Biology p 287-298 Genetic Linkage and Recombination

• In eukaryotic organisms like humans, genes reside on chromosomes within the nucleus and on DNA inside mitochondria.
• Understanding inheritance requires considering chromosome and mitochondrion transmission, as well as factors influencing chromosome structure and function.

Determination of Sex

• In mammals, genotypic sex is determined by a combination of sex chromosomes.
• Females inherit one X chromosome maternally and one X chromosome paternally, resulting in an XX pair.
• Males inherit one X chromosome maternally and one Y chromosome paternally, resulting in an XY pair.
• The Y chromosome holds fewer genes and is structurally shorter than the X chromosome.
• SRY gene, present on the Y but not X chromosome, is essential for mammalian sex determination.
• Expression of SRY leads to the development of male gonads (testes).
• SRY protein presence induces male phenotype development in fetuses.
• Without the SRY protein, fetuses develop a female phenotype.

Sex-Linked Traits

• Sex-linked inheritance arises from genes located on sex chromosomes (X and Y).
• Individuals with two X chromosomes typically exhibit female characteristics.
• Individuals with one X and one Y chromosome typically exhibit male characteristics.
• XX and XY individuals show different inheritance patterns for sex-linked traits.
• Recessive X-linked traits are more common in XY individuals because the Y chromosome cannot mask their expression.
• XX individuals are carriers if they inherit only one copy of a recessive X-linked allele.
• XX individuals must inherit the recessive alleles on both X chromosomes to exhibit a recessive X-linked trait.
• Y-linked traits are rare, appearing exclusively in individuals with a Y chromosome, due to the small number of genes unique to it.

Genetic Linkage

• Alleles on different chromosomes assort independently during meiosis.
• Genes close together on the same chromosome are linked and do not assort independently into gametes.
• Genetic linkage describes the tendency of alleles on the same chromosome to remain together.
• Linked alleles only separate during meiosis if a crossing over event occurs.
• The closer genes are on a chromosome, the less frequent crossing over is and the more tightly linked they are.

Gene Mapping

• Homologous chromosomes can exchange genetic information via crossing over or recombination during prophase I of meiosis.
• Crossing over generates new allele combinations by moving alleles between maternal and paternal copies.
• New combinations of alleles are recombinant; original combinations are parental.
• Genes close together on a chromosome are unlikely to be separated by recombination due to the small distance between them.
• Recombination frequency indicates physical distance between two genes on a chromosome.
• Gene maps, depicting relative gene positions, are constructed using recombination frequencies, with distances reported in map units (centimorgans).
• One map unit equals a 1% recombination frequency.

Biometry

• Analyzing biological data involves statistical and mathematical methods, known as biometry or biostatistics.
• Biometry is crucial for experimental design and data analysis in biological sciences.
• It has been important in genetics since Gregor Mendel's quantitative approach in the mid-1800s.
• The chi-square test is a statistical test used in biological sciences to evaluate a null hypothesis.

Extranuclear Inheritance Patterns

• Mitochondria are cellular organelles with their own DNA, containing few genes compared to nuclear chromosomes.
• Mitochondrial genes are extranuclear, inherited differently from nuclear genes.
• Sperm mitochondria are eliminated during fertilization, so mitochondrial traits are maternally transmitted.

DNA Mutations

• Mutations are alterations in DNA sequences resulting from errors in DNA replication, faulty DNA repair, and exposure to mutagens.
• Mutations are either point mutations, which alter a single nucleotide pair, or chromosomal mutations, which affect larger DNA portions.
• Frameshift mutations result from nucleotide insertion or deletion (not multiples of three), altering the reading frame.
• Missense mutations involve nucleotide substitutions that change one amino acid for another.
• Nonsense mutations involve a nucleotide change which leads to the formation of a stop codon.
• Silent mutations involve nucleotide substitutions that do not alter the amino acid sequence.
• Deletion mutations remove a DNA section from a chromosome.
• Duplication mutations repeat a section of DNA on a chromosome.
• Inversion mutations occur when a DNA section breaks off, reattaches in reverse orientation.
• Translocation mutations occur when a DNA section breaks from one chromosome and attaches to another.
• Chromosomal mutations, unlike point mutations, can disrupt numerous genes and are generally harmful.
• Some chromosomal mutations (e.g., inversions, translocations) may not directly cause loss/gain of genetic information, but can alter gene expression.
• Gene position relative to other genes and control elements can affect its expression, leading to phenotypic changes.

Mutagens

• Mutations are changes to the nucleotide sequence of an organism's genome; they can occur spontaneously or be caused by mutagens.
• Mutagens can be physical, chemical, or biological agents.
  • Physical mutagens: Heat and radiation (e.g., X-rays, gamma rays, UV light) which cause DNA damage like strand breaks and pyrimidine dimers.
  • Chemical mutagens: Substances that react directly with DNA, causing structural changes, or base analogs that are incorporated into DNA, causing mispairing and frameshift mutations.
  • Biological mutagens: Viruses (e.g., human papilloma virus) or bacteria (e.g., Helicobacter pylori), viruses insert their genome into the host cell; bacterial infections cause chronic inflammation. Movement of mobile genetic elements like transposons or retrotransposons also can cause mutations.

Somatic and Germline Mutations

• Animal cells are either germ cells (reproductive) or somatic cells (all other cells).
• Germ cells undergo meiosis, which leads to the production of gametes.
• Germline mutations, which occur in parental germ cells, get passed to offspring via gametes during sexual reproduction to form a zygote.
• Somatic mutations occur in somatic cells, and they don't get passed to offspring.

Outcomes of Mutations

• Mutations are changes in the nucleotide sequence of DNA molecules that can have varying effects on organisms.
• Natural selection is an evolutionary mechanism.
• Adaptive traits that increase fitness are more likely to get passed on.
• Beneficial mutations (found in adaptive alleles) become more common over time.
• Detrimental mutations become less common.
• Inborn errors of metabolism are genetic disorders caused by detrimental mutations in metabolic enzyme genes, leading to reduced enzyme activity.
• Decreased enzyme activity leads to build up of upstream metabolites, decreased downstream products, and negative clinical effects such as lethargy and impaired development.

Explore the principles of genetic linkage, recombination frequency, and their impact on inheritance. Understand how crossing over affects allele inheritance and gene mapping. Learn about X-linked and Y-linked traits.