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Chapter 05 - Genetics: A Review
CHAPTER
5
GENETICS: A REVIEW
CHAPTER OUTLINE
5.1.
A Code for All Life
A. History of Central Tenets of Genetics
1. Mendel described particulate inheritance.
2. Watson and Crick described nature of the coded instructions.
3. Evolutionary theory is based on common ancestral groups; genetics establishes this lineage.
4. Genes guide the organization and orderly sequence of differentiation.
5. Genetics accounts for resemblance, fidelity of reproduction, and variation.
6. Genetics is a major unifying concept of biology.
B. Mendel’s Investigations (Figure 5.1)
1. Gregor Mendel conducted his plant breeding experiments from 1856–1864.
2. His discoveries were published in 1866, but not appreciated until 1900, 16 years after his death.
3. Genes and chromosomes were as yet unknown; his experiments were based on crossbreeding.
a. Mendel carefully controlled pollination of pea plant stigmas by stamens.
b. Mendel carefully documented offspring of different parents (hybrids) and then crossed the
hybrids.
C. Chromosomal Basis of Inheritance
1. Germ cells (gametes) were recognized as providing genetic information to offspring.
2. Nuclei of germ cells, especially chromosomes, were suspected of being the hereditary material.
3. Meiosis: Reduction Division of Gametes (Figure 5.2)
a. In all animals, each body cell has two homologous chromosomes; each homolog came from a
separate parent. However, animal species differ greatly in the number of chromosomes they
have.
b. Following DNA replication, meiosis occurs. Meiosis involves two rounds of cell division that
results in gametes that each contain one chromatid from each homologous pair.
c. Body cells are diploid (containing two sets of chromosomes). Gametes are haploid
(containing a single set). Fertilization (fusion of two gametes) results in a diploid zygote.
d. The diploid (2n) number in humans is 46 chromosomes; gametes are haploid with 23.
e. In an organism’s diploid cells, there are two genes for each trait. Each gene is l;ocated on a
separate homologous chromosome.
f. Alternative forms of a gene are called alleles; one or both may have an effect on a trait and
either may be passed on to offspring.
g. “Multiple alleles” is a condition whereby numerous allelic forms of a gene exist among
individuals of a population.
h. Most unique features of meiosis occur in prophase of first meiotic division.
1) Homologous chromosomes align side-by-side to form a bivalent.
2) Each chromosome has already replicated to form two chromatids, joined at the
centromere.
3) The complex of four future chromosomes is a tetrad.
4) The location of any one gene on a chromosome is the gene locus.
5) In side-by-side contact (synapsis), the gene loci on a chromosome align.
6) In preparation for division, the centromeres holding chromatids together do not divide;
the dyads are pulled to each pole.
7) At end of first meiotic division, daughter cells contain one of each homologous
chromosome (each consisting as a dyad of sister chromatids joined by a centromere).
8) At end of second meiotic division, dyads are split and each daughter cell contains one
haploid set and one allele of each gene.
4. Sex Determination (Figures 5.3, 5.4)
a. Sex chromosomes were those that determined sex; autosomes were the remainder. (Humans
1-22)
1) The bug’s sex determination system is called XX-XO indicating the missing chromosome
as “O.”
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Chapter 05 - Genetics: A Review
b.
c.
5.2.
Humans and many others use an XX-XY system; the male has the different sex chromosomes.
1) Half the sperm carry X and half carry Y; they fertilize an X egg to produce 50% of each
sex in their offspring.
2) The Y chromosome is smaller than the X and contains fewer genes.
Birds, moths, butterflies, and some fish use an XX (or ZZ) males –XY (or ZW) female
system, in which the female is the heterogametic (ZW) sex.
Mendelian Laws of Inheritance
A. Mendel’s First Law
1. In the law of segregation, during formation of gametes, paired factors segregate independently.
a. The phenotype is the visible characteristic.
b. Tall and dwarf plants produce tall F1 progeny; hence there is no blending.
c. Self-pollinating the F1 progeny produce tall and short plants within the F2 generation in a 3:1
ratio; again there was no blending and this ratio held for crosses of six other traits.
