Topic 10: Genetics and Evolution (Notes) Flashcards
Purpose of mitosis
- To separate the genome and produce two daughter cells identical to the parent cell
- Required for growth, repair, asexual reproduction
Purpose of meiosis
- To create gametes/sex cells
- Allows production of four genetically distinct haploid daughter cells from a diploid parent cell
- Used to fulfill specialization (e.g. sperm cells -> sperm production)
Define synapsis
When two homologous chromosomes align alongside each other
Define homologous chromosomes
A set of one maternal and one paternal chromosome
Define bivalent
Composed of two chromosomes
Define tetrad
Composed of four chromatids
Define crossing over
The exchange of DNA material/DNA material between non-sister homologous chromatids
Explain the process of crossing over
- Prophase I: Homologous chromosomes pair up, in close proximity
- Non-sister chromatids get entangled and cross over
- Entanglement places stress on DNA molecules
- A section of chromatids from one chromosome may break and re-join with chromatid from other chromosome
- Catalyzed by endonuclease and DNA ligase
Define recombination and recombinant
Recombination
- Process involving breaking and re-joining of DNA to create new combinations of genetic information
Recombinant
- Chromosomes that exchanged DNA through breaking and re-joining of DNA
Define chiasma
Crossing point of DNA strands, developing an X-shaped join
Why are there an infinite range of DNA combinations during crossing over?
- Random nature of how chromatids align and where they break
- No two individuals will have exactly the same genotype (except identical twins)
What happens in meiosis I?
Homologous chromosomes separate
Explain prophase I
- DNA replicated and condensed, visible as chromosomes
- Each chromosome has two sister chromatids joined by a centromere
- Chromosomes arranged side-by-side in homologous pairs
- Centrioles migrate to opposite poles, form spindle
- Nuclear envelope breaks down, nucleolus disintegrates
Explain metaphase I
- Bivalents line up along spindle equator
- Spindler fibers attach to centromeres
- Bivalents line up by independent assortment (random orientation)
Explain anaphase I
- Homologous pairs separated
- Microtubules pull whole chromosomes to opposite ends of spindle
- Centromeres do not split
Explain telophase I
- Chromosomes arrive at opposite poles
- Spindle fibers start to break down
- Nuclear envelopes form around two groups of chromosomes, nucleoli reforms
- Some plants go straight to meiosis II without nucleus reformation
Why is meiosis I reduction division?
- Homologous chromosomes separate and move to opposite poles of the cell
- No. of chromosomes per cell reduced by factor 2
Define independent assortment/random orientation
- Production of different allele combinations in gamete cells
- Due to metaphase I: random alignment of homologous pairs along the equator of spindle
Explain prophase II
- Nuclear envelope breaks down
- Chromosomes condense
- Spindle forms at right angle to old ones
Explain metaphase II
- Chromosomes line up in single file along equator of spindle
Explain anaphase II
- Centromeres divide
- Individual chromatids pulled to opposite poles
- Sister chromatids separate
- Creates four groups of chromosomes with half the number of parent cell chromosomes
Explain telophase II
- Nuclear membranes form around each chromosome group
Explain telophase II
- Nuclear membranes form around each group of chromosomes
- Followed by cytokinesis: cytoplasm divides as new cell surface membranes form to create four haploid cells
Define unlinked genes
Genes of an organism carried on separate chromosomes, not on homologous copies
Define monohybrid cross
How alleles of one gene transfers across generations
Define dihybrid cross
- How alleles of two genes transfer across generations
- For double-heterozygous dihybrid crosses, expect 9:3:3:1 phenotypic ratio
Define loci
- The specific linear position on the chromosomes that genes occupy
- Sex-linked if gene is on sex chromosome
Define sex-linked
- Only affects one gender of a species
- Usually on X chromosome because Y chromosome contains fewer genes
- E.g. color-blindness and hemophilia affects mostly males
Define autosomal linkage
- Linked genes on chromosomes 1-22, not on sex chromosome
- Not all genes assort independently
Define interspecific and intraspecific variation
- Variation: the way organisms differ from one another
- Interspecific: Occurs between species
- Intraspecific: Occurs within the same species
Define discrete variation
- Individuals fall into clear-cut categories, no in-between
- Occurs due to genetic factors
- E.g. human blood groups
Define continuous variation
- Two or more genes affect the final characteristics
- Occurs due to interaction between genetics and environment
- E.g. height, determined by bone length, hormones, diet, etc.
Define gene pools
- Consists of all genes and different alleles in an interbreeding population
- Multiple gene pools can exist (geographically isolated)
Define allele frequencies
- Relative abundance of alleles for a particular gene
- Formula: No. of times allele of interest observed divided by Total no. of all alleles
Outline conditions of a stable gene pool
- Large population
- Equal chance of mating
- Random matings
- No selective pressure based on phenotype
Define evolution
Cumulative change in the heritable characteristics of a population
Outline factors for evolution
- Mutation: causes new alleles to form
- Selection pressures: favor certain alleles and oppose others
Define directional selection
- Population changes towards one extreme (better adapted)
- Happens with environmental changes
Define stabilizing selection
- Selects in favor of the average individual
- Happens in stable environmental conditions
- Favor individuals with advantageous alleles
- Discards extreme phenotypes
- Works mostly on polygenic traits, when characteristics influenced by >=2 genes
Define disruptive
- Selects against the average individual
- Can cause speciation and forms new species
Define reproductive isolation
Isolated from peers due to barriers, preventing inter-breeding
Define temporal reproductive isolation
- reproductive cycles at different times (seasonal)
- e.g. changes in flowering patterns
Define behavioral reproductive isolation
- Different rituals or behavioral patterns
- e.g. courtship and mating rituals
Define geographical reproductive isolation
- Natural or man-made barriers between populations
- e.g. rivers, mountains
Define sympatric
- temporal and behavioral speciation
- separately-developed species coexist in the same geographical location
Define allopatric
Two separate species diverge with complete spatial separation
Define speciation
The emergence of new and distinct species, reproductively isolated
Explain the theory of gradualism speciation
- Culmination of small changes over time
- Darwin observed vestigial structures in fossil records
- observable characteristics with no apparent function
- residual parts from ancestors, e.g. human appendix
Explain the theory of punctuated speciation
- abrupt speciation
- long periods without appreciable change, short rapid evolution
- Bateson observed sudden mass extinctions from cataclysmic events
- survivors restart reproduction with reduced gene pool (founder effect)
Define polyploidy
- when an organism has more than two sets of homologous chromosomes
- results from chromosomal mis-events e.g. mitosis or meiosis abnormalities
- e.g. egg fertilized by more than one sperm, failure of chromosomes separation in meiosis I
- can form diploid gamete and fertilize with haploid gamete to produce fertile offspring
