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Meiosis to Mendel
Chapter 9
Reproduction
• Asexual reproduction – mitosis
– Produces clones – genetically identical
individuals
• What would happen if the environment
changed?
WHY SEX ?
• Sexual reproduction combines the DNA
from two different individuals
• A gene is a section of a chromosome that
carries instructions for a specific trait
(protein)
• The greater the number of different
combinations of genes the more variation
among individuals, and the greater the
chance of survival of the species.
Sex cell formation
• Sexually reproducing organisms need to
produce specialized reproductive cells or
gametes.
• Produced from germ cells in organs called
gonads
• In females ovaries produce eggs or ova
• In males testes produce spermatozoa
Somatic (body) cells contain pairs of
chromosomes or the diploid (2n) number
of chromosomes.
Homologous pairs – each member is a
homolog
Gametes contain only one member of each
pair or the haploid (n) number of
chromosomes
• When a sperm and egg combine in the
process of fertilization, or syngamy, a
new diploid cell or zygote is formed.
• To form haploid gametes, there needs to
be a process other than mitosis – this is
called meiosis
Meiosis
• This is a two part process : meiosis I and
meiosis II
• However, the DNA is only replicated once
• Meiosis I and II both use the same four
stages of mitosis: prophase, metaphase,
anaphase and telophase
During prophase I the homologous chromosomes pair
up in synapsis. This is the longest phase of meiosis.
Crossing over may occur further increasing genetic
variation.
• In metaphase I the tetrads migrate to the
center of the cell.
• In Anaphase I the centromeres do not
break and one member of each
homologous pair (2 sister chromatids)
move to opposite ends of the cell
• Which homolog goes to which end of the
cell occurs at random.
• Telophase occurs as in mitosis.
Meiosis II
• There is no replication of chromosomes
between telophase I and prophase II
• Meiosis II proceeds just like mitosis –
during anaphase the centromeres break
and the two sister chromatids go to
opposite poles.
Cytokinesis
• Varies by which type of cell is being made
• If we are producing sperm, each of the
four cells produced by meiosis II become
sperm.
• If we are making ova, cytokinesis is
uneven and one cell takes nearly all the
cytoplasm, leaving the other cell merely a
package of discarded DNA called a polar
body.
• In humans, the cell again divides
unevenly, so at the end of meiosis II we
have formed one ovum (egg) and three
polar bodies. The polar bodies
disintegrate.
• The average woman produces one ovum
every 28 days
• Males produce 300 million sperm/ day
• If less than 20 million / ml, a man is
considered infertile.
• Fertilization is a group effort, but only one
sperm penetrates the ovum.
• Sometimes things go wrong.
• In anaphase I the separation of
homologous chromosomes is called
disjunction.
• When they do not separate it is called
nondisjunction and the resulting gametes
contain one too many or one too few
chromosomes.
• Fertilization results in a zygote with 45 or
47 chromosomes. This is an aneuploid
(vs. euploid) number of chromosomes
• Three copies of a chromosome is called
trisomy – Down syndrome is trisomy 21
• A zygote with one too few chromosomes
does not usually develop.
• Extra copies of the sex chromosomes (vs.
the autosomes) do not cause as much of a
problem
• XX is female in humans (male in birds)
• XY is male in humans
• XXY , XYY
Genetics
• A gene is a section of DNA that codes for :
– Proteins – for structures such as muscles
- or for enzymes
– Regulatory genes – areas of DNA that
regulate the expression of structural genes
• Each cell of an organism that reproduces
sexually has two copies of each
chromosome, and therefore has two
copies of every gene – one on each
member of each pair of chromosomes
(exception is the Y chromosome, which is
smaller than the X).
• The two versions of each gene are called
alleles. Alleles may be the same or
different, depending on the traits of the
parents.
• All the genes that are contained on all the
chromosomes of an individual make up
that individuals Genotype or Genome.
• Genotype can also be used to refer to the
genes for a particular trait.
• Not all genes are expressed.
• Those traits that are expressed: can be
seen (physical traits) or measured
(chemical traits) are the individual’s
phenotype.
• The phenotype is influenced by both the
genotype and the environment.
Transmission Genetics
• How traits are passed down from
generation to generation.
• Transmission of genes and the
phenotypes which come from those genes
• The phenotype determines how the
individual interacts with the world, and
it is the phenotype that is subject to
natural selection.
• If an organism has two copies of the same
allele is it said to be homozygous.
– True breeding
• If an organism has different alleles of the
same gene it is said to be heterozygous.
If an allele is dominant, it is expressed
whenever that allele is present.
A recessive trait is only expressed when the
trait is homozygous.
Dominant traits are written with capital
letters.
Recessive traits are written with small
letters.
P = purple pigment (Purple flowers)
p = no pigment (white flowers)
PP = ?
Pp = ?
pp = ?
PP and Pp = purple flowers
pp = white flowers
• Breeding two genetically distinct
organisms is called cross breeding or
crossing.
