Blueprint of Life
Topic 11: Chromosomes, Meiosis and Gamete Formation
Biology in Focus, HSC Course
Glenda Childrawi, Margaret Robson and Stephanie Hollis
DOT POINT(s)
 explain the relationship between the structure and
behaviour of chromosomes during meiosis and the
inheritance of genes
 explain the role of gamete formation and sexual
reproduction in variability of offspring
Introduction
Today we know that every species has a characteristic number of
chromosomes in every body cell (eg: 46 chromosomes in humans).
This number does not change from one generation to the next.
youshotmedown.wordpress.com
Introduction
In order to maintain the constant chromosome number from
one generation to the next, a mechanism called meiosis
(reduction division) occurs to halve the chromosome number
when gametes are produced.
anthro.palomar.edu
Every parent cell contains two sets of chromosomes— one
paternal set and one maternal set, resulting in the diploid
number.
Introduction
During meiosis, homologous
pairs of chromosomes
segregate so that each gamete
receives only one copy of every
chromosome— that is, the
gametes are haploid.
en.wikipedia.org
Introduction
The diploid number is restored
when the gametes fuse in
fertilisation to form a zygote.
www.ubqool.com
Introduction
Meiosis has the added role of mixing the paternal and maternal
chromosomes (and even the paternal and maternal genes when
crossing over occurs) and these recombined chromosomes are
passed into gametes, thereby increasing the genetic variation in
offspring produced.
www.sciencelearn.org.nz
Chromosomes
Chromosomes are compact coils of thread-like molecules called
DNA (deoxyribonucleic acid), organised around proteins called
histones. A helpful analogy is to imagine the DNA as cotton
thread that can be wound around a ‘cotton reel’ of protein.
ghr.nlm.nih.gov
Chromosomes
In this packaged form, the ‘reels of cotton’ represent
chromosomes; in the unwound form, the network of threads
created would be the chromatin material (dispersed DNA) seen
in the nucleus of a working (non-dividing) cell.
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Chromosomes
It is much easier for DNA to be transported (e.g. from one cell to
another during cell division) in its compact chromosome form.
This is why chromosomes become visible at the start of mitosis
and meiosis.
course1.winona.edu
Chromosomes
Chemically, each gene is made up of
a portion of DNA that stores
information as a coded sequence,
and each coded sequence/gene is
located at a particular site or locus
on the chromosome. The coded
information within genes determines
how living things look, behave and
function—that is, it influences
particular characteristics
(phenotypes). A chromosome can
therefore be described as a linear
sequence of genes.
biology.unm.edu
Genetic Variation
During meiosis, genetic
variation arises as a result of the
behaviour of chromosomes at
two stages:
1. during crossing over
2. when chromosomes
randomly segregate and
paternal and maternal
chromosomes assort
independently of each other.
www.studyblue.com
Genetic Variation
During meiosis I:
1. chromosomes line up in homologous pairs (one maternal and one
paternal chromosome in each pair) during prophase I
course1.winona.edu
Genetic Variation
During meiosis I:
2. crossing over occurs—arms of homologous chromosomes
exchange genetic material (during metaphase). This introduces
genetic variation— genes that occur on the same chromosome are
said to be linked. Crossing over (synapsis) ensures that linked
genes on a chromosome can be inherited independently of each
other.
course1.winona.edu
Genetic Variation
During meiosis I:
The exchange of genetic material
between homologous
chromosomes during crossing
over causes the mixing of paternal
and maternal genes and the result
is an increased number of
combinations of genes that may
be transmitted by gametes to
offspring, thereby increasing
genetic variation.
www.phschool.com
Genetic Variation
During meiosis I:
3. the chromosomes in each pair of chromosomes separate
(during anaphase I), so that one entire chromosome of each pair
moves into a daughter cell.
biology.unm.edu
Genetic Variation
During meiosis I:
The manner in which these
chromosomes separate is termed:
 independent assortment: the paternal
and maternal chromosomes sort
themselves independently of each
other.
