Ch. 10 Sexual Reproduction and Genetics
I.
Meiosis
Main idea: Meiosis produces haploid gametes
Chromosomes and Chromosome Number
1. traits (ex. Hair/ eye color, height) are controlled by
chromosomes found in the nucleus.
2. DNA located on the chromosomes is arranged in
segments that control the protein production
3. genes: DNA segments that control protein production.
4. homologous chromosomes: chromosomes that make up
a pair, one chromosome from each parent
 humans = 46 chromosomes (23 from each
parent)
 have same length, same centromere position
and carry genes for inherited traits in the
exact same position
5. Two types of chromosomes:
 Autosomes: chromosomes found in both
males and females.
 Sex chromosomes: chromosomes that
determine a person’s gender.
6. Cells
 Somatic Cell:
o A body cell
o Contains two homologous sets of
chromosomes (diploid, 2n)
 Gamete:
o A sex cell (egg or sperm)
o Contains one set of chromosomes
(haploid, n)
o When two gametes fuse (fertilization) a
diploid (2n) zygote is formed.
o 23 pairs of homologous chromosomes
are formed when two human gametes
combine
B. Meiosis I
1. meiosis: process of gamete (sex cell) formation
2. type of cell division that reduces the number of
chromosomes (reduction division) through the
separation of homologous chromosomes
 Interphase: carry out metabolic processes
including DNA replication and protein
synthesis
 Prophase I:
replicated chromosomes condense into sister
chromatids
a. homologous chromosomes:
paired sister chromatids
b. synapsis: process that binds
homologous chromosomes
together
c. crossing over: chromosome
segments are exchanged
between homologous
chromosome pairs (form
tetrads)
 Metaphase I: pairs of homologous
chromosomes line up at equator of cell
a. spindle fibers attach to centromeres of each
homologous chromosome pair
 Anaphase I: homologous chromosomes
separate and move toward opposite poles of cell
guided by spindle fibers
a. chromosome number is reduced from 2n to
n when the homologous chromosomes
separate
 Telophase I: homologous chromosomes reach
cell’s opposite poles
a. each pole contains only 1 member of the
original pair of homologous chromosomes
b. cytokinesis usually occurs after telophase I
forming a furrow in animal cells and a cell
plate in plant cells; the cell may then go into
interphase before the second set of divisions
but the DNA is not replicated again.
Meiosis II
3. Prophase II: chromosomes condense and spindle
apparatus forms
4. Metaphase II: haploid number of chromosomes line up
on equator
5. Anaphase II: sister chromatids are pulled apart and
move to opposite poles
6. Telophase II: chromosomes reach poles and nuclei
reform; four haploid cells exist following cytokinesis
D.
Factors that Contribute to Genetic Variation
 Independent Orientation (Assortment)
o Homologous chromosomes will randomly line up in the center
during meiosis II.
 Many possible combinations of chromosomes in the gametes.
 Random Fertilization
o An egg cell (with ~8 million possibilities) randomly unites with
a sperm cell (with ~8 million possibilities).
 64 trillion combinations of chromosomes.
 Crossing Over
o During synapsis in Prophase I, homologous chromosomes can
exchange homologous segments.
 sister chromatids that are no longer identical to one another.
 Genetic Recombination: the production of gene combinations
different from those carried by the original chromosomes.
E.
Sexual vs. Asexual Reproduction
1. asexual reproduction: organism inherits all its
chromosomes from a single parent (cloning)
a. most plants and simple animals
can reproduce both asexually
and sexually
2. beneficial genes (mutations) multiply faster over
time through sexual reproduction than asexually.
II. Mendelian Genetics
Main Idea: Mendel explained how a dominant allele can mask the
presence of a recessive allele.
A. Origin of Genetics
1.genetics: the science of heredity (the passing of traits to the
next generation).
2.Gregor Mendel (1866): Austrian monk that described the
inheritance of traits in pea plants; considered the father of
genetics
B.Inheritance of traits
1.Mendel found that there must be two forms of the seed
trait in pea plants (yellow-seed and green-seed)
2.allele: alternative form of a single gene passed from
generation to generation (ex: gene for yellow or green
seeds in pea plants)
3.Mendal found that the 1st filial (F1) produced all yellow
seeds and the 2nd filial (F2) generation have yellow to green
ratio of 3:1.
4.the form of the trait that appears in the F1 generation is
dominant and the form of the trait that was masked in the
F1 generation is recessive.
7. Y = allele for dominant yellow-seed form, y = allele for
recessive green-seed form
8. homozygous: organism with two of the same alleles for
a particular trait (ex: YY yellow-seed plants or yy
green-seed plants)
9. heterozygous: organism with two different alleles for a
particular trait (ex: Yy); hybrids
10.genotype: an organism’s allele pairs (YY or Yy)
phenotype: the observable characteristic or outward
expression of an allele pair (ex. yy = green seeds)
11.law of segregation: two alleles for each trait separate
during meiosis and then unite during fertilization.
a. monohybrid cross: crossing
hybrids for a single traits (ex:
YY x yy)
b. dihybrid cross: crossing hybrids
for 2 or more traits (ex: YYRR x
yyrr)
12.law of independent assortment: alleles have an equal
chance to randomly combine during gamete formation
YyRr (alleles in parent cell)
YR
Yr
yR
yr
(possible allele combinations for gametes)
H
HOOM
MO
OLLO
OG
GO
OU
USS C
CHHRROOM
MO
OSSO
OM
MEESS

Every diploid organism has two sets of homologous chromosomes. One set
comes from the female parent, one set comes from the male parent.
Homologous chromosomes bear two alleles for each characteristic (gene).
Alleles of a gene reside at the same locus on homologous chromosomes.


PPUUNNNNEETTTT SSQQUUAARREESS


A Punnett square is a tool used to determine what percentage of an offspring
generation will have certain genotypes.
Example: Two parents, both heterozygous tall
 Genotype: Tt
T
t
T
TT
Tt
t
Tt
tt



25% are TT (Tall)
50% are Tt (Tall)
25% are tt (Dwarf)

Example: Two parents, one heterozygous tall, one homozygous tall
 Genotypes TT and Tt
T
T
T
TT
TT
t
Tt
Tt



50% are TT (Tall)
50% are Tt (Tall)
0% are tt (Dwarf)
III.Gene Linkage and Polyploidy
Main Idea: The crossing over of linked genes is a source of genetic
variation.
a.
Genetic recombination: the new combinations of genes
produced by crossing over and independent assortment.
i. The number of combinations is calculated by using
the formula:
2n where n is the number of
chromosome pairs
ex. Pea plants have 7 pairs of chromosomes
27 = 128 combinations
b/c any male gamete can cross with any female gamete,
16,384 (128 x 128) combinations are possible
in humans: 223 x 223 = more than 70 trillion combinations
after fertilization
b.
Gene Linkage: genes that are located close to each other
on the same chromosome are said to be linked and
usually travel together during gamete formation.
i. gene linkage is an exception to law of independent
assortment b/c linked genes usually do not
segregate independently.
c.
ii. linked genes can sometimes separate during
crossing over
iii. chromosome maps show that crossing over occurs
more frequently between genes that are far apart
than those that are close together.
Polyploidy: the occurrence of one or more extra sets of
all chromosomes in an organism
i. ex. Triploid = 3 complete sets of chromosomes
(3n)
ii. rare in animals but sometimes occurs in
earthworms and goldfish; lethal in humans
iii. 1 in 3 flowering plants are polyploid
iv. ex. Wheat and oats (6n), sugar cane (8n)
v. polyploid plants often have increased vigor and
size.