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Chapter 5
Section 3 Meiosis
Chapter 5
Section 3 Meiosis
Bellringer
Objectives
Write a sentence to describe each of the following
terms: heredity, genotype, and phenotype. Note
how genotype and phenotype are related, and
how they are different. Is heredity necessarily a
factor in both genotype and phenotype? Why or
why not?
• Explain the difference between mitosis and meiosis.
Record your answers in your science journal.
• Interpret a pedigree.
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• Describe how chromosomes determine sex.
• Explain why sex-linked disorders occur in one sex
more often than in the other.
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Chapter 5
Section 3 Meiosis
Chapter 5
Vocabulary
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Section 3 Meiosis
Asexual Reproduction
• homologous chromosomes
• meiosis
• sex chromosome
• pedigree
• In asexual reproduction, only one parent cell is
needed. The structures inside the cell are copied, and
then the parent cell divides, making two exact copies.
• This type of cell reproduction is called mitosis. Most
of the cells in your body and most single-celled
organisms reproduce this way.
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Chapter 5
Section 3 Meiosis
Chapter 5
Section 3 Meiosis
Sexual Reproduction
Mitosis
• In sexual reproduction, two parent cells (sex cells)
join together to form offspring that are different from
both parents.
• Chromosomes that carry the same sets of genes
are called homologous chromosomes.
• Each sex cell has only one of the chromosomes
from the homologous pair.
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Chapter 5
Section 3 Meiosis
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Chapter 5
Section 3 Meiosis
Sexual Reproduction
The Steps of Meiosis
• Sex cells are made during meiosis.
• Meiosis is a copying process that produces cells
with half the usual number of chromosomes.
• Walter Sutton studied meiosis in sperm cells in
grasshoppers.
• During meiosis, chromosomes are copied once, and
then the nucleus divides twice.
• The resulting sex cells (sperm and eggs) have half
the number of chromosomes of a normal body cell.
• Using his observations and his knowledge of
Mendel’s work, Sutton proposed that: Genes are
located on chromosomes.
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Chapter 5
Section 3 Meiosis
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Chapter 5
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Chapter 5
Section 3 Meiosis
Section 3 Meiosis
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Chapter 5
Section 3 Meiosis
Meiosis and Mendel
• The steps of meiosis explain Mendel’s results.
The following slide shows what happens to a pair
of homologous chromosomes during meiosis and
fertilization.
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Chapter 5
Section 3 Meiosis
Chapter 5
Section 3 Meiosis
Meiosis and Mendel
Meiosis and Mendel
•The genes for certain disorders, such as
colorblindness, are carried on the X chromosome.
• Sex Chromosomes
carry genes that
determine sex.
•Genetic counselors use pedigrees to trace traits
through generations of a family. These diagrams can
often predict if a person is a carrier of a hereditary
disease.
• Human females have
two X chromosomes.
• Human males have
one X chromosome and
one Y chromosome.
• In selective breeding, organisms with desirable
characteristics are mated.
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Chapter 5
Section 3 Meiosis
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Chapter 5
Section 3 Meiosis
Section Summary
• In mitosis, chromosomes are copied once, and then the nucleus
divides once. In meiosis, chromosomes are copied once, and then the
nucleus divides twice.
• The process of meiosis produces sex cells, which have half the number
of chromosomes. These two halves combine during reproduction.
• In humans, females have two X chromosomes. So, each egg contains
one X chromosome. Males have both an X and a Y chromosome. So,
each sperm cell contains either an X or a Y chromosome.
A pedigree is a diagram that shows the occurrences of
genetic traits in several generations of a family.
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• Sex-linked disorders occur in males more often than in females.
Colorblindness and hemophilia are examples of sex-linked disorders.
• A pedigree is a diagram used to trace a trait through many generations
of a family.
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