50. Stage the function of each of the following. a) maternal genes, b) gap genes

Question

50. Stage the function of each of the following. a) maternal genes, b) gap genes, c) pair-rule genes d) segment polarity genes

51. State and explain the function of Hox genes

52. What are the types of stem cells (explain each)

53. Describe Mendel’s experimental approach to define the genetics of inheritance

54. Explain the Chromosome Theory of Inheritance and describe how it allows us to see the relationship between Mendel’s laws and chromosomal transmission

55. Define each of the following; a) Mendel’s Law of Segregation b) Mendel’s Law of Independent Assortment

56. To apply the basic rules of probability to predict the outcome of a genetic cross

Explanation / Answer

Q-50

During the developmental stages of Drosophila, three types of pattern regulating genes are expressed.

Maternal regulating genes: These genes are expressed prior to fertilization which is responsible for the development of the dorsoventral and anterior-posterior polarity in the egg. The dorsal part becomes the top and ventral part the bottom, anterior the head and posterior the tail of the organism.

Gap genes: The gap genes if mutated leads to the formation of a gap in the embryo's segment pattern and hence the name. The gap genes are responsible for dividing the embryo into wide regions. Majority of gap genes are transcriptional factors i.e expressed in the nucleus. the expression of these genes depends on the concentration of the gradient protein. Eg of GAP genes is the hunchback, kruppel, knirps, giant and tailless.

Pair rule genes: Pair rule is activated by the product( protein) of gap genes. The proteins made by transcription of pair-rule genes remain localized into the same cell instead of moving out. So, the embryo appears as a series of alternative strips, each representing a set of pair-rule genes. Mutation in pair-rule genes results in the disappearance of alternate segments.

Segment polarity genes: The segment polarity genes, in turn, is activated by the pair rule gene proteins. The segment polarity genes divide an embryo into 14 segments structure, thus initiating the correct orientation of the strips which will eventually represent the segments of the larval stage fly. Eg of segment genes is hedgehog, engrailed etc.

Q 51

HOX genes are the subset of homeotic genes which specifies the identity of the segment. These genes are neatly clustered in small spaces which gives us a brief look whether the segment will be a part of the head, thorax or abdomen in the adult. If we mutate or knockout the HOX gene then one body parts develop into other and thus order is disrupted. That means the head will come out of the thorax or lag will be out of the head which leads to the formation of the mutant. This tells us about the importance of the gene.

Drosophila has 8 Hox genes which are arranged in a row according to their order of expression in the body of the fly. Other organisms like human and mice also have homeotic genes for maintaining the correct polarity of their body.

Q 52

According to the potency, the stem cells can be differentiated into four types:

Totipotent: The totipotent stem cells can give rise to both embryonic and extra-embryonic cells of an organism. These cells can give rise to any cell type of an organism eg the blastula stage(8 cell stage) cells.

Pluripotent cells: The pluripotent cells can give rise to embryonic cells only. These cells can form any tissue/ organ except placenta. Eg. Inner Cell Mass

Multipotent Stem Cell: These cells can give rise to all cells of one lineage example hematopoietic stem cells can form all blood cells.

Unipotent Stem Cells: Can give rise to only one type of cells eg. hepatic stem cells can only produce hepatocytes.

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