Ch 10: Genetic Change and Variation

Variation forms the basis of evolution. There are two basic forms:

1 Continuous variation where individuals in a population shows a gradation from one extreme to the other.

2 Discontinuous variation where there is a limited

number of distinct forms within the population.

10.1 Methods of Recording Variation 10.1.1 Table of data

10.1.2 Line graph

10.1.3 Histogram

10.1.4 Bar graph

10.1.5 Kite graph 10.1.6 Pie chart

10.2 Types of Variation

10.2.1 Continuous variation Characteristics within a population vary only very

marginally between one individual and the next

a graduation from one extreme to the other examples: weight, height, IQ, EQ, etc Characteristics which show continuous variation are

controlled by the combined effect of a number of genes (polygenes) - a polygenic character

The random assortment of genes during

metaphase I of meiosis ensures that individuals possess a range of genes from any polygenic

complex:

all tall genes very tall

all short genes very short

about 1/2 tall and 1/2 short genes

intermediate height

10.2 Types of Variation

10.2.2 The normal distribution curve

The mean (arithmetic mean) is the average of a group of values.

The mode is the single value of a group which occurs most often.

The median is the central or middle value of a set of values.

10.2.2 The normal distribution curve

The standard deviation is a value which gives an indication of the range of values on either side of the mean.

10.2.3 Discontinuous (discrete) variation

characters which do not show a gradation between extremes but fall into a number of distinct forms

usually controlled by a single gene which may have 2 or more alleles

10.3 The chi-squared test (not required in syllabus) 10.4 The t-test (not required in syllabus)

10.5 Origins of Variation

1. Environment

2. Genetic change: reshuffling of genes and mutation

10.5.1 Environmental effects Phenotype is the result of its genotype and effect of the

environment. Because environmental influences are themselves very

various and often form gradations,

e.g. temperature, light intensity, etc., they are largely responsible for continuous variation

within a population.

10.5.2 Reshuffling of genes

- creating new combinations during sexual reproduction

by:

1 Mixing two different parental genotypes where cross fertilization occurs

2 Random distribution of chromosomes during

metaphase I of meiosis

3 Crossing over between homologous chromosomes

during prophase I of meiosis

10.5 Origins of Variation

Mutation Mutation is any change in the structure or the amount

of DNA of an organism. Most mutations occur in body cells and do not pass to

offspring. Only those that affect gametes can be inherited and produce sudden and distinct differences between individuals discontinuous variation

10.5.3 Changes in gene structure (point mutation)

Gene mutation (point mutation) is a change in the structure of DNA which occurs at a single locus on a chromosome

gene mutation

wrong sequence of amino acids

no enzyme

absence of a character, e.g. pigment

There are many forms of gene mutation:1. Duplication - a portion of the nucleotide chain

becomes repeated2. Addition (insertion) - an extra nucleotide sequence

becomes inserted in the chain3. Deletion – a portion of the nucleotide is removed

from the chain 4. Inversion – a nucleotide sequence separates and

rejoins at original position5. Substitution – one of the nucleotides is replaced by

another with a different base

10.5.3 Changes in gene structure (point mutation)

example: sickle-cell anaemia is the result of the replacement of just one base in the

DNA molecule causing the wrong amino acid being joined into two of the polypeptide chains which make up the haemoglobin molecule.

but the disease is resistant to malaria !

10.5.3 Changes in gene structure (point mutation)

Sickle-cell anaemia

Sickle-cell anaemia

Sickle-cell anaemia

10.5.4Changes in whole sets of chromosomes

Polyploidy is the possession of more than 2 complete sets of chromosomes.

e.g. triploid means 3 sets; tetraploid means 4 sets. Formation of tetraploid offspring:

fertilization of diploid gametes or

whole set of chromosomes doubles after fertilization Formation of triploid offspring:

Fertilization of a diploid gamete with

a normal haploid gamete

Autopolyploidy – polyploidy within the same species Autopolyploidy can be induced by colchicine

(a chemical) which inhibits spindle formation and so prevents chromosomes separating during anaphase.

Triploids are sterile because they cannot form complete homologous pairings.

If, however, a hybrid has a chromosome number which is

a multiple of the original chromosome number,

a new fertile species is formed,

e.g. wheat (n=42) is the cross between

wild grass (n=14) and emmer wheat (n=28)

Allopolyploidy –

A fertile species having a chromosome number which is

a multiple of the original haploid number Allopolyploidy is rare in animals, but relatively

common in plants, including many food plants,

e.g. wheat, coffee, banana, sugar cane, apple, tomatoes, etc The polyploid variety often have advantages,

e.g. large fruits, tomatoes have more vitamin C, etc.

10.5.5 Changes in chromosome number

Non-disjunction occurs when one of the homologous chromosomes (23 pairs) fails to segregate during meiosis, gametes formed have 22 & 24 chromosomes. This is often fatal.

Down's syndrome :

47 chromosomes (+ extra 21st chromosome)

Down's syndrome often occurs in

ova formation rather than sperms,

especially in old age pregnancies.

Turner's syndrome: Have one missing X chromosome XO with 45 chromosomes Females with small stature & sexually immature

Klinefelter's syndrome: Genotypes are XXY, XXXY or XXXXY Males with small testes but no sperms, with

breast development and female figures It indicates that Y is the cause of maleness

10.5.6 Changes in chromosome structure

This occurs in meiosis when crossing over takes place.

1 Deletion

2 Inversion

3 Translocation

4 Duplication

84-II-4

10.6 Causes of Mutations There is natural mutation rate which varies from

one species to another. Animals with shorter life cycles show a greater rate of

mutation because of more frequent meiosis. This natural mutation rate can be increased artificially

by certain chemicals e.g. colchicine, formaldehyde, nitrous acid & mustard gas

or energy sources (mutagens), e.g. uv rays, X rays,

rays, & particles and neutrons

10.7 Genetic Screening and Counselling –

To risk for a mother to have babies with certain genetic diseases could be calculated, if enough information of the disease in the family is known,

e.g. Down's syndrome, haemophilia. On the basis of this advice parents can choose whether or

not to have children. Doctors can diagnose certain genetic defects, e.g. Down's syndrome, in a foetus, by studying samples

of cells taken from the amniotic fluid which surrounds the foetus – a process called amniocentesis.

Parents can then decide to have the pregnancy terminated.

10.7.1 Gene tracking To find out on which chromosome a defective

gene is located. Blood groups are traced in families to act as gene

markers. Correlation between certain blood groups alleles and the occurrence of a genetic disease can determine whether or not the gene for the disease is on the same chromosome as that for blood groups.

If one genetic marker is not linked to the disease in question another must be tried and so on until the one which shows linkage with the disease is found.

Linked markers are then used to work out if someone carries a disease.

19. 88-II-4 (b)

What is the genetic basis of

(i) Hybrid vigour, (5 marks)

(ii) and the determination of the ABO blood groups? (4 marks)