24. Genetics II

  1. Explain what is meant by incomplete dominance, codominance, multiple alleles, epistasis, poly-genic inheritance, and pleiotropy; give an example of each.
  2. Give two examples showing how the expression of genes is affected by the external environment.
  3. Explain what is meant by a sex-linked trait, and define the term linkage group.
  4. Discuss the meaning of recombination, and explain how recombinations are possible.
  5. Discuss how chromosomes are mapped; explain how giant chromosomes can be used to study changes in hereditary patterns.
  6. Describe the differences between X-linked inheritance and sex-linked inheritance

Sex Chromosomes

XXXY

For the most part, mammals have gender determined by the presence of the Y chromosome. This chromosome is gene poor and a specific area called sex determining region on Y (SRY) is responsible for the initiation of the male sex determination. The X-chromosome is rich in genes while the Y-chromosome is a gene desert. The presence of an X-chromosome is absolutely necessary to produce a viable life form and the default gender of mammals is traditionally female.

Human X and Y chromosomes with G-Banding.

Chromosomal painting techniques can reveal the gender origin of mammalian cells. By using fluorescent marker sequences that can hybridize specifically to X or Y chromosomes through Fluorescence In Situ Hybridization (FISH), gender can be identified in cells.

Mice X Y chromosomes

The male cells have an X and a Y while the female cells have X and X combination. Credit: Janice Y Ahn, Jeannie T Lee [CC BY 2.0]

X-inactivation

The mammalian X-chromosome contains  significantly more genetic information than the Y-chromosome. This gene dosage is controlled for in females through a process called X-inactivation where one of the X-chromosomes is shut down and highly condensed into a Barr body. Inactivation of the X-chromosome occurs in a stochastic manner that results in females being cellular mosaics where a group of cells have inactivated the paternal X-chromosome and other patches of cells have inactivated the maternal X-chromosome. The most striking example of mosaicism is the calico cat. A calico cat (tortoise shell cat) is always a female. One of the genes that encodes coat color in cats resides on the X-chromosome and exist as either orange or black alleles. Due to the stochastic inactivation, the patterning of orange and black fur is a distinctive quality of calicos.

While the genetic information for the the orange or black coat color exists in all cells, they are not equally expressed. This type of heritable trait in spite of the presence of the genetic material (DNA) is called epigenetic to imply that it is “above” (epi) genetics .