4.1 chromosomes, genes, alleles & mutations

4.1 Chromosomes, Genes, Alleles and Mutations

Topic 4 Genetics

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.

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.

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.

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.

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.

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)

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.

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

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)

KaryotypingMale Karyotype Female Karyotype

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.

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