2. Dominance
a. Mendel called the tall factor (or allele) dominant. When a tall allele is present, the recessive
factor (or allele) is not expressed.
b. Recessive traits appear only in the absence of the dominant factor.
c. The alleles that represent these factors can be represented by alphabetic symbols. The same
letter is often used to represent a locus. Capitalized letters represent dominant alleles. Lowercase letters represent recessive alleles.
d. T/t represents the complete genetic constitution of the plant’s traits for height; T and t are the
possible gametes.
e. T/t and other unlike combinations form a heterozygote.
f. T/T and t/t are homozygotes.
g. T/T, T/t and t/t are the possible genotypes.
h. A cross that considers only one locus is referred to as a monohybrid cross.
3. Punnett Square
a. A Punnettt Square is a tool that represents the various combinations of alleles that can result
from gametes containing known alleles.
b. For example, a Punnett Square demonstrates how a T/t x T/t cross can result in a 3:1
phenotypic ratio of offspring.
c. Additional crosses of the progeny demonstrated that one-third of the tall was TT and twothirds were T/t.
d. The short plants, or t/t, always gave rise to short plants when self-fertilized.
4. Testcross
a. Products of a monohybrid cross of “pure”-condition tall and short parents produce tall
offspring that have both T/t and T/T genotypes.
b. To determine whether tall plants are either T/t or T/T, a testcross mates individuals of a
dominant phenotype with a “pure”-recessive.. If homozygous (T/T), the testcross yields all
tall offspring. If heterozygous (T/t), the testcross yields half tall and half short offspring.
5. Intermediate Inheritance (Figure 5.5)
a. Sometimes, neither allele is completely dominant, resulting in intermediate inheritance or
incomplete dominance.
b. Red and white homozygous four-o-clock flowers cross to form heterozygous pink flowers.
c. Chickens with black feathers crossed with splashed white feathered chickens yield blue
Andalusian chickens.
d. This appears to produce a blending of traits, but additional crosses will reveal the traits are
present and still able to be expressed with the appropriate testcross.
B. Mendel’s Second Law (Figure 5.6)
1. The law of independent assortment states that genes located on different pairs of homologous
chromosomes assort independently during meiosis.
a. This law pertains to studies of two pairs of hereditary factors at the same time.(dihybrid
crosses)
b. When tall plants with yellow seeds (both dominant traits) were crossed with dwarf plants with
green seeds, the F1 plants were all tall and yellow as expected.
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Chapter 05 - Genetics: A Review
c.
C.
D.
E.
F.
G.
When the F1 hybrids were self-fertilized, a 9:3:3:1 ratio of tall-yellow, tall-green, dwarfyellow, and dwarf-green offspring resulted, which is a combination of the two 3:1 ratios for
each set or a dihybrid cross.
d. Segregation of alleles for plant height was independent of segregation of alleles for seed
color.
2. Probability
a. All genotypes of gametes of one sex have an equal chance of uniting with all genotypes of
gametes of the other sex.
b. The probability of two independent events occurring together is the product of their individual
probabilities; this is the product rule. (Table 5.1)
c. Probability has no “memory.” That is, previous outcomes do not influence future outcomes.
Multiple Alleles
1. While only two alleles can exist at one locus, more than two types of alleles may exist in a
population.
2. For instance, a rabbit may possess two alleles from among four for coat color: C (normal), c ch
(chinchilla), ch (Himalayan) and c (albino).
3. Multiple alleles arise through mutations at the same locus over time.
Gene Interaction
1. Polygenic inheritance is a condition in which many different genes (and hence their genotypes)
may affect a single phenotype.
2. Pleiotropy is a condition in which a single gene can have multiple phenotypic effects (i.e., eye
color and other features).
3. An allele at one location that masks expression of an allele at another locus acting on the same
trait is called epistasis.
4. Polygenic characters show continuous variation between extremes (quantitative inheritance); skin
pigmentation in humans probably involves 3 or 4 genes.
Sex-Linked Inheritance (Figures 5.7, 5.8, 5.9)
1. Some traits depend on the sex of the parent carrying the gene.
a. Hemophilia is a recessive trait on the X chromosome.
b. Red-green color blindness is also a recessive trait and on the X chromosome.
c. Carriers are heterozygous for these genes and are phenotypically normal.
2.
The inheritance pattern of sex-linked alleles is unique.
a. The X-linked trait is expressed when both X chromosomes possess the recessive allele in a
female but when only one X-linked recessive allele is present in a male.
b. When the mother is a carrier (heterozygous for a X-linked trait) and the father possesses a
normal, dominant X-linked allele, half of the sons are affected.
c. X-linked recessive phenotypes are more prevalent in males because a single sex-linked
recessive gene in the male has a visible effect.
d. Hemizygous- male has one copy of gene locus.
