Week 4

• This figure only shows what happens to one homologous pair during meiosis 1. 1 of 23 pairs in human cells

  
• Phenotype


  • Turner syndrome
  • Biologically female
  • Breasts and gonads are underdeveloped
  • No menstruation
• Mendelian Genetics - Vocab


  • Gene - A distinct DNA sequence and part of a chromosome that encodes a protein
  • Allele - Different versions of a gene usually denoted by different letters or upper- and lower-case letters
  • Dominance - Refers to a phenotype that is expressed to the exclusion of the other even if the organism only contains one copy of the associated allele
  • Recessive - Refers to a phenotype that is only expressed when there are two copies of the associated allele
  • Homozygous - Refers to a genotype that has two identical alleles of a particular gene
  • Heterozygous - Refers to a genotype that has two different alleles of a particular gene

  
• Every time an organism makes haploid gametes (eggs or sperm), only one of the two copies of each gene winds up in each gamete

  
• It's a "flip of the coin" because, in meiosis I, the direction in which the long dark blue chromosome homolog moves is independent of which direction the short dark blue chromosome moves

  
• Each individual outcome is unique and based on random distribution during metaphase I
• The rules of mathematics and probability determine the chances of particular outcomes
• Key Features


  • The plants used were true breeding strains - The appearance (phenotype) of the offspring is identical to the parental
  • 7 physical features (traits) of the garden pea (Pisum sativum) were studied one or several traits at a time and these traits are associated with a single gene. Note: Traits due to single genes are rare
  • The offspring (progeny) of the experiments were counted so that phenotypic and genotypic ratios could be determined
  • Example important observation: The trait that appeared in the F1 generation (first filial generation) is dominant and the trait that did not appear is recessive

  
• The ratio of dominant to recessive phenotypes in the F2 generation was 3:1 in all seven traits Mendel studied
• The Principle of Segregation from Mendel's Experiments


  When the F1 generation (progeny) with the genotype Aa form gametes, by the principle of segregation the A and a alleles separate, so that half the gametes contain only the A allele and the other half contain only the a allele

  
• Testcross: By crossing with the true-breeding recessive strain you can learn the genotype of the unknown parent
• Starting cell before meiosis


  • There are 8 chromosomes or chromatids but the chromosomes are not replicated.
• Daughter cells after meiosis I


  • This cell shows 3 pairs of chromatids/chromosomes that experienced nondisjunction
  • This cell shows 1 pair of chromatids/chromosomes that experienced nondisjunction

  
• Daughter cells after meiosis II


  • These daughter cells have chromatids which is correct because of the chromosomes/chromatids shown in the parent cell.
  • These daughter cells have chromatids which is correct because of the chromosomes/chromatids shown in the parent cell.

  
• ¼ chance that each offspring has each genotype
• ¼ x ¼ is 1/16, the probability of each allele combination
• Each allele combination can not occur simultaneously so we add: 1/16 + 1/16 + 1/16 + 1/16 is ¼
• Recessive


  Need 2 copies of the allele
• Dominant


  Only one copy is needed
• Principle of segregation


  Individuals inherit two copies of each gene, one from the mother and one from the father, and when individuals form reproductive cells, the two copies separate equally in the eggs and sperm

  
• Principle of independent assortment


  The two copies of each gene segregate into gametes independently of the two copies of another gene
• If the trait is recessive, Julia and James are homozygous for the disease allele and Jabari is heterozygous. If the trait is dominant, Julia and James could be heterozygous for the disease allele and Jabari homozygous for the wildtype allele OR James could be heterozygous for the disease allele and Jabari and Julia homozygous for the disease allele or wildtype allele
• The Principle of Segregation from Mendel's Experiments
• When the F1 generation (progeny) with the genotype Aa form gametes, by the principle of segregation the A and a alleles separate, so that half the gametes contain only the A allele and the other half contain only the a allele
• 1st division nondisjunction violates this principle
• All seeds in the F1 generation (progeny) are yellow and round because these traits are dominant
• Principle of independent assortment


  Segregation of one set of alleles of a gene pair is independent of the segregation of another set of alleles of a different gene pair
• The two orientations are equally likely and the overall ratio from a large number of cells undergoing meiosis is 1:1:1:1

  
• Linked genes violate this principle
• Possible answers


  • Metaphase I
  • Anaphase I
  • Anaphase II
  • Telophase I
• Locus


  A particular position, point or location
• Loci


  Plural of locus. More than one location
• Crossing over


  The exchange of genetic information between homologs during meiosis

  
• Prophase I (Meiosis I): Chromosomes are condensed, shortening and thickening to allow for synapsis

  
• Synapsis: Homologous chromosomes (same set of genes) pair, form a bivalent and cross over
• After crossing over, each chromosome with recombinant non-sister chromatids is unique and contains paternal and maternal segments

  
• Genes on the same chromosome can undergo independent assortment
• Crossing over occurred between gene A with allele A or allele a and gene B with allele B or allele b
• The allele combinations that were originally together in each chromosome (A with B and a with b) have recombined (A with b and a with B) in two of the four chromatids
• Each daughter cell (haploid gamete) at the end of meiosis II receives one chromatid (chromosome) with one of the following genotypes: AB, Ab, aB or ab