Genetic Inheritance (Leaving Cert Biology): Revision Notes
Genetic Inheritance
Genetics is the study of inheritance, the passing of traits from one generation to the next. Cell continuity relies on inheritance.
Cell continuity is the ability of cells to divide and survive from one generation to the next. Every cell arises from pre-existing cells.
Fertilisation
- To produce a new individual, gametes must fuse during fertilisation.
Fertilisation is the fusion of two haploid gametes to form a diploid zygote.
Alleles and Genes
- Allele: A different form of the same gene.
- Example: Brown and blue eye colour are different alleles of the same gene.
- Alleles are found at the same position (locus) on homologous chromosomes.
- Locus: The position of a gene on a chromosome.
- If the two alleles are the same, they are called homozygous.
- If the two alleles are different, they are called heterozygous.
An allele is a different form of the same gene.
Locus is the position of a gene on a chromosome.
If the two alleles are the same, they are said to be homozygous. Homozygous: Often referred to as pure breeding, as there is no variation in the offspring.
If the two alleles are different, they are said to be heterozygous.
Dominant and Recessive Alleles
Dominant: where one allele masks the effect of the recessive allele.
Recessive: the allele is prevented from working by the dominant allele.
- Let's use B as the symbol for the allele for brown eyes and b for blue eyes.
- Genotype: The genetic make-up of an organism (e.g., BB, Bb, or bb).
- Phenotype: The physical appearance or trait expressed by the organism (e.g., brown eyes or blue eyes).
1. Homozygous Dominant (BB):
- Genotype: BB (both alleles are dominant).
- Phenotype: Brown eyes (dominant trait is expressed).
2. Homozygous Recessive (bb):
- Genotype: bb (both alleles are recessive).
- Phenotype: Blue eyes (recessive trait is expressed).
3. Heterozygous (Bb):
- Genotype: Bb (one dominant allele and one recessive allele).
- Phenotype: Brown eyes (dominant allele B masks the recessive allele b).
Representation of Alleles
- A capital letter is used for the dominant allele, and the corresponding lowercase letter represents the recessive allele.
- Example: If purple flowers are dominant over white flowers in a plant, P represents purple, and p represents white.