Download Genetic constitution of a population

Genetic constitution of a
population
A Review of Mendelian
Genetics
Lecture 2
Aim
• To review with students terms and
principles of population genetics
Lecture objectives
• By the end of the topic, students
should be able to
• Define terms related to genetic constitution
of a population
• Define genes and the link between genes
and population
• Explain gene discovery
• Explain the process of cell division and
inheritance
Definition of some genetic terms
Slide taken from: Julius van der Welf, GEN 251- Applied
Animal Breeding Notes
A Population
A breed, strain or variety or herd that
breed together and exchange genetic
material more than the rest
Genetically it is a breeding group, unlike
individuals
Breed, Variety or Strain
Breed is a homogenous sub-specific
group of domestic livestock with
definable and identifiable external
characteristics
These characteristics enable it to be
separated by visual appraisal from other
similarly defined groups within the species
Variety or strain is a subdivision of a breed
that has been developed with special
characteristics such as size, early maturity,
coat type etc
Breed, Variety or Strain
A population of chickens
A Ross broiler strain
Breed, Variety or Strain
Different breeds of goats
A population of
chickens
A Population
Must always be defined and decisions
should be made where one population
ends and another one begins
Shows continuity in time
Under similar conditions, remains
practically unchanging in both quality
and number over an indefinite time
A Population
Practically real population changes over
time
After some generations a large breeding
herd may lose its identity through
dispersal and cannot be identified as a
population any more
This may also occur in a similar way to a
breed in the course of a changing
economic environment
A Population
 In a population,
generations overlap
 A population is a
collection of individuals
 Individual is a collection of
genes
 In quantitative genetics,
the centre of interest is the
population and its qualities
or level of performance
Defining a Gene
Genes are genetic factors
These are hereditary units of animals
They are maintained in a population
And are passed from one generation to
another
Defining a Gene
When goats are mated, one expects a
goat and not sheep
Same for cattle, poultry
And these resemble each other
This shows that they share a common
property that is inherited
And this is a gene
Where are genes located?
 An animal body contains
cells, that contain nucleus
 In the nucleus, are
chromosomes – rod like
structures
 On the chromosomes are
DNA – long double strand
 It is this DNA that contains
encodes, genetic
information that is
inherited
Chromosome
A structure that is seen as rod – like during
cell division under microscope that is in
the nucleus of cells, on which genes are
located
There are several genes but few chromosomes
This gives evidence that genes are on
chromosomes
Chromosome
Chromosomes occur in pairs, a member
being called homologous chromosomes
The number of chromosomes differ
(diploid – pairs) between species
Species
Haploid (n)
Diploid (2n)
Cattle
30
60
Sheep
27
54
Goats
30
60
Pigs
19
38
Chickens
39
78
Humans
23
42
Chromosome pair
It is these genes on the chromosome that are
responsible for colour, shape, position, height
and numerous other characteristics, generally
called traits
Position on a chromosome where a particular
gene is located is called a locus (pl, loci)
Chromosome pair
Types of chromosomes
Autosomes
Are
homologus –
two copies of
each
carrying
same genes.
But these
genes may
be of
different
forms - alleles
Sex chromosomes
Are responsible for sex
characteristics
In most species, they
carry different genes
and are of different sizes
Mammals have XX in
females, XY in males
Birds have ZW in females,
ZZ in males
Cell division
Life begins with a single cell which is a
fertilised egg (zygote)
This has diploid number of chromosomes
that divides into daughter cell
Each cell receives a full complement of
chromosomes
This process is called mitosis
The process repeats and cells continue
to divide to form different parts and
tissues of the body
Cell division
What happens is that the chromosomes
double in number
Duplicate pairs then separate, one pair to
each of the daughter cells
This maintains the number of
chromosomes in all parts of the body
This process starts very early in embryonic
stage
Gamete formation
The zygote is formed by the fusion of two
special cells, the gametes
One gamete comes from each of the
parents
A sperm from the father
An unfertilised egg from the mother
Only half the number of chromosomes
appear in each gamete
i.e. only one of each member of homologous
chromosome pairs go into each gamete
Gamete formation
This process maintains number of
chromosomes from generation to
generation
This process of cell division is called
meiosis
The number of chromosomes that appear
in the gamete is called haploid number
The choice of which chromosome goes
into each gamete is random
Gamete formation
The diploid number is restored during
fertilisation
The sperm and the egg each contribute
haploid number to the fused zygote
The zygote now has normal diploid
number of chromosomes according to
the species
The zygote then starts the process of
mitosis throughout its life
Inheritance
It is chromosomes that are passed from
parents to offspring through gametes
This means those genes on the
chromosomes will be the ones transmitted
to the offspring
If homologous chromosomes contained
genes that influence a trait differently
(allele) then two offspring will look
different, hence the observed variation
Inheritance
Since chromosomes occur in pairs, genes
also function in pairs
However, on a single locus, there may be
one or more alternative forms of a gene,
and this is called an allele
These alleles have different influence on a
trait
Gene discovery
It all started by Gregor Mendel
Who was working on peas in the 19th
Century
And published his work in 1886
That there are certain characteristics that
are inherited
Based on certain rules and hypotheses
Gregor Mendel
• Studied basic genetic principles
• Used pea plants
• Theory was not well understood during his
life
• Mendel, G. 1866. Experiments on Plant
Hybridization. Transactions of the Brünn Natural
History Society.
• http://www.esp.org/foundations/genetics/classical/gm-65.pdf
Slide taken from: Dr. David Buchanan, ANSI 3433 - Animal Breeding Notes
Gregor Mendel
https://www.google.com/#q=Mendel%2C+G.+1866.+
Mandatory reading
• Chapter 2, pages 7 – 19 of Geoff
Simm ~ Genetic Improvement of
Cattle and Sheep