This separation of chromosomes, also
referred to as random segregation,
ensures the chromosome number in the
resulting gametes will be half that of the
original cell.
www.phschool.com
Genetic Variation
During meiosis I:
For example, the maternal chromosomes do not all move into one
gamete and the paternal into another. Which chromosome of each
pair ends up in a gamete is random and determined completely
independently of the separation of any other gene pair.
Genetic Variation
During meiosis I:
As a result, mixing of maternal
and paternal chromosomes
occurs and so independent
assortment leads to further
genetic variation. There are 223
possible different combinations
of chromosomes in the formation
of a human gamete.
Genetic Variation
During meiosis II:
The two daughter cells that result from meiosis I each undergo
meiosis II, which is similar to mitosis, and the behaviour of
chromosomes in the second meiotic division does not further
affect genetic variation.
biology.unm.edu
Genetic Variation
Mendel did not know about
genes and chromosomes; Sutton
and Boveri noted the similarity
between the behaviour of
chromosomes and Mendel’s laws
(1904). Units of heredity on
chromosomes were later termed
genes (1909) and their
inheritance patterns are now
used to explain the ratios
derived in Mendel’s laws.
ar.wikipedia.org
Genetic Variation
The term variability means something different to variation. In
the preliminary course, we learnt that variation is evident in
individuals (e.g. differences in colour and height). Variability in
genetics relates to the different forms of a gene within a
population (that is, the total of all alleles present in the gene pool of a
population).
evolution-khc.wikispaces.com
Genetic Variation
Both variation and variability
may have one of three origins:
■ genetic
■ environmental
■ a combination of both genes
and the environment.
Both variation and variability are
of evolutionary advantage only if
they have a genetic basis.
www.biologyaspoetry.com
Gamete Formation and Variation
We know gametes form by
meiosis, where recombination
of genetic material takes place
as a result of crossing over and
random segregation.
biology.unm.edu
Gamete Formation and Variation
Gametes that arise from genetically dissimilar parents (crossfertilisation as opposed to self-fertilisation) are likely to differ from
each other more than those produced by self fertilisation. Crossfertilisation produces a greater variety of gametes, increasing
variability.
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Fertilisation and Variability
Gametes contain different,
recombined genetic material
and the possibility of many
different combinations of
gametes fusing (that is, which
sperm cell fuses with which egg cell)
increases variation. New
combinations of genes
occurring in the offspring lead
to greater variability within a
population.
drugline.org
Fertilisation and Variability
Example:
One parent may have blue eyes and
fair hair, the other dark eyes and dark
hair. If the traits for hair colour and
eye colour assort independently
from each other, their gametes may
combine to produce offspring that
have blue eyes and dark hair or
brown eyes and fair hair. In this case,
the offspring have a different
combination of the parents’ genes,
increasing variation.
www.teach4real.com
Fertilisation and Variability
Further variability arises in a population if a greater number of
alleles are present for each gene. If within the population there are
individuals with red hair and green eyes, there is greater
variability and an even greater opportunity for more gene
combinations to arise in gametes produced by individuals.
www.bbc.co.uk
Fertilisation and Variability
Variability may be increased as a result of:
■ a recombination of genetic material
■ an increased number of alleles for a particular gene.
morningside.australialisted.com
Importance of Sexual Reproduction
Sexual reproduction is important in increasing genetic variability
(the amount by which individuals in a population vary from each other
genetically). The study of this phenomenon is called population
genetics.
ajilbab.com
Importance of Sexual Reproduction
Mechanisms of sexual reproduction, such as gamete formation
(meiosis) and fertilisation, increase gene recombination and
therefore variability in a population. Evolutionary studies show
that greater variability improves the ability of a population to
adapt to changes in the environment, resulting in an increased
chance of survival.
www.photographyblogger.net
Importance of Sexual Reproduction
If there is little or no variability within a population, the result is a
static or unchanging population that is less likely to be able to
adapt to sudden changes in the environment and is more likely to
be wiped out.
www.gizmodo.com.au
Activity/Homework
-Long PPT, NO Activity!!
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