Unit 6 review Bio

• Steps of Meiosis:

  1. Meiosis I:
     
     

     • Prophase I: Chromosomes condense, homologous chromosomes pair up (synapsis), crossing over occurs.
       
       

     • Metaphase I: Homologous pairs line up at the equator.
       
       

     • Anaphase I: Homologous chromosomes are pulled apart.
       
       

     • Telophase I: Two new cells form, each with half the chromosome number (haploid).
       
       

  2. Meiosis II:
     
     

     • Prophase II: Chromosomes condense again in the two cells.
       
       

     • Metaphase II: Chromosomes align at the equator.
       
       

     • Anaphase II: Sister chromatids are pulled apart.
       
       

     • Telophase II: Four genetically different haploid cells result.
       
       

  Combination of All Alleles in a Population:

  • Gene pool
    
    

  Normal Distribution Curve:

  • Bell-shaped; most individuals fall in the middle (average traits), fewer at extremes (rare traits).
    
    

  Convergent vs. Divergent Evolution:

  • Convergent: Unrelated species evolve similar traits (e.g., dolphins and sharks).
    
    

  • Divergent: Related species evolve different traits (e.g., Darwin’s finches).
    
    

  Coevolution:

  • Two species evolve in response to each other (e.g., flowers and pollinators, predator and prey).
    
    

  Mutations in Body Cells:

  • Not inherited; only mutations in gametes (sex cells) contribute to genetic variation in offspring.
    
    

  Chromosome Terminology:

  • Homologous: Matching chromosomes from each parent.
    
    

  • Sister chromatids: Identical copies of a chromosome connected at the centromere.
    
    

  • Centromere: Region that joins sister chromatids.
    
    

  • Gene location: Specific locus on a chromosome.
    
    

  Meiosis Products:

  • Chromosomes in product: 23 (haploid in humans)
    
    

  • Number of cells: 4
    
    

  • Name of cells: Gametes (sperm or eggs)
    
    

  • Identical or different: Different due to crossing over and independent assortment
    
    

  Steps in Meiosis:

  • Meiosis I: Homologous chromosomes separate
    
    

  • Meiosis II: Sister chromatids separate
    
    

  Crossing Over:

  • Occurs in Prophase I
    
    

  • Increases genetic variation by exchanging segments of DNA between homologous chromosomes
    
    

  Gender Determination:

  • Male determines (XY); sperm can carry X or Y
    
    

  • Female is XX
    
    

  Father of Genetics:

  • Gregor Mendel
    
    

  Patterns of Inheritance:

  1. Complete dominance: One allele completely masks another (Aa = dominant trait)
     
     

  2. Incomplete dominance: Blended phenotype (red + white = pink)
     
     

  3. Co-dominance: Both alleles are expressed (AB blood type)
     
     

  4. Multiple alleles: More than two possible alleles (blood type: A, B, O)
     
     

  5. Sex-linked: Traits on X chromosome (e.g., color blindness)
     
     

  Carrier for Trait:

  • Heterozygous individual who doesn’t express traits but can pass it on.
    
    

  More Likely Carrier:

  • Females, because they have two X chromosomes.
    
    

  Expression of Recessive Sex-Linked Disorders in Females:

  • Must have two recessive alleles (XᵃXᵃ)
    
    

  Testcross:

  • Used to determine genotype of dominant individual by crossing with a homozygous recessive.
    
    

  Dihybrid Cross:

  • Involves two traits (AaBb x AaBb) – 9:3:3:1 phenotypic ratio typical
    
    

  Polygenic Inheritance:

  • Multiple genes influence a trait (e.g., height, skin color)
    
    

  Environmental Effects:

  • Environment can influence phenotype (e.g., nutrition affects height)
    
    

  Homozygous vs. Heterozygous:

  • Homozygous: Two of the same allele (AA or aa)
    
    

  • Heterozygous: Two different alleles (Aa)
    
    

  Karyotype:

  • Picture of chromosomes; shows chromosome number, sex, and large mutations
    
    

  Pedigree:

  • Family tree showing inheritance of traits
    
    

  Genotype vs. Phenotype:

  • Genotype: Genetic makeup (e.g., Aa)
    
    

  • Phenotype: Physical appearance (e.g., tall)
    
    

  Homozygous: Same alleles; Heterozygous: Different alleles
  

Important Individuals in Genetics

• Gregor Mendel: Father of genetics, conducted experiments with pea plants and formulated the Laws of Inheritance (Law of Segregation and Law of Independent Assortment).
  
  

• Thomas Hunt Morgan: A pioneering geneticist who worked with fruit flies and discovered the concept of sex-linked inheritance and genetic linkage.
  
  

• Reginald C. Punnett: Co-developed the Punnett square for predicting genetic outcomes and worked on the study of genetic inheritance in pea plants.
  
  

Mendel’s Pea Plant Experiment:

Mendel’s experiment involved cross breeding pea plants that differed in traits such as flower color, seed shape, and plant height. By examining the ratios of traits in the F1 and F2 generations, he concluded that traits are inherited as discrete units (genes), and each parent contributes one allele for each trait to their offspring. He established the concepts of dominant and recessive alleles and the laws of inheritance.

Genetic Variation and Meiosis:

• Crossing over occurs during prophase I of meiosis, resulting in new combinations of alleles on chromosomes.

• Independent assortment occurs during metaphase I of meiosis, where homologous chromosomes are randomly distributed to gametes, increasing variation.

• Random fertilization occurs when any sperm can fertilize an egg, further increasing genetic variation.

Inheritance Patterns

1. Complete Dominance: One allele completely masks the expression of the other (e.g., TT = tall, Tt = tall, tt = short).
   
   

2. Incomplete Dominance: The heterozygote exhibits a blend of the two alleles (e.g., red + white = pink flowers).
   
   

3. Codominance: Both alleles are fully expressed in the heterozygote (e.g., red + white = red and white stripes).
   
   

4. Dihybrid Cross: Involves two traits (e.g., seed color and seed shape), requiring a 16-box Punnett square.
   
   

5. Multiple Alleles: More than two alleles exist for a gene (e.g., ABO blood types).
   
   

6. Sex-Linked Inheritance: Traits controlled by genes located on sex chromosomes, often X-linked, such as color blindness or hemophilia.
   
   

7. Polygenic Inheritance: Traits controlled by multiple genes, often resulting in a range of phenotypes (e.g., height, skin color).
   
   

8. Environmental Effects: The environment can influence the expression of genes (e.g., temperature-sensitive fur color in Siamese cats).
   
   

Testcross

A testcross is used to determine the genotype of an individual expressing a dominant trait. If the individual is homozygous dominant, all offspring will express the dominant trait. If heterozygous, about half will express the recessive trait.

Practice Problems:

• Punnett Squares: Be able to set up Punnett squares for all types of inheritance (monohybrid, dihybrid, incomplete dominance, codominance, etc.) and calculate genotype and phenotype probabilities.