1. MEIOSIS

2. In sexual reproduction, chromosomes can sometimes swap sections or cross over during the process of meiosis (cell division), thereby creating new genetic combinations and thus more genetic variation. Although DNA replication is tightly regulated and remarkably accurate, errors do occur and result in mutations, which are also a source of genetic variation. Environmental factors can also cause mutation in genes and viable mutations are inherited. Mutations may occur due to errors during DNA replication and/or environmental factors. In general, only mutations that occur in gametes (sperm and egg) can be passed to offspring. Genes have variations (alleles) that code for specific variants of a protein (or RNA), and therefore specific traits of an individual. Environmental factors also affect expression of traits, and hence affect the probability of occurrences of traits in a population. Thus the variation and distribution of traits observed depends on both genetic and environmental factors.

3. Meiosis
The form of cell division by which gametes, with half the number of chromosomes, are produced.
Diploid (2n)  haploid (n)
Meiosis is sexual reproduction.
Two divisions (meiosis I and meiosis II).
Sex cells divide to produce gametes (sperm or egg).
Gametes have half the # of chromosomes.
Occurs only in gonads (testes or ovaries).
Male: spermatogenesis
Female: oogenesis
Meiosis is similar to mitosis with some chromosomal differences.

4. Fertilization n=23 egg 2n=46 zygote
The fusion of a sperm and egg to form a zygote.
A zygote is a fertilized egg
n=23
egg
sperm
n=23
2n=46
zygote

5. Spermatogenesis n=23 n=23 2n=46 sperm haploid (n) meiosis II human
sex cell
diploid (2n)
n=23
meiosis I

6. Meiosis I (four phases) Meiosis II (four phases)
Cell division that reduces the chromosome number by one-half.
Meiosis I Meiosis II
four phases: four phases:
a. prophase I aa. Prophase II
b. metaphase I bb. Metaphase II
c. anaphase I cc. Anaphase II
d. telophase I dd. Telophase II

7. Homologous Chromosomes
Pair of chromosomes (maternal and paternal) that are similar in shape and size.
Homologous pairs (tetrads) carry genes controlling the same inherited traits.
Each locus (position of a gene) is in the same position on homologues.
Humans have 23 pairs of homologous chromosomes.
a. First 22 pairs of autosomes
b. Last -01 pair of sex chromosomes

8. Homologous Chromosomes
eye color
locus
hair color
Paternal
Maternal

9. Karyotype
A method of organizing the chromosomes of a cell in relation to number, size, and type.

10. Humans have 23 Sets of Homologous Chromosomes Each Homologous set is made up of 2 Homologues.

11. Autosomes (The Autosomes code for most of the offspring’s traits)
In Humans the “Autosomes” are sets

12. In Humans the “Sex Chromosomes” are the 23rd set
Sex Chromosomes The Sex Chromosomes code for the sex of the offspring. ** If the offspring has two “X” chromosomes it will be a female. ** If the offspring has one “X” chromosome and one “Y” chromosome it will be a male.
In Humans the “Sex Chromosomes” are the 23rd set
XX chromosome - female
XY chromosome - male

13. Crossing Over - variation
Crossing over (variation) may occur between nonsister chromatids at the chiasmata.
Crossing over: segments of nonsister chromatids break and reattach to the other chromatid.
Chiasmata (chiasma) are the sites of crossing over.

14. Another Way Meiosis Makes Lots of Different Sex Cells – Crossing-Over
Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.

15. Metaphase I Shortest phase Tetrads align on the metaphase plate.
INDEPENDENT ASSORTMENT OCCURS:
1. Orientation of homologous pair to poles is random.
2. Variation
metaphase plate OR
metaphase plate

16. Remember: variation is good!
Important to population as the raw material for natural selection.
Question:
What are the three sexual sources of genetic variation?
1. crossing over (prophase I)
2. independent assortment (metaphase I)
3. random fertilization
Remember: variation is good!

17. Mutations
Mutation – sudden genetic change (change in base pair sequence of DNA)
Can be :
Harmful mutations – organism less able to survive: genetic disorders, cancer, death
Beneficial mutations – allows organism to better survive: provides genetic variation
Neutral mutations – neither harmful nor helpful to organism
Mutations can occur in 2 ways: chromosomal mutation or gene/point mutation

18. Chromosomal mutation:
less common than a gene mutation
more drastic – affects entire chromosome, so affects many genes rather than just one
caused by failure of the homologous chromosomes to separate normally during meiosis
chromosome pairs no longer look the same – too few or too many genes, different shape

19. Examples:
Down’s syndrome – (Trisomy 21) 47 chromosomes, extra chromosome at pair #21

20. Turner’s syndrome – only 45 chromosomes, missing a sex chromosome (X)
Girls affected – short, slow growth, heart problems

21. Klinefelter’s syndrome – 47 chromosomes, extra X chromosomes (XXY)
Boys affected – low testosterone levels, underdeveloped muscles, sparse facial hair

22. Having an extra set of chromosomes is fatal in animals, but in plants it makes them larger and hardier.
Hardier

23. Gene or Point Mutation
most common and least drastic
only one gene is altered

24. Examples:
Recessive gene mutations:
Sickle cell anemia – red blood cells are sickle shaped instead of round and cannot carry enough oxygen to the body tissues – heterozygous condition protects people from malaria

25. Cystic fibrosis – mucous builds up in the lungs
Tay-Sachs Disease – deterioration of the nervous system – early death
Mutated genes produce enzymes that are less effective than normal at breaking down fatty cell products known as gangliosides. As a result, gangliosides build up in the lysosomes and overload cells. Their buildup ultimately causes damage to nerve cells.

26. Phenylketonuria (PKU) – an amino acid common in milk cannot be broken down and as it builds up it causes mental retardation – newborns are tested for this
Dominant gene mutations:
Huntington’s disease – gradual deterioration of brain tissue, shows up in middle age and is fatal
Dwarfism – variety of skeletal abnormalities

27. Detecting Genetic Disorders
picture of an individual’s chromosomes – karyotype
amniotic fluid surrounding the embryo is removed for analysis – amniocentesis
Female with Down’s syndrome