Genetics and Inheritance

Introduction to Genetics

Genetics is the study of inheritance, focusing on how traits are passed from one generation to the next. The field explores the mechanisms by which genetic information is transmitted and expressed in living organisms.

• Trait: A characteristic that is inherited from parent to offspring.

• Genes: Units of heredity made up of DNA that carry information from one generation to another.

• Information is carried along on genes—regions of DNA.

Describing an Organism's Genetic Makeup

• Phenotype: The observable traits or characteristics of an organism (e.g., flower color, height).

• Genotype: The genetic makeup of an organism; the actual alleles or genes on DNA that determine traits.

Variation and Its Determination

Variation exists among individuals due to differences in their genetic makeup. This variation is determined by the combination of alleles inherited from parents.

Mendelian Genetics

Gregor Mendel and His Experiments

Gregor Mendel (1826–1884) is known as the father of genetics. He conducted experiments with pea plants to understand how traits are inherited.

Early Genetic Observations

• Certain traits are visible, while others may reappear in later generations.

• Some traits are more highly represented than others (dominant traits).

• Traits may skip generations but can reappear in subsequent generations.

Genetic Experiments with Pea Plants (Pisum sativum)

• Easy to grow and reproduce quickly.

• Can easily self-fertilize or be cross-fertilized.

• Produce many observable characteristics (e.g., flower color, seed shape).

Mendel's Methodology

• Started with true-breeding (pure) strains, called the P generation.

• Crossed plants with contrasting traits to produce F1 hybrids (first filial generation).

• Allowed F1 hybrids to self-fertilize, producing the F2 generation (second filial generation).

• Observed the ratios of traits in each generation.

Mendel's Results

• F1 Generation: Crossing purple-flowered and white-flowered plants resulted in all purple flowers (purple = dominant trait, white = recessive trait).

• F2 Generation: When F1 plants self-fertilized, the white trait reappeared. The ratio of purple to white flowers was approximately 3:1.

Genotypic Ratio: In the F2 generation, the genotypic ratio is 1:2:1 (homozygous dominant : heterozygous : homozygous recessive).

Phenotypic Ratio: The observable trait ratio is 3:1 (dominant : recessive).

Mendel's Laws

Law of Segregation

Alleles for a trait separate (segregate) during gamete formation (meiosis), so each gamete carries only one allele for each gene.

• Explains the 3:1 ratio observed in the F2 generation.

Law of Independent Assortment

Alleles of different genes assort independently of one another during gamete formation if the genes are on different chromosomes.

• Demonstrated by dihybrid crosses (crosses involving two traits).

• Results in new combinations of traits in offspring.

Chromosomes and Exceptions to Mendelian Genetics

Sex Determination and Associated Traits

• Sex chromosomes (X and Y in mammals) determine biological sex.

• X-linked traits are associated with genes found on the X chromosome.

• Examples: Color blindness and hemophilia are X-linked recessive traits, more common in males.

X-Inactivation

• In female mammals, one X chromosome is randomly inactivated in each cell, forming a Barr body.

• Results in mosaic expression of X-linked traits (e.g., tortoiseshell cats).

Other Exceptions to Mendelian Genetics

• Incomplete Dominance: Heterozygotes show an intermediate phenotype (e.g., pink flowers from red and white parents).

• Multiple Alleles: More than two alleles exist for a gene in a population (e.g., ABO blood groups).

• Codominance: Both alleles are fully expressed in heterozygotes (e.g., AB blood type).

• Pleiotropy: One gene influences multiple traits.

• Epistasis: One gene affects the expression of another gene (e.g., coat color in Labrador Retrievers).

• Multifactorial Traits: Traits influenced by multiple genes and environmental factors.

Chromosome Structure

Chromosome Size and Shape

• Chromosomes have characteristic sizes and shapes, influenced by the position of the centromere.

Assessing Chromosome Number and Structure

• Karyotype: A display of the complete set of chromosomes of an organism, used to detect chromosomal abnormalities.

Abnormal Chromosome Structure

• Structural changes include deletion, duplication, inversion, and translocation of chromosome segments.

Abnormal Chromosome Number

Polyploidy

• Presence of extra complete sets of chromosomes at the organismal level (common in plants, rare in animals).

Aneuploidy

• Presence of an abnormal number of chromosomes (extra or missing individual chromosomes).

Genetically Transmitted Diseases

Some diseases are inherited due to mutations in specific genes. These are called genetically transmitted or inherited diseases.

• Phenylketonuria (PKU): Inability to metabolize phenylalanine, leading to intellectual disability if untreated.

• Cystic Fibrosis: Causes thick mucus in lungs and digestive tract; autosomal recessive.

• Sickle-cell Anemia: Abnormal hemoglobin causes red blood cells to become sickle-shaped; autosomal recessive.

• Tay-Sachs Disease: Progressive neurological disorder; autosomal recessive.

• Huntington's Disease: Degenerative neurological disorder; autosomal dominant.

Additional info:

• Genetic counseling and screening can help identify carriers of genetic diseases.

• Gene therapy is an emerging field aiming to treat genetic disorders by correcting defective genes.