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Transcript
Name____________________________
Chapter 11
Class __________________
Date __________
Introduction to Genetics
Summary
11–1 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
sperm cells fertilize egg cells in the same flower. The pea plants
he studied were true-breeding. True-breeding plants produce
offspring identical 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
crosses between parents with different traits. The first generation
of a cross is called the 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 of 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 also formed the principle of dominance, which
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, he let the F1 plants self-pollinate. Some of F2 plants showed
the recessive trait. The recessive alleles had not disappeared.
Instead, the dominant allele had masked them. From this, Mendel
concluded that when each F1 plant flowers and produces
gametes, or sex cells, the two alleles segregate, or separate, from
each other. As a 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.
© Pearson Education, Inc., publishing as Pearson Prentice Hall.
99
Name____________________________
Class __________________
Date __________
11–2 Probability and Punnett Squares
Probability is the likelihood that a specific event will occur.
The principles of probability can be used to 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 Exploring Mendelian Genetics
Mendel wondered if genes that determine different traits affect
one another. He did an experiment to find out. Mendel found 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 these patterns are described below.
• In incomplete dominance, one allele is not completely
dominant over another.
• In 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.
© Pearson Education, Inc., publishing as Pearson Prentice Hall.
100
Name____________________________
Class __________________
Date __________
11–4 Meiosis
Living things inherit a single copy of each gene from each parent.
These copies are separated when 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:
• Two cells form.
• Each cell has sets of chromosomes and alleles that are
different from each other and from the original cell.
Meiosis II:
• The cells divide again, but this time the DNA is not
copied first.
• Four daughter cells form.
• 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 that 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 crossover 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 then make a gene map
of the chromosome.
© Pearson Education, Inc., publishing as Pearson Prentice Hall.
101