©CNRI/Photo and the other allele is not.Alleles are often

©CNRI/Photo Researchers, Inc.MAIN IDEA
Genes influence the development of traits.6AYou may have heard about the Human Genome Project. Its goal was to find
out the sequence of the 3 billion nucleotide pairs that make up a human’s
genome. A
genome
is all of an organism’s genetic material. Unless you have
an identical twin, you have a unique genome that determines all of your traits.
Some of your traits, such as the color of your eyes, can be seen. Other traits,
such as the exact chemical makeup of your eyeball, cannot be seen.In genetics, we often focus on a single trait or set of traits. A genome is all
of an organism’s genes, but a
genotype
(JEHN-uh-TYP) typically refers to the
genetic makeup of a specific set of genes. The genotype of a pea plant includes
both of the genes that code for flower color, even if one of these genes is
masked. In contrast, the physical characteristics, or traits, of an individual
organism make up its
phenotype
(FEE-nuh-TYP). A pea plant with purple
flowers has a phenotype for purple flowers. The plant might have a hidden
gene for white flowers, but that does not matter to its phenotype.Dominant and Recessive
Alleles
If an organism is heterozygous for a
trait, which allele will be expressed?
That is, if a plant has one allele for
purple flowers and one for white
flowers, what color will the flowers
be? As Mendel learned, one allele
may be dominant over another
allele. A
dominant
allele is the allele
that is expressed when two different
alleles or two dominant alleles are
present. A
recessive
allele is the
allele that is expressed only when
two copies are present. In Mendel’s
experiments, the allele for purple
flowers was dominant to the allele
for white flowers. All F
1
plants were
purple even though they had only
one allele for purple flowers.Sometimes the word
dominant
is misunderstood. A dominant allele is not
necessarily better or stronger than a recessive allele. It does not necessarily
occur most often in the population. An allele is dominant in a heterozygote
simply because it is expressed and the other allele is not.Alleles are often represented on paper with individual letters. An organ-
ism’s genotype for a trait can be shown with two letters—one per allele.
Uppercase letters are used for dominant alleles, and lowercase letters are used
for recessive alleles. For example, the dominant allele for height in pea plants
is written as
T
, for tall. The recessive allele for short plants is written as
t
.CONNECT TO
EXCEPTIONS TOMENDEL’S LAWS
Mendel’s theory of inheritance
cannot explain all patterns of
inheritance. As you will learn in
the chapterExtending Mendelian
Genetics,
incomplete dominance,
codominance, polygenic traits,
and environmental influences all
provide exceptions.FIGURE 4.1
Polydactyly is the
condition of having more than
the typical number of fingers or
toes. The allele for polydactyly
is dominant.VISUAL VOCAB
A
dominant
allele is expressed when
two different alleles are present.A
recessive
allele is expressed only
when two copies are present.wrinkledrecessivegenotype phenotyperounddominantgenotyperecessivewrinkled
wrinkledrecessivephenotypeChapter 6:
Meiosis and Mendel
175176177Aalleles
aA
alleles
aA
A
A
aA
aa
a©John Innes ArchivesTraits and Probability
KEY CONCEPT
The inheritance of traits follows the rules ofVOCABULARYPunnett squaremonohybrid crosstestcrossdihybrid crosslaw of independent
assortmentprobabilityprobability.MAIN IDEAS
Punnett squares illustrate genetic crosses.
A monohybrid cross involves one trait.
A dihybrid cross involves two traits.
Heredity patterns can be calculated with probability.Connect to Your World
If you have tried juggling, you know it can be a tricky thing. Keeping three flaming
torches or clubs in motion at the same time is a challenge. Trying to keep track of
what organism has which genotype and which gamete gets which allele can also be a
lot to juggle. Fortunately, R. C. Punnett developed a method to keep track of all of
the various combinations graphically.MAIN IDEA
Punnett squares illustrate genetic crosses.3F, 6F, 6GShortly after Mendel’s experiments

became widely known among
scientists, a poultry geneticist named
R. C. Punnett, shown in
FIGURE 5.1,
developed the Punnett square. A

