Published by Patrick Mutisya · 14 days ago
Explain the following terms and how they relate to the inheritance of traits:
| Term | Definition |
|---|---|
| Gene | A segment of DNA that codes for a specific protein or functional RNA. |
| Locus | The fixed position of a gene on a chromosome. |
| Allele | One of two or more alternative forms of a gene that arise by mutation and are found at the same locus. |
| Dominant | An allele whose phenotype is expressed in both homozygous and heterozygous individuals. |
| Recessive | An allele whose phenotype is expressed only when an individual is homozygous for that allele. |
| Codominant | Two alleles at a locus are both fully expressed in the heterozygote, giving a phenotype that shows both traits. |
| Linkage | The tendency of genes that are close together on the same chromosome to be inherited together. |
| Test cross | A cross between an individual of unknown genotype and a homozygous recessive individual to determine the unknown genotype. |
| F1 generation | The first filial generation produced by crossing two parental (P) individuals. |
| F2 generation | The second filial generation obtained by inter‑crossing individuals from the F1 generation. |
| Phenotype | The observable characteristics of an organism resulting from the interaction of its genotype with the environment. |
| Genotype | The genetic constitution of an organism at a particular locus (e.g., AA, Aa, aa). |
| Homozygous | Having two identical alleles at a locus (AA or aa). |
| Heterozygous | Having two different alleles at a locus (Aa). |
Consider a single gene with two alleles: A (dominant) and a (recessive). The parental cross is:
\$\$
\begin{array}{c|c}
\text{Parent 1 (AA)} & \text{Parent 2 (aa)} \\
\hline
\text{Gametes: A} & \text{Gametes: a}
\end{array}
\$\$
The F1 offspring are all heterozygous (Aa) and display the dominant phenotype.
Inter‑crossing two F1 individuals (Aa × Aa) gives the classic 3:1 phenotypic ratio.
\$\$
\begin{array}{c|cc}
& A & a \\ \hline
A & AA & Aa \\
a & Aa & aa
\end{array}
\$\$
Genotypic ratio: 1 AA : 2 Aa : 1 aa
Phenotypic ratio (dominant : recessive): 3 : 1
To determine whether an individual with a dominant phenotype is homozygous (AA) or heterozygous (Aa), cross it with a homozygous recessive (aa) individual.
When two genes are on the same chromosome, they may be inherited together. The degree of linkage is expressed by the recombination frequency (θ), calculated as:
\$\$
\theta = \frac{\text{Number of recombinant offspring}}{\text{Total number of offspring}} \times 100\%
\$\$
A recombination frequency of 0 % indicates complete linkage; 50 % indicates independent assortment.
In the ABO blood‑group system, alleles IA and IB are codominant. An individual with genotype IAIB expresses both A and B antigens on the surface of red blood cells.