Download Mendel and Meiosis


Heredity- Passing on of characteristics
from parents to offspring.

TRAITS- Characteristics that are inherited.

Genetics- STUDY OF HEREDITY

GREGOR MENDEL was the first to
succeed in predicting how traits are
transferred from one generation to the
next.

Mendel used garden peas to conduct his experiments
because:
1.REPRODUCE SEXUALLY (they produce male and female
sex cells called _GAMETES)
› a. male gamete forms in the pollen grain. SPERM or male sex cell
› b. female gamete forms in female reproductive organ. OVUM or
female sex cell.

In fertilization the female gamete (ovum) unites with the
male gamete (sperm) to form a ZYGOTE which develops
into a seed.
2. In pea plants both female and male organs are in the
same plant and tightly enclosed by petals, which prevents
other pollens form entering.
 - Reproduction is PURE AND CONTROLLED

Mendel’s Experiment (fig 10.1)
 -Removed male organ
 -dusted pollen of his choice on female
organ CROSSPOLLINATION.
 -Sure of what he crossed
 -Well CONTROLLED experiment
 -Studied ONE trait at a time.
 -Analyzed data MATHEMATICALLY.

HYBRID offspring of parents that have a
different form of a trait
 For example: short x tall
 MONOHYBRID cross means that he
crossed plants that differed in only ONE
trait.


What do you get when you cross a gas
powered car with and electric car?
Original Parents : P1
 1st generation: F1 (F=filial meaning son or
daughters)
 2nd generation: F2
 RR x rr = F 1
 Rr x Rr = F 2


Mendel eventually studied 7 traits

Rule of Unit Factors: Each organism has 2
factors that control each of its traits.
› Genes exist in alternative forms called
ALLELES
› 2 alleles determine one trait. (Tt)
Recessive trait only when tt. Is present

Rule of dominanceOne trait is dominant
over another. The
trait is expressed as
long as it is present in
the gene TT or Tt.

Every individual has 2
alleles for each gene
and when gametes
are produced each
gamete only
receives one of
these alleles. During
fertilization these
gametes randomly
pair to produce 4
combinations

PHENOTYPE The way an organism looks and
behaves. (may have environmental influence)

GENOTYPE- The actual allele make up of the
organism.

HOMOZYGOUS traits have the same 2 alleles
for a trait. ( TT or tt)
HETEROZYGOUS traits have 2 different alleles
for a trait. (Tt)


Law of independent assortment- Genes for
different traits are inherited independently of
one another,

A cell with 2 of each kind of Chromosome is called a
DIPLOID cell or contains 2n for the # of chromosomes.

A cell that produces gametes (sex cells- ovum and sperm
cells), contain one kind of each chromosome know and a
HAPLOID or n number of chromosomes.

Two chromosomes with identical information are called
homologous chromosomes. They are similar structures.
They contain the genes for a specific trait, ex. Height.

Genes are arranged in the same order but because there
are different alleles (dominant and recessive) they may
not be identical.

Traits controlled by genes located on the
X and Y chromosome.
› Because X and Y are not homologus they do
not carry the corresponding allele to each
other.
› Characteristics carried on X will be expressed
and not masked by the other chromosome.
› Examples: Male pattern baldness, red-green
color blindness, hemophilia (X-linked)
Human traits are often complicated and
not expressed purely as either dominant
or recessive but rather a mixture of the
two traits.
 Incomplete dominance

› For example: Sickle cell anemia (blood
disorder that effects African American
population), blood types
Cystic Fibrosis- White Americans
 Tay-Sachs disease- Ashkenasic Jews
 Phenylketonuria (PKU)- Tested for at birth
 Lesch Nyhan

Cleft chin
 Hitchhikers thumb
 Attached earlobe
 Huntington’s disease- 30-50yoa is
expressed.

If cells reprduced gametes with a
complete set of chromosomes (for
example 14 chromosomes /7 pairs) each
generation would double chromosomes.
 F2- 28 chromosomes 14 pairs
 F3- 56 chromosomes 28 pairs
 F4- 102 chromosomes 56 pairs
 F5- 204 chromosomes or 102 pairs!!!


MEIOSIS is cell division that results in HALF
the number of chromosomes as the
parent’s body cell.
Male gamete= SPERM
Female gamete= EGGS

When the sperm cell fertilizes the egg the
result is a ZYGOTE with a diploid # of
chromosomes.

Interphase: Same as mitosis.

1.PROPHASE I
› DNA coils, homologous chromosomes line
up.
› Spindle forms
› HOMOLOGOUS structure forms called a
TETRAD each made of 2 sister chromatids. (
some crossing over occurs causing an
exchange of DNA information.)
Chromosomes in a
tetrad pair so tightly
that non sister
chromatids can
actually break and
exchange material.
This is called Crossing
over.
2.METAPHASE I
 Pulled to equator like in Mitosis except
the tetrad structure is side by side.


3.Anaphase I
› Homologous chromosomes separate (tetrad
pair separate)
› Ensures each new cell will only receive one
of the homologous chromosomes.

4.Telophase I
› New cells appear with full set of genetic
information. One homologous pair in each
cell.



Some cells undergo a short resting phase,
others go directly into meiosis II.
It is simply mitosis.
Prophase II
› Spindle forms and attach to chromosomes

Metaphase II
› Sister Chromatids line up at equator

Anaphase II
› Centromere splits apart and sister chromatids
separate and move to opposite poles.

Telophase II
› Nuclei reform, spindles break down

CYTOKINESIS – finishes process
Meiosis II
 Same as mitosis
EXCEPT
chromosomes DO
NOT REPLICATE
before they
divide. 4
HAPLOID cells
result.

Since each pair can line up at the
equator 2 different ways there are a 2N
or 2 46 or 7 x1013 or 70, 400, 000,000,000
possibilities, or 70 trillion possibilities.

Genetic recombination- reassortment of
chromosomes and the genetic
information they carry.

Nondisjunction:
› Failure of chromosomes to SEPARATE properly.
› Ex. Extra chromosomes in one cell or missing one
in another. Down’s Syndrome is Trisomy 21.
Gamete missing : Turner Female XO
Tetraploid: 2 complete sets of
chromosomes
 Polyploidy- Organisms with more than the
usual # of chromosomes. Death of zygote
in animal cells. Common in plants through
genetic engineering.