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Punnett square
is a grid system for
predicting all possible genotypes

resulting from a cross. The axes of the

grid represent the possible gamete

genotypes of each parent. The grid
boxes show all of the possible geno-

types of offspring from those two

parents. Because segregation and

fertilization are random events, each combination of alleles is as likely to be
produced as any other. By counting the number of squares with each genetic

combination, we can find the ratio of genotypes in that generation. If we also

know how the genotype corresponds to the phenotype, we can find the ratio

of phenotypes in that generation as well.
Let’s briefly review what you’ve learned about meiosis and segregation to
examine why the Punnett square is effective. Both parents have two alleles for
each gene. These alleles are represented on the axes of the Punnett square.
During meiosis, the chromosomes—and, therefore, the alleles—are separated.R. C. Punnett
FIGURE 5.1
R. C. Punnett devel-
oped the Punnett square as a way
to illustrate genetic crosses.VISUAL VOCAB
The
Punnett square
is a grid systemfor predicting possible genotypes ofoffspring.possiblegenotypesof offspringParent 2Parent 13F, 6F, 6G3F
research and describe
the history of biology and
contributions of scientists;
6F
predict possible outcomes of
various genetic combinations such
as monohybrid crosses, dihybrid
crosses and non-Mendelian
inheritance;
6G
recognize the
significance of meiosis to sexual
reproductionChapter 6:
Meiosis and Mendel
1776.5178179180181182183184185186187CHAPTERReviewINTERACTIVE
ReviewHMDScience.comGO ONLINE

Review Games

• Concept Map

• Section Self-Checks

CHAPTER VOCABULARYReviewing Vocabulary
Visualize VocabularyFor each term below, use simple shapes, lines, or
arrows to illustrate its meaning. Below each picture,
write a short caption. Here’s an example for the term
diploid:
1.
gene2.
fertilization3.
crossing over
4.
genetic linkage5.
haploidGREEK WORD ORIGINS
6.
The word
meiosis
comes from a Greek word meaning
“to diminish,” or make less. How does this word’s origin
relate to its meaning?7.
The word
haploid
comes from the Greek word
haplous,
which means “single.” The word
diploid
comes from the
Greek word
diplous,
which means “double.” Explain how
these two terms’ meanings relate to their origins.8.
The Greek prefix
homo-
means “one and the same.” How
does this relate to the words
homologous
and
homozygous
?Compare and ContrastDescribe one similarity and one difference between
the two terms in each of the following pairs.9.
monohybrid cross, dihybrid cross10.
heterozygous, homozygous
11.
genotype, phenotypeReviewing
MAIN IDEAS
12.
Each of your cells has a set of chromosomes, including
autosomes and sex chromosomes. Explain the main
differences between these two types of chromosomes.13.
A fruit fly has diploid cells with 8 chromosomes. Explain
how many chromosomes are in its haploid gametes.14.
Meiosis is a continuous process, but we can think of it
as taking place in two stages, meiosis I and meiosis II.
How do the products of meiosis I differ from those of
meiosis II?
6G15.
The foundation for our modern study of genetics began
with Gregor Mendel, who studied pea plants. What were
Mendel’s two main conclusions about how traits are
passed between generations?
3F16.
How did Mendel’s use of purebred plants—for example,
purebred white- and purebred purple-flowered peas—
contribute to his understanding of inheritance?
6F17.
How does the homozygous condition differ from the
heterozygous condition? In your answer, use the terms
gene, homologous chromosome, and allele.
18.
What does each of the following parts of a Punnett
square represent: (a) the entries on each axis of the grid
and (b) the entries in the four squares within the grid?19.
How did the results of Mendel’s dihybrid crosses lead
him to formulate the law of independent assortment?
3F, 6F20.
How does crossing over during meiosis I increase
genetic diversity?
6GREADING TOOLBOX
Diploid cells have two copies ofeach chromosome.Chapter 6: Meiosis and Mendel
18766.1
somatic cellgametehomologous chromosomeautosomesex chromosomesexual reproductionfertilizationdiploidhaploidmeiosis6.2
gametogenesisspermeggpolar body6.3
traitgeneticspurebredcrosslaw of segregation6.4
geneallelehomozygousheterozygousgenomegenotypephenotypedominantrecessive6.5
Punnett squaremonohybrid crosstestcrossdihybrid crosslaw of independent assortmentprobability6.6
crossing overgenetic linkage188189190CHAPTER(t) ©2006 Jupiterimages; (c) ©Diana Koenigsberg/Getty Images; (b) ©Gazimal/Getty ImagesExtending Mendelian GeneticsBIG IDEAMendel’s laws of inheritance do not account for the expression of all traits, which may be influenced by the number of genes involved, linkages with other genes, or the environment in which the organism lives.ONLINE BIOLOGYHMDScience.comONLINE Labs?QuickLabSex-Linked Inheritance?Codominance?Pedigree Analysis?Incomplete Dominance?Human Heredity?Examining Banding Patterns in Polytene Chromosomes?Video LabPlant Genetics190Unit 3: Genetics77. 1Chromosomes and Phenotype7.2Complex Patterns of inheritance6F7. 3gene Linkage and Mapping 3F, 6HData Analysis CONSTRUCTING BAR GRAPHS 2G7. 4Human Genetics and Pedigrees 6F, 6h191192193194195196ContentsBookmarksNotesPage ViewSearchMore