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6.1 Chromosomes and Meiosis
KEY CONCEPT
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.
body cells
sex cells (sperm)
sex cells (egg)
6.1 Chromosomes and Meiosis
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.
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 makes
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
KEY CONCEPT
During meiosis, diploid cells undergo two cell
divisions that result in haploid cells.
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.
– Pairs of homologous chromosomes (copy from mom and
copy from dad) 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.
homologous chromosomes
sister
chromatids
sister
chromatids
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 become streamlined and
motile.
– 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.
6.3 Mendel and Heredity
KEY CONCEPT
Mendel’s research showed that traits are inherited as
discrete units.
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 allowed the resulting plants to self-pollinate.
– 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
• Mendel drew three important conclusions.
– Traits are inherited as 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.
purple
white
6.4 Traits, Genes, and Alleles
KEY CONCEPT
Genes encode proteins that produce a diverse range
of traits.
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.
– Homozygous
describes two alleles
that are the same at a
specific locus.
– Heterozygous
describes two alleles
that are different at a
specific locus.
6.4 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.
6.5 Traits and Probability
KEY CONCEPT
The inheritance of traits follows the rules of
probability.
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.
6.5 Traits and Probability
6.5 Traits and Probability
• heterozygous-homozygous 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 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.
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.
number of ways a specific event can occur
• Probability =
number of total possible outcomes
• Probability applies to
random events such as
meiosis and fertilization.
6.6 Meiosis and Genetic Variation
KEY CONCEPT
Independent assortment and crossing over during
meiosis result in genetic diversity.
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