1. Meiosis

2. Inheritance of Traits
Heredity – the transmission of traits from one generation to the next
Variation – offspring differ in appearance from parents and siblings
Genetics – the scientific study of heredity and hereditary variation

3. Inheritance of Traits
Parents pass traits to their offspring in the form of “genes” which are segments of DNA.
Genes are coded information that enable cells to make enzymes and other proteins that are responsible for producing an organism’s traits.

4. Inheritance of Traits
The cells responsible for passing genetic information from parents to their offspring are “gametes.”
Most DNA is stored in chromosomes within the nucleus, (but a tiny amount is found in mitochondria and chloroplasts).
A gene’s location within a chromosome is called its “locus.”

5. Methods of Reproduction
Asexual
Sexual
One parent
Genetically identical offspring
“Clone”
Mitosis
Two parents
Genetically diverse offspring
Meiosis
Methods of Reproduction

6. Chromosomes
Human somatic cells (cells other than gametes) have 46 chromosomes – 23 from mom and 23 from dad.
Diploid = cell with 2 sets of chromosomes
2n = 2(number of chromosomes in a set) = 2(23) = 46
23rd pair are sex chromosomes
XX = female XY = male
Gametes are Haploid = n = have one set of chromosomes = 23

7. Ordered display of condensed chromosome pairs
Karyotype
Ordered display of condensed chromosome pairs

8. Meiosis and Fertilization
Meiosis = cell division to reduce the number of sets of chromosomes from 2 to 1 in gametes
Only occurs in reproductive organs
Fertilization = union of gametes in which the nuclei of the two cells fuse
Results in a haploid (2n) zygote (fertilized egg cell) which matures by mitosis

9. Variety of Sexual Life Cycles
Alternation of generations
Result (regardless of life cycle): genetic variation among offspring

10. Stages of Meiosis Two cell divisions produce 4 haploid cells
Interphase – chromosome replication
Result: 1 diploid cell with replicated chromosomes
Meiosis 1 – homologous chromosomes separate
Result: 2 haploid cells with replicated chromosomes
Meiosis 2 – sister chromatids separate
Result: 4 haploid cells with unreplicated chromosomes

12. Stages of Meiosis Prophase 1 Metaphase 1 Anaphase 1
Chromosomes condense
Crossing over
Metaphase 1
Homologous chromosomes align at metaphase plate
Anaphase 1
Sister chromatids remain attached at centromere
Homologous chromosomes move toward opposite poles
Telophase 1 and Cytokinesis
Division into two haploid cells
Chromosomes are still double
Prophase 2
Spindle apparatus forms
Metaphase 2
Chromosomes align single file on metaphase plate
Sister chromatids are not identical
Anaphase 2
Centromeres separate
Sister chromatids come apart and move to opposite poles
Telophase 2 and Cytokinesis
Nuclei form and chromosomes decondense
Each of the 4 daughter cells is genetically distinct
Stages of Meiosis

14. Sources of Genetic Variation
Fun Fact: the number of possible combinations of maternal and paternal chromosomes in one person’s gametes is about 8 million!!!
3 Ways…
1. Independent Assortment
Each pair of chromosomes is randomly oriented with its maternal or paternal homologue closer to a given pole during Metaphase 1.

15. Sources of Genetic Variation
Fun Fact: In humans, one to three crossover events occur per chromosome pair!!
3 Ways…
2. Crossing Over
In Prophase 1, as homologous chromosomes align, the DNA molecules of two non-sister chromatids are broken at the same place and then rejoined to each other’s DNA, producing chromosomes with new combinations of genes
At Metaphase 2, sister chromatids can be oriented in two alternative ways which also increases variation

16. Sources of Genetic Variation
Fun Fact: God made you genuinely unique!!
3 Ways…
3. Random Fertilization
Which sperm meets which egg?
Not counting crossing over, this would be any of about 64 trillion diploid combinations!