Topic 1 Heredity: Embryonic Development

Question 1

• There are a number of genetic changes that occur early in development that influence expression in an organism. Let’s learn about these changes!

Answer 1

Note

Question 2

• During embryonic development in female mammals, one of the two inherited X chromosomes does not uncoil into chromatin, and remains as a dark and coiled compact body, which is referred to as a Barr body. Barr bodies are therefore not expressed, and only the genes on the other X chromosome that did uncoil are expressed.
  i. It is important to understand that either one of these two inherited X chromosomes can be inactivated. This process ultimately results in all of these genes in the female to not be expressed similarly. Moreover, all of the cells in a female mammal do not necessarily have identical function.
  ii. A common example of x-inactivation is in Calico cats, where the characteristic black and orange fur coat depends on which copy of the X chromosome the cell chooses to leave

Answer 2

1. X-inactivation

Question 3

• A genetic disease that can result from X-inactivation is hemophilia, a recessive condition which results in an inability to form blood clots. X^HX^h is a normal carrier, but if X^H (the normal, functional copy) is inactivated, then only X^h is expressed, leading to disease onset

Answer 3

Note

Question 4

• Describes when one or more chromosome pairs or chromatids fail to separate during mitosis. This commonly occurs during anaphase of mitosis, when two chromatids of a single chromosome fail to separate, or during anaphase of meiosis. In meiosis, recall there are two anaphases: homologous chromosomes fail to separate during meiosis I and sister chromatids fail to separate during meiosis II.
  i. Depending on when nondisjunction occurs (in anaphase I, anaphase II, or mitotic anaphase), different outcomes can occur; therefore, knowing the following image will be incredibly helpful.
  ii. Mosaicism: a phenomenon that occurs in cells that undergo nondisjunction in meiosis during embryonic development; fraction of body cells have extra or missing chromosomes
  iii. Polyploidy - when all chromosomes undergo meiotic nondisjunction and produce gametes with twice the number of chromosomes
  a. Is common in plants

Answer 4

2. Nondisjunction