Chapter 13
Meiosis And Sexual Life
Cycle
An Introduction to Heredity
Heredity: the transition of traits from
one generation to the next
Along w/ similarities, there is also
variation- offspring differ somewhat
in appearance from parents to
offspring
Genetics: the study of heredity and
hereditary variation
An Introduction to Heredity
Parents pass information to offspring
through coded hereditary units called
genes.
- 30 to 40 thousand genes in humans
- genes are segments of DNA
Concept 13.1
Two types of reproduction:
Asexual reproduction: single parent
passes copies of all its genes to
offspring; “like begets like.”
Sexual reproduction: two parents give
rise to offspring; results in greater
variation
Concept 13.2
Life cycle: generation-to-generation
sequence of stages in the
reproductive history of an organism
The human life cycle
- each somatic cell has 46
chromosomes (23 pair); 2 copies of
each pair are called homologous
chromosomes
Concept 13.2
- chromosomes can be displayed
through a karyotype
- pairs 1-22 are called autosomes
- pair 23 are called sex chromosomes
- XX = female; XY = male
- we inherit one chromosome of each
pair from each parent
Concept 13.2
Concept 13.2
Concept 13.2
- gametes, or sex cells, contain 22
autosomes and 1 sex chromosome; a
cell with a single chromosome set is
called a haploid cell.
- haploid cells are abbreviated 1n (n =
23)
- the combining of gametes forms a
zygote; becomes a diploid cell (2n)
Concept 13.2
- The only cells of the human body not
produced by mitosis are gametes
- this process is a form of cellular
division called meiosis; meiosis
reduces the chromosome number in
half
Concept 13.2
The variety of sexual life cycles
Animal cells
- after meiosis, the gametes
undergoes no division before
fertilization
Fungi
- after meiosis, mitosis occurs and
produces a 1n multicellular organism;
forms 1n gametes that then fertilize
Concept 13.2
Plants (alternation of generations)
- has both diploid (sporophyte) and
haploid (gametophyte) multicellular
stages;
Concept 13.2
Concept 13.3
Meiosis, like mitosis, is preceded by the
replication of chromosomes; however,
the single replication is followed by 2
consecutive divisions (meiosis I and
meiosis II)
Concept 13.3
Concept 13.3
Meiosis I
- during prophase I, chromosomes
pair up in synapsis; 4 chromatids form
a tetrad
- during metaphase I, homologous
pairs line up on equator
- during anaphase I, chromosomes,
not chromatids, separate to poles
Concept 13.3
Meiosis II
- goes through the same steps as
meiosis I, but does not replicate DNA
Meiosis outcome is 4 1n gametes from a
single cell
Concept 13.4
3 mechanisms contribute to the genetic
variation arising from sexual
reproduction
Independent assortment of
chromosomes
-each homologous pair of
chromosomes is positioned
independently of the others; variation
is 223 or about 8 million
Concept 13.4
Concept 13.4
Crossing over
- when chromosomes line up along the
equator, parts of chromatids can combine
genes from parents
Random fertilization
- even w/out considering crossing over, any
two parents will produce a zygote with any
of 70 trillion combination (223 x 223)
Concept 13.4