South Tuen Mun Government Secondary School
Biology Revision Note 20
A gene is a short segment of DNA in chromosomes. The gene determines amino acid sequence of protein thus
controlling our character. Because chromosomes occur in pair, a gene has a pair of different alleles on
homologous chromosomes pair.
Mendel’s first law / Law of segregation – a character is controlled by a pair of alleles, the alleles separate
during gamete formation and recombines randomly in fertilization.
Phenotype – the observable character
Genotype – the genetic make up
Homozygous – the two alleles of a gene is the same e.g. AA, aa (homozygous recessive)
Heterozygous – the two alleles of a gene is different e.g. Aa
Dominant – the allele which is expressed in heterozygous condition
Recessive – the allele which is not expressed in heterozygous condition, it is only expressed in homozygous
recessive condition
Question
What to look for?
Explanation
(i) Which allele is dominant?
Look for 2 parents which when
The two tall parents produce short
crossed, produced a character which offspring showing that either parent
is not the same as the parent e.g. tall is heterozygous, in heterozygous
man x tall woman give birth to short parent, the dominant allele is
offspring
expressed, thus tall allele is
dominant over short allele.
(ii) What are the genotypes of an
individual?
Homozygous recessive
An individual showing recessive
Recessive allele can only be
Homozygous dominant /
Heterozygous
phenotype
Use test cross – cross an individual
with a homozygous recessive
individual
expressed in homozygous condition.
If two phenotypes are produced, the
individual is heterozygous; if one
phenotype is produced, the
individual is homozygous dominant.
(iii) Probability of an
individual with a character
Work out the genetic cross and
calculate the probability
Explain with genetic diagram
The diagram below shows a pedigree for the inheritance of a human genetic disease caused by a single gene.
(a) Which allele, the normal allele or the diseased allele, is dominant? Explain your answer without using
a genetic diagram.
(4 marks)
Normal allele (1). Individual 3 marries individual 4 and give birth to a diseased son (1), the diseased son
must receive the diseased allele from either parent, thus one of the parents is heterozygous (1). In
heterozygous condition, the dominant allele is expressed, as the parents are both normal, the normal allele is
expressed and thus is dominant (1).
(b) Deduce, with reasons,
(i) the genotype of individual 2.
(4 marks)
Individual 2 is heterozygous (1). She gives birth to a daughter who has the disease, as the diseased allele is
recessive, her daughter must be homozygous recessive (1). In homozygous recessive condition, the daughter
must receive one recessive allele from her mother, individual 2, thus individual 2 has one recessive allele
(1). Individual 2 is normal, she must also possess the normal allele (1). Thus she has one recessive diseased
allele and one normal allele, she is heterozygous.
(ii) the genotype of individual 8.
(2 marks)
Individual 8 is homozygous recessive (1) because she is diseased which is a recessive character. (1)
(c) If individual 3 and 4 are going to have another child, what is the probability of this child having the
disease? Illustrate your answer with a genetic diagram.
(5 marks)
Probability = ¼ (1).
Let B be the normal allele and b be the diseased allele (1)
Parents :
Gamete
B
Individual 3
Bb
b
Genotype of offspring
Phenotype of offspring
BB
normal
x
B
Bb
normal
Individual 4
Bb
b
Bb
normal
bb
diseased
(1)
(1)
(1)
(1)
Sex-linked gene inheritance – e.g. haemophilia and red-green colourblinded, the allele is found on X
chromosome, Y chromosome has no corresponding allele because X chromosome is longer than Y
chromosome.
Let H be the allele for normal (no haemophilia), h be the allele for haemophilia. H is dominant over h.
A carrier female has the allele of haemophilia, but is masked by the dominant normal allele, thus she does not
have haemophilia; however, there is 50% that she passes the allele to male and makes 50% of her son
haemophilia. There is no male carrier. Male can only be normal or haemophilia.
Multiple allele – there are 3 or more alleles at a gene locus e.g. blood group – there are three alleles at the same
gene locus, an individual can have two of the three alleles:
The three alleles are :
The possible blood group and genotypes are:
An example of cross diagram between AB and O
Dihybrid cross – a cross that involves 2 gene loci
Let T be the tall allele, t be the short allele; C be the coloured allele, c be the recessive allele
For example, a cross between tall, coloured parent plants (TtCc X TtCc), and the use of Punnett Square
Mendel’s second law / law of independent assortment : the separation and combination of the two alleles for
one gene is NOT affected by the separation of the two alleles for the other gene.
