Lecture 3 - Medelian Genetics

• • Mendel's peas were highly in-bred (so the alleles are homozygous)
  • Their flower structure promoted self-fertilization (therefore they are bred true, true-bred or purebred) e.g - when two tall plants were crossed, they only produced tall progeny
  • Mendel's experiments were designed so that he could study one trait at a time
• What is a gene?

  An inherited factor (encoded in the DNA) that helps determine a characteristic
• What is an allele?


  One of two or more alternative forms of a gene
• What is a locus?

  Specific plane on a chromosome occupied by an allele
• What is genotype?

  Set of alleles possessed by an individual organism
• What is a heterozygote?

  An individual organism possessing two different alleles at a locus
• What is a homozygote?


  An individual organism possessing two of the same alleles at a locus
• What is a phenotype or trait?


  The appearance or manifestation of a characteristic
• What is a characteristic or character?

  An attribute or feature possessed by an organism
• An UPPER CASE LETTER refers to the dominant (expressed factor) allele, lower case to the recessive (latent factor) allele.
• Alleles can be designed after the recessive trait.

  e.g. - a fly with a genetic mutation causes a white eye, whereas the wild-type eye is red.

  The alleles would be w=white eye; W=red eye.

  In some cases, the alleles are designated after the dominant phenotype

  e.g. - the allele R for round seeds; r for wrinkled seeds. You just have to keep them straight.
• The allelic combination is called the genotype
• The physical appearance is called the phenotype
• When peas with two different traits - round and wrinkled seeds are crossed, will their progeny exhibit one of those traits, both of those traits, or an intermediate trait?

  1. To cross different varieties of peas, Mendel removed the anthers from flowers to prevent self-fertilization.
  2. And dusted the stigma with pollen from a different plant
  3. The pollen fertilized ova, which developed into seeds
  4. The seeds grew into plants
• P=Parental Generation
• F1= First Filial Generation
• F2= Second Filial Generation
• In the P generation:

  • Mendel crossed two homozygous varieties of peas (round seeds and wrinkled seeds)
• In the F1 Generation:

  • All the F1 seeds were round. Mendel allowed plants grown from these seeds to self-fertilize
• The F2 generation:

  • 3/4 of the F2 seeds were round
  • 1/4 of the seeds were wrinkled
  • 3:1 ratio
• Analyzing further:

  • Phenotype or trait: seed texture
  • Dominant phenotype: round
  • Recessive phenotype: wrinkled
  • Allele Designation: R for round; r for wrinkled
  • Dominant genotype: RR or Rr
  • Recessive genotype: rr
  • Homozygote: RR or rr
  • Heterozygote: Rr
• Meiosis in the plant

  • Starts as heterozygous diploid cell
  • The position of the wrinkled seed trait on the chromosome is called its locus
  • Recall that meiosis is generating the haploid gametes that are in the anthers and stigma of the plant.
  • One of these haploid gametes from the anther will be combined with one of the haploid gametes in the stigma to produce a diploid seed
• P Generation:

  1. Mendel crossed a plant homozygous for round seeds (RR) with a plant homozygous for wrinkled seeds (rr)
  2. The two alleles in each plant separated when gametes were formed; one allele went into each gamete.
• F1 generation:

  3. Gametes fused to produce heterozygous F1 plants that had round seeds because round is dominant over wrinkled

  4. Mendel self-fertilized the F1 to produce the F2...

  • Gametes: 4 genetically unique daughter cells
• F2 generation:

  5. ...which appeared in a 3:1 ratio of round to wrinkled

  6. Mendel also self-fertilized the F2...
• F3 Generation:

  7. ... to produce F3 seeds

  • homozygous round peas produced plants with only round peas
  • heterozygous plants produced round and wrinkled seeds in a 3:1 ratio
  • homozygous wrinkled peas produced plants with only wrinkled peas
• What are the two principles?

  • Segregation (Mendel's First Law)
  • Independent assortment (Mendel's Second Law)
• What is the observation and state of meiosis under the principle segregation?

  1. Each individual organism possesses two alleles encoding a trait. (Before meiosis)
  2. Alleles separate when gametes are formed (Anaphase I)
  3. Alleles separate in equal proportions (Anaphase I)
• What is the observation and state of meiosis under the principle of independent assortment?

  • Alleles at different loci separate independently (Anaphase I)
• The principle of segregation states that the two alleles of genotype Rr are located on homologous chromosomes which replicate in the S phase of meiosis.
• In prophase I of meiosis, crossing over may or may not take place.
• In anaphase I, the homologous chromosomes separate.
• If no crossing over has taken place, the two chromatids of each chromosome segregate in anaphase II are identical.
• If crossing over has taken place, the two chromatids are no longer identical, and the different alleles segregate in anaphase II.
• What is part 1 of the punnet square?

  Determine the gametes
• What is part 2 of the punnett square?

  Predict the outcome

  This includes:

  • All possible gametes from one parent
  • All possible gametes from the other parent
  • All possible outcomes from crossing the different gametes
  • When we are talking about gametes we are ALWAYS referring to haploids
• What is part 3 of the punnett square?

  Generate the ratios
• Ear lobes (attached or free) is a human genetic trait. Free is considered the dominant trait and attached is considered the recessive trait. Lets call F-free and f-attached. If you have free ear lobes you could be heterozygous Ff or homozygous FF. If your earlobes are attached you are ff.
• The free ear lobe is dominant. That is, it masks the attached lobe in the heterozygote state.
• 1. Mendel's Principle of Dominance: In a heterozygote, one allele may conceal the presence of another.