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Biology LibreTexts

2.E: Chromosomes, Mitosis, and Meiosis (Exercises)

These are homework exercises to accompany Nickle and Barrette-Ng's "Online Open Genetics" TextMap. Genetics is the scientific study of heredity and the variation of inherited characteristics. It includes the study of genes, themselves, how they function, interact, and produce the visible and measurable characteristics we see in individuals and populations of species as they change from one generation to the next, over time, and in different environments.

2.1 Define chromatin. What is the difference between DNA, chromatin and chromosomes?

2.2 Species A has n=4 chromosomes and Species B has n=6 chromosomes. Can you tell from this information which species has more DNA? Can you tell which species has more genes?

2.3 The answer to question 2 implies that not all DNA within a chromosome encodes genes. Can you name any examples of chromosomal regions that contain relatively few genes?

2.4 a) How many centromeres does a typical chromosome have?

b) What would happen if there was more than one centromere per chromosome?

c) What if a chromosome had zero centromeres?

2.5 For a diploid with 2n=16 chromosomes, how many chromosomes and chromatids are per cell present in the gamete, and zygote and immediately following G1, S, G2, mitosis, and meiosis? 

2.6 Bread wheat (Triticum aestivum) is a hexaploid. Using the nomenclature presented in class, an ovum cell of wheat has n=21 chromosomes. How many chromosomes in a zygote of bread wheat?

2.7 For a given gene:

a) What is the maximum number of alleles that can exist in a 2n cell of a given diploid individual?

b) What is the maximum number of alleles that can exist in a 1n cell of a tetraploid individual?

c) What is the maximum number of alleles that can exist in a 2n cell of a tetraploid individual?

d) What is the maximum number of alleles that can exist in a population?

2.8 a) Why is aneuploidy more often lethal than polyploidy? 

b) Which is more likely to disrupt gene balance: polyploidy or duplication?

2.9 For a diploid organism with 2n=4 chromosomes, draw a diagram of all of the possible configurations of chromosomes during normal anaphase I, with the maternally and paternally derived chromosomes labelled.    

2.10 For a triploid organism with 2n=3x=6 chromosomes, draw a diagram of all of the possible configurations of chromosomes at anaphase I (it is not necessary label maternal and paternal chromosomes).    

2.11  For a tetraploid organism with 2n=4x=8 chromosomes, draw all of the possible configurations of chromosomes during a normal metaphase.

2.12   A simple mnemonic for leptotene, zygotene, pachytene, diplotene, & diakinesis is Lame Zebras Pee Down Drains. Make another one yourself.

Chapter 2 - Answers

2.1 Chromatin is the material from which chromosomes are made (mostly DNA + protein).  DNA is a component of both chromatin and chromosomes.

2.2 No.  Since chromosomes vary greatly in size, the number of chromosomes does not correlate with the total DNA content.  For reasons discussed in Chapter 1, the number of genes does not correlated closely to DNA content either.

2.3 Heterochromatic regions with repetitive DNA, centromeres, and telomeres are examples of gene-poor regions of chromosomes.

2.4 a) One, except for holocentric chromosomes.

         b) The chromosome might get pulled apart.

         c) There would have to be another way to control its movement at mitosis and meiosis.

2.5  Immediately following G1, 16 chromosomes (8 pairs) with 1 chromatid each.

         Immediately following S, 16 chromosomes with 2 chromatids each. 

         Immediately following G2, 16 chromosomes with 2 chromatids each.  

         Immediately following mitosis, 16 chromosomes with 1 chromatid each. 

         Immediately following meiosis 8 chromosomes with 1 chromatids each.   

2.6 2n=6x=42

2.7 a) Two is the maximum number of alleles that can exist  for a given gene in a 2n cell of a given diploid individual.

         b) Two is the maximum number of alleles that can exist in a 1n cell of a tetraploid individual.

         c) Four is the maximum number of alleles that can exist in a 2n cell of a tetraploid individual. .

         d) The maximum number of alleles that can exist in a population is theoretically limited only by the population size.

2.8 a) Aneuploidy can disrupt gene balance and disrupt meiosis, whereas even-numbered polyploids (e.g. tetraploid, hexaploid) can be stable through meiosis, and can retain normal gene balance.

b)  Duplication is more likely than polyploidy to disrupt gene balance since only some genes will increase their copy number following duplication of a chromosomal segment.

2.9 Maternal chromosomes are black and paternal chromosomes are grey.    

2.10

2.11 

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