Chapter 7 Foundations of Genetics
7.1
Mendel and
the Garden Pea (p. 162; Figs. 7.1, 7.2, 7.3)
A.
Heredity is the
passing along of traits from one generation to the next, and genetics is the
study of heredity.
B.
A scientist and monk
by the name of Gregor Mendel contributed to the understanding of genetics in
the 1800s by being the first to actually count numbers of offspring in crosses
involving pea plants.
C.
Early Ideas About
Heredity
1.
Over 200 years before
Mendel’s work, British farmers had tried similar crosses and noted that some
types had a stronger tendency to pass along their traits.
D.
Mendel’s Experiments
1.
Gregor Mendel was born
in 1822 and educated in a monastery.
2.
He became a monk and
later joined a science club for which he undertook a number of studies.
E.
The Garden Pea
1.
Mendel used the garden
pea because many varieties were available, previous work had been done with
these plants, peas are small and grow quickly, and left alone, the flowers
self-pollinate.
F.
Mendel’s Experimental
Design
1.
Mendel began his crosses with
true-breeding varieties that contained only one type of gene for each trait.
2.
The first generation in a
succession of crosses is the P or parental generation; their offspring are the
F1 generation or first filial generation.
3. Offspring of two members of the F1 generation comprise the F2 generation.
7.2
What Mendel
Found (p. 164; Figs. 7.4, 7.5, 7.6)
A. Mendel repeatedly came up with the same results when examining seven pairs of contrasting traits.
B. The F1 Generation
1. Mendel called the trait expressed in the F1 plants the dominant trait and the trait not expressed was recessive.
C. The F2 Generation
1. When the F1 plants were allowed to self-fertilize, Mendel found 3:1 dominant to recessive phenotype in the F2 generation.
2. Mendel’s studies were unique because he actually counted the number of offspring with each trait.
D. A Disguised 1:2:1 Ratio
1. When F2 plants were allowed to self-fertilize, Mendel found a 1:2:1 ratio of true-breeding dominant to not true-breeding dominant to true-breeding recessive.
7.3
Mendel Proposes
a Theory (p. 166; Table 7.1)
A. In his crosses, Mendel found dominant traits expressed in his F1 generations and recessive traits showing up in the F2 generation.
B. Each individual has two genes (Mendel called these “factors”) for each trait.
C. When both genes are the same, the individual is said to be homozygous for that trait. If the two genes are different, the individual is heterozygous.
D. Alternate types of genes for each trait are alleles.
E. The genetic makeup of an individual is the genotype.
F. Phenotype refers to the outward expression of the genes.
7.4
Analyzing
Mendel’s Results (p. 167; Figs. 7.7, 7.8, 7.9)
A. Punnett Squares
1. A modified multiplication table, called a Punnett square, is an easy way to organize all possible genotypes when conducting a genetic cross.
B. The likelihood of getting one particular genotype in the offspring is expressed as a probability.
C. Analyses using Punnett squares demonstrate that Mendel’s results reflect independent segregation of gametes.
D. The Testcross
1.
When Mendel did not
know the genotype of an individual expressing a dominant trait, he did a
testcross by crossing the individual with a homozygous recessive for the trait.
2. Testcrosses can also be used to determine the genotype of an individual when two genes are involved.
7.5
Mendel’s Laws
(p. 169; Fig. 7.10)
A. Mendel’s First Law: Segregation
1. Mendel's First Law, or the Law of Segregation, says that only one of a pair of alleles is passed to a gamete, and gametes join randomly when uniting to form offspring.
B. Mendel’s Second Law: Independent Assortment
1. Mendel's Second Law was determined when he worked with two traits at a time in dihybrid crosses. This law, called the Law of Independent Assortment, states that genes located on different chromosomes are inherited independently.
7.6
Epistasis
(p. 170; Fig. 7.11)
A. Epistasis is an interaction between the products of two genes in which one of the genes modified the phenotypic expression produced by the other.
7.7
Multiple
Alleles (p. 171; Fig. 7.12)
A. Sometimes more than two alleles, or multiple alleles, exist for a given trait in a population of individuals.
B. An example is the human ABO blood designation.
C. In the human ABO blood group, A and B are equally dominant and can occur together as codominants.
7.8
Other
Modifications of the Genotype (p. 172; Figs. 7.13, 7.14, 7.15)
A. The expressions of genotype are not always straightforward.
B. Incomplete Dominance
1. A condition known as incomplete dominance is seen when offspring exhibit a phenotype intermediate to that of both parents.
