Gabriel Krotkov
XVI. Genetics
Biology Notes
3/18/14
A. Mendelian Genetics
1. Gregor Mendel
a. Austrian Monk (now Czech Republic)
b. Scientific Approach identifying laws of inheritance
i. Chose to work with peas because they have a large variety of heritable
features. Those that varied among indivudals were called characters. Each variant
for a character is called a trait.
ii.
c. 7 Experimental traits
i. Seed Shape
ii. Seed Color
iii. Flower Color
iv. Pod Shape
v. Pod Color
vi. Flower position
vii. Stem Length
d. Generations in Mendelian Crosses
i. P-parental
ii. F1 – Filial 1
iii. F2 – Filial 2
e. Mendel’s Conclusion
i. Alleles – Varied forms of a gene
ii. Homozygous – two identical alleles
iii. Heterozygous – One dominant, one recessive (often)
2. Cross types
a. Specific types of crosses
i. Monohybrid: Only exactly one trait
ii. Bihybrid: Examining two traits
iii. Polyhybrid: Examining three or more traits
iv. Trihybrid Crosses (special rules): Rule of multiplication allows for a
shortcut, make three monohybrid crosses, multiply them together. Can apply for any
polyhybrid cross.
3. Mendelian Laws of genetics
a. Law of Segregation
i. States that allele pairs are separated during gamete formation
ii. Allows for variance in genetics
b. Law of independent assortment
i. Traits are passed on independently of each other
ii. Means that traits do not have association
iii. Each parent passes on one allele for each trait
iv. NOTE THAT THIS IS TRUE FOR CHROMOSOMES; but recombination
and linkage prove this partially false for genes on the same chromosome
c. Law of dominance
i. States that one allele will be dominant over another and will determine
phenotypic expression, even if the other allele is present: thus, a homozygous
genotype will result in a dominant phenotype
ii. This law does not always hold, but it is the most common form of
inheritance.
B. Types of Inheritance
1. Incomplete Dominance
a. Combination (Blending) of any two alleles
b. Multiple dominant alleles
2. Codominance
a. Two traits represented at once
b. Not a blend!
3. Multiple Alleles
a. A trait with more than two alleles
b. Based on a whole population, not one organism
4. X- linked traits
a. A trait carried on only the X-chromosome
i. This causes males to be determined by only the x given by their maternal
parent.
b. It is more damaging to males, because they have no second x chromosomes
to cover up a disorder
c. All x-linked traits are recessive, if deadly diseases (dominant kill
themselves out)
d. Eg.
i. red-green colorblindness
ii. Hemophilia – One in 10,000 males, 1 in 100000000 females
5. Polygenic Inheritance
a. Controlled by more than one gene
6. Epistasis
a. A type of inheritance where the expression of a gene at one locus changes
the phenotypic expression of a gene at another locus
b. For example, when one gene can “turn another on or off”; resulting in more
than 2 possible expressions.
7. Pleiotropy
a. The opposite of polygenic inheritance, when a single gene will control
multiple seemingly unrelated phenotypes
C. Genetic Code
1. Sequence of 4 bases (A, T/U, C, G)
2. Read in Codons
a. Codons: A triplet in mRNA; codes for one amino acid
3. 64 possible codons, with only 20 amino acids
a. Lots of redundant code
D. Sexual Life Cycles
1. 3 Mechanisms for genetic variation
a. Independent assortment of chromosomes
i. Random orientation of pairs of homologous chromosomes at metaphase
of meiosis I; 50% chance for maternal chromosome, 50% chance for paternal
chromosome
ii. Each pair of homologous chromosomes is positioned independently, and
thus the number of combinations possible is 2^n, where n = haploid # chromosomes
b. Crossing over
i. During metaphase I, paternal/maternal chromosomes form recombinant
chromosomes, mixed-up versions of the original 2. Usually 1 to 3 crossover events
occur per pair.
c. Random Fertilization
i. Locus of chromosomes
ii. Produces zygot with about 70 trillion diploid combinations.
