University of North Carolina

BIO 101

1)In humans, we have 46 chromosomes in any somatic cell. Each of these is a pair. For example, you have two copies of chromosome number 1. You inherited one from your mother (in the egg cell) and one from your father (in the sperm cell). What do we call these pairs?

2. A diploid organism has 22 chromosomes in its somatic cells. How many chromosomes will be in the gametes?

3. Contrast the term diploid from haploid:

4. How many divisions take place in mitosis?_______ How many in meiosis?

5. If a fruit fly cell has 8 chromosomes, how many chromosomes will be in the daughter cells after a cell goes through mitosis?

6. How many chromosomes are present in daughter cells after meiosis in a fruit fly?

7. Crossing over takes place during prophase I. What does this mean exactly?

8. During anaphase I, what is being pulled apart?

9. During anaphase II, what is being pulled apart?

10. Are the cells produced just after meiosis I considered haploid or diploid and why?

11. Why is another division needed after meiosis I?

12. Examine figure 8.14, what makes metaphase I different from metaphase of mitosis?

13. Why are the chromosomes colored red and blue in this figure? What does red or blue represent?

14. Describe for yourself or draw (based on fig 8.15) why independent orientation of homologs at metaphase I produces variation.

15. Explain why crossing over between homologous chromosomes produces variation.

16. Imagine a chromosome that has alleles “R” and “g” on it. The homologous chromosome has different versions of these genes and contains the “r” and “G” versions. What are the possible gamete combinations that can occur?

Non-disjunction

Guided Reading Qs

(Reading: Section 8.18-8.22)

Reading Objectives:

• Draw cells in which non-disjunction occurs in anaphase I in a specific pair of chromosomes and explain the outcome for these specific cells.
• Describe how a karyotype is performed and what can be learned from it.
• Describe the consequences of non-disjunction in the sex chromosomes in humans.

1. 1. Draw a cell in which 2n= 4 and show nondisjunction in anaphase I for one of the chromosome pairs. What do the resulting gametes look like?

1. 2. Now draw a cell in which 2n= 4 and show nondisjunction in anaphase II for one of the chromosome pairs. What do the resulting gametes look like?

1. 3. What is a karyotype? What can you learn from it?

4.Why are cells in the middle of mitosis used to obtain chromosomes from?

5.Why is a hypotonic solution used in preparing cells for karyotyping?

6. What is risk of having a child with Down Syndrome at age maternal age 40 vs. 45?

7. Dr. Hogan’s grandmother was 50 years old when she gave birth to a son with Down Syndrome. What was the probability of that for her age?