Linkage and Recombinant Types

• Types of Linkage


  • Complete linkage
  • Incomplete linkage
• Complete linkage


  • Genes are very close to each other thus transmitted together
  • All parental types (100%)
  • No recombinant types (0%)
• Complete linkage test cross

  1. pr+ vg+ x pr vg

  2. Gametes: pr+ vg+, pr vg

  3. All progeny are parental types, no recombinants
• Incomplete linkage


  • Genes are far from each other thus crossing-over is possible
  • Parental types >50%
  • Recombinant types <50%
• Incomplete linkage test cross

  1. pr+ vg+ x pr vg

  2. Gametes: pr+, vg+, pr, vg

  3. Parental types and recombinant types obtained
• Linkage group


  • Physical association of genes on a chromosome
  • Number of linkage groups is equal to the haploid number
• Number of linkage groups in different organisms
• Linkage mapping


  • Linear arrangement of non-allelic genes on the chromosome
  • Distance between genes based on % recombination
• Steps in gene mapping


  1. Make a three-point test cross

  2. Establish the proper gene order

  3. Compute the distance between genes based on % recombination
• Gene mapping in corn


  • Genes: C, Sh, Wx
  • Testcross data and analysis
  • Linkage map
• Coefficient of coincidence (cc)


  • Measure of strength of linkage
  • cc = 0-1
  • Interference (i) = 1 - cc
• Calculating frequencies of test cross progeny


  DCO, SCOI, SCOII, Parentals
• Sex linkage


  Sex chromosomes carry other genes aside from sex determination genes
• 1. linked inheritance


  • Colorblindness, hemophilia, etc.
• 1. linked inheritance


  • Webbing of toes, hypertrichosis
• Types of sex determination


  • Genetic sex determination
  • Environmental sex determination
  • Chromosomal sex determination
• Barr body and X-inactivation (Lyonization)
• Recombination occurs when crossing over takes place between homologous chromatids during prophase I of meiosis.
• Genes located closer together on a chromosome have a higher chance of being inherited together, while those farther apart can be separated during recombination.
• The closer two genes are on a chromosome, the less likely they are to be separated during the process of chromosomal crossover that occurs during meiosis.
• Genetic linkage refers to the tendency of certain genes to be inherited together because they are located near each other on the same chromosome.
• The probability that two genes are linked is determined by the distance between them on the chromosome.
• The probability that two genes are linked is determined by the distance between them.
• True or False: Genes assort independently in linkage.


  False
• This theory proposes that the sex-determining genes are located throughout the genome and it is the balance of the two kinds of determiners that dictate the direction of the sex differentiation.

  Theory of Genic Balance
• This theory by Hartman proposes that every cell of reproducing organisms are bisexual that have the capability to have an equal potential to be a female or a male.

  Sexuality Theory
• She proposed that the Barr body is an inactive X chromosome, and that every person only has one active X chromosomes out of all X chromosomes in one's body.

  Mary Lyon
• heterozygous females for X-chromosome genes containing patches of cells expressing one or the other heterozygous allele

  mosaics
• what happens to the inactive X chromosome during interphase?


  it will remain condensed (it goes through mitosis normally) during interphase and becomes threadlike at the wrong time
• Adjustment where there is the same effective dosage in two sexes (single active X chromosome having the same expression in males and females).

  Dosage compensation
• To distinguish linked genes from independently assorted genes, the notation for heterozygote AaBb is...?


  AB/ab
• Phase of linkage where linked genes are being located on the separate chromosome as they enter the zygote separately. For example, AaBb's notation is Ab/aB.

  Repulsion phase
• What happens to the crossing over as the maternal age increases?

  decreases
• True or False: Heterogametic sex of a species has higher crossover frequencies.

  False, lower.
• True or false: Genes close to the centromere tend to show reduced crossing over.

  True