Understanding Heredity: DNA and Genes

Fundamental Concepts of Heredity

• Heredity involves traits passed from parents to offspring
• Understanding heredity requires knowledge of DNA, chromosomes, genes, and traits

DNA: The Blueprint of Life

• DNA (deoxyribonucleic acid) codes for traits and is a type of nucleic acid
• Nucleotides are the building blocks of DNA
• Nucleotides consist of deoxyribose sugar, a phosphate group, and a nitrogenous base
• DNA includes a sugar-phosphate backbone
• There are four types of nitrogenous bases in DNA: adenine (A), thymine (T), cytosine (C), and guanine (G)
• A pairs with T, and C pairs with G; this base pairing is key for DNA structure and replication
• DNA has a double-helix shape, with two nucleotide strands held by hydrogen bonds
• The sequence of DNA bases codes for traits
• DNA base sequence variations exist between species and individuals
• An organism's entire DNA code resides in the nuclei of nearly all body cells

Genes: Units of Heredity

• Genes are DNA portions
• Genes code for a trait or characteristic
• Many genes code for proteins, influencing traits like eye color pigment
• Some DNA is noncoding, and not all genes code for proteins
• Genes can be turned on/off through gene regulation, not used in all body cells at all times

Chromosomes: Organized DNA

• DNA forms a chromosome when compacted and organized
• Chromosomes are DNA wrapped around protein structures
• Humans have 46 chromosomes in most body cells, arranged in 23 pairs
• 23 chromosomes are inherited from each parent, which makes each pair contains 1 from each parent
• Sex chromosomes are the last pair and determine the sex (XX for females, XY for males)
• Autosomes are all non-sex chromosomes

Alleles: Variations of Genes

• Alleles are different versions/forms of a gene
• Individuals inherit two alleles per gene, one from each parent
• Two alleles for a trait (at a locus on a chromosome) can be the same or different

Mendelian Inheritance: Dominant and Recessive Alleles

• Alleles can be dominant or recessive
• A dominant allele is expressed with at least one copy in the genotype, and are represented by capital letters
• A recessive allele is expressed only with two copies and no dominant allele, are represented by lowercase letters
• Genotype is an individual's genetic makeup (allele combination), such as TT, Tt, and tt
• Phenotype refers to an expressed physical characteristic based on genotype (e.g., PTC tasting)
• Homozygous genotypes have two identical alleles (e.g., TT or tt)
• Heterozygous genotypes have two different alleles (e.g., Tt)
• The ability to taste PTC shows that tasting PTC is a dominant trait (T), and the inability to taste PTC is a recessive trait (t)
• Individuals with TT or Tt genotypes can taste PTC
• Individuals with tt genotype cannot taste PTC
• Parents that taste PTC(Tt) can have a child who cannot (tt)

Non-Mendelian Inheritance: Beyond Simple Dominance

• Non-Mendelian inheritance patterns exist
• Incomplete dominance: a dominant allele is not fully expressed, resulting in an intermediate phenotype, such as pink snapdragons (RR x rr = Rr)
• Codominance: Both alleles are fully expressed simultaneously, such as speckled chickens (BB x WW = BW)
• Polygenic Traits: Many genes contribute to one trait, such as human height and skin color, interaction of genes determine phenotype; environmental factors can also have an influence
• Epistasis: One gene depends on another gene's expression; in llamas, a wool color gene might not be expressed if an epistatic gene causes no pigment production, resulting in albino
• Multiple Alleles: Some genes have more than two alleles in the population, although an individual still inherits only two; the ABO blood type system in humans is an example, with three alleles that determine the four phenotypes(A,B, AB, and O)
• Sex-Linked Traits: Genes for these traits are located on the sex chromosomes, most commonly the X chromosome because it is larger and contains more genes than the Y chromosome
  • In sex-linked recessive traits, a female needs two copies of the recessive allele (on both X chromosomes) to express the trait, such as XhXh for hemophilia
  • Males only need one copy of the recessive allele on one X chromosome to express the trait, (XhY); recessive sex-linked traits are more common in males
  • Females with one recessive and one dominant allele are carriers, such as XHXh for hemophilia
  • In sex-linked dominant traits, only one copy of the dominant allele on the X chromosome is needed for the trait to be expressed in both males and females, such as XDXD, XDXd, and XDY

