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Summary Genetics, ISBN: 9780077229726 Genetics
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Chapter 2 mendelian inheritance2.1 Mendel’s study of pea plants
Mendel studied the pea plants for 8 years. He did not immediately receive recognition for his
findings. Only around the 1900 his work was rediscovered.
Mendel chose pea plants as his experimental organism
He did hybridization experiments, In which he crossed parent to create hybrids. The male gametes
produced within pollen grains that form in anthers. Female gametes produced within ovules
that from in ovaries. Mendel used self-fertilization and cross-fertilization.
Mendel studied seven characteristics that bred true
General characteristics of organisms = characters. A specific property of a character = traits/ variant.
Mendel started with a true-breeding line = variety that continues to produce same trait after
generations.
2.2 Law of segregation
Mendel followed the outcome of a single character for two generations
Mendel used an empirical approach = no hypothesis, but analyses of mathematical relationship. His
first proposal was that one variant is dominant to the other. His data were consistent with a
particulate theory of inheritance genetic traits are inherited as units that remain unchanged. He
proposed genes segregate.
Mendels 3:1 ratio is consistent with law of segregation
Mendel’s law of segregation states: 2 copies of a gene segregate during process that gives rise to
gametes. The phenotypic ratio in F2 = 3:1, the genotypic ratio = 1:2:1.
Punnett square can be used to predict outcome of crosses and self-
fertilization experiments
The genotypes of the parents must be known. The possible gametes should be formed and the
square can be filled in. The square shows the genotypic ratio.
2.3 Law of independent assortment
To formulate this second law, two-factor crosses needed to be done. Two characters studied.
Mendel also analyzed crosses involving two different characters
Because of independent assortment, 4 kinds of gametes could be produced in a two factored cross.
The occurrence of non parental variants contradicted the linked-assortment hypothesis. The law of
independent assortment: 2 different genes will randomly assort their alleles during process that
gives rise to gametes. The phenotypic ratio becomes 9:3:3:1 in a heterozygote plant. When offspring
receives nonparental variant genetic recombination.
Multiplication and forked-line methods can be used to solve independent
assortment
Any cross bigger than 2 factor will be a lot of work to make in a Punnett square. Each gene can have
their own punnet square, multiply the three phenotypic outcomes together.
Modern geneticists are often interested in relationship between molecular
exoression and outcome of traits
Recessive alleles ca be loss-of-function alleles.
,2.4 Studying inheritance patterns in humans
A pedigree is specifically organized. Vertical lines connect generations, horizontal lines connect
parents. Males are square and females round. The filled in figures are the diseased. Sometimes
figures are also partly filled in, this indicates the individual is a carrier of the disease. However many
times the pedigree is based on the phenotypes and carriers are not mentioned.
2.5 Probability and statistics
The product rule can be used to calculate problems with independent outcomes.
Probability is the likelihood an outcome will occur
Probability = number of times particular outcome occurs / total number of possible outcomes. The
deviation between observed and expected outcomes = random sampling error.
Product rule is used to predict probability of independent outcomes
First use a Punnett square to calculate individual probability of affected offspring. Multiply the
outcomes with each other. The rule can also be used to calculate the chance of a specific genotype.
,Chapter 3 Chromosome transmission during cell
division and sexual reproduction
3.1 General features of chromosomes
The most basic observation a cytogeneticist can make is the examination of chromosomal
composition while the cell is actively dividing. Somatic cell = any cell of the body that is not a gamete
or precursor gamete. Germ cells = gametes (sperm and egg cells).
To examine cells, they are treated with a chemical to stimulate cell division treated with another
chemical that halts cell division during mitosis the solution is mixed with hypotonic solution to
make the cells swell and more visible cells treated with fixative that freezes cells, chromosomes
stop moving around treated with dye so the chromosomes become visible.
Eukaryotic chromosomes are inherited in sets
Most Eukaryotic species are diploid. The members of one chromosomes pair = homologs. The
homologs may carry different versions of genes alleles. The base sequences of homologs differ for
less than 1%.
3.2 Cell division
Asexual reproduction = a existing cell divides to produce 2 new cells, mostly bacterial cells.
