Understanding Genetics: Locus, Alleles, and Inheritance
Understanding Basic Genetic Concepts
1. Defining Locus and Allele
Locus: The specific, fixed position on a chromosome where a particular gene is located.
Allele: One of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome. Each allele controls a specific trait or characteristic.
2. Phenotype vs. Genotype: Observing Similarities
When observing similarities between a baby and their parents, we are looking at the phenotype. The phenotype is the observable expression of the genotype, representing the set of characteristics that an individual displays.
3. Heterozygous and Homozygous Phenotypes
A heterozygous individual can display the same phenotype as a homozygous individual if the allele in question is dominant. For example, if ‘A’ is dominant and ‘a’ is recessive:
- AA (homozygous dominant) = Phenotype A
- Aa (heterozygous) = Phenotype A
- aa (homozygous recessive) = Phenotype a
Therefore, both AA and Aa individuals will show the same phenotype (A).
4. Reappearance of Parental Phenotypes in Second Generation
The reappearance of parental phenotypes in the second generation can be explained by Mendel’s Third Law, the Law of Independent Assortment. This law states that different alleles are inherited independently of each other and combine randomly in the offspring.
5. Sex-Related Diseases on Chromosome X
Sex-related diseases located on the X chromosome appear more frequently in men. This is because males have only one X chromosome (XY), while females have two (XX). A recessive gene on the X chromosome will always be expressed in males, as there is no second X chromosome to potentially carry a dominant allele to mask it.
6. Novel Traits in Offspring
A sexually reproducing individual can exhibit a trait that does not appear in either parent if the trait is determined by a recessive allele. In a heterozygous individual (Aa), the recessive allele (a) can be hidden by the dominant allele (A). If both parents are heterozygous (Aa), there is a chance for the offspring to inherit two recessive alleles (aa) and thus express the trait.
7. Genotype Examples
- a) Heterozygous: Aa
- b) Homozygous dominant: AA
- c) Homozygous recessive: aa
- d) Dihybrid heterozygous: AaBb
8. Proteins, Amino Acids, and Genetic Information
Proteins are chains of simpler molecules called amino acids. The sequence of amino acids is determined by:
- A) Gene
9. Tetraploidy After Fertilization
If sex cells carried pairs of chromosomes (instead of single chromosomes), fertilization would result in a tetraploid individual (having four sets of chromosomes). In humans, this condition is generally not viable. However, tetraploidy is more common in plants.
10. Bacteria in Early Genetic Engineering
Bacteria were among the earliest tools of genetic engineering because of their rapid replication rate.
11. Solar Radiation as a Mutagen
Solar radiation is a mutagen. In women, it can affect the ovaries, and similarly, it can affect the reproductive cells in men.
12. X-rays and Pregnancy
Pregnant women are advised to avoid X-rays because they can cause mutations in the developing fetus.
13. Disadvantages of Cloning
The main disadvantages of cloning include:
- Lack of genetic variation
- Potential for premature aging and death
14. Benefits of Genetic Engineering in Clinical Diagnostics
Genetic engineering has provided clinical diagnostic tests that are:
- Faster
- More efficient
- Capable of detecting previously undetectable diseases
15. Polymerase Chain Reaction (PCR)
PCR (Polymerase Chain Reaction) is a technique used to amplify specific DNA sequences. It allows for the identification of individuals from even small samples like a drop of blood. DNA fragments are separated into bands by passing an electric current through them, creating a unique genetic pattern.
16. The Human Genome Project
The Human Genome Project was an international research effort with the primary goal of determining the complete sequence of nucleotides in human chromosomes. Its objectives included:
- Identifying all human genes and their locations.
- Sequencing each gene.
- Determining the function of each gene.
Some key findings include:
- The human genome contains approximately 2.9 billion base pairs.
- Many human genes appear to have originated from viruses and bacteria (fossil DNA).
- Each gene can be involved in the synthesis of multiple proteins.