Transcript Presentation

KEY CONCEPT - Section 1
Gametes have half the number of
chromosomes that body cells have.
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.
body cells
sex cells (sperm)
sex cells (egg)
Your cells have autosomes and sex chromosomes.
• Your body cells have 23 pairs of
chromosomes.
– Homologous pairs of
chromosomes have the same
structure.
– For each homologous pair, one
chromosome comes from each
parent.
• Chromosome pairs 1-22 are
autosomes.
• Sex chromosomes, X and Y,
determine gender in mammals.
Body cells are diploid (2n)
Gametes are haploid (1n)
• 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.
• Haploid (n) cells have one copy of every
chromosome.
– Gametes are haploid.
– Gametes have 22 autosomes and 1 sex chromosome.
• 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 makes
more diploid cells.
• Meiosis makes haploid cells from diploid
cells.
– Meiosis occurs in sex cells.
– Meiosis produces gametes.
KEY CONCEPT –section 2
During meiosis, diploid cells undergo two cell
divisions that result in haploid cells.
Cells go through two rounds
of division in meiosis.
• Meiosis does two things:
– reduces chromosome number
– creates genetic diversity.
• Why are these things important?
• Meiosis I and Meiosis II each have four phases,
similar to those in mitosis.
homologous chromosomes
sister
chromatids
sister
chromatids
– Pairs 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.
• Meiosis I occurs after DNA has been replicated.
• Meiosis I divides homologous chromosomes in
four phases.
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
• Meiosis II divides sister chromatids in four
phases.
• DNA is not replicated between meiosis I and
meiosis II. Why not?
• Meiosis differs from mitosis in significant ways.
1. Meiosis has two cell divisions while mitosis has one.
2. In mitosis, homologous chromosomes never pair up.
3. Meiosis results in unique, haploid cells; mitosis
results in identical, diploid cells.
Haploid cells develop into mature gametes.
• Gametogenesis is the
production of gametes.
• Gametogenesis differs between
females and males.
– Sperm become
streamlined and mobile.
– Sperm primarily
contribute DNA to an
embryo.
– Eggs contribute DNA, cytoplasm, and
organelles to an embryo.
– During meiosis, the egg gets most of the
contents; the other cells form polar bodies
– The body gets rid of polar bodies
Summary
1. What is the major difference between
Metaphase I and Metaphase II?
2. What is the major difference between
Anaphase I and Anaphase II?
3. List the key differences between meiosis I
and II.
4. Why is an egg cell much larger than
sperm?
KEY CONCEPT – Section 3
Mendel’s research showed that
traits are inherited as discrete units.
Review terms before we begin…
• Zyg = pair
• Homo = same
• Hetero= different; other
So how would you define the following words?
Homozygous and Heterozygous
So which are homozygous? heterozygous?
Aa
aa
AA
rr
RR Rr
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.
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
For example, a gene that is
coding for height may
have contrasting traits:
either tall or short
• Mendel used pollen to fertilize selected pea
plants.
– P generation crossed to produce F1 generation
– interrupted the self-pollination process by removing
male flower parts
Mendel controlled the
fertilization of his pea plants
by removing the male parts,
or stamens.
He then fertilized the female
part, or pistil, with pollen from
a different pea plant.
• 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
• Mendel observed patterns in the first and second
generations of his crosses.
• Mendel drew three important conclusions.
1. Traits are inherited as discrete units.
– The last two are the Law of Segregation:
2. Organisms inherit two copies of each gene, one from each
parent.
3. The two copies segregate
during gamete formation.
purple
white
KEY CONCEPT – Section 4
Genes encode proteins that
produce a diverse range of traits.
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.
• 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.
– Homozygous
describes two alleles
that are the same at a
specific locus.
– Heterozygous
describes two alleles
that are different at a
specific locus.
Genes influence the development of
traits.
• All of an organism’s genetic material is called the
genome.
• Genotype refers to the makeup of a specific set
of genes.