Bacteria reproduce asexually by binary fission
Binary fission = two daughter cells become separated from each other by formation of septum. The
cells are usually genetically identical. FtsZ important protein assembles in a ring to recruits
producing of new cell wall. Evolutionary related to tubulin,
Eukaryotic advance through a cell cycle to produce genetically identical
daughter cells
G = gap, S=synthesis, M=mitosis. G1, S and G2 interphase. If a cell is in G0 temporarily not
advancing in cell cycle. During G1 cell may prepare to divide and may accumulate molecular
changes, which helps advance through the stages = restriction point. cell advances through S
phase (chromosomes replicated).
Kinetochore = group of proteins bound to centromere, help hold the chromatids together and
chromosome sorting. During G2 cell accumulates materials needed for cell division.
3.3 Mitosis and cytokinesis
The mitotic spindle apparatus organizes and sorts eukaryotic
chromosomes
Mitotic spindle apparatus = formed from microtubule-organizing centers (MTOCs), structures from
which microtubules grow. In animal cells spindle apparatus is formed from 2 MTOCs centrosomes
= each one located at a spindle pole and a pair of centrioles at right angles to each other. Plant cells
do not have centrosomes, the nuclear envelope is the MTOC. The animal cell spindle apparatus has 3
types of microtubules: Astral microtubule (=outward of centrosome to the membrane), Polar
microtubules (=toward chromosomes) and kinetochore microtubules (=attachements to
kinetochores, proteins bound to centromere).
The transmission of chromosomes during division of eukaryotic cells
requires mitosis
Prophase – chromosomes have replicated, chromatids become condensed into compact strucures,
the centrosomes move apart and spindle apparatus begins to form
, Prometaphase – centrosomes establish two spindle poles, nuclear membrane is fragmented into
vesicles, allowing spindle fibers to interact. The ends are captured by kinetochore.
Metaphase – sister chromatids align themselves in metaphase plate and each pair of chromatids
attached to both poles by kinetochore microtubules.
Anaphase – The connection between the chromatids is broken and The kinetochore microtubules
are shortening and chromatids move to the sides.
Telophase – chromosomes reach their poles and decondense. Nuclear membrane reforms to make 2
nuclei.
Cytokinesis – mitosis is followed by cytokinesis, nuclei are segregated into daughter cells. In animals:
A contractile ring, myosin and actin filaments, assemble at cytoplasmic surface. Myosin hydrolyzes
ATP shortens the rind form cleavage furrow. In plants: Formation of cell plate begins in the
middle.
3.4 meiosis
Meiosis produces cells that are haploid
Meiosis 1 and meiosis 2 are both divided in the 5 phases
Prophase (1) – subdivided into 5 phases:
leptotene = chromosome condensation
zygote = recognition process, synapsis, of homologous chromosomes.
Pachytene = the associated chromatids, bivalents, have been completely aligned. Crossing-
over occurs, a lack of crossing-over can be associated with diseases. Connection that results
from crossing over chiasma
Diplotene = synaptonemal complex largely dissociates
Diakinesis = synaptonemal complex completely disappears
Prometaphase (1) – spindle apparatus complete, chromatids attached via kinetochore microtubules.
Metaphase (1) – the random arrangements of homologs is consistent with Mendel’s law of
independent assortment. There are 2^23 = over 8 million possibilities. Second feature Is the
attachment of kinetochore microtubules, one homologous pair is linker to 1 side.
Anaphase (1) – Each pair of chromatids moves to 1 side, the two dyads within a tetras separate.
Telophase (1) – in most species decondensation occurs.
Meiosis 2 – Meiosis 2 is the same as mitosis, only the amount of genetic material differs. Only half of
the genetic material in comparison with mitosis.
Mitosis produces to two diploid daughter cells, meiosis created 4 haploid daughter cells.
3.5 sexual reproduction
Gametes are produced by gametogenesis. When the gametes are morphologically similar =
isogamous. Most eukaryotic species heterogamous.
In animals, spermatogenesis produces 4 haploid sperm cells oogenesis 1
eggcell
Testes contain spermatogonial cells that divide via mitosis to produce 2 cells one spermatogional
cell and one primary spermatocyte. advances through meiosis 1 and 2 to form 4 haploid
spermatids. become sperm cells with head and flagellum. The head contains the haploid nucleus
and an acrosome = contains digestive enzymes for entering egg cell.
Early in the development of the ovary oogonia initiates meiosis to produce primary oocytes. 1
million produced before birth, they enter a dormant phase at prophase of meiosis 1 until the woman
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