• The offspring of such crosses are called
hybrids.
• The parents are called the parental or
P generation
• The offspring of these parents are called
the F1 generation (first filial)
• What would we get if we crossed a
homozygous purple flower (PP) with a
homozygous white flower (pp)?
• To find out, we can use a Punnett square –
named after Reginald Punnett
Parent 1
P
a
r
e
n
t
2
P
P
p
Pp
Pp
p
Pp
Pp
Gregor Mendel
• 1856 -1863
• monk, Czech Republic
• Studied 7 traits in pea plants, Pisum sativum
– Established basic rules of transmission genetics
• Good science, but ignored for >30 years
• Why peas?
– Many varieties with contrasting traits
– Self-pollinating, with true-breeding varieties
– easy to snip parts to cross pollinate
– Need little space, produce lots of offspring
His experiments would not have worked out
except:
1) He chose traits that were all dominant
or recessive
2) He chose traits that were all located on
different chromosomes (pea plants
have 7 chromosomes)
Pretty amazing since he had no idea how
these traits were passed on – he called
them “unit factors”
• What would happen if we crossed
members of the F1 generation?
P
P
p
p
Genotypes: 1: 2 : 1
PP : Pp : pp
What if we had a plant that had a dominant
characteristic and wanted to know if it was
homozygous or heterozygous ?
We could do a back cross or test cross –
breed the individual with a homozygous
recessive individual.
Genotypic ratio 1:1 Pp : pp
If the purple plant was homozygous, the F1
generation would all be purple - Pp
But, not all traits show simple dominantrecessive relationships. There is also partial
dominance where both traits are expressed.
Some traits show incomplete dominance.
Snap dragons have genes for red flowers (R1)
and white flowers (R2).
A heterozygous flower (R1R2) would be
Pink!
This type of trait gave early scientists the idea
that traits blended in offspring of different
individuals.
Notice that both
traits are given
capital letters, and
the F2 generation
shows a 1:2:1 ratio
of phenotypes as
well as genotypes.
Other traits show Codominance where both
alleles are equally expressed.
Blood types: A B O blood groups
Isoantigens – particular proteins on the cell
membrane. IA, IB and i (O is recessive,
and this is a case of multiple alleles)
IAIA = Type A IBIB = Type B
IAi = Type A
IBi = Type B
IAIB = Type AB
ii = Type O
Mendel’s Crosses Showed:
• Principle of segregation: each sexually
reproducing organism has two genes for
each characteristic, and these two genes
segregate or separate during the
production of gametes.
• Principle of independent assortment:
traits which lie on different chromosomes
are passed on independently of each
other.
With these rules in mind, we can cross
individuals that have two different traits.
Dihybrid (vs. monohybrid) cross.
We can cross peas that have green pods (G)
which are inflated (I) with peas that have
yellow pods (g) which are constricted (i).
GGII X ggii = GgIi
parents
F1
The only “trick” to a dihybrid cross is setting
up the Punnett square.
GgIi →
GI, Gi, gI, gi
Like making a snack – take one of each
GI
Gi
gI
gi
GI
GGII
GGIi
GgII
GgIi
Gi
GGIi
GGii
GgIi
Ggii
gI
GgII
GgIi
ggII
ggIi
gi
GgIi
Ggii
ggIi
ggii
Genotypes :
1 GGII : 2 GgII : 2 GGIi : 4 GgIi
1 GGii : 2 Ggii 1 ggII : 2 ggIi
1 ggii
Phenotypes:
9 Green inflated
3 Green constricted
3 Yellow inflated
1 Yellow constricted
16
Sex-Linked traits
• Some genes carried on the X chromosome
are missing from the Y chromosome.
• These traits show up in different ratios in
males and females and are called sexlinked traits
• Males are said to be hemizygous for these
traits since they can only have one gene
and a recessive gene will always be
expressed.
Color blindness is carried on the X
chromosome (X’) Normal color vision (X)
X
X
XX
Y
XY
X’
XX’
X’Y
The females all have
normal color vision, but
half the males are color
blind.
X
X’
X’
XX’
X’X’
Y
XY
X’Y
Here half the females
and half the males are
color blind.
• In 1902 Mendel’s work was rediscovered
and William Bateson and Walter Sutton
realized that the behavior of chromosomes
during meiosis explains Mendel’s
principles of segregation and independent
assortment.
• Walter Sutton published his own 6 rules of
inheritance.
What happens when two traits are
located on the same chromosome?
They tend to be passed on together – this is
called genetic linkage.
Can these two traits be inherited separately?
The likelihood that two genes on the same
chromosome will be inherited separately
depends on the distance between them.
A map unit is defined as the distance
between two genes that produces a 1
percent recombination in gametes.
What is the greatest possible distance
(in map units) between two genes?
50 map units.
If the genes were on separate
chromosomes they would end up in
different gametes 50 % of the time.