7.9
Chromosomes
Are the Vehicles of Mendelian Inheritance (p. 174; Figs. 7.16, 7.17)
A. The Chromosomal Theory of Inheritance
1. Observations that similar chromosomes paired with each other during meiosis led to the chromosomal theory of inheritance.
2. Diploid individuals have two copies of each heritable gene, and gametes each have one.
B. Segregation and independent assortment were observed, and were consistent with Mendel’s model. Problems with the Chromosomal Theory
1. The observation that the number of traits that assorts independently in an organism exceeds the number of chromosome pairs the organism possesses made scientists wonder whether the chromosomal theory was correct.
C. Sex Linkage Proves the Chromosomal Theory
1. Male fruit flies have only one X chromosome while female fruit flies have two X chromosomes.
2. Eye color in this instance was sex-linked, which explained why males were white-eyed and females had red eyes.
3. Morgan’s experiments were important because they illustrated that genes are carried on chromosomes, and Mendel’s laws are true.
7.10
Human
Chromosomes (p. 176; Figs. 7.18, 7.19, 7.20, 7.21)
A. Humans have 23 pairs, or 46, chromosomes that vary by size, shape, and appearance.
B. Photographing the chromosomes produces a karyotype.
C. Nondisjunction
1. Sometimes during meiosis, the homologous chromosomes or the sister chromatids do not separate properly, a mistake known as nondisjunction.
2. This leads to aneuploidy, which means having an abnormal number of chromosomes.
3.
Humans have 23 pairs
of chromosomes, of which 22 pairs are called autosomes.
4.
Monosomics have only
one of a pair of a particular set of chromosomes, and trisomics have three
copies of a chromosome, rather than the normal two.
5.
Down syndrome is an
example of a trisomic condition in which the individual is born with an extra
copy of chromosome 21.
6. This condition results in mental impairment and a host of other physical defects.
D. Nondisjunction Involving the Sex Chromosomes
1. Humans have 23 pairs of chromosomes, of which 22 pairs are called autosomes; the remaining pair determines the sex of the individual and are the sex chromosomes.
2. Two kinds of sex chromosomes exist, an X chromosome and a Y chromosome.
3. Human females have two X's, and males have an X and a Y.
4. It is the human male, then, that determines the sex of his offspring.
5. Just as aneuploidy can occur with the autosomes, so it can also occur with the sex chromosomes, resulting in a number of abnormal conditions.
7.11
The Role of
Mutations in Human Heredity (p. 178; Table 7.2)
A. Many human hereditary disorders reflect the presence of mutations within human populations.
B. Family trees, or pedigrees, can indicate the mode of inheritance of the mutation.
7.12
Hemophilia
(p. 179; Figs. 7.22, 7.23)
A. Hemophilia, an autosomal or sex-linked recessive trait, results from mutations of genes encoding blood-clotting proteins.
7.13
Sickle-Cell
Anemia (p. 180; Figs. 7.24, 7.25)
A. Sickle
cell anemia is due to a recessive mutation of hemoglobin that is common in
7.14
Other
Disorders (p. 181; Figs. 7.26, 7.27)
A. Tay-Sachs Disease
1. Tay-Sachs disease is an incurable hereditary disorder that progressively destroys the brain of those affected.
8.1
The
A. He found that the virulent strain’s polysaccharide coat was necessary for infection.
B. He experimented further and found that the information specifying the polysaccharide coat could be passed from dead, virulent bacteria to coatless, nonvirulent strains.
C. Hereditary information could thus be passed from dead cells to live ones, transforming them.
8.2
The Avery
Experiments (p. 189; Fig. 8.2)
A.
Avery’s experiments with the transforming principle
from
The Hershey-Chase Experiment (p. 190;
Fig. 8.3)
A. This was additional evidence that DNA was the genetic
material.
8.3
Discovering
the Structure of DNA (p. 192; Figs. 8.5, 8.6, 8.7)
A. By the end of the 1950s, then, it was accepted that the genetic material was DNA or RNA, but the structure of these nucleic acids had yet to be determined.
B. Chargaff found that DNA always had equal amounts of purines (adenine and guanine) and pyrimidines (thymine and cytosine).
C. He also found that the amount of adenine equaled the amount of thymine and that the amount of cytosine was the same as the amount of guanine, a phenomenon now called “Chargaff’s rule.”
D. Watson and Crick then connected the ideas of a helix with base-pairing to further elucidate the structure of DNA.
E. The DNA molecule has a sugar-phosphate backbone with base-pairing on its interior, and is twisted into a double helix.
