Name: ___________________________________
Heredity:
VCE Biology- Unit 4
Mainly Chromosomes
1. Name the material of which genes and chromosomes are made.
Deoxyribonucleic acid (DNA)
2. Describe the relationship between DNA, genes and chromosomes.
A chromosome is a long molecule of DNA. Chromosomes can be divided into sub-units called genes. Each
gene is a section of DNA that codes for the production of a particular protein. Genes are made of DNA and a
series of linked genes forms a chromosome.
3. What is the difference between the terms genotype and phenotype? What factors determine each?
The genotype refers to the alleles an individual possesses - literally the type of genes present. Often
genotypes are written as pairs of letters. Each letter represents a member of an allele pair. E.g. a genotype
for eye colour is BB where ‘B’ represents the allele responsible for production of brown pigment. An
individual with this genotype has a particular appearance or phenotype – in this case brown pigments in the
eye. Phenotype refers to the characteristic e.g. the production of a particular enzyme but its presence is part
of an organism’s phenotype. The phenotype is influenced by the alleles inherited (genotype) and the
environment.
4. How many chromosomes do humans have in:
a) normal body cells (somatic cells)? 46 (Mature red blood cells are exceptions as they possess no
chromosomes).
b) sex cells (gametes)? 23
5. How do eukaryotic and prokaryotic genes differ?
Prokaryote
Eukaryote
One promoter may initiate the transcription of a
number of polypeptide coding regions (genes)
One promoter for each polypeptide coding region
(gene)
No introns
Introns present
6. Which cells of your body are diploid?
All body cells will be diploid except for sex cells and odd examples such as red blood cells, which lack
nuclei. Diploid describes cells where the chromosomes are in pairs. These pairs are matching or homologous
with one exception. Human males have one non-matching pair, the sex chromosomes.
7. Why is meiosis significant to sexually-producing organisms? You should be able to make several points.
The significance of meiosis:
Meiosis results in a reduction in number of chromosomes in the gametes (ova and sperm). Sexual
reproduction involves the fusion of gametes. If gametes were diploid, each time fertilisation occurred the
chromosome number would double. Meiosis results in gametes containing one of each pair of chromosomes.
Fertilisation restores the diploid number in the new individual.
Meiosis results in variations in the offspring. Variation in populations allows species to survive changing
environments. Meiosis results in new combinations of alleles which may be expressed as new phenotypes.
During meiosis the following lead to variation in gametes:


Random assortment and separation of homologous chromosomes during the first stage of meiosis.
Crossing over during prophase I of meiosis.
Gametes that contain unique combinations of alleles that are different to those found in either parents’ cells
are the result of meiosis.
Note: Meiosis is a shuffling process and does not add new alleles to gametes. This is the role of mutation.
Name: ___________________________________
VCE Biology- Unit 4
8. How are the terms ‘segregation of alleles’ and ‘independent assortment of chromosomes’ related?
Genes are found on chromosomes. They code for the production of proteins. Different forms of a gene are
called alleles. They code for slightly different proteins. In diploid organisms chromosomes exist in pairs
(homologous) that carry the same genes. One allele for each gene is carried on each chromosome of the
homologous pair. During meiosis, homologous pairs line up and separate so that one chromosome of each
pair is distributed to the gametes. As each gene has one allele on each chromosome, as the chromosomes
separate so do the alleles for a particular gene. This separation and distribution of alleles into different
gametes is called ‘segregation of alleles’.
There are many different pairs of homologous chromosomes in diploid cells. Each pair of homologous
chromosomes carry different genes. Genes that are carried on different homologous chromosomes are said
to be unlinked. During meiosis the different homologous chromosomes line up randomly and are separated
so that one of each pair is distributed to the gamete. Because they line up randomly there is no relationship
between one homologous pair and another. This is referred to as ‘independent assortment of chromosomes’.
Genes that are unlinked show no relationship to the inheritance of each other. This is sometimes referred to
as ‘independent assortment of genes’.
