Heredity, Genes, and Variation

• Heredity is the passing on of traits from parents to offspring
• It involves the transmission of genetic information from one generation to the next, ensuring continuity and variation in living organisms
• Genetics is the branch of biology that studies heredity and variation

Variation

• Variation refers to the differences in traits among individuals within a population
• These differences can be physical, physiological, or behavioral
• Variation arises due to several factors, including genetic recombination, mutation, and environmental influences
• Genetic recombination occurs during sexual reproduction, where genes from two parents are combined to produce offspring with unique combinations of traits
• Mutation is a sudden change in the genetic material that can introduce new traits into a population
• Environmental factors such as diet, climate, and exposure to toxins can also influence the expression of traits
• Variation is essential for evolution

Genes and Chromosomes

• Genes are the basic units of heredity, composed of DNA and located on chromosomes
• They contain the instructions for building specific proteins or performing specific functions in the body
• Each gene has a specific location on a chromosome, called its locus
• Chromosomes are thread-like structures made of DNA and proteins that carry the genetic information in the form of genes
• In eukaryotic cells, chromosomes are found in the nucleus
• Humans have 46 chromosomes arranged in 23 pairs
• Each parent contributes one chromosome to each pair, so offspring inherit half of their chromosomes from their mother and half from their father

Alleles

• Alleles are different versions of a gene
• Individuals typically inherit two alleles for each gene, one from each parent
• If the two alleles are the same, the individual is homozygous for that gene
• If the two alleles are different, the individual is heterozygous for that gene
• Dominant alleles mask the expression of recessive alleles in heterozygous individuals
• The phenotype is the observable characteristics of an organism, which is determined by the genotype (the genetic makeup) and environmental factors

Mendel's Laws of Inheritance

• Gregor Mendel was an Austrian monk who is considered the father of genetics, because he conducted experiments on pea plants to study inheritance
• Mendel proposed several laws of inheritance based on his experiments:
  • Law of Segregation: During gamete formation, the two alleles for each trait separate, so that each gamete receives only one allele
  • Law of Independent Assortment: The alleles of different genes assort independently of one another during gamete formation; this holds true for genes located on different chromosomes or far apart on the same chromosome
  • Law of Dominance: In a heterozygous individual, the dominant allele will mask the expression of the recessive allele

Monohybrid Cross

• A monohybrid cross is a cross between individuals that involves one pair of contrasting traits
• Mendel's experiments with monohybrid crosses led to the formulation of the Law of Segregation
• When Mendel crossed purebred tall pea plants with purebred dwarf pea plants, all the offspring in the first generation (F1) were tall
• However, when he allowed the F1 plants to self-pollinate, the offspring in the second generation (F2) showed a 3:1 ratio of tall to dwarf plants
• This led Mendel to conclude that the trait for tallness was dominant over the trait for dwarfness, and that the alleles for tallness and dwarfness segregated during gamete formation

Dihybrid Cross

• A dihybrid cross is a cross between individuals that involves two pairs of contrasting traits
• Mendel's experiments with dihybrid crosses led to the formulation of the Law of Independent Assortment
• When Mendel crossed pea plants with round, yellow seeds with pea plants with wrinkled, green seeds, the offspring in the first generation (F1) all had round, yellow seeds
• However, when he allowed the F1 plants to self-pollinate, the offspring in the second generation (F2) showed a 9:3:3:1 ratio of round, yellow seeds to round, green seeds to wrinkled, yellow seeds to wrinkled, green seeds
• This led Mendel to conclude that the alleles for seed shape and seed color assorted independently of one another during gamete formation

Sex Determination

• Sex determination is the process by which the sex of an organism is determined
• In humans, sex is determined by the sex chromosomes, X and Y
• Females have two X chromosomes (XX), while males have one X chromosome and one Y chromosome (XY)
• During meiosis, females produce eggs that contain only X chromosomes, while males produce sperm that contain either X or Y chromosomes
• If a sperm containing an X chromosome fertilizes an egg, the resulting offspring will be female (XX)
• If a sperm containing a Y chromosome fertilizes an egg, the resulting offspring will be male (XY)
• Therefore, the sex of the offspring is determined by the sperm

Sex-Linked Traits

• Sex-linked traits are traits that are determined by genes located on the sex chromosomes
• In humans, most sex-linked traits are located on the X chromosome
• Because males have only one X chromosome, they are more likely to express recessive sex-linked traits than females
• Examples of sex-linked traits include color blindness and hemophilia

Evolution

• Evolution is the change in the heritable characteristics of biological populations over successive generations
• It occurs as a result of several processes, including natural selection, genetic drift, mutation, and gene flow
• Natural selection is the process by which individuals with traits that are better suited to their environment are more likely to survive and reproduce, passing on their traits to their offspring
• Genetic drift is the random change in the frequency of alleles in a population due to chance events
• Mutation introduces new traits into a population
• Gene flow is the movement of genes between populations, which can introduce new traits into a population or alter the frequency of existing traits

Speciation

• Speciation is the process by which new species arise
• It occurs when populations of a species become reproductively isolated from one another, preventing them from interbreeding
• Reproductive isolation can occur due to several factors, including geographic isolation, behavioral isolation, and genetic incompatibility
• Over time, the isolated populations may diverge genetically and evolve into distinct species