Transcript Chapter 6

Chapter 6
6.1 Chromosomes and Meiosis
 Gametes have half the number of chromosomes that body
cells have.
6.1 Chromosomes and Meiosis
 You have body cells and
gametes
 Body cells are also called
somatic cells
 Germ cells develop into
gametes
 Germ cells are located in
the ovaries and testes
 Gametes are sex cells: egg
and sperm
 Gametes have DNA that
can be passed to offspring
6.1 Chromosomes and Meiosis
 Your cells have autosomes
and sex chromosomes
 Your body has 23 pairs of
chromosomes.
 Homologous pairs of
chromosomes have the
same structure.
 For each homologous pair,
one chromosome comes
from each parent.
6.1 Chromosomes and Meiosis
 Chromosome pairs 1-22
are autosomes
 Sex chromosomes, X and
Y, determine gender in
mammals
6.1 Chromosomes and Meiosis
 Body cells are diploid;
gametes are haploid
 Fertilization between egg
and sperm occurs in sexual
reproduction.
 Diploid (2n) cells have two
copies of every
chromosome.
 Body cells are diploid
 Half the chromosomes
come from each parent
6.1 Chromosomes and Meiosis
 Haploid (n) cells have one
copy of every
chromosome.
 Gametes are haploid
 Gametes have 22
autosomes and 1 sex
chromosome.
6.1 Chromosomes and Meiosis
 Chromosome number must
be maintained in animals.
 Many plants have more than
two copies of each
chromosome.
 Mitosis and meiosis are types
of nuclear division that make
different types of cells.
 Mitosis make more diploid
cells.
6.1 Chromosomes and Meiosis
 Meiosis makes haploid
cells from diploid cells
 Meiosis occurs in sex cells
 Meiosis produces gametes
6.2 Process of Meiosis
6.2 Process of Meiosis
 Cells go through two
rounds of division in
meiosis.
 Meiosis reduces
chromosome number and
creates genetic diversity.
6.2 Process of Meiosis
 Meiosis I and meiosis II each
have four phases, similar to
those in mitosis
 Paris of homologous
chromosomes separate in
meiosis I
 Homologous chromosomes
are similar but not identical
 Sister chromatids divide in
meiosis II
 Sister chromatids are
copies of the same
chromosome.
6.2 Process of Meiosis
 Meiosis I occurs after DNA
has been replicated
 Meiosis I divides
homologous
chromosomes in four
phases.
6.2 Process of Meiosis
 Meiosis II divides sister
chromatids in four phases
 DNA is not replicated
between meiosis I and
meiosis II.
6.2 Process of Meiosis
 Meiosis differs from
mitosis in significant ways
 Meiosis has two cell
divisions while mitosis has
one.
 In mitosis, homologous
chromosomes never pair
up. Meiosis results in
haploid cells; mitosis
results in diploid cells.
6.2 Process of Meiosis
 Haploid cells develop into mature
gametes.
 Gametogenesis is the production
of gametes.
 Gametogenesis differs between
females and males.

Sperm primarily contribute DNA to
an embryo.

Sperm become streamlined and
motile.

Eggs contribute DNA, cytoplasm,
and organelles to an embryo.