Autosomal Linkage and Crossing Over
1. Linkage
a. Not all factors segregate as stated in Mendel’s second law.
b. Genes on the same chromosome are linked, and the traits are inherited together.
2. Traits on the same chromosome are coded as letters without a slash mark (i.e., AB/ab).
Crossing Over (Figure 5.10)
a. Linkage is not absolute; some separation of alleles on the same chromosome occurs due to
crossing over.
b. During protracted prophase of meiosis I, some paired homologous chromosomes break and
exchange equivalent portions.
c. Crossing over exchanges genes between homologous pairs with great frequency; crossing
over occurs nearly of 100% each meiotic cycle for longer chromosomes.
d. Because more distant loci are likely to be separated by crossing-over, one can consider. the
frequency in crossing over can facilitate mapping the location of genes on chromosomes.
Chromosomal Aberrations
1. Structural and numerical deviations from the norm that affect many genes are chromosomal
aberrations.
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Chapter 05 - Genetics: A Review
2.
5.3.
5.4.
It is estimated that five of every 1,000 humans are born with a serious genetic defect from
chromosomal anomalies.
3. Euploidy is the addition or deletion of whole sets of chromosomes; polyploidy, the possession of
three or more complete sets (homologs) of chromosomes, is most common in plants but animals
cannot tolerate this type of chromosomal aberration.
4. Aneuploidy is the addition or deletion of a single chromosome.
a. It is usually caused by failure of chromosomes to separate during meiosis (nondisjunction).
b. This results in one gamete or polar body having an extra chromosome and one lacking a
chromosome.
c. The monosomic animal (n-1) rarely survives due to uneven balance of genetic instructions.
d. Trisomy (n+1) is more common; Down syndrome often occurs within individuals that are
trisomy-21, i.e., these individuals have an extra 21st chromosome.
5. Structural aberrations involve whole sequences of genes within a chromosome.
a. Inversions reverse the order of a segment of genes.
b. Translocation is the movement of a section of genes.
c. Deletion is loss of a single, or block of genes.
d. Duplication adds an extra section of chromosome; they may add additional genetic
information and allow new functions.
6. Genetic Nondisjunction and Syndromes: Klinefelter syndrome and Turner syndrome are
the result of genetic nondisjunction.
Gene Theory
A. Gene Concept
1. W. Johannsen coined the term “gene” in 1909 to name the hereditary factors described by Mendel.
a. Originally, genes were thought to be indivisible units.
b. Alleles are now known to be divisible by recombination; portions are separable.
c. Parts of eukaryote genes are separated by introns, which are sections of DNA that do not
specify a product.
B. One Gene–One Polypeptide Hypothesis
1. Phenotypic expression of genes appears to follow: gene  gene product  phenotypic
expression.
2. Gene products are usually proteins; proteins can act as enzymes, antibodies, hormones and
structures.
Storage and Transfer of Genetic Information (Figures 5.11; Table 5.2)
A. Nucleic Acids: Molecular Basis of Inheritance
1. Nucleotides
a. Both DNA and RNA are polymers built of nucleotides; a nucleotide contains a sugar, a
nitrogenous base and a phosphate group.
b. DNA contains a 5-carbon sugar called deoxyribose. RNA contains a 5-carbon sugar called
ribose.
c. Nitrogenous bases are either pyrimidines (a single, 6-membered ring) or purines (two fused
rings). (Figure 5.12)
d. Purines in both DNA and RNA are adenine and guanine.
e. Pyrimidines in DNA are thymine and cytosine; in RNA they are uracil and cytosine.
f. The DNA backbone is built of phosphoric acid and deoxyribose.
g. The 5' end of the backbone has a free phosphate group on the 5' carbon of the ribose and the 3'
end has a free hydroxyl group on the 3' carbon. (Figure 5.13)
h. DNA is two complementary chains precisely cross-linked by specific hydrogen bonding
between purine and pyrimidine bases. (Figure 5.14)
i. The number of adenines in a molecule of DNA is equal to the number of thymines, and the
number of guanines is equal to the number of cytosine’s. This suggests that these bases are
paired. (Figure 5.15)
j. The DNA ladder is twisted into a double helix; ten base pairs occur per turn. (Figure 5.16)
k. The two DNA strands are antiparallel; the 5' end of one is bonded to the 3' end of the other.
l. Strands are complementary; sequence of bases of one strand specifies sequence of the other.
m. RNA is similar to DNA except it has a single polynucleotide chain, has ribose instead of
deoxyribose, and has uracil instead of thymine.
