukaryotic cells.
Gregor Mendel (conducted experiments 1856-1863; results published 1866) is regarded as the founder of classical genetics for his systematic breeding experiments on pea plants. Mendel's observations led to general principles that describe how traits are inherited.
Basic terms and concepts
- Gene: A unit of heredity; a DNA sequence coding for a trait or part of a trait.
- Allele: Alternate forms of a gene (for example, A and a).
- Genotype: The genetic constitution of an organism (e.g., AA, Aa, or aa).
- Phenotype: The observable physical or biochemical characteristics resulting from genotype plus environment.
- Homozygous: Carrying two identical alleles for a trait (AA or aa).
- Heterozygous: Carrying two different alleles for a trait (Aa).
- Dominant allele: An allele that is expressed in the heterozygote and masks the recessive allele.
- Recessive allele: An allele expressed phenotypically only when homozygous (aa).
- Gametes: Sex cells (sperm and egg) that carry a single allele for each gene.
- Mutation: A heritable change in DNA sequence that can alter phenotype.
- Genetically modified organism (GMO): An organism whose genome has been altered using genetic engineering techniques.
Mendelian Laws of Inheritance
Law of Segregation
Each individual has two alleles for each trait, which segregate (separate) during gamete formation so that each gamete receives one allele. At fertilisation, offspring receive one allele from each parent.
Law of Independent Assortment
Alleles of different genes assort independently of one another during gamete formation when the genes are on different chromosomes or are far apart on the same chromosome. This results in new combinations of traits in the progeny. Note: this law does not apply when genes are closely linked on the same chromosome.
Monohybrid cross
A monohybrid cross examines inheritance of a single trait. Mendel crossed true-breeding (homozygous) parents differing in one trait and observed the progeny across generations.
- Classic result: a cross between two heterozygotes (Aa × Aa) gives genotype ratio 1 : 2 : 1 (AA : Aa : aa) and phenotype ratio 3 : 1 when A is dominant to a.
- Example: Seed colour in peas where yellow (Y) is dominant to green (y). Cross Yy × Yy gives progeny phenotypes 3 yellow : 1 green.
- Punnett square is a convenient diagram to predict genotypes and phenotypes of offspring.
Dihybrid cross
A dihybrid cross examines inheritance of two independent traits simultaneously. Mendel's classic dihybrid cross was between plants differing in seed shape and seed colour.
- When two heterozygotes for both traits are crossed (AaBb × AaBb) and the genes assort independently, the phenotypic ratio in the F2 generation is 9 : 3 : 3 : 1.
- Example: Round (R) dominant to wrinkled (r) and yellow (Y) dominant to green (y). F2 phenotypes: 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green.
Worked example (monohybrid)
Consider a trait with dominant allele A and recessive allele a. Cross two heterozygotes (Aa × Aa).
Parental gametes: each parent produces gametes carrying A or a in equal frequency.
Possible zygotes: AA, Aa, aA, aa.
Genotype counts: AA (1), Aa (2), aa (1) → genotype ratio 1 : 2 : 1.
Phenotype counts (A dominant): dominant : recessive = 3 : 1.
Non‐Mendelian inheritance and important exceptions
- Incomplete dominance: Heterozygote shows intermediate phenotype (e.g., red × white → pink in some plants). Genotypic and phenotypic ratios often match (1 : 2 : 1).
- Codominance: Both alleles are expressed (e.g., human ABO blood group: IA and IB are codominant; IAIB genotype expresses AB phenotype).
- Multiple alleles: More than two alleles exist in a population for a gene (ABO blood group has IA, IB, i).
- Polygenic inheritance: Many genes contribute to a single trait (e.g., human height, skin colour); these show continuous variation.
- Epistasis: One gene masks or modifies the effect of another gene (gene‐gene interaction affecting phenotype ratios).
- Linkage and recombination: Genes located close together on the same chromosome are linked and tend to be inherited together; crossing over can produce recombinants at a frequency proportional to distance between genes.
- Sex‐linked inheritance: Genes on sex chromosomes (X or Y) show patterns such as X‐linked recessive traits being more common in males (e.g., haemophilia, colour blindness).
Chromosomal basis of inheritance
Chromosomes carry genes in a linear order. During meiosis, homologous chromosomes segregate and recombine, explaining Mendel's observations at a cytological level. The number and structure of chromosomes vary among species. In humans, somatic cells have 46 chromosomes (23 pairs): 22 pairs of autosomes and 1 pair of sex chromosomes.
Mutations and genetic variation
- Mutation: A change in DNA sequence; can be point mutation, insertion, deletion, duplication, or chromosomal rearrangement.
- Mutations are a primary source of genetic variation and can be neutral, deleterious or occasionally beneficial.
- Mutations in germ cells can be inherited; somatic mutations affect only the individual.
Applications and examples
- Human genetic disorders: Many follow Mendelian patterns (e.g., cystic fibrosis is autosomal recessive; Huntington's disease is autosomal dominant).
- ABO blood group: Illustrates codominance and multiple alleles; important in transfusion medicine.
- Genetic engineering and GMOs: Techniques such as recombinant DNA, CRISPR and transgenic methods are used to modify organisms for agriculture, medicine and research.
- Pedigree analysis: Family trees are used to infer modes of inheritance and predict risks of genetic conditions.
- Population genetics (basic): Study of allele frequency changes in populations; Hardy-Weinberg equilibrium provides a null model for non‐evolving populations.
Common techniques and tools
- Punnett square: Diagram to predict genotypic and phenotypic outcomes of crosses.
- Pedigree charts: Used for tracking inheritance in families.
- Chromosome staining (karyotyping): Visualises chromosomal number and large structural changes.
- Molecular methods: DNA sequencing, PCR, gel electrophoresis, and cytogenetic assays used to identify genes and mutations.
Important historical experiments and terms to remember
- Mendel's pea experiments: Demonstrated particulate inheritance and led to formulation of segregation and independent assortment.
- F1 and F2 generations: F1 - first filial generation (offspring of parents); F2 - progeny of F1 crosses.
- P, F1, F2 notation: Parental generation (P), first filial (F1), second filial (F2).
- Recombinant frequency: Measure of genetic distance; percentage of offspring showing recombinant phenotypes.
Study tips and exam‐oriented points
- Memorise key ratios for monohybrid (3 : 1), dihybrid (9 : 3 : 3 : 1) and genotypic 1 : 2 : 1 for monohybrid crosses between heterozygotes.
- Practice constructing Punnett squares and pedigree charts; these skills are frequently tested.
- Be able to explain exceptions to Mendel (linkage, epistasis, incomplete dominance, codominance) with examples.
- Understand chromosome behaviour during meiosis and how it explains Mendelian laws.
Summary
Genetics explains how traits are transmitted through genes and chromosomes. Mendel's laws provide the foundation for understanding inheritance patterns, while additional phenomena such as linkage, codominance, multiple alleles and polygenic inheritance expand the classical framework. Modern genetics combines Mendelian concepts with molecular tools to study DNA, mutations and their applications in medicine, agriculture and research.