During meiosis, the egg gets most
of the contents; the other cells form
polar bodies.
6.3 Mendel and Heredity
6.3 Mendel and Heredity
 Mendel laid the groundwork
for genetics.
 Traits are distinguishing
characteristics that are
inherited.
 Genetics is the study of
biological inheritance
patterns and variation.
 Gregor Mendel showed that
traits are inherited as discrete
units.
 Many in Mendel’s day
thought traits were blended.
6.3 Mendel and Heredity
 Mendel’s data revealed
patterns of inheritance.
 Mendel made three key
decisions in his
experiments.
 use of purebred plants
 control over breeding
 observation of seven
“either-or” traits
6.3 Mendel and Heredity
 Mendel used pollen to
fertilize selected pea
plants.
 P generation crossed to
produce F1 generation
 interrupted the selfpollination process by
removing male flower
parts
Mendel controlled the
He then fertilized the female
fertilization of his pea plantspart, or pistil, with pollen from
by removing the male parts, a different pea plant.
or stamens.
6.3 Mendel and Heredity
 Mendel allowed the
resulting plants to selfpollinate.
– Among the F1
generation, all plants
had purple flowers
– F1 plants are all
heterozygous
– Among the F2
generation, some plants
had purple flowers and
some had white
6.3 Mendel and Heredity
6.3 Mendel and Heredity
 Mendel observed
patterns in the first and
second generations of
his crosses.
6.3 Mendel and Heredity
6.3 Mendel and Heredity
 Mendel drew three
important conclusions.
– Traits are inherited as
purple
white
discrete units.
– Organisms inherit two
copies of each gene, one
from each parent.
– The two copies
segregate
during gamete formation.
– The last two conclusions
are
called the law of
segregation.
6.4 Traits, Genes and Alleles
6.4 Traits, Genes, and Alleles
 The same gene can have
many versions.
 A gene is a piece of DNA
that directs a cell to make
a certain protein.
 Each gene has a locus, a
specific position on a pair
of homologous
chromosomes.
6.4 Traits, Genes, and Alleles
 An allele is any alternative
form of a gene occurring at a
specific locus on a
chromosome.
 Each parent donates one
allele for every gene.
 Homologous describes two
alleles that are the same at
a specific locus.
 Heterozygous describes
two alleles that are
different at a specific locus.
Traits, Genes, and Alleles
 Genes influence the
development of traits
 All of an organism’s
genetic material is called
the genome.
 A genotype refers to the
makeup of a specific set of
genes.
 A phenotype is the
physical expression of a
trait.
6.4 Traits, Genes, and Alleles
 Alleles can be represented
using letters.
 A dominant allele is
expressed as a phenotype
when at least one allele is
dominant.
 A recessive allele is
expressed as a phenotype
only when two copies are
present.
 Dominant alleles are
represented by uppercase
letters; recessive alleles by
lowercase letters.
6.4 Traits, Genes, and Alleles
 Both homozygous
dominant and
heterozygous genotypes
yield a dominant
phenotype.
 Most traits occur in a range
and do not follow simple
dominant-recessive
patterns.
6.5 Traits and Probability
 Punnett squares illustrate
genetic crosses
 The Punnett square is a grid
system for predicting all
possible genotypes resulting
from a cross.
 The axes represent
the possible gametes
of each parent.
 The boxes show the
possible genotypes
of the offspring.
 The Punnett square yields
the ratio of possible
genotypes and phenotypes.
6.5 Traits and Probability
 A monohybrid cross
involves one trait.
 Monohybrid crosses
examine the inheritance of
only one specific trait.
 homozygous dominant-
homozygous recessive: all
heterozygous, all
dominant
6.5 Traits and Probability
– heterozygous-
heterozygous—1:2:1
homozygous dominant:
heterozygous:
homozygous recessive;
3:1 dominant: recessive
6.5 Traits and Probability
•
heterozygoushomozygous recessive—
1:1 heterozygous:
homozygous recessive;
1:1 dominant: recessive
• A testcross is a cross
between an organism
with an unknown
genotype and an
organism with the
recessive phenotype.
6.5 Traits and Probability
 A dihybrid cross involves two
triats
 Mendel’s dihybrid crosses
with heterozygous plants
yielded a 9:3:3:1 phenotypic
ratio.
 Mendel’s dihybrid crosses
led to his second law,
the law of independent
assortment.
 The law of independent
assortment states that
allele pairs separate
independently of each
other during meiosis.
6.5 Traits and Probability
 Heredity patterns can be
calculated with probability.
 Probability is the likelihood
that something will happen.
 Probability predicts an
average number of
occurrences, not an exact
number of occurrences.
 Probability applies to random
events such as meiosis and
fertilization.
 Record equation on board.
6.6 Meiosis and Genetic Variation
 Sexual reproduction creates
unique combinations of
genes.
 Sexual reproduction creates
unique combination of genes.
 independent assortment of
chromosomes in meiosis
 random fertilization of
gametes
 Unique phenotypes may give
a reproductive advantage to
some organisms.
6.6 Meiosis and Genetic Variation
 Crossing over during
meiosis increases genetic
diversity.
 Crossing over is the
exchange of chromosome
segments between
homologous
chromosomes.
 occurs during prophase I
of meiosis I
 results in new
combinations of genes
6.6 Meiosis and Genetic Variation
 Chromosomes contain many
genes.
 The farther apart two genes
are located on a
chromosome, the more likely
they are to be separated by
crossing over.
 Genes located close
together on a chromosome
tend to be inherited together,
which is called genetic
linkage.
 Genetic linkage allows the
distance between two genes
to be calculated.
6.6 Meiosis and Genetic Variation