Genes are on chromosome pairs• Inheritance happens through

o Sexual reproduction: a cell containing genetic information from the mother and a cell containing genetic information from the father combine into a completely new cell, which becomes the offspring

• Genes are on chromosome pairso Processes are coded for by genes: a

unit of heredity that occupies a specific location on a chromosome and code for a particular product

o Heredity is the passing of genes from parents to offspring

o Genes code for the expression of traits Traits are not inherited: the gene code

for the trait is. Many genes can code for one trait

Genes are on chromosome pairs

• Cells contain pairs of chromosomes• One chromosome of each pair

coming from each of two parents• Homolog: chromosomes in a pair

o Sites where specific genes are located

Homologous Chromosomes

There is a gene code for a trait at a specific site on a chromosome

EX: plant height may be at site A on a pair of homologs Both homologs have the gene for height at site A, but the

genes may be different (tall vs short) EX: flowers color may be at site B on a pair of homologs

Both homologs have the gene for color at site B, but the genes may be different (purple vs white)

Chapter 4: Patterns of Heredity

4.1 Living things inherit traits in patterns4.2 Patterns of heredity can be predicted

4.3 DNA is divided during meiosis4.4 Cells use DNA and RNA to make proteins

4.3 Before you learned:Mitosis produces two genetically identical cellsIn sexual reproduction, offspring inherit traits from both parentsGenetic traits are inherited in predictable patterns

4.3 Now, you will learn:Why meiosis is necessary for sexual reproductionHow cells and chromosomes divide during meiosisHow meiosis differs from mitosis

Meiosis is necessary for sexual reproductionbody cells vs reproductive cells

• Cells containing genetic information from each the mother and father combine to form a completely new cell: becomes the offspring– The offspring has some genetic material from each parent

• Body cells (most human cells) contain 46 chromosomes (23 pairs)– this is a 2n cell, or a diploid cell

• n tells you the # of chromosomes in one set• Diploid means two sets of chromosomes: 2 x 23

– Humans: 23 sets of chromosomes, 2 x 23 = 46 total– Fruit fly: 4 sets of chromosomes, 2 x 4 = 8

Meiosis is necessary for sexual reproductionbody cells vs reproductive cells

• Gamete (reproductive cell): cells that contain half the usual number of chromosomes (46/2 = 23) – one from each pair: this is a 1n cell, or a haploid cell– Only found in the reproductive organs of plants and animals

– Just the female gamete: egg cell = 1n– Just the male gamete: sperm cell = 1n– Fertilized egg cell = 2n develops into an offspring

– Ex: gamete of human: n cells, 23 unpaired chromosomes– Ex: gamete of fruit fly: n cells, 4 unpaired chromosomes

Fertilization Two gametes combine to become a 2n cell that can grow into a new offspring.

humans: egg (23 chromosomes) + sperm (23 chromosomes) = fertilized egg cell (46 chromosomes)

Body cells (most human cells) contain 46 chromosomes: this is a 2n cell, or a diploid cell

Gamete: cells that contain half the usual number of chromosomes: this is a 1n cell, or a haploid cell

Just the female gamete - egg cell = 1nJust the male gamete – sperm cell = 1nFertilized egg cell = 2n develops into an offspring

Meiosis• Body cells divide by mitosis

– Produces two daughter cells, each containing exact copies of the chromosomes in the parent cell

– Each daughter cell is a standard diploid cell: 2n

To produce gametes (haploid), need a different kind of division: Meiosis•Produces haploid, 1n, cells•A single cell goes through two divisions:

•Meiosis I•Meiosis II

•Only in the reproductive tissues of an organism

Meiosis I• BEFORE Meiosis: DNA in each chromosome of the

parent cell is copied• a cell ready to divide has twice the usual amount

of DNA…therefore, for a cell to have half the amount of DNA, two divisions are necessary

• Four steps in Meiosis I:1. Prophase I: Chromosomes pair up with their

partner• Two sets of each of the chromosome pairs

in the parent cell• Each chromosome pair is attached together

in sets of doubled homologs2. Metaphase I: each set of chromosome pairs lines

up along the center of the cell3. Anaphase I: The two copies of one homolog

are pulled apart from the two copies of the other homolog

4. Telophase I and Cytokinesis: A new cell membrane forms at the center of the cell; parent cell divides two daughter cells

Meiosis II• In Meiosis I, two daughter cells formed

– Chromosomes of these two cells are not copied before Meiosis II

– Both these cells divide during Meiosis II 4 daughter cells

5. Prophase II: the two copies of each n chromosome are attached together

– Metaphase II: The chromosomes line up along each cell’s center

– Anaphase II: The two attached copies of each chromosome separate and are pulled to opposite poles in each cell

– Telophase II and Cytokinesis: A new cell membrane forms in the center of each cell, as each cell divides into two 1n daughter cells 4: 1n cells

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter28/animation__how_meiosis_works.html

Meiosis

• One cell in an organism’s reproductive system divides twice to form four 1n cells

• In males, these gametes become sperm

• In females, at least one of these cells becomes an egg– In some species (humans) only one of four daughter

cells becomes and egg• The rest dissolve back into the organism or are never

produced

Meiosis vs Mitosis

• Only cells that are to become gametes go through meiosis

• A cell that divides by meiosis undergoes two divisions, but the chromosomes are not copied before the 2nd division…in mitosis, chromosomes are always copied before dividing

• Daughter cells produced by meiosis (haploid/1n) contain only ½ the genetic material of the parent cell (one of each chromosome)

• Daughter cells produced by mitosis (diploid/2n) contain exactly the same genetic material as the parent (pairs of chromosomes)

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter28/

animation__unique_features_of_meiosis.html

• try it

• Meiosis

How Cells Divide