Inheritance (b)
Monohybrid inheritance
Unit 1
Sub-topic 7
Cell Function and Inheritance
Monohybrid inheritance
The science of inheritance aims to explain the way characteristics are passed from
generation to generation. In simple terms this involves studying crosses between parent
organisms and examining the offspring produced.
1 a) Distinguish between the following pairs of words:
(i) gene and allele
(ii) genotype and phenotype
(iii) dominant and recessive
(iv) homozygous and heterozygous
b) What do the following symbols stand for: F1 =
F2 =
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2 Give reasons why organisms such as fruit flies (Drosophila) and pea plants are suitable
for studying inheritance?
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3 Fruit fly body can be grey or ebony, grey being the dominant phenotype. Using symbols
of your choice, summarise a cross between a homozygous grey fruit fly and a fruit fly
with an ebony body and then a second cross between members of the F1 offspring.
original cross
……………
gametes
……………
F1 genotype
X
………………
……………..
………………
F1 phenotype
…………………………………………….
F1 self-cross
……………
F1 gametes
X
…………………
………………
………..…………
Punnett square
of F2 genotypes
F2 genotypes
………..
………..
F2 phenotypes
…………………….
…………
……………………
…………
……………………
…………………….
phenotype ratio
………..…………… : …………………………
4 a) What is always the expected F2 ratio of dominant to recessive phenotypes in a truebreeding monohybrid cross?
b) Explain why this ratio rarely happens in real life.
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Monohybrid inheritance in humans
5 Tongue rolling is a well-cited example of monohybrid inheritance in humans.
a) Briefly describe these examples of monohybrid inheritance in humans.
Rhesus D-antigen
Albinism
b) A number of medical disorders are also a result of monohybrid inheritance.
Name and describe three of these.
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c) Apart from late expression of the symptoms, identify what else is different about
Huntington’s chorea compared to other genetic disorders, and explain the significance
of this difference on sufferers and their children.
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Incomplete dominance and co-dominance
So far we have looked at the inheritance of traits that have two alleles, one of which is
completely dominant over the other. In some traits, an allele is not completely dominant,
or both are equally dominant. Both cases result in intermediate phenotypes.
Incomplete dominance and sickle cell anaemia
6 a) Draw a diagram of the two types of red blood cell and briefly describe the
effects of sickle cell anaemia.
b) (i) Why are the alleles H and S described as being incompletely dominant to one
another? Explain how this affects the phenotype.
(ii) What condition results from both alleles being present and why can this condition
be beneficial to the sufferer?
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c) With reference to their genotype and phenotype and using the symbols H and S,
draw a table to compare a person with normal haemoglobin, a sufferer of sickle cell
trait and a sufferer of sickle cell anaemia.
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Co-dominance and the MN blood group system
7 a) Describe what is meant by the term co-dominance of alleles.
b) The MN blood group system is an example of this. Briefly describe why this system is
an example of co-dominance.
Multiple alleles
8 a) What is meant when a gene is described as having multiple alleles?
b) The ABO blood group system is an example of. Describe the relationships in terms of
dominance of each of the alleles, A, B and O.
c) With reference to the ABO blood group system, identify:
(i) a phenotype that can result from either of two different genotypes
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(ii) a phenotype that can only result from one genotype
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Sex-linked inheritance
There are 22 pairs of autosomes and one pair of sex chromosomes in the chromosome
compliment of humans. Due to their nature there are a number of genetic disorders linked
to the sex chromosomes.
1 a) Identify and label which human sex chromosomes is X and which
Y on the diagram, and explain the significance of the difference
in size.
b) What term is used to refer to genes that are present on the X chromosome but
absent from the Y chromosome?
c) Show how sex-linked alleles are represented symbolically using an example.
d) If a human male inherits the recessive allele of a sex-linked gene, it is always
expressed in his phenotype. Explain why?
Red-green colour blindness, haemophilia and muscular dystrophy are all sex-linked
conditions found in humans.
2a) Why are heterozygous females for these conditions called carriers?
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b) Represent symbolically the genotype(s) of:
(i) a colour-blind male (ii) a human female with normal vision who is a carrier (iii) the offspring that could be produced if (i) and (ii) were their parents.
c) Why are sex-linked conditions expressed much less frequently in the phenotype of
females compared with males?
Polygenic inheritance
Many characteristics are controlled by the alleles of more than one gene and display a
wide range of phenotypes from one extreme to another.
4a)(i) Note the meaning of the terms continuous and discontinuous variation giving an
example of each in your answer.
(ii) Which of these forms of variation is controlled by alleles of more than one gene?
(iii) What term is used to refer to this type of inheritance?
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(iv) What relationship exists between the number of genes involved and the number of
intermediate phenotypes that can be produced? Explain why this is.
b)(i)Apart from genetic factors, what other type of factors influence the wide range of
phenotypic expression shown by some characteristics?
(ii) Give an example of such an effect
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