Studying Inheritance (AQA A-Level Biology): Revision Notes
Studying Inheritance
Understanding how traits pass from parents to offspring has fascinated scientists for centuries. The foundation of modern genetics was established through the pioneering work of Gregor Mendel in the 19th century, who discovered that characteristics are inherited according to specific patterns. This knowledge helps us understand genetic variation within populations and forms the basis of inheritance studies.
Genotype and phenotype
Two essential concepts form the foundation of inheritance studies: genotype and phenotype.
Genotype refers to the complete genetic makeup of an organism. This encompasses all the inherited genetic material (alleles) that an individual possesses. The genotype sets the genetic potential for various characteristics, such as determining that a person might have the genetic capacity to reach 1.8m in height.
Phenotype describes the actual observable or measurable characteristics of an organism. These traits result from the interaction between an organism's genetic makeup and environmental factors.
Worked Example: Height Expression
- Genotype: Genetic potential for 1.8m height
- Environmental factors: Nutrition, calcium availability during development
- Resulting phenotype: Actual height achieved (may be less than 1.8m if environmental factors are limiting)
The environment plays a crucial role in phenotype expression, meaning that identical genotypes can produce different phenotypes under varying environmental conditions.
Genes and alleles
Genes are specific DNA sequences composed of nucleotide bases that typically code for particular polypeptides. These polypeptides often function as enzymes in biochemical pathways that produce observable characteristics. For instance, a gene might code for an enzyme that produces brown pigment in the iris of an eye.
Each gene occupies a specific position called a locus on a DNA molecule. Genes can exist in different versions or forms.
Alleles represent the different variants of the same gene. In pea plants, the gene controlling seed pod colour has two alleles: one for green pods and another for yellow pods. These different alleles arise from variations in the DNA sequence of the same gene.
In diploid organisms, chromosomes exist in pairs called homologous chromosomes. Since each chromosome in a pair carries one copy of each gene, diploid organisms possess two alleles for every gene - one inherited from each parent.
Types of genetic inheritance
The combination of alleles an organism possesses determines how traits are expressed:
- Homozygous dominant: When both alleles at a gene locus are identical and dominant (e.g., both alleles for green pods). The dominant trait appears in the phenotype.
- Heterozygous: When the two alleles at a gene locus are different (e.g., one allele for green pods, one for yellow pods). In most cases, only the dominant allele expresses itself in the phenotype.
- Homozygous recessive: When both alleles are identical and recessive (e.g., both alleles for yellow pods). The recessive trait appears in the phenotype because no dominant allele is present to mask it.
The key principle: In heterozygous individuals, the dominant allele typically masks the expression of the recessive allele, even though both alleles are present in the genotype.
Dominance and recessiveness
When different alleles are present in a heterozygous organism, typically only one expresses itself in the phenotype:
- Dominant alleles express themselves even when paired with a different allele
- Recessive alleles only express themselves when paired with an identical recessive allele
The recessive allele's effect becomes apparent in the phenotype only in diploid organisms when it occurs alongside another identical recessive allele (homozygous recessive state).
Codominance
In some inheritance patterns, both alleles contribute equally to the phenotype when present together. This is called codominance.
Examples of Codominance:
- Shorthorn cattle: Display roan coat colour (mixture of red and white hairs from red and white alleles)
- ABO blood group system: Both A and B antigens appear in AB blood type
Multiple alleles
While each diploid individual can only possess two alleles for any gene, some genes exist in more than two different forms within a population. This creates multiple alleles for that characteristic.
Since homologous chromosome pairs only allow for two alleles per individual, only two of the multiple existing alleles can be present in any single organism.
Example: Human ABO Blood Group System
- Population level: Three alleles exist (A, B, and O)
- Individual level: Each person can only have two of these alleles
- Possible combinations: AA, AO, BB, BO, AB, OO
Key Points to Remember:
- Genotype is your genetic makeup; phenotype is what you actually observe
- Dominant alleles mask recessive ones in heterozygous individuals
- Recessive traits only appear when both alleles are recessive (homozygous recessive)
- Codominance occurs when both alleles contribute equally to the phenotype
- Multiple alleles exist in populations, but individuals can only carry two alleles per gene