Introduction to Heredity and Mendelian Genetics

Introduction to Heredity

• Heredity is the passing of traits from parents to offspring.
• Traits are determined by genes, which are segments of DNA.
• Genes provide instructions for building and maintaining an organism.
• Inheritance involves complex mechanisms.

Mendelian Genetics

• Gregor Mendel's pea plant experiments formed the basis for understanding inheritance.
• He observed predictable inheritance patterns for traits.
• Mendel's laws are fundamental principles of inheritance:
  • Law of Segregation: Each parent contributes one allele for each trait to offspring, separating alleles during gamete formation.
  • Law of Independent Assortment: Traits are inherited independently of each other.
• Alleles are different versions of a gene.
• Dominant alleles mask recessive alleles.
• Genotype is the genetic makeup (e.g., AA, Aa, aa).
• Phenotype is the observable physical trait (e.g., purple flowers, white flowers).

Beyond Mendelian Genetics

• Not all traits follow simple Mendelian patterns.
• Incomplete dominance: Heterozygotes show an intermediate phenotype between the two homozygous phenotypes.
• Codominance: Both alleles are expressed in the heterozygote, showing traits from both (e.g., AB blood type).
• Multiple alleles: More than two alleles for a gene exist in a population, resulting in more than two genotypes.
• Polygenic inheritance: Multiple genes influence a single trait, creating a wide range of phenotypes (e.g., height, skin color).
• Pleiotropy: A single gene affects multiple traits, causing complex interactions.

Chromosomes and Gene Expression

• Chromosomes are thread-like DNA structures.
• Humans have 23 pairs of chromosomes (22 autosomes, 1 sex chromosome).
• Genes are located at specific positions (loci) on chromosomes.
• Gene expression is tightly regulated, controlled by processes dictating when and how genes are used.
• Environmental factors affect gene expression, influencing phenotypes.
• Epigenetics studies heritable changes in gene expression without altering the DNA sequence.

Sex-Linked Inheritance

• Sex-linked genes are located on sex chromosomes (X and Y).
• Sex-linked traits show different inheritance patterns in males and females due to X chromosome presence/absence.
• Color blindness and hemophilia are examples of sex-linked traits.

Genetic Disorders

• Mutations in DNA lead to genetic disorders.
• Disorders can arise from single gene changes (e.g., cystic fibrosis) or chromosomal abnormalities (e.g., Down syndrome).
• Genetic counseling provides information and support to families affected by or at risk of genetic disorders.

Modern Techniques in Genetics

• Advances in molecular biology offer powerful gene and inheritance study tools.
• Techniques like PCR, DNA sequencing, and genetic engineering are revolutionizing research and leading to new medical applications (e.g., genetic testing).