Understanding Genetics: Genes, Alleles, and Inheritance

Understanding Genetics: Key Concepts

Genetics is the branch of biology that studies the laws governing the transmission of traits.

Basic Genetic Terms

• Gene: The unit of genetic transmission.
• Genotype: The complete set of genes of an individual.
• Phenotype: The observable characteristics of an individual.
• Allele: A variant form of a gene.
• Dominant Allele: An allele that prevents the manifestation of the alternative allele for the same trait.
• Recessive Allele: An allele that is only manifested when the dominant allele is not present.

Mendel's Laws of Inheritance

1. 1st Law (Law of Segregation): When two heterozygous individuals are crossed, all descendants of the first filial generation are equal in both genotype and phenotype.
2. 2nd Law (Law of Independent Assortment): When crossing individuals from the first filial generation, the second filial generation is integrated by two types of phenotypes.
3. 3rd Law (Law of Independent Assortment): When two individuals differing in more than one trait are crossed, the transmission of each trait is independent of the others.

Linked Genes: Genes located on the same chromosome are linked and tend to be inherited together.

Chromosome Determination of Sex

Heterosomes are the chromosomes that differ between males and females; autosomes are common to both.

• A) XX (female) / XY (male)
• B) ZZ (male) / ZW (female)
• C) XX (female) / X0 (male) - only one X chromosome

Sex determination can also depend on:

• Relationship between X chromosomes and autosomes: In Drosophila melanogaster, sex is determined by the ratio of X chromosomes to sets of autosomes.
• Haploidy/Diploidy: In bees, sex determination depends on the total number of chromosomes. Diploid individuals are female, and haploid individuals are male.

Sex-Linked Inheritance

When a trait is governed by a gene on the X chromosome, it is said to have linkage with the X chromosome. Heterozygous females may not show a recessive trait but can pass it on to offspring, making them carriers. Males cannot be carriers because they have only one allele for the trait.

Mutations

Mutations are changes in the genetic material.

Types of Mutations:

• Affected Cells: Somatic (body cells), Sexual (germ cells)
• Causes: Spontaneous, Induced
• Effects: Beneficial, Neutral, Negative
• Resulting Alleles: Dominant, Recessive
• Genetic Alteration: Gene, Chromosome, Genome

Types of Genetic Alterations:

• Gene Mutations: Alterations affecting the chemical structure of genes.
• Chromosomal Mutations: Alterations affecting the structure of chromosomes.
• Numerical or Genomic Mutations: Alterations affecting the number of chromosomes.

Mutagenic Agents:

• Physical: Radiation (e.g., X-rays) can damage genetic material.
• Chemical: Dyes, pesticides, and pollutants can cause changes in genes.
• Biological: Some viruses can increase the frequency of gene mutations by interfering with the cell's genetic material.