Therefore ‘segregation of alleles’ refers to the separation of the alleles of one gene during meiosis whereas
‘independent assortment of chromosomes’ relates to the independent inheritance of alleles of different
genes.
9. Describe what may happen to linked genes during the ‘crossing over’ process? Why is crossing over
important?
During crossing over homologous chromosomes pair or synapse. Pieces of homologous chromosomes of
equal length can be swapped between the two homologous chromosomes. This gives the possibility for new
arrangements of alleles. These new combinations of alleles may be more advantageous to offspring than the
original combinations possessed by their parents.
10. Which are most likely to be affected by crossing over – closely or distantly linked genes?
Crossing over can occur at any position along a chromosome and is a random process. If two genes are close
together it is unlikely that a crossover will occur between them. If two genes are a long distance apart, for
example near opposite ends of the chromosome, then it is more likely that a crossover will occur between
them.
Name: ___________________________________
VCE Biology- Unit 4
11. Can genes on different chromosomes become linked? Carefully explain.
Usually crossing over occurs within a homologous pair of chromosomes. Genes on different chromosomes
do not become linked. An exception to this occurs with translocation. In translocation, a piece of one
chromosome breaks and becomes attached to a chromosome that is not its homologue. Therefore genes that
were originally on different chromosomes can become linked.
12. The chromosome sets of normal human males and females are similar but different. Explain.
In human male and female diploid cells, there are normally 23 pairs of chromosomes. Chromosome pairs 122 are the same in males and females. The 23rd pair differs. Human females have a matching pair of X
chromosomes, but males have one X chromosome and a smaller Y chromosome.
13. Distinguish between the terms homologous, homozygous and heterozygous.
The term homologous refers to the size and shape of chromosomes. If two chromosomes are the same length
and contain genes for the same characteristics they are homologous. The term homozygous and
heterozygous refer to the alleles of a particular gene carried on homologous chromosomes. If the alleles are
identical the organism is said to be homozygous for the alleles of that gene. If the alleles are different the
organism is described as heterozygous.
14. A normal woman’s chromosomes are homologous. Is the same true for a normal man?
A man’s X chromosome and Y chromosome are partly homologous. They carry some genes for the same
characteristics. The Y chromosome is, however, shorter than the X chromosome. This means it does not
carry all the genes found on the X chromosome. Also the Y chromosome carries some unique genes not
carried on the X chromosome. Research has shown that these genes are responsible for sex determination.
15. A gene’s locus is on the X chromosome. Does this affect its inheritance patterns?
If a gene’s locus is on the X chromosome, its pattern of inheritance depends on the sex of the individual.
To express an X-linked recessive characteristic, a man need only inherit one copy of the allele as he only
has one X chromosome. A woman has two X chromosomes, therefore must inherit one copy of the allele
from both parents.
X-linked dominant characteristics will always be passed from father to daughter as the father has only one
X chromosome. A mother may be heterozygous or homozygous to express the dominant trait. If she is
homozygous, all of her sons and daughters would be expected to inherit the characteristic. If she is
heterozygous, 50% of her children would be expected to inherit the sex-linked characteristic.
16. What differences in chromosome structure are used to sort chromosomes for a karyotype?



Position of the centromere
Size of chromosomes
Staining patterns visible as a series of light and dark bands
17. How can features visible with a microscope be used to distinguish a human X chromosome from a
human Y chromosome?
The X chromosome is much larger than the Y chromosome. The centromere of the X chromosome is near
the middle of the chromatid. In the Y chromosome, one arm is much longer than the other (the centromere is
not central).
18. Give an example of a condition that arises as a result of non-disjunction. Explain what must have
happened during meiosis.
Down syndrome is a condition that results from non-disjunction. During meiosis in the mother’s ovary the
21st pair of homologous chromosomes fails to separate. At the end Meiosis I, one cell has 24 chromosomes
and the other 22. This mistake will be continued in Meiosis II which results in ova with two chromosome
21s or ova with no chromosome 21. If ova with two chromosome 21s fuse with a sperm, a zygote with 47
chromosomes will result.
