Nerve activates contractionIn asexual reproduction, a single individual passes along copies of all its genes to its offspring.
Single-celled eukaryotes reproduce
produce two identical daughter cells.
Even some multicellular eukaryotes,
cells produced by mitosis.
asexual vs. sexual reproduction
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
Fig. 13.2
Fertilization restores the diploid condition by combining two haploid sets of chromosomes.
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Fig. 13.4
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.
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Fig. 13.6
Mitosis produces two identical daughter cells, but meiosis produces 4 very different cells.
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Fig. 13.8
independent assortment
crossing over
random fertilization
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
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.
Female
Male
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.
Evolutionary adaptation depends on a population’s genetic variation
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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
Fig. 14.9
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
offspring, but very light
and very dark individuals
are possible as well.
The range of phenotypes
Fig. 14.12
Fig. 14.14
Pedigree problems
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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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
Linked genes tend to be inherited together because they are located on the same chromosome
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Sex chromosome
X-Y system of mammals
Other options include the X-0 system, the Z-W system, and the haplo-diploid system.
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Fig. 15.8
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.
Sex-linked genes have unique patterns of inheritance
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Fig. 15.9
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.
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Fig. 15.10
Mitosis produces two identical daughter cells, but meiosis produces 4 very different cells.
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Fig. 13.8