GENETICS REVISION- SOLUTIONS
1.
Give definitions of the following terms:
Chromosome
 The shape DNA folds into (usually an X shape) prior to replicating itself.
Dominant/Recessive Inheritance Pattern

When one form (allele) of a gene is displayed in preference to another allele. Thus if at
least one dominant allele is present the phenotype will relate to that allele (e.g. black
fur in some rabbits is dominant whereas white fur is recessive)
Co-dominant Inheritance Pattern

When both alleles are displayed in the offspring, but clearly separate (e.g. roan cattle
have one red and one white parent and have red and white hairs in their coat)
X-Linked or Sex-linked Inheritance Pattern
 Any trait inherited from a gene located on the 23rd pair of chromosomes in human
beings (may be a different numbered chromosome in other animals). In this case many
traits are only passed on by mother as the smaller Y chromosome in men has less DNA
and is missing a portion covered by the X chromosome
Phenotype
 The appearance of a trait
Genotype
 The alleles of the genes present which determine a trait (shown by letters e.g. BB, Bb)
Allele
 Alternative forms of a gene
Homozygous
 Two of the same alleles present in the genotype (e.g. BB, bb)
Heterozygous
 Two different alleles present in the genotype (e.g. Bb)- also called a hybrid
Hybrid
 As heterozygous above- genotype contains two different alleles
Sex chromosome
 One set of chromosomes codes for sex (amongst other things). Females are XX, males
are XY
Autosomes/ autosomal inheritance
 All chromosomes except the sex chromosomes are called the autosome. Autosomal
inheritance relates to traits which derive from genes on any chromosomes except the
sex chromosomes).
Centromere
 The connection point where chromosomes are held together (like the fulcrum in a pair
of scissors)
DNA

Molecule of genetic inheritance consisting of repeating units which consist of a sugar a
phosphate and a base. Sugar and phosphate units make up the backbones of the
molecule (like the sides of a ladder) with bases in the centre (like the rungs of a ladder).
The molecule, when folded is called a double helix (think of a twisted ladder).
Gene
 A section of DNA which contains the code to make one protein
Karyotype
 The complete set of DNA contained in one normal cell of an organism
1.
2.
3.
4.
5.
6.
Chromosomes occur in pairs, explain why this is the case.
 One is inherited from mum, one from dad
How many chromosomes are found in human cells?
 46, in 23 pairs in a normal cell (23 single chromosomes in sex cells)
How many chromosomes does a normal human being have?
 46, in 23 pairs
Which gamete (egg or sperm) determines the sex of the offspring?
 The sperm determines the sex of the offspring. It can either contain an X (makes a
female) or a Y (which makes a male)
What are the three components of a nucleotide?
 A sugar, a phosphate or a base
Draw a diagram to show the overall structure and shape of DNA. Include the nitrogen
bases, ribose sugar and phosphate group. Also show the base pairing rules in your
diagram.
7.
Show diagrams showing the processes of meiosis and mitosis.
8.
What are the purposes of meiosis and mitosis and where are the daughter cells of each
of these processes found?
Mitosis- duplication of cells- parent duplicates to produce two daughter cells identical to
the parent (used for cell replacement and growth in an organism)
Meiosis- division of parent cell into four non-identical daughter cells with half the
number of chromosomes to a normal cell (used to produce sex cells used in reproduction
of a new organism)


9.
The genes that determine if a mouse is albino come from
the male parent only
the female parent only
one from each parent
two from each parent?
In the garden pea, white flower (w) is recessive to purple flower (W).
What is the phenotype of the 'Ww' plant?
 Purple
b. What is the genotype of a plant with white flowers?
 ww
c. What is the genotype of a plant with purple flowers?
 WW or Ww
a
b.
c.
d.
10.
a.
11.
Two agouti mice have a litter of six pups, all of them agouti. Which statement below is
correct. ('A' represents agouti, 'a' albino) Justify your answer by using punnet grids to show
possible outcomes.
a.
Both parent mice must be'AA'
b.
At least one of the parents must be 'AA'
c.
Both parents may be 'Aa'
d.
If one parent is 'Aa', the other is 'AA'
12.
In Shorthorn cattle, when a red bull (RR) is crossed with a white cow (WW)
the first generation offspring have a coat that is a mixture of red and white hairs.
a.
Show by means of a punnet grid why this is the case.
R
R
RW
RW
W
RW
RW
W
All offspring have the mixed coat colour
b.
What type of inheritance is shown for the coat colour?
 Codominance
c.
When a red-white (RW) cow is mated with a red-white bull (RW),
what are the expected genotypic and phenotypic ratios?
R
W
RR
RW
R
RW
WW
W
Genotype ratio:
RR: RW: WW = 1:2:1
Phenotype ratio
Red:Mixed (Roan): White = 1:2:1
13. Using the letters 'B' or 'b' to represent the gene for coat colour, predict the results of the
following crosses. Draw punnet grids to show your predictions.
i. a pure breeding black mouse is mated with a hybrid black mouse.
B
B
BB
BB
B
Bb
Bb
b
All mice will be black (half heterozygous black, half heterozygous)
ii. a pure breeding black mouse is mated with a pure breeding white mouse
B
B
Bb
Bb
b
Bb
Bb
b
All mice are black (all heterozygous)
iii. Is black dominant to white or white to black. Explain your answer.
 Black is dominant to white due to the lettering system used (capital letter is used
to indicate the dominant gene)
14. Explain the way in which DNA replicates itself, and how a gene works to produce
proteins. Use the terms: codons, amino acids and proteins in your answer.
 Replication- The DNA ladder separates down the middle (between bases) to
form two separate single strands. Nucleotides containing the complimentary
bases match up to form two new double stranded DNA molecules.
 Proteins are made by two processes- transcription and translation. First the DNA
segment (gene) to be decoded unzips between the bases. An RNA copy matches
to one strand in the transcription process. This copy then leaves the cell nucleus
and the DNA zips up again. The RNA copy travels to the main part of the cell
(cytosol) where it connects with an organelle called a ribosome. In the translation
process the RNA is decoded in groups of three bases (codons), resulting in
joining of amino acids in a specific sequence. Each codon codes for one amino
acid.
15. Sometimes diseases are caused by genetic mutations. Explain what is meant by the
terms: single gene mutations and whole chromosome mutations, and give examples of
the diseases that can be caused in each of these cases.


Single gene mutations- relate to a mistake in the DNA or the RNA copy for one gene.
Depending on the type of mutation a minor or major change in the protein structure will
result. Often the protein will be non-operational as its structure will be incorrect
Chromosome mutations relate to issues such as excess DNA on a chromosome, missing
information or wrong numbers of chromosomes. These mutations often lead to significant
diseases and disorders.