2. Inheritance of genes
a. Genes are a hereditary unit that parents pass along to their progeny
b. Gametes are the reproductive cells in plants & animals that transmit genes
from one generation to the next; as opposed to somatic cells, which are everything
else.
c. Asexual/Sexual Reproduction
i. In asexual reproduction, a single individual is a sole parent and passes
copies of all its genes (virtual copies; almost clones) to its offspring without the
fusion of gametes – mitotic cell division
ii. Sexual reproduction requires 2 parents, in which the progeny receives
random genes from both parents, varying genetically from both the parents and
their siblings.
3. Fertilization and meiosis alternation
a. Definition of a life cycle:
i. Generation-to-generation sequence of stages in the reproductive history
of an organism, from conception to production of its own offspring.
b. Diploid/Haploid
i. A diploid cell is a cell with two chromosome sets, with a diploid number
of chromosomes. This is abbreviated: 2n
ii. A haploid cell is a cell with one chromosome set, abbreviated n
iii. Each sexually reproducing species has a characteristic diploid and
haploid number of chromosomes in their cells.
c. Gametophyte/Sporophyte
i. A gametophyte is a multicellular organism that is haploid
ii. A sporophyte is a multicellular organism that is diploid
4. 3 Main sexual life cycles
a. Animals (includes humans)
i. Gamete w/n chromosomes added to another gamete
ii. Fertilization
iii. Zygote (2n chromosomes)
iii. Mitosis results in diploid multicellular organism
iv. Meiosis produces gametes of n chromosomes
v. Repeat cycle
b. Plants and some algae (Alternation of Generations)
i. Haploid gametophyte produces gametes by mitosis
ii. Fertilization of gametes
iii. Zygotes (diploids) use mitosis to grow into a sporophyte
iv. Sporophyte grows via mitosis
v. Sporophyte uses meiosis to split, giving 2 gametophytes
vi. Repeat cycle
c. Most fungi and protists
i. Haploid unicellular or multicellular organism
ii. Mitosis grows organism into gametes
iii. Fertilization produces zygotes from gametes
iv. Zygotes use meiosis to produce haploid unicellular organisms.
5. Variety of Sexual Life Cycles
a. Commonalities
i. Meiosis and fertilization alternate in all sexual organisms; meiosis
reduces from 2n  n, and fertilization
b. Alternation of generations
i. Life cycle of plants and some species of algae that includes both
multicellular diploid and haploid organisms
ii. The multicellular diploid stage is called the sporophyte
iii. Sporophyte produces spores, haploid cells produces from the
sporophyte by meiosis.
iv. The spores divide mitotically, creating an entirely different organism:
the gametophyte.
v. Fertilization of haploid spores creates a diploid zygote.
E. Evolutionary Significance
1. Mutations are the original source of different alleles, which get distributed
around by the mechanisms of variation (see above)
2. Stable environments favor asexual reproduction (stick with what works),
while unstable ones favor sexual reproduction (try something new)
F. Chromosomal theory of inheritance
1. Definition
a. States that mendelian genes have specific loci along chromosomes, and it is
chromosomes, not the genes themselves, that undergo segregation and independent
assortment.
b. This has all sorts of implications, which are detailed below.
2. Scientific Background
a. Thomas Hunt Morgan
i. Experimental embryologist at Columbia who produced experiments that
supported Mendelian theory. Also set precedent of using Fruit Flies as genetic
experimentation tools. (prolific breeders, quick maturation, identifiable traits, only
4 pairs of chromosomes)
ii. Disovered that fly eye color was related to sex (Breeading a white-eyed
fly with a red-eyed female produces 3:1 phenotype ratio, but the white type only
existed in males)
iii. Discovered that fly body color and wing type were linked genes
b. Morgan’s nomenclature
i. A gene takes its symbol from the first mutant (non-wild) allele
discovered.
ii. A superscript + identifies the wild type.
iii. Wild type: Most common phenotype
iv. Mutant Type: Not a wild type phenotype.
3. Sex-linked genes
a. Sex chromosome overview
i. In most animals, each organism has a pair of sex-linked chromosomes
ii. These chromosomes are either an X or a Y. The X chromosomes are
significantly larger than those of a male, and are homologous to the Y chromosome
only at a short segment at either end of the Y-chromosome, allowing the two to act
as homologous pairs during meiosis
iii. An organism with XX is female, while an organism with XY is male.
