GENETICS 1-4

• Diploid


  Nuclei possessing pairs of homologous chromosomes (2n)
• Diploid cells


  • Possess two gene copies (alleles) for each trait
  • All somatic (body) cells in the organism will be diploid
  • New diploid cells are created via mitosis
  • Present in most animals and many plants

  
• Haploid


  Nuclei possessing only one set of chromosomes (n)
• Haploid cells


  • Possess a single gene copy (allele) for each trait
  • All sex cells (gametes) in the organism will be haploid, and are derived from diploid cells via meiosis
  • Present in bacteria (asexual) and fungi (except when reproducing)

  
• Sexually reproducing organisms inherit their genetic sequences from both parents
• Homologous chromosomes


  Maternal and paternal chromosome pairs that share the same structural features and the same genes at the same loci positions (while the genes are the same, alleles may be different)
• Genetics is the branch of biology that deals with heredity and variation of organisms
• Chromosomes carry the hereditary information (genes)
• Genetic information flow


  DNA -> RNA -> Proteins

  
• Chromosomes (and genes) occur in pairs
• New combinations of genes occur in sexual reproduction
• Mendel published Experiments in Plant Hybridization


  1866
• Mendel established his three Principles of Inheritance
• Mendel's work was largely ignored for 34 years, until 1900, when 3 independent botanists rediscovered it
• Mendel was the first biologist to use mathematics to explain his results quantitatively
• Mendel predicted the concept of genes that genes occur in pairs and that one gene of each pair is present in the gametes

  
• Monohybrid cross


  Parents differ by a single trait
• Punnett square


  A useful tool to do genetic crosses and predict the genotypes and phenotypes of the offspring

  
• Steps in using a Punnett square

  1. Determine the genotypes of the parent organisms

  2. Write down the "cross" (mating)

  3. Draw a Punnett square

  4. Split the letters of the genotype for each parent and put them "outside" the Punnett square

  5. Determine the possible genotypes of the offspring by filling in the Punnett square

  6. Summarize the results (genotypes & phenotypes of offspring)

  
• The secret to the Punnett square is determining the gametes of each parent
• Possible Punnett square results

  • All heterozygous - 1:2:1
  • All homozygous - 4
  • 1 homozygous, 1 heterozygous - 2:2

  
• Mendel's Principle of Dominance


  One allele masks another, one allele is dominant over the other in the F1 generation
• Mendel's Principle of Segregation

  When gametes are formed, the pairs of hereditary factors (genes) become separated, so that each sex cell (egg/sperm) receives only one kind of gene
• Mendel's Principle of Independent Assortment


  Members of one gene pair segregate independently from other gene pairs during gamete formation, allowing for many gene combinations
• Dihybrid cross


  Matings that involve parents that differ in two genes (two independent traits)
• There's a correlation between the movement of chromosomes in meiosis and the segregation of alleles that occurs in meiosis
• Karyotype


  46 total human chromosomes, 23 homologous pairs
• Chromosome types


  • Autosomes (first 22 homologous pairs)
  • Sex chromosomes (23rd pair)
• Autosomal recessive trait


  Requires two recessive alleles to be present to express the trait

  
• Autosomal dominant trait


  If the dominant allele is present, the individual will express the trait
• Genotypes for autosomal recessive traits

  • aa (affected)
  • AA or Aa (unaffected, carrier)

  
• Genotypes for autosomal dominant traits

  • AA and Aa (affected)
  • aa (unaffected)

  
• Close relatives who reproduce are more likely to have affected children with autosomal recessive traits

  
• Both males and females are affected with equal frequency for autosomal recessive and dominant traits
• Sex-linked traits


  Produced by genes only on the X chromosome, can be dominant or recessive
• Pedigree


  Shows the relationship between parents and children over generations and how a trait is passed down

  
• More males than females are affected by sex-linked recessive traits
• Sex linked Dominant trait


  Trait that is expressed even if only one copy of the gene is present
• Sex linked Recessive trait


  Trait that is only expressed if both copies of the gene are present

  
• More males than females are affected by sex-linked traits