BIOL 139 Midterm Review
Artificial selection purposeful matings
Phenotype = observable characteristic; genotype = genetic make up
Monohybrid cross matings between individuals that differ in only one trait; Dihybrid cross
matings between individuals that differ in two traits; multihybrid cross matings between
individuals that differ in three of more traits
Alleles are versions of a gene; a polymorphic gene is a gene that has several alleles that normally
occur in a populations; a monomorphic gene is a gene that has only one allele that is normally
present in a population. A maximum of two alleles can pre present in one gene of a diploid
The law of product: the probability of two or more independent events occurring together is the
product of the probabilities that each event will occur by itself
The law of sum: the probability of either of two mutually exclusive events occurring is the sum
of their individual probabilities
Gregor Mendel performed genetic crosses in a systematic way, using mathematics to analyze
the data he obtained and to predict outcomes of other experiments.
Mendels law of segregation states that alleles of genes separate during gamete formation and
then come together randomly at fertilization. The Punnet square is one tool for analyzing allele
behaviour in a cross.
In the analysis of a genetic cross, the product rule multiplies probabilities to predict the chance
of a particular fertilization event (if, then). The sum rule adds probabilities to predict the
proportion of progeny that share a particular trait such as pea colour (either, or).
The results of a test cross, in which an individual showing the dominant phenotype is crossed
with an individual showing the recessive phenotype, indicate whether the individual with the
dominant phenotype is a homozygote or heterozygote.
The law of independent assortment states that the alleles of genes for different traits segregate
independently of each other during gamete formations.
Branched-line diagrams or direct calculations of probabilities are often more efficient methods
than Punnet squares for the analysis of genetic crosses involving two or more genes.
Modifications of Mendelian Ratios
Incomplete dominance the F1 hybrid resembles neither pure bred parent (often an
intermediate phenotype); codominance alternative traits are both visible in the F1 hybrid
Allele frequency the percentage of the total number of copies of a gene in a population
represented by a particular allele. Wild type allele greater than 1% in population. Mutant allele
less than 1% in population
Multifactorial inheritance arising from the action of two or more genes (polygenic), or from
interactions between genes and the environment Complementary gene action two or more genes can work in tandem, in the same biochemical
pathway to produce a particular trait (9:7)
Heterogeneous trait a mutation at any one of a number of genes can give rise to the same
Epistasis a gene interaction in which the effects of an allele at one gene hide the effects of
alleles at another gene. Epistatic control over another gene. Recessive epistasis (9:3:4) and
Dominant epistasis (12:3:1 or 13:3)
Penetrance percentage of the population with a particular genotype that demonstrates the
expected trait. Expressivity the degree or intensity with which a particular genotype is
expressed in a phenotype within a population
Sex linked traits due to genes on the x or y chromosome. Sex-limited traits affect a structure
or process that is found in one sex but not the other. Sex-influenced traits show up in both
sexes but their expression may differ between the two sexes
Conditional lethality occurs with a particular type of allele which is lethal under only certain
conditions. Permissive (allowing life) vs restrictive (restricting life) conditions
In cases of incomplete dominance or codominance, mating of F hybr1ds produces an F 2
generation with a 1:2:1 phenotypic ratio. The reason is that heterozygotes have a phenotype
different from that of either homozygote.
Within a population, a gene may have multiple alleles, but any one individual can have at most
two of these alleles. Considered in pairs, the alleles can exhibit a variety of dominance
Genes and alleles can be classified according to allele frequencies. A monomorphic gene has a
single common allele referred to as the wild-type allele; a polymorphic gene has several
common variants. Rare or newly arisen alleles of any gene are mutant alleles.
A mutant allele can disrupt many biochemical processes; as a result, mutations often have
pleiotropic effects that can include lethality at various times in an organisms life cycle.
F2phenotypic ratios of 9:3:3:1 or its derivatives indicate the combined action of two
independently assorting genes. For heterogeneous traits caused by recessive alleles of two or
more genes, a mating between affected individuals acts as a complementation test, revealing
whether they carry mutations in the same gene or different genes.
Because modifier genes, the environment, and chance events can affect phenotypes, the
relationship of a particular genotype and its corresponding phenotype is not always absolute: An
alleles penetrance can be incomplete, and its expressivity can be variable.
The action of a handful of genes, combines with environmental effect, can produce an
enormous range of phenotypic variation for a particular trait.
Autosomal one that is conferred by a gene residing on a chromosome not involved in sex
determination. Sex-linked one that is conferred by a gene residing on the X or Y chromosome
If an individual is affected by a rare dominant trait, the trait should also affect at least one of
that persons parents, one of that persons grandparents, and so on. The idea that genes reside on chromosomes was verified by experiments involving sex-linked
genes in Drosophila and by the analysis of pedigrees showing X-linked patterns of inheritance in
humans. The chromosome theory provides a physical basis for understanding Mendels laws.
Chromosomal theory of inheritance hereditary information is on genes and genes are located
on chromosome. Egg and sperm contribute equally to the genetic endowment of offspring
through their nuclei
Mitosis nuclear division that results in 2 daughter cells each containing identical numbers of
chromosomes to the parent cell. Meiosis nuclear division that results in each egg and sperm
containing half the number of chromosomes found in other cells (somatic)
Homologous chromosomes contain the dame genes, vary in alleles, pair at meiosis and are
called homologs. Non-homologous chromosomes contain different genes and do not pair at
Microscopic studies suggested that the nuclei of egg and sperm contribute equally to the
offspring by providing a single set of n chromosomes. The zygote formed by the union of haploid
gametes is diploid (2n).
Karyotyping, the analysis of stained images of all the chromosomes in a cell, reveals that
different species have different numbers and shapes of chromosomes.
In many species, the sex of an individual correlates with a particular set of chromosomes termed
the sex chromosomes. The segregation of the sex chromosomes during gamete formation and
their random reunion at fertilization explains the 1:1 ratio of the two sexes.
Mitosis the process of nuclear division in cells that produces daughter cells that are genetically
identical to each other (cloning cells)
G1 cell grows and carries out normal metabolism and organelles duplicate. S DNA replication
begins through semi-conservative replication and chromosomes duplicate. G2 cell grows and
prepares for mitosis
Prophase, metaphase, anaphase, telophase
Centrosome microtubule organizing center, the core of which is seen as two darkly staining
bodies in animal cells, known as centrioles
Kinetochore site at which chromosome is attached to microtubules
Cell division = cytokinesis
The essence of mitosis is the arrangement of chromosomes at metaphase. The kinetochores of
sister chromatids are connected to fibres from opposite spindle poles, but the sister chromatids
remain held together by their connection at the centromere.
After mitosis plus cytokinesis, the sister chromatids of every chromosome are separated into
two daughter cells. As a result, these two cells are genetically identical to each other and to the
original parent cell. Meiosis
Somatic cells mitotically divide and make up the vast majority of an individuals tissues. Germ
cells produce gametes (egg/sperm) and undergo meiosis
Prophase stages leptotene, zygotene, pachyetene, diplotene, diakineses
Centromere remains intact during meiosis
Oogenesis egg formation in humans
During prophase I, homologous chromosomes pairs, and recombination occurs between
nonsister chromatids of the paired homologs.
The essence of the fir