Transcript Mendel and Meiosis

Mendel and Meiosis
Chp 10 Pp. 252-279
Contents
• 10-2 Meiosis
• 10-1 Mendel
Chromosome
Rod shaped
structures made of
DNA and proteins
Carrier of genetic
material
Located in the
nucleus
Copied and passed
from generation to
generation
Chromosomes
Sex chromosomes:
chromosomes that
determine the sex
of an organism
Humans X and Y
Females: X X
Males: X Y
Autosomes: all other types of chromosomes
Chromosomes
Homologous Chromosomes
2 copies of each autosome
Same size and shape
Carry genes for the same trait
Ex. If 1 homologous chromosome
contains gene for eye color the other
homologous chromosome will too.
Homologous Chromosomes
Diploid Cell
Cell with 2 sets of chromosomes
Contains chromosomes for each
homologous pair
Somatic Cells= Body Cells: Diploid
All human cells except sex cells are diploid.
One from each parent
2n
Humans 2n = 46
Haploid
Cell containing one of each kind of
chromosome
Sperm and egg cells = Gametes
1 set of chromosomes
Half the number of chromosomes of diploid
n
Sperm + Egg = Zygote
n + n = 2n
Mitosis vs. Meiosis
• Mitosis: t = two: Diploid
2n=46
• Meiosis: o = one: Haploid
»
n = 23
Meiosis
Gametes are produced in specialized
body cells
Produces Sperm and Egg Cells
2 divisions: Meiosis I and Meiosis II
Meiosis occurs in sex cells, not body
cells
Results in 4 daughter cells
Each cell has half the chromosomes of
the parent
Meiosis I
Interphase occurs: the
cell grows and DNA
replicates
Meiosis I begins
Original cell produces
two new cells
Prophase I
DNA Coils tightly into
chromosomes
spindle fibers appear
Each chromosome lines up next to
the homologue
Synapis occurs: pairing of
homologous chromosomes
Tetrad: Each pair of homologous
chromosomes
Crossing Over
•Crossing Over: why we do not look
exactly
like our parents.
Portions of the chromatid breaks off and
attaches to adjacent chromatids on the
homologous chromosome
Permits the exchange of genetic material
between maternal and paternal
chromosomes
•Occurs during Prophase
Genetic Recombination
Crossing over
produces a new
mixture of
genetic material
• Occurs during
Prophase
Causes of Variation
Chromosomes are assorted randomly
Crossing over may occur
Cells do not have identical genetic info as
each other or the parent
Good: more chance of survival and
evolution
Bad: mistakes more likely
Metaphase I
Tetrads line up
randomly along
the mid-line
Spindle fibers
attach to
centromeres
Anaphase I
Homologous
chromosomes
move to the
opposite poles
Random
separation or
Independent
Assortment results
in separation of
maternal and
paternal
chromosomes.
Telophase I
Chromosomes
reach opposite
ends of cell
Cytokinesis
begins
cell is haploid
Meiosis II
Occurs in each
cell formed in
Meiosis I
Interphase does
not occur again
Prophase II
Spindle fibers
form and move
the
chromosomes
to the mid-line
of the dividing
cell
Metaphase II
Chromosomes
move to the midline of the
dividing cell
facing opposite
poles of the
dividing cell
Anaphase II
Chromatids
separate and
move to
opposite poles
of the cell
Telophase II
Nuclear membrane forms around the
chromosomes in each of 4 new cells
Cytokinesis II
Cytoplasm divides
Cell Membrane closes off
End Result:
Four new cells that contain half of the
original cells number of chromosomes
4 sex cells are created
Meiosis Animation
• Meiosis Animation
Haploid = one of each kind
Diploid = two of each kind
• 2n = diploid
n = haploid
WHY DO WE NEED HAPLOID?
Female gamete
23
Male gamete
•
EGG
23
SPERM
Fertilization restores the diploid number
1n
fertilization
1n
meiosis
2n
2n
Mitosis and
cell growth
2n
Gametes
formed by meiosis
haploid reproductive cells
humans: meiosis occurs in the testes and
ovaries
Spermatogenesis: Male
Oogenesis: Female
One Mature
Egg Cell
Human
Karotype
Asexual Reproduction
Production of offspring from one parent
Does not involve meiosis
Offspring are genetically identical to
parent
Sexual Reproduction
Production of offspring through meiosis
and the union of sperm and egg
Offspring are genetically different form
parents
Genes are combined in new ways in
meiosis
Evolutionary advantage is that it enables
species to rapidly adapt to new conditions
Mitosis vs. Meiosis
• Mitosis vs. Meiosis Animation
Nondisjunction
• Failure of homologous chromosomes to
separate properly during meiosis.
• Both chromosomes of a homologous
pair move to the same pole of the cell.
• 1 gamete has an extra chromosome
• Or
• 1 gamete is missing a chromosome
Nondisjunction Animation
• Nondisjunction Animation
10-1 Mendel
• Gregor Mendel
• Austrian monk
• Studied how traits are
inherited from parents
to offspring
• Father of heredity
• Chose garden peas
for his meticulous
experiments
Garden Peas
• Reproduce sexually
• W/ male & female
gametes (sex cells)
• Fertilization results
in zygote
• Becomes seed
• Pollination male
pollen transferred to
female pistil
Monohybrid Cross
• Hybrid- offspring
of parents with
different forms of a
trait
• Tall or short
• crossed true-bred
tall plants w/ truebred short plants to
get heterozygous
offspring which
then self-pollinated
Some Genes are Dominant
• Some Genes Are Dominant click to play
Seven Traits of Peas
Mendel’s Rules
• Alleles- different
gene forms
• Rule of Dominance
– Dominant –observed
trait
– Recessivedisappearing or hidden
trait
Mendel’s Rules
• Law of Segregation
• Every individual has 2
alleles of each gene
w/ each gamete
receiving 1.
• During fertilization,
gametes randomly
pair to produce four
combinations
Phenotypes & Genotypes
• Appearance=
Phenotype
• Allele combination=
Genotype
• Homozygous-Both
alleles alike
• Heterozygousdifferent alleles, one
dominant & one
recessive
Mendel’s Dihybrid Cross
Law of Independent Assortment
• Says that genes for
different traits are
inherited
independently from
each other.
• The alleles can
recombine in four
different ways.
Punnett Squares
1BB: 2Bb:1bb
• Shorthand way to find
possible genotypes
from crossing two
known parents.
• Two heterozygous
parents produce 1
homozygous
dominant: 2
heterozygous:1
homozygous
recessive