Biol 114 Spring 2025 Week 2 (1/30) Cell Division and Heredity

Genetic Heritability

• Heritability of phenotypes is key to understanding natural selection as "descent with modification".
• Darwinian evolution requires heritability; without it, selected variants would be lost.
• Darwin acknowledged a lack of understanding regarding variation and trait inheritance.
• Phenotypes (measurable traits) are caused by genotypes (genes).
• Genetic variation encodes for phenotypic variation, which allows the environment to select for or against members of a population.
• Genes are "selfish" and aim to replicate themselves (Richard Dawkins, The Selfish Gene).
• Phenotypes serve as a mechanism for genes to get to the next generation; "instinct" is genes using phenotypes for self-preservation.

Genes

Definitions:

• Genome: All hereditary information within an individual (including non-gene DNA).
• Gene pool: All alleles of all genes within a population.
• Genotype: All alleles of all genes within an individual, or a specific set under study.
• Genes: DNA sections on chromosomes encoding for a polypeptide, causing a trait (phenotype), and may regulate other genes.
• Locus: The location of a gene on a chromosome.
• Alleles: Particular versions of a gene at the same locus on homologous chromosomes.

Does Genotype Really Equal Phenotype?

• Similar genotypes can result in different phenotypes due to environmental pressures.
  • Example: Identical plants in different light conditions.
  • Identical twins have slight differences due to developmental environments affecting regulator genes.
• Similar phenotypes can result from different genotypes.
  • Example: Convergent evolution of gray wolf and Tasmanian wolf.
    • Tasmanian wolf (marsupial) and true wolves share similarities like sharp teeth, powerful jaws, raised heels, and body form, despite distant common ancestry.
• Truly identical phenotypes require identical genotypes and homogeneous developmental conditions, nearly impossible outside of lab settings.

Variation in Eukaryotes

Characteristics:

• Genes are carried on chromosomes (DNA and proteins).
• Chromosomes occur in sets; species have multiple chromosomes within a set.
• Eukaryotes may have multiple sets of chromosomes; ploidy refers to the number of sets.
• Homologous pairs of chromosomes: one copy from each parent (maternal/paternal) in the same nucleus (2n = diploid).
  • They have the same genes but can differ in gene expression based on alleles.
• Individuals may be haploid (1n) or diploid (2n).
  • Examples: ants, bees, and wasps (haploid males, diploid females); mosses (mostly haploid); organisms alternating between sexual (haploid) and asexual (diploid) generations (Alternation of Generations).
• Dominance: one allele is expressed even with another (recessive) allele present.

Meiosis

• Meiosis: production of gametes (egg and sperm) by halving chromosome number.
• Haploid gametes join to create a diploid zygote, which develops into a reproductive individual.
• Meiosis involves two cell divisions (Meiosis I and II) to increase variation.
  • Homologous pairs exchange genetic material during early meiosis I.
  • Pairs of chromosomes (sister chromatids) come together with homologues (one from each parent).
  • The mechanism of attraction is unknown.
  • Resulting tetrad consists of four chromatids: two sisters from the paternal homolog, and two sisters from the maternal homolog.
  • Chromatids from parental homologs (maternal and paternal) are referred to as non-sister chromatids.
• Tetrad: structure formed when homologous chromosomes pair during Prophase I of meiosis.
  • Consists of 4 chromatids: 2 sister chromatids from the paternal chromosome, 2 sister chromatids from the maternal chromosome.
  • Homologous chromosomes align and undergo crossing over, exchanging DNA between non-sister chromatids.

Areas of Variation in Meiosis:

1. Non-sister chromatids link at a chiasma and exchange genetic information via crossover.
2. Tetrads migrate and align randomly along the center of the cell during meiosis I; daughter cells can have all paternal, all maternal, or a combination of homologs.
3. Sister chromatids separate into gametes with a single copy of each gene during meiosis II; alignment before separation is random.
4. Mate selection is another source of variation.

Other Sources of Variation:

1. Mutations from nucleotide sequence changes.
   • Caused by environmental mutagens (radiation, chemicals).
   • Most mutations are not deleterious, occurring in junk DNA and not expressed; used to measure genetic relationships of taxa.
   • Expressed mutations are usually harmful and removed by natural selection.
2. Mutations from replication errors occur and are expressed more frequently.
3. Nondisjunction: cell ends up with both pairs of homologs of one chromosome (meiosis I) or both sister chromatids (meiosis II).
   • Results in gametes with an extra chromosome (n+1) or one too few (n-1).
     • Example: Down Syndrome is trisomy 21 (3 copies of chromosome 21).
       n-1 is monosomy.
   • Too many or too few chromosomes are referred to as aneuploidy.
   • Happens in 10% of meiotic divisions, but most end as fetal death.
4. Unequal crossing over: one arm of a chromatid ends up longer than its sister.
   • Example: Huntington’s Disease

Asexually Reproducing Organisms

The Problem:

• Asexual organisms (e.g., bacteria) reproduce by splitting (similar to mitosis).
• Mature cells are haploid. They copy their genetic makeup and split.
• Each generation would look the same, meaning sexual reproduction and different types of asexual reproduction would not have evolved.

Sources of Variation in Asexual Organisms:

• Mutations can occur.

Transformation:

• Bacterial cells take up DNA fragments from other cells and incorporate them into their own DNA or as a separate plasmid.
• Used in the lab to create modified organisms (e.g., golden rice, oil-eating bacteria).

Transduction:

• Viral capsids attach to bacterial cells and inject their DNA, fragmenting the host DNA.
• Host cell lysis releases fragments into the environment.
• Other bacterial cells encounter capsids with viral DNA, viral + bacterial DNA, or just bacterial DNA; new bacterial DNA replicates with the new host DNA.

Conjugation:

• Cytoplasmic tube forms between cells; a plasmid is transferred from a cell with a plasmid to one without.
• Sometimes just a fragment of the plasmid is transferred, introducing a new variant.