Fundamentals of Genetics: Inheritance, Chromosomes, and Mutations
1. Basic Concepts of Genetics
„h Trait (or hereditary character): A feature passed through generations.
„h Gene: A unit of hereditary material determining a trait.
„h Genotype: The set of genes in a cell for a character or organism.
„h Phenotype: Observable characteristics of an organism.
„h Allele: An alternative form of a gene.
„h Homozygote: An individual with identical alleles for a trait (pure-breeding).
„h Heterozygote: An individual with two different alleles for a trait (hybrid).
Types of hybrids:
- Monohybrid: Differs at one hereditary character.
- Dihybrid: Differs in two pairs of alleles.
- Polyhybrid: Differs in more than two pairs of alleles.
„h Dominant allele: Masks the recessive allele in heterozygotes.
„h Recessive allele: Masked by the dominant allele in heterozygotes.
„h Co-dominant alleles: Both contribute equally to the phenotype.
„h Co-dominance or intermediate inheritance: Heterozygote phenotype is intermediate.
2. Early Studies in Genetics
„h Gregor Mendel systematically studied trait transmission.
„h He used pea plants (Pisum sativa) for experiments.
„h He introduced statistical techniques.
„h He deduced rules for offspring production (genes).
„h Results: Mendel’s Three Laws.
2.1. Mendel’s Laws
„h Mendel’s First Law (Law of Uniformity): F1 generation from pure-breeding parents are identical.
„h Mendel’s Second Law (Law of Segregation of Alleles): F2 generation shows a 3:1 phenotypic ratio.
„h Mendel’s Third Law (Law of Independence of Characters): Traits are inherited independently.
Mendel’s discoveries:
- Two factors (alleles) for each trait.
- Only one factor is present in each gamete.
- One factor (dominant) masks the other (recessive).
- Factors for different traits are inherited independently.
3. Special Genetic Cases
Some transmissions deviate from Mendel’s Laws.
3.1. Multiple Allelism
Most genes are mono-allelic (one allele). Some are di-allelic (two alleles). Few are poly-allelic (more than two alleles), e.g., ABO blood groups.
3.2. Gene Interaction
One gene pair influences the expression of another, e.g., albinism masking skin color genes.
3.3. Lethal Genes
Cause premature death, modifying phenotypic proportions.
3.4. Quantitative Inheritance
Traits show a continuous gradient of phenotypes, controlled by multiple genes, e.g., human skin color.
4. Gene Location
Mendel didn’t link factors to chromosomes.
4.1. Chromosome Theory of Inheritance
Scientists linked genes to chromosomes.
Summary:
- Genes are located on chromosomes (DNA fragments).
- Each gene has a specific locus on a chromosome.
- Two alleles are on homologous chromosomes.
4.2. Linked Genes
Genes on the same chromosome are linked.
- Independent genes: On different chromosomes, inherited independently.
- Linked genes: On the same chromosome, inherited jointly.
- Linked genes with recombination: Crossing over can separate linked genes.
4.3. Chromosome Mapping
Graphic representation of gene positions on chromosomes.
5. Sex Inheritance
Sex determination and sex-linked inheritance.
5.1. Sex Determination
Biological systems determine sexual characteristics.
a) Genetic sex determination:
„h Chromosomal determination system:
- XX/XY System (mammals, some fish, amphibians): XX = female, XY = male.
- ZZ/ZW System (birds, reptiles): ZZ = male, ZW = female.
- XX/X0 System (some insects): XX = female, X0 = male.
„h X-to-autosome ratio determination system (e.g., Drosophila).
„h Haplodiploid sex-determination system (bees, ants, wasps): Diploid = female, Haploid = male.
b) Environmental sex determination:
„h Temperature (reptiles), age (some fish).
5.2. Sex-Linked Inheritance
Genes on sex chromosomes show different phenotypic ratios.
„h X-linked inheritance: Gene on X chromosome. Women can be carriers.
„h Y-linked inheritance: Gene on Y chromosome. Only in males.
6. Mutations
Alterations in genetic material. Source of variation for natural selection and evolution.