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asexual vs. sexual reproduction
• In asexual reproduction, a single individual passes
along copies of all its genes to its offspring.
• Single-celled eukaryotes reproduce
asexually by mitotic cell division to
produce two identical daughter cells.
• Even some multicellular eukaryotes,
like hydra, can reproduce by budding
cells produced by mitosis.
• Sexual reproduction results in greater variation
among offspring than does asexual reproduction.
• Two parents give rise to offspring that have unique
combinations of genes inherited from the parents.
• Offspring of sexual
reproduction vary
genetically from
their siblings and
from both parents.
Fig. 13.2
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• Fertilization restores the diploid condition by
combining two haploid sets of chromosomes.
Fig. 13.4
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• Meiosis reduces
chromosome number by
copying the chromosomes
once, but dividing twice.
• The first division, meiosis
I, separates homologous
chromosomes.
• The second, meiosis II,
separates sister chromatids.
Fig. 13.6
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• Mitosis produces two identical daughter cells, but
meiosis produces 4 very different cells.
Fig. 13.8
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genetic variation among offspring
• Three mechanisms contribute to genetic variation:
• independent assortment
• crossing over
• random fertilization
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• Independent assortment of chromosomes
contributes to genetic variability due to the random
orientation of tetrads at the metaphase plate.
• There is a fifty-fifty chance that a particular daughter
cell of meiosis I will
get the maternal
chromosome of a
certain homologous
pair and a fifty-fifty
chance that it will
receive the paternal
chromosome.
Fig. 13.9
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• chromosome would be
exclusively maternal or
paternal
• However, crossing over
produces recombinant
chromosomes which
combine genes inherited
from each parent.
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• The random nature of fertilization adds to the
genetic variation arising from meiosis.
• Any sperm can fuse with any egg.
• An ovum is one of approximately 8 million possible
chromosome combinations (actually 223).
• The successful sperm represents one of 8 million
different possibilities (actually 223).
• The resulting zygote is composed of 1 in 70 trillion (223
x 223) possible combinations of chromosomes.
• Crossing over adds even more variation to this.
• Mutations create a population’s diversity of genes.
Evolutionary adaptation depends on a
population’s genetic variation
• Darwin recognized the importance of genetic
variation in evolution via natural selection.
• A population evolves through the differential
reproductive success of its variant members.
• Those individuals best suited to the local
environment leave the most offspring, transmitting
their genes in the process.
• This natural selection results in adaptation, the
accumulation of favorable genetic variations.
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• As the environment changes or a population moves
to a new environment, new genetic combinations
that work best in the new conditions will produce
more offspring and these genes will increase.
• The formerly favored genes will decrease.
• Sex and mutations are two sources of the continual
generation of new genetic variability.
• Gregor Mendel, a contemporary of Darwin,
published a theory of inheritance that helps explain
genetic variation.
• However, this work was largely unknown for over 40
years until 1900.
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• Alleles
• dominance vs. recessive alleles
• Tay-Sachs disease lack a functioning enzyme to
metabolize gangliosides (a lipid) which accumulate in
the brain, harming brain cells, and ultimately leading to
death.
• Children with two Tay-Sachs alleles have the
disease.(homozygous recessive)
• Heterozygotes with one working allele and homozygotes
with two working alleles are “normal” but heterozygotes
produce less functional enzyme (carriers).
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• Complete and incomplete dominance alleles.
• incomplete dominance flower color of
snapdragons.
• A cross between a
white-flowered plant
and a red-flowered
plant will produce all
pink F1 offspring.
• Self-pollination of the
F1 offspring produces
25% white, 25% red,
and 50% pink
Fig. 14.9
offspring.
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• Codominance in which two alleles affect the
phenotype in separate, distinguishable ways.
• For example, the M, N, and MN blood groups of
humans are due to the presence of two specific
molecules on the surface of red blood cells.
• People of group M (genotype MM) have one type of
molecule on their red blood cells, people of group N
(genotype NN) have the other type, and people of group
MN (genotype MN) have both molecules present.
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• Because an allele is dominant does not necessarily
mean that it is more common in a population than
the recessive allele.
• For example, polydactyly, in which individuals are born
with extra fingers or toes, is due to an allele dominant to
the recessive allele for five digits per appendage.
• However, the recessive allele is far more prevalent than
the dominant allele in the population.
• 399 individuals out of 400 have five digits per
appendage.
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• Polygenic - cross between two AaBbCc
individuals (intermediate skin shade) would
produce offspring covering a wide range of shades.
• Individuals with
intermediate skin shades
would be the most likely
offspring, but very light
and very dark individuals
are possible as well.
• The range of phenotypes
forms a normal
distribution.
Fig. 14.12
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• Pedigrees provide information
Fig. 14.14
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Pedigree problems
1. Freckles is a dominant gene (F)
and no freckles is recessive (f).
Sarah has freckles and is married to Sam who has no freckles.
They have two children, Tom with freckles and Tina without.
Indicate the genotype or possible genotype for each.
2. Cystic fibrosis affects lung function and is caused by a
recessive gene (c). Normal lung function is dominant (C).
Harry and Hannah have normal lung function and have two
children. Their daughter, Kristy has normal lung function but
Kit their son has Cystic fibrosis.
Mendelian inheritance has its physical basis
in the behavior of chromosomes during
sexual life cycles
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Linked genes tend to be inherited together
because they are located on the same
chromosome
• Each chromosome has hundreds or thousands of
genes.
• Genes located on the same chromosome, linked
genes, tend to be inherited together because the
chromosome is passed along as a unit.
• Results of crosses with linked genes deviate from
those expected according to independent assortment.
• Linked genes used for gene mapping
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• Sex chromosome
• X-Y system of mammals
• Other options include the
X-0 system, the Z-W
system, and the haplodiploid system.
Fig. 15.8
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Sex-linked genes have unique patterns of
inheritance
• In addition to their role in determining sex, the sex
chromosomes, especially the X chromosome, have
genes for many characters unrelated to sex.
• These sex-linked genes follow the same pattern of
inheritance as the white-eye locus in Drosophila.
Fig. 15.9
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• Several serious human disorders are sex-linked.
• Duchenne muscular dystrophy affects one in
3,500 males born in the United States.
• Affected individuals rarely live past their early 20s.
• This disorder is due to the absence of an X-linked gene for a key muscle
protein, called dystrophin.
• The disease is characterized by a progressive weakening of the muscles
and loss of coordination.
• Hemophilia is a sex-linked recessive trait defined
by the absence of one or more clotting factors.
• Individuals with hemophilia have prolonged bleeding because a firm clot
forms slowly.
• Bleeding in muscles and joints can be painful and lead to serious damage.
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• X inactivation to balance XX vs. XY
• orange and black pattern on tortoiseshell cats is
due to patches of cells expressing an orange allele
while others have a nonorange allele.
Fig. 15.10
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