Understanding Genes, Alleles, and Mendelian Genetics
A gene is a fragment of DNA that contains the information needed to make a protein. Qualitative characters are determined by a single gene, while quantitative characters are influenced by many genes.
Alleles, Genotypes, and Phenotypes
Alleles are different versions of the same gene within a population. An individual is homozygous for a specific gene if they have two identical alleles on their homologous chromosomes. They are heterozygous if they possess two different alleles for that gene.
- Genotype: The complete set of genes present in an individual’s chromosomes.
- Phenotype: The observable characteristics of an individual, resulting from the expression of their genotype.
Dominant and Intermediate Inheritance
In dominant inheritance, the effect of one allele prevails over the others. In intermediate inheritance, the heterozygous individual expresses a blended or intermediate effect of the two different alleles.
Mendelian Genetics
Mendelian genetics applies Mendel’s methodology, which involves crossing individuals with known phenotypes to deduce the genotypes of the individuals involved.
Mendel’s Laws
- Mendel’s First Law (Law of Segregation): When crossing two purebreds for a specific trait, all descendants will be identical.
- Mendel’s Second Law (Law of Independent Assortment): The two alleles for a single trait present in a hybrid do not merge but maintain their independence and segregate during gamete formation.
- Mendel’s Third Law (Law of Dominance): Genes for different traits are transmitted independently and can combine in various ways in the offspring.
Chromosomes and Sex-Linked Traits
Genes are distributed among the 46 chromosomes present in our cells.
- Autosomes: Chromosomes containing genes related to somatic features common to both women and men.
- Sex Chromosomes: Chromosomes that determine an individual’s sex.
Sex-Linked Hereditary Diseases
- Hemophilia: A sex-linked hereditary disease characterized by the blood’s inability to clot properly.
- Color Blindness (Daltonism): A sex-linked inherited disease affecting vision, causing difficulty distinguishing between green and red colors.
Protein Synthesis
Macromolecules called proteins are formed by amino acid units. Protein synthesis from the information in a gene occurs in two stages in eukaryotic cells: transcription in the nucleus and translation in the cytoplasm.
RNA Types
- Messenger RNA (mRNA): Carries instructions from DNA to ribosomes for translation.
- Transfer RNA (tRNA): Carries amino acids to the ribosome.
- Ribosomal RNA (rRNA): A constituent of the ribosome itself.
During transcription, the information contained in one strand of DNA is used to construct a unique strand of RNA. During translation, the information in mRNA is read in triplets by the ribosome.
Mutations
Mutations are alterations in the nucleotide sequence of DNA or the number/structure of chromosomes.
Types of Mutations
- Natural Mutations: Occur accidentally during cell processes involving genetic material.
- Artificial Mutations: Caused by mutagens that induce changes in genetic material.
Mutagen Types
- Physical Mutagens: Various types of radiation that can damage DNA structure.
- Chemical Mutagens: Chemical substances that interact with DNA and cause compositional changes.
Mutation Categories
- Gene Mutations: Changes in the nucleotide sequence.
- Chromosomal Mutations: Changes affecting the structure or number of chromosomes.