IB BIOLOGY
TOPICS 4 AND 10: GENETICS
I. DNA Structure
A. Elements are carbon, hydrogen, oxygen, nitrogen and phosphorus
B. Monomers are called nucleotides
Figure 1. Structure of a nucleotide and the nitrogenous bases.
http://www.yellowtang.org/images/structure_of_nucleo_c_la_784.jpg
1. Nucleotide is made of a sugar (ribose or deoxyribose), one or more phosphate groups and a nitrogenous base
*note that a nucleoside is the sugar and the nitrogenous base, no phosphate groups
a. The carbon with the nitrogenous base is called the 1’ (one prime) carbon
b. The carbon with the H in deoxyribose or the OH in ribose is called the 2’ (two prime) carbon
c. The carbon with the phosphate group is called the 5’ (five prime) carbon
d. When 2 nucleotides join, the 5’ phosphate of one nucleotide bonds wtih the 3’ carbon of the next nucleotide
e. Think of the 5’ carbon as being the front (“f” for five and “f” for front) of a train with other nucleotides joining to the 3’ carbon at the back of the train (usually more cars are attached to the back end of the previous car, not the front end)
*helpful for the processes of replication and transcription
f. Nitrogenous bases are either purines or pyrimidines
i. purines
a. adenine and guanine
b. structure has 2 rings
ii. pyrimidines
a. cytosine, thymine and uracil
b. structure has 1 ring
C. Arrangement of nucleotides
Figure 2. Complementary base pairing with detail of hydrogen bonding.
http://www.starsandseas.com/SAS%20OrgChem/SASNucleic.htm
Figure 3. Space-filling model of DNA.
0560|00590|00010|00020|00030|00040|00050|00060|00070|00120|00080|00090|00100|00110|01000|02000|03000|04
Figure 4. A nucleotide represented with simple shapes as outlined in the IB syllabus.
Figure 5. Part of one strand of DNA represented with simple shapes as outlined in IB syllabus.
1. 2 chains/strands of nucleotides
a. Each chain/strand has alternating deoxyriboses and phosphates that act as a “backbone” with the nitrogenous bases sticking out between the chains
b. Chains held together by hydrogen bonds between the nitrogenous bases
i. complementary base pairing: a purine always bonds to a pyrimidine
a. A always binds to T
b. C always binds to G
c. important to replication, transcription and translation
c. The 2 strands run anti-parallel to each other: the 3’ end of one strand is at the same end as the 5’ end of the other strand
II. Chromosome Structure
Figure 6. Levels of structure of chromatin.
http://www.nature.com/scitable/resource?action=showFullImageForTopic&imgSrc=18847/pierce_11_5_FULL.jpg
Figure 7. Levels of structure of chromatin and reduction in length.
http://www.biochem.arizona.edu/classes/bioc462/462a/NOTES/Nucleic_Acids/nucacid_structure.html
Figure 8. Electron micrograph of chromatin from chicken red blood cell.
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/N/Nucleus.html#Nucleosomes
Question: What is odd about the preceding Figure? Read the title carefully!
______
A. Made of units called nucleosomes
1. DNA wrapped around 8 histone proteins and held together by another
histone protein
a. Each nucleosome has about 165 DNA base pairs
b. Nucleosomes join together to form chromatin
c. Each chromosome contains 100’s of 1000’s of nucleosomes, joined by the DNA that runs between them (about 20 base pairs)
i. called linker DNA
2. Nucleosome formation helps to make the DNA compact enough to fit in the nucleus
a. allows supercoiling of the DNA
i. Regulates gene transcription because the RNA polymerase cannot
easily access the gene
a. the DNA must be partially uncoiled at the relevant gene for
transcription to occur
i. done by modifying the histones: adding acetyl, methyl or
phosphate groups; displacing histone with chromatin
remodelling complexes which exposes the underlying DNA
sequences to polymerases etc.
Figure 9. Supercoiling of DNA.
http://oregonstate.edu/instruction/bb492/lectures/StructureIII.html
For some information about the size of each chromosome and what genes are on it:
http://genomics.energy.gov/gallery/chromosomes/gallery-01.html
III. Meiosis
Figure 10. The stages of meiosis .
http://www.britannica.com/bps/media-view?1
A. Differences between meiosis and mitosis:
1. The number of chromosomes in the daughter cells is ½ of the number of chromosomes in the parent cell
2. 4 daughter cells are produced
3. (i.e. a cell that is 2n (diploid) produces 4 cells that are n (haploid))
4. There are 2 divisions (meiosis I and meiosis II)
5. In metaphase I, the chromosomes line up at the equator in pairs but in metaphase II, the chromosomes line up at the equator in single file
B. Formation of Sex Cells/Gametes
Figure 11. Spermatogenesis and oogenesis.
http://www.bio.miami.edu/dana/104/gametogenesis.jpg
C. Crossing Over
Really fast animation: http://www.tokyo-med.ac.jp/genet/anm/mimov.gif
Figure 12. Crossing over.
