Pre Med III Genetics
Guri Tzivion, PhD
tzivion@windsor.edu
Extension 506
Winter 2016
Windsor University School of Medicine
Questions on Mendelian
genetics?
Basic terminology:
Generations:
P: parental generation
F1: 1st filial generation, progeny of the P generation
F2: 2nd filial generation, progeny of the F1 generation (F3 and so on)
Crosses:
Monohybrid cross: cross of two different true-breeding strains
(homozygotes) that differ in a single trait.
Reciprocal cross: sexes for the two strains are reversed (and if
the results are the same, trait is not sex-linked).
Dihybrid cross: cross of two different true-breeding strains
(homozygotes) that differ in two traits.
F1 x F1 Punnett square:
F2 genotypes:
1/4 SS
1/2 Ss
1/4 ss
F2 phenotypes:
3/4 smooth
1/4 wrinkled
Test Crosses
Dihybrid
cross:
F2 generation
phenotypic
ratio:
9:3:3:1
Summary of Mendel’s 3 Principles:
 Mendel’s Principle of Uniformity in F1:
F1 offspring of a monohybrid cross of true-breeding strains
resemble only one of the parents (complete dominance)
 Mendel’s Principle of Segregation:
Recessive characters masked in the F1 progeny of two truebreeding strains, reappear in a specific proportion of the F2
progeny.
Two members of a gene pair segregate (separate) from each
other during the formation of gametes.
Inheritance is particulate, not blending as previously believed.
 Mendel’s Principle of Independent Assortment:
Alleles for different traits assort independently of one another.
Genes on different chromosomes behave independently in
gamete production.
Exceptions To Mendel’s Principles
Incomplete dominance
Pleiotrophy
Co-dominance
Multiple alleles
Environmental effects
on gene expression
Polygenic traits
Linkage
Epistasis
Sex linkage
Sample Questions
An individual with a dominant phenotype (A) is crossed with
an individual with a recessive phenotype (a), 4 of their 9
offspring show the recessive phenotype. What is the
genotype of the first parent?
A) AA
B) Aa
C) aa
D) AA or Aa
E) The answer cannot be determined from this
information.
Sample Questions
An individual with a dominant phenotype (A) is crossed with
an individual with a recessive phenotype (a), 4 of their 9
offspring show the recessive phenotype. What is the
genotype of the first parent?
A) AA
B) Aa
C) aa
D) AA or Aa
E) The answer cannot be determined from this
information.
Sample Questions
Which of the following crosses would always result in
offspring that only display the dominant phenotype?
A) TT x tt
B) Tt x Tt
C) TT x TT
D) Tt x Tt
E) Both TT x tt and TT x TT
Sample Questions
Which of the following crosses would always result in
offspring that only display the dominant phenotype?
A) TT x tt
B) Tt x Tt
C) TT x TT
D) Tt x Tt
E) Both TT x tt and TT x TT
Pre Med III Genetics
Class 3
Genes Chromosomes and
Heredity
2. Cell Cycle, Mitosis and Meiosis
Cell Division
Mitosis & Meiosis
Prokaryote
versus
Eukaryote
cell
Mitosis
Mitosis is a single cell division with the aim to
produce 2 identical daughter cells
It is used for reproduction in prokaryotes and
asexually reproducing organisms
In eukaryotes, mitosis is used for growth and
repair
Mitosis does not increase variability as the
daughter cells are identical to the parent cell
Definitions
Term
Definition
Chromatin
A complex of macromolecules found in cells, consisting of DNA,
protein and RNA
a single piece of coiled double-stranded DNA, containing many
genes, regulatory elements and other non-coding DNA
Chromosome
The prokaryotes—bacteria and archaea—typically have a single
circular chromosome.
In eukaryotes, nuclear chromosomes are packaged by proteins
into a condensed structure called chromatin.
Chromatid
One copy of a duplicated chromosome, which is generally joined
to the other copy by a single centromere
Centromere
The part of the chromosome that links sister chromatids
Replicated Chromosome
The Cell Cycle
Interphase: the cell spends the majority of its life here,
growing and functioning. During the S phase of the cell
cycle, the DNA replicates, in anticipation of Mitosis
G1: the first
part of
Interphase,
(Growth
Phase)
Check point
S phase: the
Second part of
Interphase
(Synthesis
Phase – the
DNA
duplicates)
G2: the third
part of the Cell
Cycle, serves as
a checkpoint to
make sure that
the duplication
process is error
free.
Mitosis:
Cell Division
The last part of
the Cell Cycle is
called Mitosis,
M phase, and
has 4/5 phases
during which
the cell divides
into 2 cells
Stages of Mitosis…(IPMATC)
Interphase
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis
M
Stages of Mitosis - IPMATC
Interphase: DNA replicates (genetic material
doubles) and centrosomes duplicate
Stages of Mitosis - IPMATC
Prophase: chromatin condenses to form chromosomes
with a centromere. The nuclear membrane disappears. The
centrosomes move away from each other to either side of
the cell.
In the early stage of prophase the chromosomes
condense, centrioles move to the poles of the nucleus,
and spindle fibers develop
Pair of
Centrioles
Chromosomes
consisting of 2
sister
chromatides
Spindle fibers
Stages of Mitosis - IPMATC
Metaphase: The chromosomes move to the equator of
the cell. The mitotic spindle attaches to the centromere on
each side of the chromosome.
Stages of Mitosis - IPMATC
Anaphase: The mitotic spindle pulls one sister chromatid
of each chromosome to one end of the cell.
Stages of Mitosis - IPMATC
Telophase: The chromosomes start disappearing as the
nuclear membrane forms around the DNA. The cells begin
to divide at the equator.
cleavage
furrow
Stages of Mitosis - IPMATC
Cytokinesis: The cleavage furrow pinches the cell in two.
