Journal Entry:
5/10
Write a sentence to describe each of the following
terms: heredity, genotype, and phenotype. Is
heredity necessarily a factor in both genotype and
phenotype? Why or why not?
Objectives:
1. Explain the difference between mitosis and meiosis.
2. Describe how chromosomes determine sex.
Asexual Reproduction

 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.
Cell Cycle

 Interphase
 G1
 S
 G2
 Mitosis




Prophase
Metaphase
Anaphase
Telophase
Mitosis

Sexual Reproduction

 In sexual reproduction, two parent cells (sex cells)
join together to form offspring that are different
from both parents.
Chromosomes

 Chromosomes: Structure
consisting of DNA that transmits
genetic information from the
previous generation of cells and
organisms to the next generation.
 Chromosomes that carry the same
sets of genes are called
homologous chromosomes.
Sexual Reproduction,
Continued

 Body Cells
 Human body cells have 46 chromosomes or 23
homologous chromosome pairs.
 Human body cells are referred to as somatic cells.
•
Examples: Liver cells, heart cells
 Sex cells
 Sex cells have only one of the chromosomes from the
homologous pair (total of 23 chromosomes per cell)
 Sex cells are made during meiosis.
 Sex cells are referred to as gametes.
•
Examples: Sperm and Egg
Sexual Reproduction,
Continued

 Genes and Chromosomes
 Walter Sutton studied meiosis
in sperm cells in grasshoppers.
 Using his observations and his
knowledge of Mendel’s work,
Sutton proposed that:
 Genes are located on
chromosomes.
Meiosis

 Meiosis is a copying process that produces sex cells
(sperm and egg) with half the usual number of
chromosomes.
 Why? When the sperm (23 chromosomes) combines
with the egg (23 chromosomes) it produces a cell with
46 chromosomes.
DO NOW:
5/9
What do you think would happen if something went
wrong during cell division and the sperm or egg cell
ended up with either too few or too many
chromosomes?
Objectives:
1. Explain why sex-linked disorders occur in one sex more
often than in the other.
2. Interpret a pedigree.
The Steps of Meiosis

 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.


Mitosis vs. 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.

Meiosis and Mendel,
Continued

 Sex chromosomes carry
genes that determine sex.
 Human females have two
X chromosomes.
 Human males have one X
chromosome and one Y
chromosome.
Meiosis and Mendel,
Continued

 Sex-Linked Disorders
 The genes for certain genetic
disorders, such as
colorblindness, are carried
on the X chromosome.
 Since the gene for these
disorders is recessive, men
are more likely to acquire
the genetic disorder.
Meiosis and Mendel,
Continued

 Genetic Counseling
 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.
 Selective Breeding
 In selective breeding,
organisms with desirable
characteristics are mated.