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Chapter 05 - Genetics: A Review
DNA is replicated precisely before placed into daughter cells; each strand of a parent cell’s
DNA is a template for the new complementary strand. (Figure 5.16)
o. Ribosomal, transfer, and messenger RNAs are the most abundant and well-known types of
RNA, but many structural and regulatory RNAs, such as micro RNAs, have been reported.
DNA Coding by Base Sequence (Figure 5.17)
a. The DNA coding sequence is collinear with the sequence of amino acids in a protein.
b. The four kinds of DNA nucleotides cannot individually code for each of the identified 20
amino acids.
c. Sequences of 3 bases provides 64 (43) combinations, enough to code for the 20 amino acids.
d. Later work confirmed the triplet coding sequence with redundancy. (Table 5.3)
e. DNA is stable but subject to chemical and radiation damage.
f. Excision repair uses enzymes to separate pyrimidines covalently bonded by UV radiation.
g. DNA polymerase synthesizes the missing strand according to base-pairing rules.
h. DNA ligase joins the end of the new strand to the old one.
i. DNA polymerase only synthesizes new strands in the direction of 5' to 3'.
The parent DNA strands are antiparallel, so synthesis along one of the strands is continuous,
and the other is performed in a series of fragments running 5' to 3'.
Transcription and the Role of Messenger RNA (Figures 5.18, 5.19)
a. DNA codes for proteins but does not participate directly in protein synthesis.
b. An intermediary, messenger RNA (mRNA) is used.
c. DNA is transcribed into mRNA with uracil substituting for thymine. (Table 5.3)
d. RNA polymerase makes a mRNA that is complementary to one strand of DNA.
e. A different RNA polymerase is used to produce ribosomal, transfer and messenger RNA.
f. Only one of the two DNA strands, the “sense” strand, is used as a template for RNA
synthesis. The strand not used as a template is called the “antisense” strand.
g. Genes were thought to be continuous stretches of DNA until introns, sections that do not code
for a product, were discovered.
h. Genes coding for many proteins may be discontinuous; genes coding for histones and
interferon are continuous.
i. Some genes are rearranged during development to code for different proteins.
j. Some RNA can self-catalyze the excision of introns; since it changes in the reaction, this is
not technically an enzyme.
Translation: Final Stage in Information Transfer (Figures 5.20, 5.21, 5.22)
a. Translation takes place on ribosomes composed of protein and ribosomal RNA (rRNA).
b. Ribosomes consist of large and small subunits; together they form a functional unit.
c. Many ribosomes may attach to a single mRNA to form a complex called a polyribosome or
polysome; thus, several molecules of the same protein can then be synthesized on a mRNA at
once, one per ribosome.
d. Assembly of proteins requires large transfer RNA molecules.
e. The tRNA collects free amino acids and delivers them to the polysome.
f. There is a unique tRNA for each amino acid.
g. Each tRNA has a specific tRNA synthetase to sort and attach amino acid to the end of each
tRNA, called charging.
h. On the tRNA, a sequence of three bases (anticodon) forms base pairs with complementary
bases (codon) in mRNA.
n.
2.
3.
4.
C. Genetic Sources of Phenotypic Variation
1. There are several sources of phenotypic variation.
a. Natural selection preserves favorable phenotypes thus increasing populations of alleles,
leading to adaptive evolution.
b. Independent assortment of chromosomes, crossing over and random fusion of gametes
reshuffle and amplify the genetic material present.
c. Gene mutations and chromosomal aberrations provide new genetic variation.
2. Gene Mutations
a. Chemical or physical changes in genes result in alteration of the sequence of bases in DNA.
b. A codon substitution results in incorrect amino acids causing sickle cell anemia.
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Chapter 05 - Genetics: A Review
c.
d.
3.
Once a gene is mutated, it faithfully reproduces itself.
The environment imposes a screening process (natural selection) that continues the beneficial
and eliminates the harmful.
e. A population carries a reservoir of mutations unexpressed in heterozygotes.
Frequency of Mutations
a. A long gene is more likely to have a mutation than a short gene.
b. Every person carries approximately one new mutation; most are recessive and not expressed.
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