Important Considerations

• Many traits, especially disorders, are influenced by multiple genes and may not follow Mendelian patterns
• Environmental factors can influence trait expression, such as nutrition affecting height, sun exposure affecting skin color
• Unless there are clues, don't assume a non-Mendelian inheritance pattern; introductory problems usually are autosomal with simple dominance
• Inheritance patterns vary in different organisms; birds have Z and W sex chromosomes; some animals have different sex chromosome numbers

Heredity and the Role of DNA

• Heredity is the passing of traits from parents to offspring; understanding the roles of DNA, chromosomes, and genes explains how traits are inherited, determined by genes on chromosomes

The Structure and Function of DNA

• DNA carries genetic instructions for an organism's development, functioning, growth, and reproduction
• DNA's basic structure is a double helix resembling a twisted ladder, composed of two nucleotide strands
• Each nucleotide consists of a deoxyribose sugar, a phosphate group, and a nitrogenous base
• The sugar and phosphate molecules form the backbone of each strand
• Bases pair up in the middle (A with T, C with G) held by hydrogen bonds

Genes and Their Role in Traits

• Genes are specific DNA segments with instructions for building and maintaining an organism
• Genes code for proteins and influence traits like eye color or the ability to taste PTC
• Some genes are non-coding but have regulatory roles
• The DNA base sequence determines the specific functional RNA molecule or resultant protein is produced

Chromosomes: Organization of DNA

• Chromosomes are highly organized DNA structures wound around proteins
• Chromosomes condense and organize DNA for managing cell division
• Humans have 46 chromosomes in body cells, arranged in 23 pairs
• One chromosome is inherited from each parent to ensure offspring receives half of their genetic information from each parent
• Genes are located at specific positions (loci) on chromosomes

Alleles and Genotype vs. Phenotype

• Alleles are different versions of a gene
• Each individual inherits two alleles per gene, one from each parent
• Genotype is the allele combination, representing the individual's genetic makeup for a trait (e.g., TT, Tt, tt for PTC tasting)
• Phenotype is the observable physical or biochemical characteristic resulting from the genotype (e.g., ability to taste PTC)
• Allele interaction determines which phenotype is expressed

Mendelian Inheritance: Dominant and Recessive Traits

• In simple Mendelian inheritance, some alleles are dominant and will be expressed if present
• Recessive alleles will only be expressed when two copies are present
• Tasting allele 'T' is dominant, and the non-tasting allele 't' is recessive for PTC tasting
• Individuals with TT or Tt genotypes can taste PTC
• Only those with tt genotype can not taste PTC
• Those who taste PTC(if they are heterozygous, Tt) can have a child who cannot (tt)

Beyond Mendelian Inheritance: Non-Mendelian Patterns

• Non-Mendelian inheritance has several types:
  • Incomplete dominance occurs when the dominant allele is not fully expressed, resulting in an intermediate phenotype, typically pink snapdragons from red and white parents
  • Codominance happens when both alleles are fully expressed, such as speckled chickens with both black and white feathers
  • Polygenic traits are influenced by multiple genes, such as human height and skin color.
  • Epistasis is when one gene affects another gene for pigment color.
  • Multiple alleles exist when a gene has more than two alleles, such as ABO blood types
  • Sex-linked traits are genes located on the sex chromosomes, such as hemophilia and colorblindness in X chromosome

Sex-Linked Inheritance and its Implications

• Sex-linked traits are determined by genes on the sex chromosomes, primarily the X chromosome
• Recessive sex-linked traits often appear more frequently in males
• Males have only one X chromosome (XY) that will be expressed if the recessive allele is inherited
• Females have two X chromosomes so a recessive trait will only be expressed if they inherit two copies
• Females with only one copy are carriers and can pass the recessive allele to offspring