Reproduction
• A method of copying genes to pass them on to offspring.
• Two main types:
• Asexual reproduction
• Sexual reproduction

Asexual Reproduction
• Parent passes all of its genes to its offspring.
• Uses mitosis.
• Also known as cloning.

Advantages
• Only need 1 parent
• Don’t spend energy finding mate
• Good genetic traits are conserved and reproduced.

Disadvantages
• No new DNA combinations for evolution to work on.
• Clones may become extinct if attacked by a disease or pest.

Sexual Reproduction
• Two parents contribute DNA to an offspring.

Advantages
• Offspring has a unique combination of DNA which may be an improvement over both parents.
• New combination of DNA for evolution to work with.

Disadvantages
• Need two parents.
• Good gene combinations can be lost.

Question ?
• Do parents give their whole
DNA copy to each offspring?
• What would happen to chromosome number if they did?

Result
• Chromosome number would double each generation.
• Need a method to reduce the chromosome number but still preserve genes

Meiosis - Purpose
• To reduce the number of chromosomes by half.
• Create new combinations of genetics in the offspring

Diploid
• A diploid cell (2 n ) has two sets of chromosomes.
• For humans, the diploid number is 46 (2 n = 46).
• (somatic cells)

Haploid
• A haploid cell (n) has 1 set of chromosomes
• For humans, the haploid number is 23 (n=23)
• Gametes (sperm and egg)

Human Chromosomes
• In a diploid cell, the two sets of chromosomes are all homologous chromosomes
• Homologous chromosomes have
• The same length
• The same genes
• The same locus
• Can have different alleles!
• They are NOT identical

Genes
• The DNA for a trait.
• ex: eye color gene
• Locus - the physical location of a gene in a chromosome.

Allele
• Definition: one of two or more alternative forms of a gene that are found at the same place on a chromosome.
• Ex: The eye color gene has multiple alleles:
• Blue
• Brown
• Green

• Diploid human 2n = 46
• 23 from mom, 23 from dad
• 23 homologous pairs, 1 in each pair comes from mom, the other from dad.

• A karyotype is an ordered display of the pairs of chromosomes from a cell.

• A gamete (sperm or egg) contains a single set of chromosomes, and is haploid
( N ).
• For humans, the haploid number is 23 ( N = 23).

Meiosis - Uniqueness
• Three events are unique to meiosis, and all three occur in meiosis l:
1. Crossing over in Meiosis 1:
Homologous chromosomes physically connect and exchange genetic information.

Continued…
2. At the metaphase plate, there are paired homologous chromosomes
(tetrads), instead of individual replicated chromosomes.
3. During Meiosis I, it is homologous chromosomes, instead of sister chromatids, that separate.

Meiosis/Mitosis Preview of differences
• 1st division separates PAIRS of chromosomes, not duplicate chromosomes.

Meiosis
• Has two cell divisions.

Meiosis - Results
• 4 cells produced.
• Chromosome number halved.
• Gametes or sex cells made.
• Genetic variation increased.

Sexual Sources of Genetic
Variation
1. Independent Assortment of
Chromosomes during Meiosis.
2. Random Fertilization.
3. Crossing Over.

Independent Assortment
• There are 23 pairs of chromosomes in humans.
• The chance to inherit a single chromosome (maternal or paternal) of each pair is 1/2.

Gamete Possibilities
• With 23 pairs of chromosomes, the number of combinations of chromosome types
(paternal and maternal) are:
2 23 or 8,388,608

Random Fertilization
• The choice of which sperm fuses with which egg is random.

Random Fertilization
• Therefore, with 8,388,608 kinds of sperms and 8,388,608 kinds of eggs, the number of possible combinations of offspring is over
.

Result
• Is it any wonder that two offspring from the same human parents only resemble each other and are not identical twins?

Crossing-Over
• The exchange of sister chromatid material during synapsis.
• Occurs ONLY in Meiosis I.

Importance
• Breaks old linkage groups.
• Creates new linkage groups increases genetic variation.

Importance
• Very common during meiosis.
• Frequency can be used to map the position of genes on chromosomes.

Comments
• With crossing over, offspring can never be 100% like a parent if sexual reproduction is used.
• Multiple cross-overs are common, especially on large chromosomes.

Summary
• Know how the chromosomes separate during Meiosis.
• Know how Meiosis differs from
Mitosis.
• Know how sexual reproduction increases genetic variation.