Genetics revision notes

Mendel's Experimental Approach

Gregor Mendel used pea plants for his experiments due to their clear, distinguishable traits, short generation time, and ease of cross-pollination. He studied characteristics like seed shape, seed colour, flower colour, and plant height. He began by creating 'pure-breeding' lines for each trait. He then cross-pollinated these lines, carefully tracking the inheritance patterns over several generations. His quantitative analysis of the results was key to his discoveries.

Key Discoveries and Principles

Mendel discovered that traits are passed on as discrete units (factors, now called genes) rather than blending. He observed dominant and recessive forms of these traits. His work led to the Law of Segregation, stating that allele pairs separate during gamete formation and randomly unite at fertilisation. He also proposed the Law of Independent Assortment, which states that alleles for different genes assort independently of one another during gamete formation (for unlinked genes). These principles formed the basis of modern genetics.

Challenges Before Mendel

Before Mendel, understanding inheritance was difficult because scientists believed in 'blending inheritance', where offspring traits were an average of parental traits. This theory couldn't explain why some traits skipped generations or why variations persisted. There was no concept of discrete hereditary units, and experimental approaches were often less systematic and quantitative than Mendel's, leading to confusion and a lack of clear explanatory models.

Genetic Variants in Coding Regions

Genetic variants are alterations in DNA sequence that can occur anywhere in the genome. When they happen within the coding region of a gene, they may change the sequence of codons used during translation, affecting which amino acids are included in the protein.

Effect on Amino Acid Sequence and Protein Structure

Changes to codons in the coding DNA can cause the wrong amino acid to be inserted into the polypeptide chain. Since protein function depends on precise folding (driven by its amino acid sequence), even a single change can disrupt folding and protein activity, leading to altered phenotype.

Outcomes of the Human Genome Project

The HGP successfully sequenced the entire human genome, providing a complete map of all human genes. This achievement led to the identification of approximately 20,000-25,000 genes and their locations on chromosomes. It established a universal reference sequence for human DNA, accelerating biological research worldwide. The project also spurred the development of advanced sequencing technologies and computational tools for analysing vast amounts of genetic data, significantly contributing to the field of bioinformatics.

Medical Applications

The HGP's findings have numerous medical applications. They enable better understanding and diagnosis of genetic diseases by identifying disease-causing gene mutations. This knowledge supports the development of new treatments, including gene therapy. Furthermore, it underpins personalized medicine, allowing healthcare professionals to tailor drug dosages and select treatments based on an individual's unique genetic makeup, optimising effectiveness and minimising adverse reactions. It also aids in pharmacogenomics, studying how genes affect a person's response to drugs.

Ethical, Legal, and Social Implications (ELSI)

Beyond its scientific achievements, the HGP also highlighted important ethical, legal, and social implications. These include concerns about genetic discrimination in employment or insurance, the privacy of genetic information, and the societal impact of genetic testing and gene editing technologies. Addressing these ELSI aspects has become an integral part of ongoing genomic research and its clinical applications.

Sex-Linked (X-Linked) Genetic Disorders

Sex-linked genetic disorders are caused by alleles found on the X chromosome. Males (XY) are more likely to be affected because they have only one X chromosome. Females (XX) need two faulty alleles to display the disorder, while one faulty and one normal allele makes them a carrier.

Inheritance Patterns and Carriers

For X-linked recessive disorders, a male with a faulty allele will be affected, but a female generally needs two faulty alleles. Carrier females can pass the faulty allele to their children, and if a son inherits the faulty X, he will have the disorder. Fathers pass their X chromosome to all daughters, potentially making them carriers.