Test cross
 a genetic cross between an individual with the dominant phenotype with a recessive phenotyped parent
(genotype : homozygous recessive)
 the purpose is to find out the genotype of the individual with the dominant character
For example :
Let T be the allele for tall allele, t be the short allele. Tall allele is dominant over short allele.
A tall plant can have genotype TT or Tt.
As seen from the cross diagrams, if the tall plant is TT, all the offspring will be tall, for Tt, the phenotypic ratio
Tall : short = 1 : 1
Variation
(i) Continuous variation – character that cannot be divided into sharp distinct groups, there is always intermediate
phenotypes between two distinct groups
Examples : Weight, height, length of fingers, I.Q.
Continuous variation are characters which are strongly affected by the environment e.g. food supply, they are
normally controlled by a large number of genes.
(ii) Discontinuous variation – character that can be separated into sharp distinct groups, no intermediate between
groups
Examples : tongue rolling, ear-lobed
Discontinuous variation are characters which are not affected by the environment, they are normally
controlled by one or a few genes. Their pattern of inheritance can be followed by Mendel's law.
(iii) The causes of variation
 Independent assortment of chromosomes in meiosis produces a large number of different gametes.
 Random fertilization of different gametes by chance makes the offspring of the same parents different.
 Mutation – an inherited change in the genetic material/DNA
(iv) Environment
Chemical structure of DNA (deoxyribonucleic acid)
DNA is a polymer made up of a basic unit, nucleotide. A nucleotide has 3 parts : phosphate, deoxyribose (5-C
sugar) and nitrogen base. There are 4 types of nitrogen bases : adenine (A), thymine (T), guanine (G) and
cytosine (C).
DNA is made up of 2 strands of polynucleotide. One strand is running opposite to the other. The two strands run
spirally, thus it is called double helix. “A” always bind with “T” and “G” always bind with “C” by hydrogen
bondings, thus it is called complementary base pair.
DNA replication
Protein synthesis
(i) Transcription of DNA inside the nucleus
One of the two strands in DNA is used
as a template for making mRNA.
(Note : RNA has no thymine but it has
uracil [U], “A” binds with “U” and “C”
binds with “G”.
RNA polymerase is the enzyme that
catalyses the formation of mRNA.
The base sequence of DNA determines
the base sequence of mRNA.
(ii) Translation of mRNA to make protein
tRNA has anticodon that is
complementary to the codon
of mRNA.
The anticodon determines
a specific amino acid
carried by the tRNA. Thus
the base sequence of mRNA
determines the amino acid
sequence of the protein.
Codon
 3 consecutive bases on the mRNA, it is determined by the triplet genetic code of DNA
 the codons on one mRNA is non-overlapping
 the codons are degenerate [there are 20 amino acids determined by 43 = 64 codons, thus one amino acid is
determined by 2 or more codons], in the codons that determine the same amino acid, the first two bases are
the same while the third base can be different.
 the codons are universal [all living organisms use the same codon for the same amino acid]
 some codons do not determine any amino acids, they are stop signals e.g. UAG and UAA, translation stops
at these codons

the codon AUG acts as a start signal because translation starts at this codon, it determines the amino acid
“methionine”.
Triplet genetic code
 3 consecutive bases on the DNA
Recombinant DNA technology
A vector is used to transfer a gene / a segment of
DNA from the donor into another organism e.g.
virus DNA (viral DNA), bacterial plasmid. In
this example: plasmid acts as the vector.
A plasmid is a smaller circular / ring of
extrachromosomal DNA in bacteria.
Restriction enzyme [endonuclease / restriction
eodonuclease]
 recognizes specific base sequence and cut
the DNA at specific site


some restriction enzymes (not all) produce
sticky ends when cutting the DNA [Sticky
end is a single strand base sequence of a
DNA that readily binds to a single strand
base sequence of another DNA that are
complementary.
restriction enzymes are specific
Ligase is an enzyme that joins DNA together.
The recombinant plasmid contains a foreign DNA. It is then put into a host cell e.g. in this example, E. coli is
the host cell. E. coli is now a Genetically Modified Organism (GMO). When the GMO is an organism that
produces a food for human, the food is called GM food.
Advantage of human insulin as a drug:
 Less side effects e.g. chance of rejection, allergy is lower
 Less impurities
 A large quantity can be produced in a shorter time  cheaper