Question

1. In humans, X and Y chromosomes are technically not homolous Explain how X and Y chromosomes pair up in cells of males prior to cell division 2. Familial breast cancer is caused by mutations in the BRCA1 gene which is located on human chromosome 17. Females with a mutation in BRCA1 gene have an 80% lifetime risk of developing breast cancer. Therefore, out of 100 women who carry the disease-causing mutation, only 80 will develop breast cancer. Why is it that the other 20 women do not develop breast cancer even though they carry the mutant allele? 3. While our genetics play a large role in determining our phenotype, the environment does as well. This concept is well demonstrated when looking at the coat coloration on Siamese cats. How has the environment impact their coat color? 4. Achondroplasia is a common cause of dwarfism in humans. All individuals with achondroplasia are thought to be heterozygous at the locus that controls this trait. When two individuals with achondroplasia mate, the offspring occur in a ratio of 2 achondroplasia:l normal. What is the most likely explanation for these observations?

Answer 1

1. In mammals, the timing of meiosis differs greatly between males and females. Male germ cells, or spermatogonia, do not enter meiosis until after puberty. Even then, only limited numbers of spermatogonia enter meiosis at any one time, such that adult males retain a pool of actively dividing spermatogonia that acts as a stem cell population. On the other hand, meiosis occurs with quite different kinetics in mammalian females. Female germ cells, or oogonia, stop dividing and enter meiosis within the fetal ovary. Those germ cells that enter meiosis become oocytes, the source of future eggs. Consequently, females are born with a finite number of oocytes arrested in the first meiotic prophase. Within the ovary, these oocytes grow within follicle structures containing large numbers of support cells. The oocytes will reenter meiosis only when they are ovulated in response to hormones. Human females, for example, are born with hundreds of thousands of oocytes that remain arrested in the first meiotic prophase for decades. Over time, the quality of the oocytes may deteriorate; indeed, researchers know that many oocytes die and disappear from ovaries in a process known as atresia.

2.Since the discovery of the BRCA1 and BRCA2 in 1994 and 1995 about 80% of the women who inherit mutated forms of these genes will develop breast cancer in their lifetime, usually at relatively early age and woman with BRCA1 mutations have a high risk of developing ovarian cancer as well.