1. Occurs during prophase I
2. As the chromatids are tightly entwined, produces stress on the DNA of the chromatids and they can break
3. Repair is not perfect because the genetic sequences of homologous chromosomes are so similar so that a fragment of one chromatid is rejoined with the broken chromatid of the neighbouring chromosome rather than with the original chromatid
4. The site of the crossing over is called a chiasma (plural chiasmata)
D. Non-disjunction
1. Non-disjunction: an error in chromosome sorting during either meiosis I or meiosis II
a. In meiosis I, result is 2 daughter cells that have 2 copies of the chromosome and 2 cells that are missing the chromosome altogether
Figure13. Non-disjunction in meiosis I and the result of fertilization.
http://www.medgen.ubc.ca/robinsonlab/mosaic/intro/tri_how.htm
b. In meiosis II, the sister chromatids do not separate and travel to the same cell
Figure 14. Non-disjunction in meiosis II and the result of fertilization.
http://www.medgen.ubc.ca/robinsonlab/mosaic/intro/tri_how.htm
2. Autosomal trisomies that can produce a viable offspring in humans (other trisomies do not produce viable offspring)
a. Down syndrome/Trisomy 21
i. affects 1:700 children
ii. produces characteristic facial features, short stature, heart defects,
susceptibility to respiratory disease, shorter lifespan, prone to
Alzheimer’s and leukemia, possible sexual underdevelopment and
sterility, some degree of intellectual challenge
iii. correlated with age of mother but can also be the result of non-
disjunction in father
b. Patau syndrome/Trisomy 13
i. 1:5 000 live births
ii. serious eye, brain and circulatory defects, cleft palate
iii. children rarely live more than a few months
c. Edward’s syndrome/Trisomy 18
i. 1: 10 000 live births
ii. almost every organ system affected
iii. children rarely live longer than a few months
3. Non-disjunction of sex chromosomes
a. Klinefelter syndrome/XXY males
i. have male sex organs
ii. unusually small testes, sterile
iii. some feminine body characteristics such as breast enlargement
iv. normal intelligence
b. Supermale syndrome/XYY males
i. somewhat taller than average
ii. often have below normal intelligence
iii. once thought to be more likely to be criminally aggressive but that
has been disproven
c. XXX females/trisomy X
i. 1: 1 000 live births
ii. healthy and fertile
iii. usually can not be distinguished except by karyotype
d. Monosomy X/Turner’s syndrome/XO females
i. 1:5 000 live births
ii. the only viable monosomy in humans
iii. do not mature sexually and are sterile
iv. short stature
v. normal intelligence
IV. Theoretical Genetics
A. Essential Definitions
1. Gene: a heritable factor that controls a specific characteristic. Each type of
chromosome has a number of genes located on it. Since human
somatic cells are diploid, each human cell has two copies of each
type of chromosome and therefore two copies of each gene. These
copies may be of the same form or different forms from each other.
2. Allele: one specific form of a gene differing from other alleles by only one
or a few bases and occupying the same gene locus as other alleles
of the gene
3. Genome: the whole of the genetic information of an organism
4. Genotype: the alleles of an organism
5. Phenotype: the characteristics of an organism, controlled by genotype
6. Dominant allele: an allele that has the same effect on the phenotype
whether it is present in the homozygous state or
heterozygous state
7. Recessive allele: an allele that only has an effect on the phenotype when
present in the homozygous state
Table 1. The dominant and recessive traits that were studied by Mendel.
Pea Plant Characteristic / Dominant Trait / Recessive TraitSeed shape / Round / Wrinkled
Seed colour / Yellow / Green
Flower colour / Purple / White
Pod shape / Inflated / Constricted
Pod colour / Green / yellow
Flower position / Axial / Terminal
Plant height / Tall / Short
8. Codominance/Codominant alleles: pairs of alleles that both affect the
phenotype when present in a
heterozygote
9. Incomplete dominance*: term used to refer to the affect of two
codominant alleles when their effects are blended
in the phenotype e.g. pink flowers in snapdragons
10. Partial dominance*: term used to refer to the affect of two codominant
alleles when both of their effects are separately
expressed in the phenotype e.g. roan fur in cattle
*According to IB, these terms are no longer used.