Name: ___________________________________
VCE Biology- Unit 4
19. a) Distinguish between the terms complete dominance and partial dominance.
For a gene where there are at least two possible alleles, one allele may be expressed and mask the other
allele, i.e. one allele has determined the phenotype. When heterozygous or homozygous individuals have the
same phenotype, the trait is described as dominant. To inherit the recessive condition two copies of an allele
must be present. In a situation of partial dominance three phenotypes (e.g. red, pink or white-coloured
petals) are possible. Individuals can be homozygous for either allele or heterozygous. Heterozygous
individuals will have a different phenotype to either homozygous individual.
b) Which of the above is more common in living organisms?
Partial dominance.
20. List the possible genotypes and phenotypes for an example of complete dominance.
Given D represents the allele for the expression of the dominant characteristic and d represents the allele for
the expression of the recessive characteristic;
Phenotype – the dominant characteristic. Genotypes are DD or Dd.
Phenotype – the recessive characteristic. Genotype is dd only.
21. List the possible genotypes and phenotypes for an example of co-dominance.
e.g. ABO blood typing. IA and IB show co-dominance: If they are present, they mask the action of i.
Blood group
A
B
AB
O
Genotypes
IAIA or IAi
IBIB or IBi
IAIB
ii
Phenotypes
Antigen A produced
Antigen B produced
Antigen A & B produced
Neither antigen produced
22. What is the major difference between monohybrid and dihybrid cross problems?
A monohybrid cross involves the alleles of only one gene. A dihybrid cross involves the alleles of two
different genes.
23. Explain the meaning of the phrase ‘an independent event’ in genetics. Relate this to a man and woman
producing several children. Does the genotype of one child influence the genotype of future children?
Events are said to be independent if the outcome of one event does not affect the outcome of other events.
E.g. the probability of producing a girl or boy is a ½. If a couple have 3 girls, the probability that a fourth
child will be a girl is still a ½. Each child is an independent event and the sex of one child does not influence
the sex of further children.
24. A man and a woman are heterozygous for a gene. The gene has two possible alleles and there are two
phenotypes. Use ‘D’ to represent the allele whose expression results in the dominant phenotype and ‘d’ to
represent the allele that results in a recessive phenotype.
Calculate the following probabilities:
a) the two individuals produce a heterozygous dominant offspring.
b) the two individuals produce a homozygous dominant offspring.
c) the two individuals produce a homozygous recessive offspring.
d) the two individuals produce two homozygous recessive offspring.
e) the probability the child is heterozygous given the two individuals produce a child with the dominant
phenotype?
f) the expected ratio for dominant to recessive phenotypes?
Name: ___________________________________
The relevant cross is: Dd x Dd
VCE Biology- Unit 4
a) Probability of a heterozygous dominant offspring (Dd) is a ½.
b) Probability of a homozygous dominant offspring (DD) is a ¼ .
c) Probability of a heterozygous recessive offspring (dd) is a ¼ .
d) Two children result from two independent events. The probability of the two events happening is the
product of the individual events’ probabilities. Therefore probability of two homozygous recessive
individuals is: ¼ x ¼ = 1/16
e) ½ divide ¾ = 2/3 (i.e. number of heterozygotes / number of dominant phenotype = 2/3)
f) 3 dominant to 1 recessive.
25. In the previous question, complete dominance was the mode of inheritance. Using the same allele
symbols, show a cross involving a homozygous recessive man and a woman heterozygous for the gene.
The relevant cross is: Dd x dd
F1 genotypes
F1 phenotypes
What are the possible genotypes and phenotypes of their offspring and in what proportions?
There are two possible genotypes and phenotypes. The probability of each is a ½. The Dd genotype is a
dominant phenotype. The dd genotype is expressed as the recessive phenotype. The phenotypic ratio is 1:1.
