Inheritance and Genetics: Key Concepts in Biology
Mendel’s Laws of Inheritance
For his experiments on inheritance, Gregor Mendel chose the common pea plant. His findings laid the foundation for modern genetics.
- Mendel’s First Law (Law of Segregation): When crossing two varieties of plants or animals that differ in one character, all the first-generation hybrids exhibit the dominant character. The recessive character reappears in the second generation.
- Mendel’s Second Law (Law of Independent Assortment): When crossing two varieties of plants that differ by two characters, each character is transmitted independently of any other.
Variations and Specific Features
The classification system of living beings is based on the characteristics that these organisms possess. Individuals belonging to a given species possess characteristics common to them all that differentiate them from other species.
- Genotype: The genetic material, consisting of the genes in each cell.
- Phenotype: All observable morphological and physiological characteristics in an organism.
Genetics: The Study of Heredity
Genetics is the branch of biology that studies the genotypic and phenotypic characteristics of organisms. The study of genetics relies on performing crosses between organisms carrying different genotypes and phenotypes and analyzing the offspring.
The Fruit Fly: A Model Organism in Genetics
The fruit fly (Drosophila melanogaster) is only 1 mm in length, is easy to grow and maintain, and produces a new generation every two weeks. These traits make it a suitable model organism for genetic research.
Chromosome Theory of Inheritance
Thomas Hunt Morgan conducted experiments with Drosophila, crossing wild-type red-eyed flies with white-eyed mutants. His observations led to the following conclusions:
- Red eye color is dominant, while white eye color is recessive.
- In the F2 generation, 3/4 of the flies had red eyes, and 1/4 had white eyes.
- All white-eyed flies were male, indicating a sex-linked trait.
Morgan also studied the chromosomes of Drosophila, which has four pairs of chromosomes: three pairs are identical in males and females, and one pair differs. In females, the sex chromosomes are straight and elongated (XX), while in males, there is one X chromosome and one Y chromosome shaped like a cane.
Each chromosome contains several genes arranged linearly, each occupying a specific site called a locus.
Sex Chromosomes and Sex Determination
The fruit fly has four pairs of chromosomes. Females have two identical sex chromosomes (XX), while males have different sex chromosomes (XY).
Linkage and Chromosome Maps
Linkage measures the degree to which alleles of two genes segregate independently during meiosis or genetic crosses.
Chromosome maps show the locations of genes responsible for the main mutant characters relative to the normal characters present in a wild-type organism.
Giant Chromosomes
There are two types of giant chromosomes:
- Polytene Chromosomes: Present in various organs of Diptera, in the macronucleus of some ciliates, and in certain cells of bean seeds.
- Plumose Chromosomes: These are the only types of chromosomes that allow for simple light microscopy observation of chromosomal regions operating in a particular cell.
What is a Gene?
A gene is essentially defined as the unit of recombination in classical genetics. It is also considered the unit of change and the smallest unit whose alteration by mutation affects the phenotype over which it exercises control.
Crossovers
Crossovers are the process by which two chromosomes pair and exchange sections of their DNA. Synapsis starts before the synaptonemal complex develops and is not complete until near the end of prophase 1.