Download Section 3 Meiosis

Warm Up
Answer the following questions:
1. Chromosomes contain DNA. What is
DNA?
2. What do you think is an important
function of DNA?
Chapter 6
Section 3 Meiosis
What You Will Learn
• In sexual reproduction, offspring receive half
of their genetic material from each parent.
• Homologous chromosomes contain the same
genes but may have different alleles for each
gene.
• Meiosis results in the production of haploid
cells that have half the number of
chromosomes that diploid cells do.
Chapter 6
Section 3 Meiosis
Chromosome Numbers
• Meiosis and sexual reproduction
increase the number of possible genetic
combinations in a population.
• There is a carefully controlled process
that ensures sexual reproduction leads
to the proper number of chromosomes
in the offspring.
Chapter 6
Section 3 Meiosis
• Each species has a normal number of
chromosomes in their body cells.
• Human body cells have 46 chromosomes,
corn cells have 20, and dogs cells have 78.
• Each chromosome in a body cell has a
matching pair with the same genes but
different alleles. Chromosomes that carry
the same sets of genes are called
homologous chromosomes.
Chapter 6
Section 3 Meiosis
Chapter 6
Section 3 Meiosis
Chromosomes in Reproduction
• Cells that have homologous pairs of
chromosomes are called diploid. Body
cells are diploid.
• Before sexual reproduction can occur,
an organism must make sex cells. The
male sex cells are sperm, and the
female sex cells are eggs.
Chapter 6
Section 3 Meiosis
• Sex cells do not have homologous pairs and
are called haploid. Human sex cells have
23 chromosomes.
• Sex cells are haploid so that when they
combine during fertilization, they produce an
individual with the normal number of
chromosomes.
• Human body cells have 46 chromosomes.
Chapter 6
Section 3 Meiosis
Meiosis
• Sex cells are made during a process called
meiosis.
• Meiosis is a copying process that produces
cells that have half the usual number of
chromosomes.
• When a cell undergoes meiosis, it divides
twice. This produces four cells, each with half
the number of chromosomes as the parent
cell.
Down’s Syndrome
Chapter 6
Section 3 Meiosis
Chapter 6
Section 3 Meiosis
Chapter 6
Section 3 Meiosis
Meiosis and Mendel
• Meiosis can help explain the results first
observed by Mendel.
• If the true breeding plant for wrinkled seeds
provided the sperm, all of the sperm cells
would have the same recessive allele, r.
• If the true breeding plant for smooth seeds
provided the eggs, all of the eggs would have
the dominant allele R.
Chapter 6
Section 3 Meiosis
Meiosis and Mendel, continued
• The only genotype possible in the
offspring in this cross is Rr.
• The only phenotype possible is smooth
seeds, since all of the offspring would
have the dominant allele.
Chapter 6
Section 3 Meiosis
Chapter 6
Section 3 Meiosis
• So in sexual reproduction, half of the
offspring’s genetic material comes from the
mother’s egg, and half comes from the
father’s sperm.
• Mitochondria, which have their own DNA,
come from the mother’s egg. So all of the
mitochondria in your body can be traced
back to your mother.