11. Locus: the particular position on homologous chromosomes of a gene
12. homozygous: having two identical alleles of a gene
13. heterozygous: having two different alleles of a gene
14. carrier: an individual that has one copy of a recessive allele that causes a
genetic disease in individuals that are homozygous for the allele
15. test cross: testing a suspected heterozygote by crossing it with a known
homozygous recessive
16. Punnett Square/Grid: a device used in genetic analysis, does not have to
be a square but can be rectangular
Gametes of Parent 1
Allele in first type of gameteP1A1 / Allele in second type of gamete
P1A2
Allele in first type of gamete
P2A1 / Genotype of offspring 1
(P1A1, P2A1) / Genotype of offspring 2
(P1A2, P2A1)
Allele in second type of gamete
P2A2 / Genotype of offspring 3
(P1A1, P2A2) / Genotype of offspring 4
(P1A2, P2A2)
17. Sex chromosome: chromosomes that determine gender in humans,
denoted X and Y. Females have 2 X chromosomes,
males have 1 X chromosome and one Y chromosome.
18. Sex-linked trait: a trait governed by a gene that is found on a sex
chromosome, usually the X chromosome.
For HL:
19. Autosome: any chromosome that is not a sex chromosome
20. dihybrid cross: a cross that involves 2 traits (genes)
21. (Mendel’s) Law of Independent Assortment: the inheritance of an allele of
one gene does not influence which allele of a second gene is inherited
22. linkage: the situation where two or more genes are found on the same
chromosome and are “linked”; therefore certain combinations of
alleles tend to be inherited together when predicting the result of
mating
For a longer, more detailed explanation: http://www.nature.com/scitable/topicpage/some-genes-are-transmitted-to-offspring-in-6524945
23. linkage group: a group of genes that is found on the same chromosome;
therefore the gene combinations of each parent tend to be
inherited together (i.e. do not follow the Law of
Independent Assortment)
Figure 13. Some linked genes in Drosophila (fruit fly). (Note: I have not checked
what the original source of this figure is.)
http://www.biologycorner.com/APbiology/inheritance/12-2_gene_linkage.html
Here is a worked example: http://www.biologycorner.com/APbiology/inheritance/12-2_gene_linkage.html
Example: A fly that is heterozygous for long wings (Ll) and heterozygous for long aristae (Aa) is crossed with another fly of the same type. AaLl x AaLl. In both cases the dominant allele is located on the same chromosome.
24. recombinants: can refer to chromosomes that result from crossing over,
offspring that have genotypes that are different from
either of the parents’ genotypes or to organisms that have
been genetically engineered to have DNA from 2 different
species
25. crossing over: the phenomenon where homologous chromosomes
exchange fragments during either meiosis I or meiosis II.
For animation of definitions 20, 21, 24, and 25: http://bcs.whfreeman.com/thelifewire/content/chp10/1002s.swf
Figure 14: An example of the results of crossing over in fruit flies.
http://www.biologycorner.com/APbiology/inheritance/12-2_gene_linkage.html
Note that the proportion of offspring that result from the recombinant chromosomes is small.
26. polygenic inheritance: describes the inheritance of a trait that is controlled
by more than one pair of genes and usually results
in continuous variation in the trait. Examples
include human skin colour, human height and
wheat kernel colour, plant height in tobacco, Rh
factor in humans
Table 2. Polygenic inheritance in people showing a cross between two mulatto parents (AaBbCc x AaBbCc). The offspring contain seven different shades of skin color based on the number of capital letters in each genotype.
http://waynesword.palomar.edu/lmexer5.htm
*This website has other examples of polygenic traits with tables showing the combinations of alleles as well.
Multiple Gene (Polygenic) Inheritance
Gametes / ABC / ABc / AbC / Abc / aBC / aBc / abC / abcABC / 6 / 5 / 5 / 4 / 5 / 4 / 4 / 3
ABc / 5 / 4 / 4 / 3 / 4 / 3 / 3 / 2
AbC / 5 / 4 / 4 / 3 / 4 / 3 / 3 / 2
Abc / 4 / 3 / 3 / 2 / 3 / 2 / 2 / 1
aBC / 5 / 4 / 4 / 3 / 4 / 3 / 3 / 2
aBc / 4 / 3 / 3 / 2 / 3 / 2 / 2 / 1
abC / 4 / 3 / 3 / 2 / 3 / 2 / 2 / 1
abc / 3 / 2 / 2 / 1 / 2 / 1 / 1 / 0
B. Performing Genetic Crosses
*To learn how to perform crosses, please follow the examples below.
Example 1: Complete dominance.
In humans, the allele for the ability to roll one’s tongue in the shape of a “U”