Mendelian genetics, named after the pioneering work of Gregor Mendel in the 19th century, describes the patterns of inheritance of traits controlled by single genes with two alleles. Mendel's experiments with pea plants revealed fundamental principles of inheritance that laid the foundation for modern genetics. Here are the principles of Mendelian genetics along with examples of genetic disorders and their inheritance patterns:
Principles of Mendelian Genetics:
1. Principle of Dominance:
In a heterozygous individual (having two different alleles for a gene), one allele (the dominant allele) masks the expression of the other allele (the recessive allele).
Example: In Mendel's pea plants, the allele for yellow seed color (Y) is dominant over the allele for green seed color (y).
2. Principle of Segregation:
During gamete formation, the two alleles for each gene segregate (separate) from each other, with each gamete carrying only one allele for each gene.
Example: In a heterozygous individual with genotype Yy, half of the gametes produced will carry the Y allele, and the other half will carry the y allele.
3. Principle of Independent Assortment:
Genes located on different chromosomes assort independently of each other during gamete formation.
Example: Mendel's experiments with pea plants showed that the inheritance of seed color (Y/y) and seed texture (R/r, round or wrinkled) were independent of each other.
Examples of Genetic Disorders and Their Inheritance Patterns:
1. Cystic Fibrosis:
Cystic fibrosis is a genetic disorder caused by mutations in the CFTR gene, which encodes a protein involved in ion transport across cell membranes.
Inheritance Pattern: Cystic fibrosis is inherited in an autosomal recessive manner. Individuals must inherit two copies of the mutated CFTR gene (one from each parent) to develop the disorder.
Example: If both parents are carriers (heterozygous) for the cystic fibrosis allele, their children have a 25% chance of inheriting two copies of the mutated gene and developing the disorder.
2. Huntington's Disease:
Huntington's disease is a neurodegenerative disorder caused by a mutation in the HTT gene, which leads to the progressive breakdown of nerve cells in the brain.
Inheritance Pattern: Huntington's disease is inherited in an autosomal dominant manner. Individuals who inherit a single copy of the mutated HTT gene from one parent will develop the disorder.
Example: If one parent carries the Huntington's disease allele, each child has a 50% chance of inheriting the mutated gene and developing the disorder.
3. Sickle Cell Anemia:
Sickle cell anemia is a genetic disorder characterized by abnormal hemoglobin molecules, leading to the production of sickle-shaped red blood cells.
Inheritance Pattern: Sickle cell anemia is inherited in an autosomal recessive manner. Individuals must inherit two copies of the mutated HBB gene (one from each parent) to develop the disorder.
Example: If both parents are carriers of the sickle cell allele (heterozygous), each child has a 25% chance of inheriting two copies of the mutated gene and developing the disorder.
These examples demonstrate how genetic disorders can follow Mendelian inheritance patterns, with the principles of dominance, segregation, and independent assortment influencing the transmission of disease alleles from parents to offspring.
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