Genes, Alleles and Phenotypes

• Genes are DNA sequences determining traits passed from parents to offspring.
• These sequences signal the production of molecules like proteins and RNA.
• A human cell contains an estimated 120,000 genes, but not all are physically expressed.
• Gene expression is cell-specific; a brain cell contains but cannot express genes for hair pigment.

Functional Units of a Gene

• A gene comprises a promoter region and a coding region.
• The promoter region signals when a gene is expressed, with specific nucleotide sequences read by appropriate cells.
• The coding region contains information for assembling molecules like amino acids and proteins.
• Genes can contain various codes for possible trait expressions.

Alleles and Phenotypes

• Alleles are different forms of a single gene, signaling different phenotypes.
• Phenotypes are physical gene expressions, e.g., hair color.
• Individuals can possess genes for multiple traits but only express one.

Genetic Disorders

• Broken genes or alleles lead to genetic disorders.
• Genetic abnormalities in an organism's genetic material leads to abnormal phenotypes.
• A defective allele for oxygen-carrying proteins in red blood cells can cause sickle cell anemia.
• This results in rigid blood cells that get stuck in capillaries, causing tissue damage and organ failure.
• Genes are packaged into structures called chromosomes.
• Chromosomes are thread-like structures of DNA strands tightly wound around histones.

Chromosomal Disorders

• These involve mistakes in chromosomal assortment within gametes.
• Translocation occurs when a chromosome section breaks off and attaches to another during sex cell creation.
• Translocation results in sex cells with extra or missing genetic material.
• Chromosome types include metacentric, submetacentric, acrocentric, and telocentric (not in humans), differing in arm lengths extending from centrosomes.
• Genetic disorders are classified as monogenetic, chromosomal, and multifactorial inheritance disorders.

Monogenetic Disorders

• These are caused by the malfunction or mutation of a single gene.
• They are characterized by birth defects and developmental issues in sensory, motor, and intellectual functions.
• Inheritance patterns can be dominant or recessive.
• Dominant malfunctioning allele results in the disorder's expression.
• Recessive disorders require homologous recessive genes for expression.
• Individuals with one recessive allele are carriers.
• Treatments like gene therapy, bone marrow transplants, and stem cell transplantation are being explored.
• Examples of monogenic disorders include cystic fibrosis, retinoblastoma, albinism, hemophilia, sickle cell disease, and fragile X syndrome.

Chromosome Disorders

• These disorders are caused by errors during meiosis.
• Meiosis separates homologous chromosomes into haploid gametes (sex cells).
• Homologous chromosomes contain similar genetic information in the same order from opposite parents.
• During meiosis, chromosomes are organized into four separate gametes, each with half the DNA of the parent cell.
• Nondisjunction occurs when chromosomes fail to separate during anaphase of meiosis, resulting in gametes with missing or extra chromosomes.
• Translocation during meiosis transfers a chromosome portion from one chromosome to a nonhomologous chromosome.
• Chromosome disorders often have noticeable phenotypes.
• Trisomy is having three additional chromosomes; Down syndrome is a common form with three copies of chromosome 21.
• Symptoms of Down syndrome include cognitive impairment and physical birth defects.
• Other chromosome disorders include Turner's syndrome and Klinefelter's syndrome.

Multifactorial Inheritance Disorders

• These are polygenic disorders caused by environmental factors and gene mutations.
• Gene mutations result from exposure to mutagens like radiation, which damages genetic material.
• Symptoms require specific genetic and environmental factors.
• High blood pressure is linked to multiple genes, with environmental factors like diet and stress influencing symptoms.
• Other examples include heart disease, obesity, diabetes, cancer, Alzheimer's disease, and arthritis.

Genes are DNA sequences that determine traits passed down from parents. Genes comprise promoter and coding regions and can have different forms called alleles. Alleles signal the expression of phenotypes, which are physical expressions of genes.