162.101
Basic Principles of Genetics
This worksheet is aimed primarily at those who haven’t done any genetics before, and for
those who have had a lot of trouble understanding how to do genetics problems. Attempt
the following questions and problems to check your understanding of genetics principles
and then check the answers. If you have problems with these introductory questions then
you need to attend the genetics tutorials. If you get all of these correct, or understand
where you went wrong, proceed to Genetics Questions A (Essentials Worksheet 8a) for
more practice. This question set includes some important concepts that will help you
understand the principles of genetics.
A. Definitions and Terminology
Keep these definitions in your head at all times when thinking about genetics
1. What is a gene?
2. What is the product of a gene?
3. What process(es) are used to make a gene product?
4. What is an allele?
5. How might the products of two different alleles differ from each other?
6. If there are two alleles of gene B, called B and b; that produces colour in flowers of
a plant species. The B allele is dominant and results in blue flowers. The b allele
is recessive: flowers are white because no blue colour is produced. Complete the
table below to show the genotype and the resulting phenotype (flower colour In
this case).
genotype
phenotype
homozygous dominant
heterozygous
homozygous recessive
© Rose Motion & Rosie Bradshaw, Institute of Fundamental Sciences, Massey University 2017
Note: Remember that plant and animal breeders and doctors studying genetic
diseases actually see only the phenotype of an organism or person. While it is
now sometimes possible to determine the genotype using the methods of DNA
technology, in the past the actual genotype had to be determined by test crosses or,
for people, from family trees (‘pedigrees’). This is the basis of many of the genetics
questions and exercises we will give you for practice. These methods are still
useful and valid (genetic tests are expensive, and the genes responsible for some
phenotypes are still not known).
B. Meiosis Revision
1.
Meiosis occurs in special cells in the _g_________ and produces _g___________.
2.
If there are n different chromosomes (numbered 1 to n) found in a certain species
of organism, and most cells of the organism contain 2n chromosomes, then
We call the 2n cells __________ and the n cells __________
In animals, those cells with only n chromosomes are _____________
2n cells contain a pair of each numbered chromosome. These pairs are called
_____________________ chromosomes
3.
In the production of gametes, the purpose of the process of meiosis is to ……
4. There are two divisions in meiosis. Look up the two-page diagram of meiosis in
Campbell and write a brief description of what happens in each stage of each
division. Write no more than two sentences for each stage, and concentrate
mostly on what is happening with the chromosomes.
Meiosis I
Prophase I
MI
AI
T I & Cytokinesis I
Meiosis II
etc
© Rose Motion & Rosie Bradshaw, Institute of Fundamental Sciences, Massey University 2017
How do the chromosomes line up in:
(a) Metaphase I
(b) Metaphase II
Are the cells n or 2n after:
(c) Metaphase I
(d) Metaphase II
(e) When does recombination (crossing over) take place?
(f) In your own words, briefly describe what happens during crossing over (no
more than one sentence)
(g) Look at metaphase I in the diagram in Campbell. The chromosomes of each
homologous pair are coloured red or blue to indicate which parent of the
organism they originally came from. During metaphase I, do they line up so
that all the red ones move to the same pole during anaphase and all the blue
ones move to the other? What are the consequences of this?
(h) When do the homologous chromosomes start to separate?
C. Genes and Chromosomes
1. Each gene is part of a chromosome found at a specific position called a
__________.
2. A gene is made of ____________.
3. How many alleles of one gene are found on each chromosome?
© Rose Motion & Rosie Bradshaw, Institute of Fundamental Sciences, Massey University 2017
4. If there are known to be fifteen different alleles of a specific gene, how many of
these can be found in a specific individual diploid animal? How are new alleles
made?
5. What happens in DNA replication? In what stage of the cell cycle does this occur?
What happens to the chromosomes when this happens?
6. Draw the structure of a pair of homologous chromosomes at the metaphase stage
of meiosis I.
Use different colours or shading to represent the different chromosomes of the
homologous pair, and shaded circles to represent centromeres.
Assume that this chromosome carries the gene “A”, and that your diagram
represents an individual that is heterozygous for this gene. Add appropriate labels
to your diagram to show the individual’s genotype, assuming that no recombination
has taken place.
D. A quick quiz to check if you are on track with these concepts
1. State whether the following are true or false; for any false statements, rewrite them to
make them true.
a) Gametes contain two copies of each gene.
b) Gametes are haploid whereas somatic cells are diploid.
c) A recessive allele masks the effect of a dominant allele.
d) An individual who is heterozygous for a gene will have the recessive allele in 50%
of their gametes.
© Rose Motion & Rosie Bradshaw, Institute of Fundamental Sciences, Massey University 2017
E. Typical Genetics problems
The key to tackling classical genetics problems successfully is to look for all the clues in
the question. Usually there are statements about the phenotypes (appearance) of some of
the parents &/or offspring. Each of these statements provides important information – you
need to work out what each tells you – i.e. unpack the question. If you learn to do this,
you will find that the problems aren’t so daunting after all.
A Guide to Notation
Use a capital (e.g. A) for a dominant allele, and lower case (a) for a recessive allele of the
same gene.
Genotypes
AA denotes homozygous dominant, Aa heterozygote, aa homozygous recessive.
If you have a dominant phenotype and you are unsure if the genotype is AA or Aa, then
you should write A_. Sometimes a problem asks you to work out the genotype from the
information given to you, but while you are unsure, use A_.
1. Assume that the allele for straight hair is recessive to the allele for wavy hair. A father
has wavy hair and his son has straight hair.
a) What are their genotypes?
b) What can you say for sure about the mother’s genotype? Her phenotype?
If you have completed this set of questions and managed the last question, you should
now go to Genetics question set A (Self-Check Worksheet 8) and try the questions there
for practice. If you didn’t understand any parts of this worksheet or the answers, please
ask in a tutorial or come and see Rose during Office hours.
© Rose Motion & Rosie Bradshaw, Institute of Fundamental Sciences, Massey University 2017