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Chapter 11: Intro to Genetics
The work of Gregor Mendel:
Every living thing inherits traits, or characteristics, from its parents.
People have long wondered how these traits are passed from one
generation to the next. Genetics is the scientific study of heredity.
Gregor Mendel did experiments with pea plants to study inheritance.
Pea plants are usually self-pollinating, meaning that the sperm cells
fertilize egg cells in the same flower. The pea plants he studied were
true breeding. True breeding plants produce offspring indentical to
themselves.
Mendel wanted seeds that inherited traits from two different parent
plants. He crossed two plants with different forms of the same trait. A
trait is a specific characteristic, such as height or seed color. Mendel
then grew plants from the seeds formed by each cross. These plants
were hybrids. Hybrids are the offspring of the crosses between parents
with different traits. The first generation is called F1 generation. The
second generation is called F2, and so on.
Each group of Mendel’s hybrid plants looked like only one of its
parents. In one case, all of the offspring were tall. In another, all the
offspring had yellow seeds. From these results, Mendel drew two
conclusions:
 Biological inheritance is determined by factors that are
passed from one generation to the next.
These factors are called genes. One gene with two different
forms controlled each trait. Each form of the gene is called
an allele_.
 Mendel’s principle of dominance
o States that some alleles are dominant and others are
recessive_.
A living thing with a dominant allele for a trait always shows the trait.
Recessive alleles are not seen if the dominant allele is present.
Mendel wondered what happened to the recessive allele. To find out,
Mendel let the F1 plants self- pollinate. Some of the F2 plants showed
the recessive trait. The recessive alleles had not disappeared. Instead,
the dominant allele had “masked” them. Mendel concluded:
 When each F1 plant flowers and produces gametes, or sex
cells, the two alleles segregate, or separate, from each
other. The result, each gamete carries only a single copy of
each gene.
 Therefore, each F1 plant produces two types of gametes –
those with the allele for tallness and those with the allele
for shortness.
Probability and Punnett Squares:
Probability is the likelihood that a specific event will occur. The
principles of probability can predict the outcomes of genetic crosses.
This is because the ways in which alleles segregate is completely
random. There are two important points to remember with
probabilities:
 Past outcomes do not affect future events.
 Probabilities predict the average outcome of many events. They
do not predict what will happen in a single event. Therefore, the
more trials there are, the closer the numbers will get to the
predicted values.
Punnett Squares are diagrams that model genetic crosses.
Punnett Squares can be used to predict and compare the genetic
variations that will result from a cross. They help predict the chances
an offspring will be homozygous or heterozygous for a trait.
 Organisms that have two identical alleles for a particular trait are
called homozygous.
 Organisms that have two different alleles for the same trait are
called heterozygous.
11-3:
Mendel wondered if genes that determine different traits affect one
another. He did an experiment to find out. Mendel found out that the
gene for seed shape did not affect how the gene for seed color sorted.
He summarized his conclusions as the principle of independent
assortment. The principle of independent assortment states that genes
for different traits can segregate independently during the formation of
gametes. Independent assortment helps account for the many genetic
variations observed in plants, animals, and other organisms.
Not all genes show simple patterns of dominant and recessive alleles.
Some alleles are neither dominant nor recessive, and many traits are
controlled by multiple alleles or multiple genes. Some of the patterns
are described below:
 Incomplete dominance, one allele is not completely dominant
over the other.
 Codominance, both alleles appear as part of the phenotype of the
heterozygous offspring
 Genes that have more than two alleles are said to have multiple
alleles.
 A single trait can be controlled by more than one gene. These are
called polygenic traits.
Genes do not control all characteristics. Some are due to interactions
between genes and the environment.
11-4: Meiosis
Living things inherit a single copy of each gene for each parent. These
copies are separated when the gametes form. The process in which
this happens is called meiosis. Meiosis is a process in which the
number of chromosomes per cell is divided in half through the
separation of homologous chromosomes in a diploid cell.
 A cell that has both sets of homologous chromosomes is said to
be diploid. Diploid means “two sets”.
 Gametes have half the number of chromosomes as their parent
cells. Cells that have only one set of chromosomes are said to be
haploid. Gametes are genetically different from the parent cell
and from one another.
Before meiosis begins, cells undergo DNA replication forming duplicate
chromosomes. Meiosis then occurs in two stages;
 Meiosis I:
o Two cells form
o Each has sets of chromosomes and alleles that are
different from each other and from the original cell
 Meiosis II:
o The cells divide again, but this time the DNA is not copied
first
o Four daughter cells form
o Each daughter cell contains half the number of
chromosomes as the original cell
Although they sound the same, meiosis and mitosis are different.
Mitosis makes two identical cells. These cells are exactly like the parent
cell. Meiosis, however, forms four cells. Each cell has only half the
number of chromosomes as the parent cell. The cells are also
genetically different from one another.
11-5: Linkage and Gene Maps
Some genes are usually inherited together. These genes are linked. A
chromosome is a group of linked genes. When gametes form, it is the
chromosomes sort independently, not individual genes. The location of
genes on a chromosome can be determined by studying crossover
events. The farther apart two genes are, the more likely they will be
separated by cross-over events. Scientists collect data on how often
crossing-over separates certain genes. These data are used to find the
distance between the genes on a chromosome. Scientists can make a
gene map of the chromosome.