1. Modified from: Kim Foglia,
Chapter 13.
Meiosis &
Sexual Reproduction
Modified from: Kim Foglia,
Explore Biology

2. Human female karyotype

3. Human male karyotype

4. How do we make sperm & eggs?
reduce 46 chromosomes  23 chromosomes
half the number of chromosomes 23 46 23 46
egg
meiosis 23 46 23
fertilization
sperm

5. Meiosis: production of gametes
Alternating processes, alternating stages
chromosome number must be reduced
diploid  haploid
2n  n
humans: 46  23
meiosis reduces chromosome number
fertilization restores chromosome number
haploid  diploid
n  2n

6. Homologous chromosomes
Paired chromosomes
both chromosomes of a pair carry genes
control same inherited characters
homologous = same information
diploid 2n
homologous chromosomes
double stranded homologous chromosomes

7. Double division of meiosis
DNA replication
1st division of meiosis separates homologous pairs
2nd division of meiosis separates sister chromatids

8. Steps of meiosis Meiosis 1 Meiosis 2 interphase prophase 1 metaphase 1
anaphase 1
telophase 1
Meiosis 2
prophase 2
metaphase 2
anaphase 2
telophase 2
1st division of meiosis separates homologous pairs
(2n  1n)
2nd division of meiosis separates sister chromatids
(1n  1n)
* just like mitosis *

11. Crossing over During Prophase 1
homologous pairs swap pieces of chromosome
sister chromatids intertwine
crossing over
tetrad
synapsis

12. What are the advantages of sexual reproduction?
Crossing over
3 steps
cross over
breakage of DNA
re-fusing of DNA
New combinations of traits

13. Genetic variation
Meiosis & crossing over introduce great genetic variation to population
drives evolution
Consider the greater variation with 23 pairs of chromosomes = mixing and matching

14. The value of meiosis Meiosis introduces genetic variation
gametes of offspring do not have same genes as gametes from parents
genetic recombination
random assortment in humans produces 223 (8,388,608) different combinations
new gametes
made by offspring
from Mom
from Dad

15. And more variation… Crossing over
creates completely new combinations of traits in next generation

16. Random fertilization
Any 2 parents will produce a zygote with over 70 trillion (223 x 223) diploid combinations

17. Sources of genetic variability
Genetic variability in sexual reproduction
independent assortment
homologous chromosomes in Meiosis 1
crossing over
between homologous chromosomes in prophase 1
random fertilization
random ovum fertilized by a random sperm
metaphase1

18. Mitosis Asexual Reproduction Fusion Meiosis Cell A Cell B Cell C
Cell A has 4 chromosomes and Cell D has 2 chromosomes. What process created Cell D?
Mitosis
Asexual Reproduction
Fusion
Meiosis
Cell A
Cell B
Cell C
Cell D 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 10

19. Cell A is considered _______ while Cell D is considered ________.
Haploid; Diploid
Tetrad; Diploid
Diploid; Haploid
Homologous; Haploid
Cell A
Cell B
Cell C
Cell D 10 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

20. Cell A is considered _______ while Cell D is considered ________.
somatic cell; gamete
gamete; somatic cell
stem cell; somatic cell
stem cell; gamete 30

21. How can we best describe the diagram below?
Two sister chromatids
Homologous chromosomes
One replicated chromosome
Haploid chromsomes 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 10

22. What process is best demonstrated by the diagram below?
Anaphase I
Segregation of alleles
Crossing over
Independent assortment 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 10

23. Which of the following leads to genetic variation during meiosis?
Crossing Over
DNA replication
Independent assortment
Mitotic spindle formation
All of the above (A, B, C, D)
A & B
A & D
A, B, & D
A & C 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 10

24. Mitosis vs. Meiosis Mitosis Meiosis 1 division
daughter cells genetically identical to parent cell
produces 2 cells
2n  2n
produces cells for growth & repair
no crossing over
Meiosis
2 divisions
daughter cells genetically different from parent
produces 4 cells
2n  1n
produces gametes
crossing over

25. Mitosis vs. Meiosis

26. Changes in Chromosomes Number
Euploidy = correct # of chromosomes
Aneuploidy = a change in chromosomes number due to non-disjunction during meiosis
Monosomy- only 1 copy of an individual chromosome
Trisomy- 3 copies of an individual chromosome

27. Primary nondisjuction = Meiosis I Secondary nondisjuction = Meiosis II

28. Trisomy 21 : Down Syndrome
Delayed mental and social skills
Decreased muscle tone at birth
Asymmetrical or odd-shaped skull
Small skull
Small mouth with protruding tongue
Broad short hands
Increased risk of developing
Leukemia and Alzheimer’s later in life

30. Trisomy 18 : Edward’s Syndrome
Most children die in the first year of life, some have lived 10 years
Growth deficiency
Feeding difficulties
Breathing difficulties
Developmental delays
Mental Retardation
Overlapped, flexed fingers
Webbing of the second and third toes
Clubfeet
Structural heart defects at birth

31. Trisomy 13 : Patau Syndrome
Mental retardation, severe
Seizures
Small head
Scalp defects
Cleft lip and/or palate
Eyes close set (hypotelorism) –may fuse
Extra digits (polydactyl)
Hernias
Undescended testicle
Children die in the first year of life

32. Karyotype
A visual display of the chromosomes arranged by size, shape, and banding pattern
Used to identify aneuploid conditions

33. Procedure: Amniocentesis and Karyotyping

34. Karyotyping

35. Changes in Sex Chromosome #
Turners Syndrome (XO) – missing Barr Body
Kleinfelter’s Syndrome (XXY)
Swyer Syndrome (XY female)
La Chapelle Syndrome (XX male)
Poly-X Females (XXX, XXXX)
Jacob’s Syndrome (XYY males)
-SRY gene (located on short arm of Y chromosome)
-hormone= testis-determining factor
-Barr Body – Inactive X chromosome (XX)

36. Chromosomal Mutations

37. Deletion Syndromes
Williams Syndrome (deletion of a piece of chromosome 7)
Cri du chat (cat’s cry) (deletion of a piece of chromosome 5)

38. Translocation Syndromes
Alagille syndrome – Chromosomes 2 and 20 exchange segments
Cancers
Chronic myelogenous leukemia (2 and 9)
Burkitt lymphoma (8 and 14)