Genetics: Key Concepts and Heredity Principles
Gene: A gene is the basic unit of heredity, transmitting genetic information.
Genotype: The complete set of genes within an individual.
Phenotype: The observable characteristics of an individual, resulting from the interaction of genotype and environment.
Homozygous: Having two identical alleles for a particular gene.
Heterozygous: Having two different alleles for a particular gene.
Dominant Allele: An allele that expresses its trait even when paired with a recessive allele.
Recessive Allele: An allele that only expresses its trait when paired with another recessive allele.
Codominant Alleles: When two alleles both have the same capacity for expression, resulting in both traits being visible.
Mendel’s Laws
Mendel’s First Law: Law of Uniformity. When two purebred individuals are crossed, the resulting offspring (first filial generation) will exhibit uniform genotypes and phenotypes.
Mendel’s Second Law: Law of Segregation. During gamete formation, allele pairs segregate randomly, so that each gamete receives only one allele of each pair.
Mendel’s Third Law: Law of Independent Assortment. Genes for different traits are sorted separately from one another so that the inheritance of one trait is not dependent on the inheritance of another.
Multiple Alleles: When there are more than two alleles possible for a particular trait.
Lethal Genes: Genes that, when expressed, cause the death of the individual, modifying the expected phenotypic and genotypic ratios.
Quantitative Inheritance: Inheritance patterns involving multiple gene pairs, where the effects of each gene are additive.
Chromosome Theory of Heredity
- Genes are located on chromosomes. A gene is a specific segment of DNA that makes up a chromosome.
- Each gene occupies a specific location on a chromosome, called a locus. Genes are arranged linearly along the chromosome.
- The two alleles for a character are located on homologous chromosomes, which are chromosomes of the same size and appearance.
Linked Genes: Genes located on the same chromosome are linked and tend to be inherited together.
Metacentric Chromosome: A chromosome with two arms of equal length.
Submetacentric Chromosome: A chromosome with two arms of unequal length.
Telocentric Chromosome: A chromosome with the centromere located at one end, resulting in one very short arm.
Chromosome Determination of Sex
In many animals, sex is determined by specific sex chromosomes, which differ between males and females. Chromosomes common to both sexes are called autosomes.
XX/XY System: Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).
ZZ/ZW System: Males have two identical sex chromosomes (ZZ), while females have two different sex chromosomes (ZW). This system is typical of birds and reptiles.
XX/XO System: In this system, there is only one type of sex chromosome (X). Females have two X chromosomes (XX), while males have only one (XO).
Sex-Linked Inheritance
When a trait is governed by a gene located on the X chromosome, it is said to be sex-linked. Heterozygous females can be carriers of the trait. Males, having only one X chromosome, will express the trait if they inherit the allele.
Mutations
Mutations are alterations in the genetic material.
Types of Mutations:
- Criterion: Affected cells (somatic, germline)
- Cause: (spontaneous, induced)
- Effects: (beneficial, neutral, detrimental)
- Alleles Result: (dominant, recessive)
- Genetic Alteration: (gene, chromosome, genome)
Gene Mutation: Affects the chemical structure of a gene.
Chromosome Mutation: Affects the structure of a chromosome.
Genomic Mutation: Affects the number of chromosomes.
Mutagenic Agents:
Physical Mutagens: Radiation, such as X-rays, can cause damage to genetic material.
Chemical Mutagens: Industrial dyes, pesticides, and pollutants can cause changes in genes.
Biological Mutagens: Some viruses can increase the frequency of gene mutations.