• Phenotype is the physical expression of a trait.
So which is the
genotype? Phenotype?
• 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.
• Both homozygous dominant and heterozygous
genotypes yield a dominant phenotype.
• Most traits occur in a
range and do not follow
simple dominantrecessive patterns.
KEY CONCEPT – Section 5
The inheritance of traits follows
the rules of 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.
A monohybrid cross involves one trait.
• Mono = One
• Monohybrid crosses examine the inheritance of only one
specific trait.
– homozygous dominant (two capital letters)-homozygous
recessive (two lower case letters): all heterozygous genotypes
What would all
of these plants
look like? What
would their
phenotype be?
Heterozygous-Heterozygous—
Genotype = 1:2:1 homozygous dominant: heterozygous:
homozygous recessive
Phenotype = 3:1 dominant:recessive
• heterozygous-homozygous recessive—
– Genotype = 1:1 heterozygous:homozygous recessive;
– Phenotype = 1:1 dominant:recessive
The Unknown
• What if you have an unknown
genotype?
• A testcross is a cross between an
organism with an unknown genotype
and an organism with the recessive
phenotype.
• You have brown eyes but that means
you have two possible genotypes
– BB or Bb
– If you cross with someone with blue eyes
and have a child with blue eyes, what
must your genotype be?
A dihybrid cross involves two traits.
• 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.
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 =
number of ways a specific event can occur
number of total possible outcomes
• Probability applies
to random events
such as meiosis and
fertilization.
Ch 7 .1 KEY CONCEPT
The chromosomes on which genes are located
can affect the expression of traits.
Two copies of each autosomal gene
affect phenotype.
• Mendel studied autosomal
gene traits, like hair texture.
• Mendel’s rules of inheritance apply to
autosomal genetic disorders.
– A heterozygote for a recessive disorder is a carrier.
– Disorders caused by dominant alleles are uncommon.
(dominant)
Males and females can differ in sexlinked traits.
• Genes on sex chromosomes are called sex-linked genes.
– Y chromosome genes in mammals are responsible for male
characteristics.
– X chromosome genes in mammals affect many traits.
• Male mammals
have an XY
genotype.
– All of a male’s sexlinked genes are
expressed.
– Males have no
second copies of
sex-linked genes.
Think about it…
• Why are sex-linked disorders such as
color-blindness more common in males
than females?
• Female mammals have an XX genotype.
– Expression of sex-linked genes is similar to
autosomal genes in females.
– X chromosome inactivation randomly “turns
off” one X chromosome.
Ch 7.2 KEY CONCEPT
Phenotype is affected by many different factors.
Phenotype can depend on interactions of alleles.
• In incomplete dominance, neither allele is
completely dominant nor completely recessive.
– Heterozygous phenotype is intermediate between the two
homozygous phenotypes
– Homozygous parental phenotypes not seen in F1 offspring
• Codominant
alleles will both
be completely
expressed.
– Codominant
alleles are
neither
dominant nor
recessive.
– The ABO blood
types result
from
•
codominant
alleles.
Many genes have more
than two alleles.
Many genes may interact to produce
one
trait.
• Polygenic traits are
produced by two or
more genes.
Order of dominance:
brown > green > blue.
• An epistatic gene is a single gene that, if
present, overrides all other genes
– Example: Mouse fur and Albinism
The environment interacts with
genotype.
Phenotype is a combination of genotype and environment.
• The sex of sea turtles
depends on both genes
and the environment-eggs
buried in warm climates = female;
in cooler climates = male
• Height is an example of a
phenotype strongly affected
by the environment. How might
environment cause a difference?
1. (pg. 207) How do multiple alleles differ
from polygenic traits?
Multiple alleles are traits influenced by
several different versions of one gene;
polygenic traits are influenced by multiple
genes.
2. Do identical twins have identical genes?
Do they have identical fingerprints?
Yes, but finger prints will vary because when in the
womb, they touch the walls and the environment
affects the ridges