Eukaryotic chromosomes are made of DNA and proteins. A gene is a heritable factor that controls a specific characteristic. Alleles are different forms of the same gene that occupy the same locus. A genome is the complete set of genetic material of an organism. Gene mutations, such as base substitutions, can occur which may lead to genetic disorders like sickle-cell anemia caused by a mutation replacing glutamic acid with valine.

1. 4.1 Chromosomes, Genes, Alleles
and Mutations
Topic 4 Genetics

2. Genetics
 4.1.1 State that eukaryote chromosomes are made of DNA
and proteins.
 The names of the proteins (histones) are not required, nor
is the structural relationship between DNA and the
proteins.

3. Genetics
 4.1.2 Define gene, allele and genome.
 Gene: a heritable factor that controls a specific
characteristic. (The differences between structural genes,
regulator genes and genes coding for tRNA and rRNA are
not expected at SL).
 Allele: one specific form of a gene, differing from other
alleles by one or a few bases only and occupying the same
gene locus as other alleles of the gene.
 Genome: the whole of the genetic information of an
organism.

4. Genetics
 4.1.3 Define gene mutation.
 The terms point mutation or frameshift mutation will not
be used
 4.1.4 Explain the consequence of a base substitution
mutation in relation to the processes of transcription and
translation, using the example of sickle-cell anemia.
 GAG has mutated to GTG causing glutamic acid to be
replaced by valine, and hence sickle-cell anemia.

5. Genetics
 Aim 8: There is a variety of social issues associated with
sickle-cell anemia, including the suffering due to anemia,
personal feelings if one has either inherited or passed on
the sickle-cell allele, questions relating to the desirability
of genetic screening for the sickle-cell allele before having
children, and the genetic counselling of carriers of the
allele.
 There are also ethical issues relating to screening of
fetuses and abortion of those found to have a genetic
disease.

6. Genetics
 Genetics is the study of variation and inheritance.
 The basic unit of inheritance is the gene.
 A gene is a heritable factor that controls a specific
characteristic.
 A gene is a segment of chromosome that contain the code
required to direct the manufacture of a polypeptide or
RNA molecule.
 All the genes of an organism are known collectively as the
genome.

7. Alleles
 The locus of a gene is its position on a chromosome. The
same gene will have the same locus on the same
chromosome.
 Different forms of a gene are called alleles. An allele is a
form of a gene differing from other alleles of the gene by
a few bases at most and occupying the same locus as the
other alleles of that gene.
 One of the most studied sets of alleles are the alleles
controlling blood type:
 Controlled by 3 alleles A (IA
), B (IB
) O (i)

9. Chromosomes
 Genes are made up of DNA (deoxy ribonucleic acid).
 Genes are part of a larger molecule called chromosome.
 Eukaryotic chromosomes are are made up of DNA and
proteins.
 When DNA coils up to form a chromosome, it wraps
around special proteins called histones to form a
nucleosome.
 A nucleosome consists of DNA wrapped around 9 histone
proteins.

10. Human Chromosomes
 Chromosomes are visible when the cell starts to divide.
 Human body cells have 46 chromosomes;
 23 pairs of chromosomes
 one set from the mother and one set from the father
 22 pairs of autosomes (non sex chromosomes)
 1 pair of sex chromosomes
 XX –female
 XY – male

11. Karyotyping
 A complete set of chromosomes is called a karyotype.
 Each chromosome has genes specific for that
chromosome making it identifiable.
 Karyotyping is arranging the chromosomes in pairs
according to their structure.
 The chromosomes are arranged depending upon:
 Their length
 The position of their centromere
 Karyotyping can be used to detect chromosome
aberrations in foetuses.
 eg: An amniocentesis to check for Downs syndrome (47
Chromosomes)

12. Karyotyping
Male Karyotype Female Karyotype

13. Gene Mutations
 Genes are nearly always passed from parent to offspring
without mistakes.
 Occasionally a change will occur in a gene.
 This is called a Gene Mutation.
 Gene mutations can be:
 Base substitution – one base is changed for another
 Frameshift mutation:
 Insertions - insertion of one or more bases
 Deletions – deletion of one or more bases
 An example of a base substitution is Sickle-cell anaemia.

14. Sickle-cell anaemia
 Mutation in hemoglobin gene
 GAG has mutated to GTG (a base substitution)
 In translation, this causes causing glutamic acid to be
replaced by valine
 Hemoglobin has a different shape
 RBC becomes sickle-shaped
 Can block capillaries

15. Genetics
 4.1.1 State that eukaryote chromosomes are made of DNA
and proteins.
 The names of the proteins (histones) are not required, nor
is the structural relationship between DNA and the
proteins.

16. Genetics
 4.1.2 Define gene, allele and genome.
 Gene: a heritable factor that controls a specific
characteristic. (The differences between structural genes,
regulator genes and genes coding for tRNA and rRNA are
not expected at SL).
 Allele: one specific form of a gene, differing from other
alleles by one or a few bases only and occupying the same
gene locus as other alleles of the gene.
 Genome: the whole of the genetic information of an
organism.

17. Genetics
 4.1.3 Define gene mutation.
 The terms point mutation or frameshift mutation will not
be used
 4.1.4 Explain the consequence of a base substitution
mutation in relation to the processes of transcription and
translation, using the example of sickle-cell anemia.
 GAG has mutated to GTG causing glutamic acid to be
replaced by valine, and hence sickle-cell anemia.

18. Genetics
 Aim 8: There is a variety of social issues associated with
sickle-cell anemia, including the suffering due to anemia,
personal feelings if one has either inherited or passed on
the sickle-cell allele, questions relating to the desirability
of genetic screening for the sickle-cell allele before having
children, and the genetic counselling of carriers of the
allele.
 There are also ethical issues relating to screening of
fetuses and abortion of those found to have a genetic
disease.