The nuclear membrane has reformed, containing the DNA
in the form of chromatin.
Mitosis
Summary of mitosis phases
Meiosis
Only occurs in sexually reproducing organisms
The goal of meiosis is to produce gametes
2 cell divisions result in 4 daughter cells
The chromosome number is halved
Meiosis increases variability as no 2 gametes are
the same
Meiosis 1
Meiosis 1: the cell undergoes the same IPMATC
stages as in mitosis, but the chromosomes
behave differently in some of the stages:
recombination (cross-over)
Meiosis 1 - Prophase
As the chromatin condenses to
form chromosomes, the
chromosomes line up in pairs
Crossing-over occurs between
adjacent sister chromatids,
with each homologous pair
having one or more chiasmata
Meiosis 1…
Prophase 1
 Leptotene
 Zygotene
 Pachytene
 Diplotene
Metaphase 1
Anaphase 1
Prophase 1…(LZPD)
Leptotene: chromosomes become visible
Zygotene: (synapsis, conjugation)
 chromosomes come together
 Point to point contact of homologous pairs
Pachytene: chromosomes become shorter and
thicker forming chromatids
Diplotene: bivalent pair consist of 4 chromatids.
 Crossing-over occurs
Meiosis…
A homologous pair of
parental chromosomes
Gene re-assortment by
crossing-over
In meiosis I, each chromosome duplicates,
producing two sister chromatids
Crossing-over
(Recombination)
meiosis II
Meiosis 1 - Metaphase
During metaphase, the
chromosome pairs line
up together at the
equator of the cell
Meiosis 1 – Anaphase
The spindles will now attach to the centromere and pull
the whole chromosome to one end of the cell
Metaphase I
Spindle fibers attach to the chromosomes
Homologous chromosomes line up in the middle
of the cell
Anaphase I
Fibers pull the homologous chromosomes toward
opposite ends of the cell
Telophase I and Cytokinesis
Nuclear membranes form
Cell separates into two new cells
Interkinesis
Resting period between Meiosis I and Meiosis II
DNA DOES NOT REPLICATE AGAIN HERE!
Meiosis II
The daughter cells from Meiosis I divide again
WITHOUT replicating their chromosomes
That leads to 4 gametes, each with half the
number of chromosomes (haploid) as the
original “mother” cell
Prophase II
 Spindle fibers form and move chromosomes to center
Metaphase II
 Spindle fibers attach to the chromosomes
 chromosomes line up in the middle of the cell – similar to
how they do in Mitosis
Anaphase II
• Fibers pull the sister chromatids toward
opposite ends of the cell
Telophase II and Cytokinesis
• Nuclear membranes form
• Both cells separate – forming 4 new haploid cells
Meiosis 2…
At the end of Meiosis the
individual Gamete cell has
divided from one cell to
four.
Males produce 4 viable
sperms.
Females produce 1 viable
egg and 3 non functioning
polar bodies.
MEIOSIS:
The process of creating gametes
Oogenesis – meiosis in human female
reproductive cells – makes eggs (ovum)
• Total of 4 cells
produced:
• Forms one
usable egg cell
with a large
supply of stored
nutrients.
• The other 3 cells,
called polar
bodies,
disintegrate.
Spermatogenesis – meiosis in human male
reproductive cells to make sperm (spermatozoa)
• Produces 4
viable gametes
• All 4 gametes
produce a long
whip-like tail
Meiosis:
 Cell division necessary for sexual reproduction
 Produces 4 daughter cells
 Daughter cells are Haploid
 Daughter cells are gametes (sexual reproductive cells)
 2 nuclear/cellular divisions
 Crossing over = opportunity for genetic variability
Differentiate
Mitosis
Meiosis
 Asexual reproduction
 Used for sexual reproduction
 Produces 2 daughter cells
 Produces 4 daughter cells
 Daughter cells are diploid
 Daughter cells are Haploid
 Daughter cells are
 Daughter cells are genetically
identical to each other
and to parent cell
 Produces somatic cells
 One cell/nuclear division
different from each other,
and from parent cell
 Produces gametes
 Two nuclear/cellular divisions
Mitosis vs. Meiosis
Quick Review – Place Cells in Mitosis Order
A
B
D
C
E
C: Interphase
E: Prophase
A: Metaphase
D: Anaphase
B: Telophase
End
Cell Division Terms
Term
Definition
Interphase
the phase of the cell cycle in which the cell spends the majority of its time and prepares for
cellular division
G0 phase
the cell functions normally; occurs in cells that do not divide often (or ever)
G1 phase
the cell grows and functions normally
G2 phase
the cell resumes its growth in preparation for division
S phase
the cell duplicates its DNA
Mitosis
the process by which cells replicate
Prophase
a stage of mitosis in which the chromatin condenses into chromosomes
Prometaphase
the nuclear membrane breaks apart into numerous "membrane vesicles,” and the chromosomes
inside form protein structures called kinetochores
Metaphase
chromosomes align in the equator of the cell before being separated into the two daughter cells
Anaphase
chromosomes are split move to opposite poles of the cell
Telophase
the final stage in both meiosis and mitosis
Centrosome
an organelle that serves as a regulator of cell-cycle progression
Mitoic spindle
the subcellular structure that segregates chromosomes between daughter cells
Kinetochore
the protein structure on chromatids where the spindle fibers attach during cell division
Cytokinesis
the process in which the single eukaryotic cell is divided to form two daughter cells
Cleavage furrow
the indentation of the cell's surface that begins the progression of cleavage, by which some cells
undergo cytokinesis
Cell plate
the plate that causes cytokinesis in plant cells
Three organelles replicate:
Mitochondria
Chloroplasts
Centrioles