Beyond Mendel—Part 2

The Human Karyotype

• A karyotype is an image of an individual's stained chromosomes, organized so one can see the physical structure.
  • Chromosomes are numbered approximately in order of size from largest to smallest (chromosome 21, not 24, is actually the smallest).
  • 22 chromosomes come in pairs and are called autosomes.
  • X and Y chromosomes (23 and 24) are the sex chromosomes, responsible for the sex of the individual.
• Heterochromatin: dark regions that are very compact
• Euchromatin: light regions that are less tightly packed and contain genes
• Trivia: chromosome 19 has the greatest number of genes.

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Gene Linkage

• Mendel's Second Law
  • Traits will assort independently from one another.
  • This is true if and only if the genes are found on different chromosomes.
• Each chromosome contains many genes.
  • Example: humans have 23 pairs of chromosomes, but 23 000 genes.
  • Therefore, many genes must be located on the same chromosomes, which means they will not independently assort.
  • Since they are inherited together, such genes are said to be linked.
• Linked genes (genes on the same chromosome) show inheritance patterns that do not follow Mendel's Law of Independent Assortment.
• Note: gene linkage is not the same as sex linkage.

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Example

• In Drosophila, the trait for body color and wing length are on the same chromosome.
  • Body color
    • Gray = b⁺ (normal color)
    • Black = b (mutant color)
  • Wing length
    • Normal = vg+ (normal length)
    • Vestigial = vg (short, “vestigial” length)
• Suppose you cross male fly that is heterozygous for each trait (b⁺bvg⁺vg) with a female fly that is homozygous recessive for each trait (bbvgvg).

  • If these traits were on separate chromosomes they would assort independently:

    

    Via Mr. Hammack's mad drawing skills

    In this case (the traits are not linked), one would get a 1:1:1:1 ratio as shown below.

    

    Via Mr. Hammack's mad drawing skills

  • In reality, these traits are located on the same chromosome, producing a 1:1 ratio as shown here:
    

    Via Mr. Hammack's mad drawing skills

• Linked traits do not exhibit Mendelian ratios.
  • Mendel must have noticed this, but didn't understand it—he got strange ratios he couldn't explain—so he ignored such traits.
  • The traits Mendel ended up studying were either on separate chromosomes or far apart on the same ones, so they were not linked.

Conclusion

If you have two sets of traits (like body color and wing length in Drosophila) and you are doing a test cross (homozygous recessive) with an individual that is heterozygous for both traits, there are two expected outcomes:

• If you get a 1:1:1:1 ratio, then the traits are completely independent.
• If you get a 1:1 ratio, then the traits are completely linked.

Crossing Over

Strange Ratios

• Based on what we know about linkage, we would expect this cross to give a 1:1 ratio (gray–normal: black–vestigial).
• But when T. H. Morgan (the fly guy at Columbia University) did these crosses and counted 2300 offspring, he got this strange data:

Figure 15.4, page 265, Campbell's Biology, 5th Edition

The Explanation: Crossing Over Occurs

• Homologous chromosomes exchange portions of their chromosomes (so that alleles can be recombined, i.e. move between chromosomes).
• This event, called crossing over, occurs during prophase I of meiosis.
  • Remember the formation of chiasmata.
  • Crossing over is very frequent: every homologous chromosome undergoes at least one crossover event during prophase I of meiosis (generally it is more like 2 or 3).
• More information is available at the DNA Learning Center.

Figure 15.5, page 266, Campbell's Biology, 5th Edition

Chromosome Mapping

• One of Thomas Morgan's graduate students noticed that the frequency of crossing over was not constant.
  • For example, it isn't always 17 % like in the previous example.
  • It depends on the distance between the traits on the chromosome
    • Farther apart: More crossing over
    • Closer together: Less crossing over
• This resulted in the idea that you could map the location of alleles (genes) on the chromosome based on crossover frequency.
• If enough crosses are done, you can create a fairly accurate map.

Figure 15.6 and 15.7, page 267, Campbell's Biology, 5th Edition

Sex-Linked Traits

• Specialized chromosomes determine gender, as explained at the DNA Learning Center.
• The X chromosome carries some important genes unrelated to sex.
  • Males have only one copy of the X chromosome.
  • So, they are vulnerable to harmful recessive alleles that could be masked by the second X chromosome in females.
• Result: males are more prone to genetic diseases and infirmities associated with the X chromosome.
  • Hemophilia
  • Color blindness
• The DNA Learning Center explains sex-linked inheritance.