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Topic 4 Genes, Chromosomes
Offspring acquire genes from
parents by inheriting
Each gene in an organism’s
DNA exists at a specific locus
on a certain chromosome.
We inherit one set of
chromosomes from our
mother and one set from our
Comparison of Asexual and Sexual
In asexual reproduction,
a single parent produces
genetically identical
offspring by mitosis.
Sexual reproduction
combines sets of genes
from two different
parents, forming
genetically diverse
Fertilization and Meiosis alternate
in sexual life cycles
Sets of chromosomes—
Normal human somatic cells
are diploid. They have 46
chromosomes made up of two
sets of 23. --- one set from
each parent. There are 22
pairs of autosomes, each with
a maternal and paternal
homolog. The 23rd pair , the
sex chromosomes, determine
gender (XX) or (XY).
Behavior of chromosome sets in
the Human Life Cycle
At sexual maturity, ovaries
and testes (the gonads)
produce haploid gamete by
meiosis, each gamete
containing a single set of 23
chromosomes (n=23). During
fertilization, an egg and
sperm unite, forming a diploid
(2n = 46) single celled zygote,
which develops into a
multicellular organism by
Meiosis reduces the number of
chromosome sets from diploid to
The stages of meiosis
produce four haploid
daughter cells. The
number of chromosome
sets is reduced from two
(diploid) to one (haploid)
during meiosis I, the
reductional division.
A comparison of Mitosis and
Meiosis is distinguished from
mitosis by three events of
meiosis I
Prophase I: each homologous
pair undergoes synapsis and
crossing over.
Metaphase I: Chromosomes
line up as homologous pairs
on metaphase plate.
Anaphase I: Homologs
separate from each other;
sister chromatids remain
joined at the cetromere.
Meiosis II What’s left?
Separation of sister
Genetic variation produced in
sexual life cycles contributes to
Three events in sexual
reproduction contribute
to genetic variation in a
Independent assortment of
chromosomes during meiosis.
chromosomes can line
up in no particular order
of paternal or maternal
Crossing over during meiosis I
In prophase I of meiosis I,
the replicated homologous
pair of chromosomes
comes together in the
process called synapsis,
and sections of the
chromosomes are
exchanged. You can see
that after crossing over,
the resulting
chromosomes are neither
entirely maternal nor
entirely paternal, but
contain genes from both
parents. Synapsis and
crossing over occur only
Random fertilization of egg cells by
During random
fertilization, any one of
the 8,388,608 possible
combinations of
gametes .... give rise to
sperm cells, which will
fertilize an egg and
result in the
Do to sister chromatid
cohesion, crossing over
leads to chiasmata,
which hold homologs
Evolutionary Significance of
Genetic Variation Within
Genetic variation is the raw
material for evolution by
natural selection.
Mutations are the original
source of this variation.
The production of new
combinations of variant genes
in sexual reproduction
generates additional genetic
Mendel’s Law of Independent
States that when gametes are
formed, the separation of one
pair of alleles between the
daughter cells is independent
of the separation of another
pair of alleles.
One allele does not follow
another when it is passed on
to a gamete—they will sort
Independent Assortment and
Why do traits get passed
on independently of one
1. The orientation of
bivalents during
metaphase 1
Compare Mitosis to Meiosis
DNA replication
Occurs during
interphase before
Occurs during
interphase before
meiosis I begins
Number of divisions
One, including
anaphase and
Two, each including
Synapsis of
Does not occur
Occurs during
prophase I with
crossing over
Number of
daughter cells
and genetic
Two, each diploid
(2n) and
identical to the
parent cell
Four, each haploid (n)
, containing half as
many chromosomes
as the parent cell;
genetically different
from the parent cell
and from each other
Role in the body
Enables multicellular
adult to arise from a
zygote, produces
cells for growth,
repair, and in some
species, asexual
Produces gametes,
reduces number of
chromosomes by half,
and introduces
genetic variability
among the gametes