Variation in Traits

1. Variation in Traits • Two important mechanisms that occur during meiosis introduces variation in traits among offspring:

2. Crossing Over in Prophase I • During prophase I, homologous chromosomes pair up • Each chromosome and its homologue ___________ together so that they are tightly aligned • Nonsister chromatids exchange segments in a process called _____________ _________ • Exchange a segment of the DNA molecule at the same place along their length

3. Crossing Over in Prophase I • Because alleles for the same trait can vary, crossing over allows chromosomes to swap certain genes • Results in a new combination of alleles not present in either parental chromosome, which is one source of genetic variation • Leads to __________ _______________ • Crossing over is a common event • In each cycle of meiosis in humans: • This rate varies among species and chromosomes

4. Crossing Over Figure 9.6, pg 144

5. Crossing Over Figure 9.6, pg 144

6. Crossing Over Resulting chromosome containing a new combination of alleles Figure 9.6, pg 144

7. Metaphase I Alignments • During metaphase I, homologous chromosomes ____________ line up at the spindle equator • Microtubules attach to the first chromosome they contact, regardless of whether it is maternal or paternal • During anaphase I, the homologous chromosomes separate into two haploid cells • Each cell has a ______________________________ ____________________________________________ • This chromosome shuffling is another source of genetic variation for each new generation

8. 1 2 3 Metaphase I Alignment combinations possible or or or Figure 9.7, pg 145

9. Metaphase I Alignment • For a species with just three pairs of homologues: • Eight possible combinations (23 ) for possible gametes • That means that in humans, a sperm or egg has:

10. From Gametes to Offspring • Every species has a distinctive life cycle • Gametes play an important role in these life cycles and bridge generations of individuals • For now, we are going to focus on the life cycle of plants and the life cycle of animals

11. Gamete Formation in Plants • Plant cycles usually involve two kinds of multicelled bodies • _____________________ are multicelled plant bodies (diploid) that produce haploid spores • Spores are haploid cells that undergo mitosis and give rise to multicelled haploid __________________ • Haploid gametes-producing bodies are formed in gametophytes, and gametes are formed • Gamete-producing bodies and spore-producing bodies develop during the life cycle of plants • Example: • Pine trees are sporophytes • Male and female gametophytes develop in different types of pine cones on each tree

12. _______ Life Cycle Figure 9.8, pg 146

13. Plant and Animal Life Cycles Figure 9.8, pg 146

14. Gamete Formation in Animals: ____ • In males: meiosis and gamete formation is called spermatogenesis • Sequence of events in sperm formation: • Germ cells (2n) • Primary spermatocyte (2n) • MEIOSIS I • Two secondary spermatocytes (n) • MEIOSIS II • Four spermatids (n) • Each __________ develops a tail to become a mature sperm

15. __________________ : Figure 9.9, pg 147

16. Gamete Formation in Animals: _______ • In females: meiosis and gamete formation are called ________ • Sequence of events in oogensis: • Germ cell (2n) • Primary oocyte (2n) • MEIOSIS I • Secondary oocyte (n, and large in size) + polar body (n, and small in size) • MEIOSIS II • One large ovum (n) plus three polar bodies (n, small) • The single ovum (egg) is the ONLY cell capable of being fertilized by a sperm • The polar bodies wither and die

17. _________________: Figure 9.9, pg 147

18. Fertilization • The chromosome number of the parent is restored at __________________ • Fertilization is the fusion of two haploid nuclei of two gametes • Of all the genetically diverse gametes produced, ________ will determine which two will meet at fertilization • Many gametes are formed, but only two are needed to produce a zygote • Meiosis allows species to maintain their chromosome number; without it, an offspring would have twice as many chromosomes as their parent

19. Variation in Offspring • Three events cause new combinations of alleles in offspring: • Crossing over during prophase I (meiosis) • Random alignment of maternal and paternal chromosomes at metaphase I (meiosis) • Chance meeting of gametes at fertilization • All three contribute to variation in traits of the offspring

20. Comparing Mitosis and Meiosis Mitosis Meiosis • Results in clones • Occurs in single-celled eukaryotes as a means of asexual reproduction • All eukaryotes engage in mitosis for growth and tissue repair • Occurs in immature reproductive cells giving rise to gametes • Sexual reproduction Both processes of meiosis and fertilization give rise to genetic variation

21. An Ancestral Connection? • Despite the differences, there are many similarities in the steps of mitosis and meiosis • The molecular machinery of mitosis may have been recruited for meiosis • For example: • Proteins the recognize and repair breaks in DNA are used in both • If the separation of anaphase in mitosis did not occur properly, the result would be that of anaphase I in meiosis • Did sexual reproduction result through mutations that caused changes in the mechanisms important before and during mitosis?

22. Chapter 9 Homework • Self Quiz 1-9 • Critical Thinking: 1, 3, and 5 (for only the Fruit fly)