Unit 5: Heredity

• Genetics - the study of Heredity

• Foundation of genetics is that traits are produced by genes found on chromosomes

• Diploid organisms carry two copies of every gene, one on each of two homologous chromosomes, one that came from their mother and one that came from their father

• Homozygous - same alleles

• Heterozygous - different alleles

• Genes - genotype of an organism

  • Physical expression of the traits is the phenotype

• The allele in heterozygous individuals that directs the phenotype is the dominant allele, and the one that doesn’t show is the recessive allele

• Transformation - When a cell (typically when we ) takes in foreign genetic material and incorporates it, resulting in genetic alteration

• Transduction - Specific to bacteriophages and how they use bacteria to reproduce

• Some viruses have reverse transcriptase

  • Enzyme that defies the central dogma of biology

  • Takes mRNA and brings it back to DNA

• Whenever the question says “wild type“ it means nonmutant DNA

🧪 Labs

• pglo

  • Take a gene and insert it to e coli plasmid DNA

  • Chromosomal DNA (bacteria) - single loop of DNA

  • Plasmid DNA - can be single stranded or double stranded

    • Generally smaller than Chromosomal DNA

• Gel Electrophoresis

  • Used to compare the DNA of two individuals

  • We take two samples (liquids) containing the DNA of the two individuals

    • Add restriction enzymes to cut up the DNA

  • put the samples in wells on one side (negative)

  • The DNA will go to the other side (positive) across the gel because DNA is negatively charged due to the Sugar-Phosphate backbone

  • Some DNA will be faster than others

    • Longer sections of DNA are slower, and will lag behind

  • The DNA will split into different “bands“ across the gel based on how fast they are

  • The “bands“ can be used to compare if the DNA is the same or different

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🌱 Mendelian Genetics

• Simple genetics with dominants and recessives was illuminated by Gregor Mendel

• Three laws:

  • Law of Dominance

  • Law of Segregation

    • Each gamete gets only one of two copies

    • Gametes are haploid

  • Law of Independent Assortment

    • How one trait gets split up into gametes has no bearing on how the other trait gets split up

    • This is because chromosomes segregate independently, homologous chromosomes are split left or right during Meiosis I

    • Alleles of different genes can mix and match

    • Use Dihybrid cross punnet square (4×4 looking thing)

      • Memorize the phenotype ratios ex:

      • 9 tall and green

      • 3 tall and yellow

      • 3 short and green

      • 1 short and yellow

      • 9:3:3:1 only when two heterozygotes for two genes are crossed

      • 6:3:3:2:1:1 when there is incomplete dominance & complete dominance on a dihybrid cross

• Monohybrid Cross - when two individuals are crossed and one gene is being studies

  • Use punnet squares

• Humans have 23 pairs of homologous chromosomes for a total of 46

  • 22 pairs are autosomes and 2 pairs are sex chromosomes

    • Females have XX, males have XY

    • Because females have two X chromosomes, one is inactivated in each cell and condenses into a Barr body

  • Most sex-linked traits are on the X-chromosome “X-linked“

  • Barr bodies - one of the two X linked chromosomes in each female cell is inactivated, creating this

🩸 Blood types

• 4 blood types:

  • Type A

  • Type B

  • Type AB

  • Type O

• A and B are codominant

• O is recessive

👁 Non-Mendelian Genetics

• Non-Mendelian genetics refers to a situation in which traits do not follow the Mendelian laws

• Can be due to being governed by more than one gene

• If two genes are on the same chromosome and close together, they are linked

  • cannot segregate independently, violating the law of independent assortment

• Completely linked alleles on the same chromosome are locked together and travel together during gamete formation

• Crossing over can separate linked alleles, though unlikely

  • separation of linked alleles leads to more predictable inheritance patterns with minimal variability

• Crossing over can also lead to recombination, allowing for new genetic combinations that seems like it wouldn’t be possible due to linked alleles

• Recombination frequency - can be used to measure how far apart two genes are on a chromosome

  • can be calculated by adding up the recombinants and dividing by the total number of offpsring

    • Ex: two linked genes A and B recombine with frequency of 9/90 = 10%

    • Distance on a chromosome is measured in map units/centrimorgans

    • Ex above: A and B are 10 units apart

• Offspring formed form recombination events (crossing over)

  • Ex of Non Mendelian Genetics:

    • incomplete dominance

      • Traits blend together to produce an intermediate phenotype

    • codominance

      • heterozygous alleles in a gene pair are simultaneously expressed

      • neither dominant nor recessive

    • polygenic inheritance

      • Traits controlled by many genes

        • Ex: our height is controlled by over 400 genes

👪 Pedigrees

• Traits that appear more in one sex than the other are usually sex-linked

• Traits that skip generations are usually recessive

🧬 Meiosis

• AKA gametogenesis

  • If sperm is produced, it is called spermatogenesis (oogenesis if egg is produced)

  • 4 cells are produced for each diploid cell

• Meiosis produced four genetically distinct haploid gametes (sperm or egg cells)

  • Involved two rounds

  • Meiosis I - homologous chromosomes separate, and cross over some DNA

    • Prophase I

      • Crossing over takes place

    • Metaphase I

      • Law of independent assortment takes place

    • Anaphase I

      • Law of segregation takes place

    • Telophase I

  • Meiosis II - sister chromatids separate

    • Prophase II

    • Metaphase II

    • Anaphase II

    • Telophase II

  • Mutations, such as nondisjunction events or whole chromosome translocations, can lead to genetic diseases

• Meiotic errors

  • Nondisjunction - chromosomes failed to separate properly during meiosis, producing the wrong number of chromosomes

  • Translocation - segment of a chromosome moves to another non homologous chromosome

    • Occurs when crossing over (recombination) occurs incorrectly

Mitosis vs. Meiosis

• Mitosis

  • Occurs in somatic (body) cells

  • Produces identical cells

  • Diploid cell → diploid cells

  • 1 cell becomes 2 cells

  • Number of divisions: 1

• Meiosis

  • Occurs in germ (sex) cells

  • Produced gametes

  • Diploid cell → haploid cells

  • 1 cell becomes 4 cells

  • Number of divisions: 2