Name: ___________________________________
VCE Biology- Unit 4
26. Coat colour variation in cattle shows a ‘co-dominance’ inheritance pattern. Cattle can be red, white or
roan. Roan cattle have a mixture of red hairs and white hairs. Use ‘CR’ as the symbol for the allele that
causes the production of red hair pigments and ‘CW’ for the allele that causes the production of white hair
pigments. If a roan cow (CRCW) and roan bull (CRCW) mate, calculate the following probabilities:
a) the cattle produce a heterozygous calf.
b) the cattle produce a white calf.
c) the cattle produce a red calf.
d) the cattle produce two roan calves.
e) the cattle produce three red calves.
The relevant cross is: CRCW x CRCW
F1 Genotypes
F1 Phenotypes
a) The probability of a roan calf is ¼ + ¼ = ½.
b) The probability of a while calf is ¼.
c) The probability of a red calf is ¼.
d) The probability of producing two roan calves is ½ x ½ = ¼.
e) The probability of producing three red calves is ¼ x ¼ x ¼ = 1/64.
27. Explain why sex-linked recessive conditions, like colour blindness, are more common in men than
women.
X-linked recessive characteristics are more common in males as they require only one copy of the
allele for it to be expressed. For a female to express a recessive characteristic she requires two
recessive alleles which is less likely.
28. Referring to recessive conditions (sex-linked or autosomal) what is meant by a carrier?
A carrier is an individual who carries an allele for a recessive condition that is not expressed in their
phenotype. They are heterozygous individuals.
29. A woman with a colour blind father has children with a man with normal vision.
a) What is the probability that a child is colour blind?
b) What is the probability that a son is colour blind?
c) What is the probability that a daughter is colour blind?
Allele symbols:
XC = X chromosome carrying allele for normal vision
Xc = X chromosome carrying allele for colour-blindness
Y = Y chromosome
Relevant cross:
XCXc
x
XCY
Name: ___________________________________
VCE Biology- Unit 4
a) ¼
b) ½
c) 0
30. Consider two genes with two alternative alleles. Both are on different chromosomes. A gene with
the alleles B (abnormally high cholesterol) and b (normal cholesterol levels) is on chromosome 19.
Another gene controls Rhesus blood type (D- Rhesus positive and d- Rhesus negative). Its locus is on
chromosome 1.
A couple are both heterozygous for the two genes. What is the expected phenotypic ratio amongst
their offspring given they produce a huge number of children.
Relevant cross is: BbDd x BbDd
Name: ___________________________________
VCE Biology- Unit 4
31. Describe the drawing of pedigrees. What symbols are used and how are generations numbered?
Circles are used to represent females.
Squares are used to represent males.
Shading indicates an individual shows the characteristic being studied.
Roman numerals are used to indicate generation.
Individuals are numbered left to right within a generation.
32. What patterns do you look for in a pedigree to distinguish between autosomal recessive
inheritance and autosomal dominant inheritance?
Looking at the figure above you will note that two unaffected parents produce affected children. To
produce children of a different phenotype, the parents must be heterozygous and the affected children
homozygous recessive. To determine whether the trait is expressed as a dominant or recessive, look
for two parents with the same phenotype that produce at least one child with a different phenotype.
The different phenotype is only expressed in the homozygous genotype.
33. What patterns do you look for in a pedigree to distinguish between X-linked recessive inheritance
and X-linked dominant inheritance?
For X-linked recessive inheritance males will be more likely to inherit the condition than females. A
female that is affected will pass the condition on to all sons. Carrier females are expected to produce
50% affected sons. Daughters will not inherit an X-linked recessive condition unless the father has it
and the mother is at least a carrier. This is unlikely. For X-linked dominance, a male with the
condition will pass it on to all daughters. An affected woman could pass it on to sons or daughters
irrespective of the father’s phenotype.
Males do not inherit a trait from their father in an X-linked pattern of inheritance.