Occasionally this is not the genotype (XYY, XXY, etc.) in which case genetic disorders
result. Note that in this system, the sex chromosome present in sperm determines
the gender of the organism
iv. There are other systems of determining sex in animals: in some insects,
XX = Female and X(nothing) = Male. Sex of the offspring is determined by whether
the sperm cell contains an X chromosome or no sex chromosome. In some birds &
fish, ZW = Female and ZZ = Male, where the sex chromosome in the egg, rather than
the sperm, determines the sex of the offspring
v. In the diplo-haploid system of bees and ants, there are no sex
chromosomes. Rather, females develop from fertilized eggs and are diploid, while
males develop from unfertilized eggs and are haploid.
vi. Any gene located on either sex chromosome is designated a “sex-linked
gene”; while you can also be more specific and say “X-linked gene” if you like.
b. Inheritence of X-Linked Genes
i. X-Linked genes, unlike Y-linked genes, are intended for characteristics
unrelated to sex (y is almost entirely sex-related, and passed down almost
identically from father to son [probably only changes via crossing over of
homologous sections at each end of Y-chromosome, and only rarely])
ii. Fathers pass x-alleles to all of their daughters, and none of their sons.
Mothers, however, can pass either of their alleles to either gender offspring.
iii. Because males only have one locus of x-chromosomes, the terms
homozygous and heterozygous don’t quite fit – hemizygous is used instead. Because
of the hemizygous male condition, many more males have disorders than do
females.
iv. In females, X-chromosomes mostly become inactive during embryonic
development, in order to prevent females from making twice as many X-linked
proteins as males do. The inactive chromosome is called a “Barr body”; and these
genes are not expressed.
v. Selected of which chromosome will form the Barr body occurs randomly
and independently in each embryonic cell, ad all descendants of that cell follow that
pattern, (so why does the law of dominance apply?) which causes female bodies to
be a mosaic of two types of cells: those with active X derived from the father and
those with active X derived from the mother.
4. Gene linkage
a. Genes located near each other on the same chromosome tend to be
inherited together, such genes are called “linked genes”
i. Note: they are not always inherited together; just often.
b. Genetic Recombination
i. Definition: The production of offspring with combinations of traits that
differ from those found in either parent.
ii. The recombinant type is a phenotypic type that combines phenotypes
that are not seen in conjunction in either parent (eg. Cross
trait1RECESSIVEtrait2DOMINANT with trait1DOMINANTtrait2RECESSIVE =
trait1DOMINANTtrait2DOMINANT. In this example, the offspring is a recombinant
type)
iii. The percentage of recombinant occurrence in a testcross observing two
genes can give the “recombination frequency”, which is equal to “map units” (see
later) that are used to measure distance between two genes on a chromosome.
c. Crossing over
i. Crossing over occasionally breaks the physical connection between
specific alleles of genes on the same chromosome; accounting for the recombination
of linked genes
d. Mapping
i. Genetic map: an ordered list of genetic loci along a particular
chromosome. Linkage map: a genetic map based on recombination frequencies.
ii. The farther apart two genes are, the higher the probability that a
crossover will occur between them and therefore the higher the recombination
frequency between those two genes.
iii. The recombination frequency between two genes can be converted into
map units, which express the distance between genes. One map unit = 1%
recombination frequency. 50 map units is the maximum value that two genes can
have – indicating that they are as infrequently inherited together as genes on
separate chromosomes.
iv. Because a linkage map is based only on recombination frequency, it
only gives a rough picture of the chromosome: frequency of crossing over is not
uniform over the length of the chromosome, and thus map units do not correspond
to physical distances; but rather to order of genes on chromosome
G. Exceptions to Mendelian inheritance
1. Genomic imprinting
a. A few traits vary based on whether the alleles were supplied by the male or
female parent
b. During gamete formation, a particular allele on certain genes is silenced,
and because these genes are imprinted differently in males and females, only
either the male or female will be expressed (whichever was not silenced)
2. Inheritance of Organelle Genes
a. Mix in endosymbiotic theory with inheritance.
b. Organelles reproduce themselves and transmit their genes to daughter
organelles; more like bacteria than like eukaryotes.