Introduction to Genetics
Chapter 11
The Work of Gregor Mendel
Genetics
• The scientific study of heredity.
Vocabulary to Know
• Trait: a specific characteristic varying among
individuals
ex: eye color
• Gene: a portion of DNA determining a trait;
found on the chromosomes
ex: the gene for eye color
Vocabulary (continued)
• Hybrid: the offspring of
two parents with different
traits
• Gametes: reproductive
cells (aka: sex cells;
sperm & egg)
Allele: the form of a gene
• Allele: the different forms of a
gene
* Represented by a letter.
ex: Widow’s Peak = W
* Organisms have two alleles
for each trait, one inherited from
the mother and one from the
father.
ex: WW
Forms of Alleles
Dominant Allele: trait will be expressed with
only one copy present
* Represented with capital letters.
ex: W (Widow’s peak)
Recessive Allele: trait will only be expressed
when no dominant alleles are present
* Represented with lower case letters.
ex: w (No widow’s peak)
Gregor Mendel
• Considered the father of genetics.
• Mid-1800s
• Monk who experimented with pea
plants in the monastery garden.
Mendel’s Conclusions
1. Inheritance is determined by factors
passed from one generation to the
next.
2. Principle of dominance: Some alleles
are dominant and others are
recessive.
Mendel’s Conclusions
3. Segregation: A gamete carries only one
copy of each gene.
4. Principle of independent assortment:
Genes for different traits segregate
independently from one another.
Applying Mendel’s Principles
Allele Combinations
• Homozygous: both alleles are the same
ex: WW or ww
• Heterozygous: alleles are different
ex: Ww (capital letter is always 1st!)
• Genotype: the actual allele combination; what
the genes say
ex: WW, Ww, ww
• Phenotype: the trait observed
ex: Widow’s Peak
Punnett Squares
• Used to predict the genotypes of offspring
when the genotypes of both parents is
known.
Other Patterns of Inheritance
Incomplete Dominance
 The heterozygous phenotype (Ww) is somewhere
between both homozygous phenotypes (WW & ww).
 Ex: pink flowers
Codominance
• Both alleles contribute to the phenotype;
neither allele is dominant .
•Ex: roan cattle
Multiple Alleles
 More than two allele possibilities.
 Ex: blood type
Polygenic Traits
 Traits controlled by more than one
gene.
 Ex: skin color
Meiosis
Words to Know
 Haploid: “one set”; cells that contain one set of each
chromosome (gametes); 1n
 Example: in humans n=23, so human haploid cells
(gametes) have 1(23) or 23 total chromosomes
 Diploid: “two sets”; cells that contain two sets of
each chromosome ; 2n
 Example: in humans n=23, so human diploid cells
have 2(23) or 46 total chromosomes
Homologous Chromosomes
 The pair of chromosomes that have
the genes for the same traits.
 A copy is inherited from each parent.
Meiosis
 A type of cell division that produces
gametes containing half the number of
chromosomes as a body cell.
 Contains two separate rounds of division,
called Meiosis I & Meiosis II.
 For example, during meiosis in humans,
diploid cells with 46 chromosomes are
divided into 4 haploid sex cells (gametes)
with 23 chromosomes each.
 Animation/
Phases
of
Meiosis
 Interphase: cell grows, replicates
its chromosomes and prepares to
divide.
 Prophase I: chromosomes take
shape; homologous chromosomes
pair up, forming a tetrad
 Crossing over can occur!
Chromosomes touch &
exchange genes. This can mix
up linked genes. (ones found
on the same chromosome that
occur together).
Metaphase I, Anaphase I, Telophase I
•Chromosomes
line up across
the cell’s
center.
•Homologous
chromosome
pairs separate.
•Nuclear membrane
reforms &
cytokinesis occurs.
Meiosis II
•Similar to Mitosis, except 4 haploid
daughter cells are created.
Prophase II:
• No Replication
occurs!
Metaphase II:
• Chromosomes
line up across the
cell’s center.
Meiosis II (continued)
Anaphase II:
•Chromosomes
separate.
Telophase II:
• Nuclear
membrane
reforms &
cytokinesis
occurs.
The end result:
 In males, all 4 of the
daughter cells
become sperm.
 In females, the
division of the
cytoplasm is
uneven, so only 1
egg is produced.
The other three
cells, called polar
bodies are not used
for reproduction.
Genetic Recombination
 The reassortment of chromosomes and the
genetic information that they carry by
segregation and crossing over.
 For example:
 In humans, n = 23, so the number of different
gametes that can be produced is 223 (more
than 8 million).
 When fertilization occurs, 223 x 223, or 70 trillion
different zygotes are possible and this is
without accounting for crossing over!
 Zygote: the fertilized egg
Mitosis vs. Meiosis
Mitosis
• Two identical daughter cells
are formed.
• Each daughter cell has one
complete set of
chromosomes. (diploid)
Meiosis
• Four different gamete cells
are created (although in
females, only 1 is viable).
• Each gamete cell has half of
the total number of
chromosomes. (haploid)
http://www.pbs.org/wgbh/nova/miracle/divi_flash.html
Linked genes
 Genes that are located on the same
chromosome & thus, tend to be
inherited together.
Human Heredity
Genome: the full set of genetic
information carried in an organism’s
DNA
 Humans have 46 chromosomes in their
genome.
Karyotype: a picture of all diploid pairs
of chromosomes, arranged in order of
decreasing size
Chromosome Types
Autosomes: the remaining 44 human
chromosomes other than
the sex chromosomes
Sex chromosomes: the pair of
chromosomes determining gender
 Males have XY.
 Females have XX.
 In female cells, most of the genes in 1 X are
randomly switched off (called Barr bodies).
 Ex: female calico cats
Sex-linked genes
 A gene located on a sex
chromosome.
 Genes on the Y chromosome are
only found in males.
 Genes on the X chromosome are
found in both genders.
 For recessive traits on the X, they
are more common in males than
females.
 Ex: color-blindedness, hemophilia
Pedigree
 A chart used to show the presence or
absence of a trait within a family.
Human Genetic Disorders
Nondisjunction
 When homologous chromosomes fail to
separate during meiosis.
 Creates monosomy and trisomy.
http://learn.genetics.utah.edu/content/chro
mosomes/diagnose/
WHEN A MISTAKE IS MADE…