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Meiosis Introduction
Production of the Sperm and Egg
Genetics and Heredity
• The transmission of traits from one generation
to the next is called heredity or inheritance.
• However, offspring differ somewhat from
parents and siblings, demonstrating variation.
• Genetics is the study of heredity and variation.
Genes
• Parents pass on genes to their offspring
• Genes program specific traits that emerge as
we develop from fertilized eggs into adults.
– Example: Your may have a gene for blue eyes,
which you inherited from your mother.
• Genes are segments of DNA.
• In plants and animals, sperm and ova
(unfertilized eggs) transmit genes from one
generation to the next.
Chromosomes
• Sex chromosomes: determine an individuals
sex
– XX = Female
– XY = Male
• Autosomes: any chromosome that is not a sex
chromosome
Types of cells
• Somatic cell: any cell other than the sperm or
ovum
– Contains 46 chromosomes
– diploid: has two sets of chromosomes (2n)
• Gametes: reproductive cells – the sperm and
ovum
– Contains 23 chromosomes
– haploid: only have one set of chromosomes (1n)
Homologous chromosomes
• Homologous chromosomes: The
chromosomes that make up a pair
• Pairs are due to sexual reproduction: we
inherit one chromosome of each pair from
each parent.
– One from mommy one from daddy!  
• They carry the same genes controlling the
same inherited characteristics
• They have the same length,
centromere position, staining
pattern
• Also known as “homologues”
Purpose of Meiosis
• The purpose of meiosis
is to produce haploid
cells: the sperm and
ovum
• This way, when the
sperm and ovum fuse
the baby will have a full
set of chromosomes
• Sperm (23) + ovum (23)
= baby (46
•The fertilized egg (zygote) now
chromosomes)
has two chromosomes bearing
genes from the maternal and
paternal family lines.
Life Cycle
• Fertilization and
meiosis
alternate in
sexual life
cycles.
• Karyotype: a display of the 46 chromosomes
(shows 23 pairs of chromosomes)
Normal Karyotype
Meiosis Gone Wrong
• What happens if meiosis does not work
correctly?!
Down Syndrome
characteristic facial features, short stature; heart defects
susceptibility to respiratory disease, shorter lifespan
prone to developing early Alzheimer's and leukemia
often sexually underdeveloped and sterile, usually some
degree of mental retardation.
Patau Syndrome
Results in serious eye,
brain, circulatory defects
as well as cleft palate.
1:5000 live births.
Children rarely live more
than a few months.
Edward Syndrome
Almost every organ
system affected 1:10,000
live births. Children with
full Trisomy 18 generally
do not live more than a
few months.
Klinefelter Syndrome
Male sex organs; unusually
small testes, sterile. Breast
enlargement and other
feminine body characteristics.
Normal intelligence.
Turner’s Syndrome
1:5000 live births; the only viable monosomy in
humans - women with Turner's have only 45
chromosomes!!! XO individuals are genetically female,
however, they do not mature sexually during puberty
and are sterile. Short stature and normal intelligence.
(98% of these fetuses die before birth)
Conclusion
• It is terribly important
that meiosis works
correctly!
• The homologous
chromosomes need to
be separated perfectly
so the sperm and
ovum have 23 different
chromosomes.
• Let’s see how this
takes place…