1. Specialized Cells are Divided into Two Groups
(Body Cells)
(Sex Cells)
Examples of somatic (body) cells: spleen, kidney, eyeball
The DNA in these cells are NOT passed on to your offspring!
Examples of gametes (sex cells): ova/egg and sperm
DNA in your gametes can be passed on to offspring!

2. Meiosis Reduces Chromosome Number
Imagine how the chromosome number would increase with each generation if chromosome reduction did not occur!

3. Meiosis Reduces Chromosome Number
somatic cells
gametes (egg and sperm)

4. Homologous Chromosomes (in humans)
Somatic cells (body cells) have 23 pairs of different chromosomes, for a total of 46 chromosomes.
(chromosomes not directly
involved in determining
gender)
Each of the 23 pairs consists of two homologous chromosomes.
Homologue from parent A
Homologue from parent B

5. Homologous Chromosomes (in humans)
Each pair of homologous
chromosomes comes from the parents.
(chromosomes not directly
involved in determining
gender)
Cells that contain two sets of chromosomes are diploid (2n).
Homologue from parent A
Homologue from parent B

6. Gametes – Reproductive Cells (in humans)
Unlike body cells, human
gametes (egg and sperm)
contain only one set of
chromosomes, 23 total.
Cells that contain one set of chromosomes are said to be haploid (n).

7. Gametes – Reproductive Cells (in humans)
The fusion of two haploid
gametes (fertilization)
forms a diploid zygote (2n).
A zygote is a fertilized egg cell. The first cell of a new individual.

8. Homologous Chromosomes and Sister Chromatids

9. Homologous Chromosomes “Cross Over” During Prophase I of Meiosis

10. Separation of homologous chromosomes into two haploid cells.
Meiosis I
Separation of homologous chromosomes into two haploid cells.

11. Separation of sister chromatids into four haploid cells.
Meiosis II
Separation of sister chromatids into four haploid cells.

12. Haploid Cells Develop into
Mature Gametes
The sperm cell, the male gamete, is much smaller than the egg. Spermatogenesis, sperm production, occurs in the testes.
Sperm formation starts with a round cell and ends by making a streamlined cell that can move rapidly.

13. Haploid Cells Develop into
Mature Gametes
DNA is tightly packed and much of the cytoplasm is lost. The sperm cell develops a whip-like flagellum and connecting neck region packed with mitochondria that drive the cell.

14. Haploid Cells Develop into
Mature Gametes
Oogenesis, the formation of an egg, begins before birth. This process occurs in the ovaries of the developing female.
An egg not only gives its share of DNA to an embryo, but also contributes the organelles, cytoplasm, and nutrients an embryo needs to begin life.

15. Haploid Cells Develop into
Mature Gametes
Only one of the four cells produced by each round of meiosis actually makes an egg.
In many species, including humans, the other cells produced by Meiosis I do not undergo Meiosis II.

16. Gregor Mendel Heredity – passing traits from parents to offspring
Gregor Mendel was born in Austria in A monk, Mendel discovered the basic principles of heredity through experiments in his monastery's garden.
His work showed that the inheritance of certain traits in pea plants follows particular patterns.

17. Gregor Mendel Heredity – passing traits from parents to offspring
These studies became the foundation of modern genetics, the study of heredity.
200 years earlier British farmers had performed similar breeding experiments but Mendel applied
the principles of the scientific method by collecting and analyzing data.

18. Mendelian Theory of Heredity
For each inherited trait, an individual has two copies of the gene
– one from each parent.
2. There are alternative versions of genes. For example, the gene
for flower color in peas can exist in a “purple” version or a
“white” version. Different versions of a gene are called alleles.

19. Gregor Mendel and Heredity
Mendelian Theory of Heredity
3. When two different alleles
occur together, one of them may
be completely expressed
(dominant trait), while the other
may have no observable effect
(recessive trait) on the
organism’s appearance.
Gregor Mendel and Heredity
Purple flower color is dominant.
White flower color is recessive.

20. Alleles and Phenotype
What makes one allele dominant over another? It depends on the protein that is, or is not, made.
For example, pigment gives cells color. If P directs flower cells to make pigments, the flower may look purple. If p directs the cells not to make pigment, the flower looks white.
So P codes for pigment to be present, but p codes or nothing, the absence of pigment.

21. Mendel’s Findings in Modern Times
The set of alleles (the different versions of a gene) that an individual
has is called its genotype. The
physical appearance of a trait is called a phenotype.
Individuals with the phenotype of freckles may have the genotype FF or Ff. A phenotype of no freckles is expressed with the genotype ff.
What is the genotype of an individual with a cleft chin?
CC or Cc
What is the phenotype of an individual
who is cc?
No cleft chin

22. Genetic Variation Genetic variation is the raw material for evolution.
The more genetically diverse a species is the more likely the population will survive.
Three processes of meiosis that contribute to genetic variation are:
1. Crossing-Over (Prophase I)
2. Independent Assortment (Anaphase I)
3. Random fertilization of an egg (ova) by sperm.

23. Importance of Genetic Variation
Meiosis and the joining of gametes are essential to evolution. No genetic process generates variation more quickly.
The pace of evolution is sped up by genetic recombination. The combination of genes from two organisms results in a third type, not identical to either parent.