F. Watson and Crick also suggested a mechanism by which DNA was able to copy itself.
8.4
How the DNA
Molecule Replicates (p. 194; Figs. 8.8, 8.9)
A. Each individual chain of a DNA molecule is complementary to its pair.
1. If one chain has the bases ATTGCAT, its partner will have the complementary sequence of TAACGTA.
B. When DNA copies itself, it is said to have semiconservative replication.
C. How DNA Copies Itself
1. An enzyme called DNA polymerase oversees the operation, and the DNA molecule “unzips” first, revealing an area called the replication fork.
2. The polymerase places into position the correct complementary nucleotide until the entire replication fork has been copied.
3. A mechanism for DNA repair ensures that very few mistakes are made in the replication process.
4. The process is called semiconservative replication because in each new DNA molecule, one strand is “new” DNA, and the complementary strand is the parent DNA molecule.
8.5
Transcription
(p. 196; Figs. 8.10, 8.11)
A. Transcription is the process whereby a messenger RNA (mRNA) molecule is synthesized from a portion of the DNA molecule in the nucleus, and is the first step in gene expression.
B. The second step, called translation, occurs when the mRNA leaves the nucleus of the cell and directs the production of a protein molecule.
C. The Transcription Process
1. Transcription uses an enzyme called RNA polymerase that binds to the DNA molecule at a specific site called the promoter and then moves along the DNA molecule.
2. A strand of mRNA is produced whose nucleotide sequence is complementary to that of the DNA.
8.6
The Genetic
Code (p. 197; Fig. 8.12)
A. The genetic code is written such that a three-nucleotide sequence codes for a given amino acid, the building blocks of proteins.
B. The mRNA sequence that corresponds to the three-nucleotide sequence on DNA is called a codon.
C. There are 64 different possible codons in the genetic code dictionary, and the same genetic code is employed, for the most part, by every living creature.
8.7
Translation
(p. 198; Figs. 8.13, 8.14, 8.15, 8.16, 8.17)
A. In translation, organelles called ribosomes use the mRNA transcript to direct the synthesis of a protein.
B. The Protein-making Factory
1. Translation occurs in the cytoplasm in conjunction with ribosomes, which are made up of proteins and ribosomal RNA (rRNA).
2. Ribosomes hold the mRNA in position so translation of the code can occur.
C. The Key Role of tRNA
1. A third type of RNA, called transfer RNA (tRNA) has on one end an anticodon, which is a sequence of three nucleotides complementary to an mRNA codon.
2. On the other end of the tRNA molecule, is the amino acid that corresponds with the codon of the mRNA.
D. Making the Protein
1. The role of tRNA is to bring the appropriate amino acid into position along the mRNA molecule held by the ribosome.
2. As the ribosome proceeds along the mRNA, the next amino acid is added to the growing peptide chain.
3. When the process is finished, the ribosome complex falls apart, and the completed protein is released into the cell.
8.8
Architecture
of the Gene (p. 201; Fig. 8.18)
A. Introns
1.
Prokaryotic DNA is made up of
a continuous sequence of genes with no interruptions.
2.
Eukaryotic DNA is constructed
differently because it possesses gene sequences that code for amino acids,
called exons, plus intervening, nonusable sequences of nucleotides, called
introns.
3. Intron sequences must be removed from mRNA before translation can occur.
8.9
Mutation
(p. 204; Fig. 8.22)
A. In the very large amount of DNA in each cell, mistakes during DNA replication are bound to happen.
B. Mistakes Happen
1. A change in a cell’s genetic message is a mutation.
2. Mutations are the raw material for evolution.
C. The Importance of Genetic Change
1. Evolution can be viewed as the selection of particular combinations of alleles from a pool of alternatives.
2. The rate of evolution is limited by the rate of mutation.
3. Genetic change through mutation and recombination provides the raw material for evolution.
8.10
Kinds of
Mutation (p. 205; Table 8.1)
A. Most mutations are detrimental and their effects may be minor or catastrophic.
B. Mutations in Somatic Tissues
1. Changes in somatic cells are not passed on from generation to generation.
2. A somatic mutation may have drastic effects on the individual in which it occurs.
C. Point Mutations
1. Point mutations are changes in the hereditary message of an organism that involve only one or a few base pairs of the coding sequence.
2. Sometimes the changes involve a base substitution, an insertion or deletion, or a frame-shift mutation.
3. Some mutations may arise spontaneously, while others are the result of exposure to mutagens.
8.11
Cancer and
Mutation (p. 206; Table 8.2)
A. Agents thought to cause cancer are carcinogens.
B. The suspicion that chemicals contribute to the incidence of cancer is called the chemical carcinogenesis theory.
C. Carcinogens Are Common
1. Numerous chemicals have been found to be carcinogenic.