Chapters 10 and 12


1st studies of heredity  genetics
Traits  characteristics that are inherited





Contrasting traits were easily seen
Short generation time
Many offspring per generation
Sexes on 1 flower  control of pollination
Mathematical analysis of data


Physical appearance
Expressed in words  tall, short





Genetic makeup
Expressed in terms of alleles
Allele  form of a gene for a trait
Dominant allele  always expressed (T 
tall)
Recessive allele  if present, may not be
expressed (t  short)



Homozygous  2 identical alleles (TT, tt)
Heterozygous (hybrid)  2 different alleles
(Tt)
Different genotypes can have the same
phenotype

Tall  TT or Tt
A single trait



Original parents  P generation
Offspring  F1 generation
All F1 were tall


F2  75% were tall, 25% short
3:1 ratio

Each organism has 2 factors (alleles) that
control each trait


When an individual is hybrid for a pair of
contrasting traits, only the dominant trait can
be seen
TT x tt  Tt


**recessive trait is masked**
Crossing 2 hybrids always results in 3:1



Pairs of alleles for a trait are separated during
the formation of gametes and are recombined
during fertilization
Tt  T (egg) + t (sperm)  Tt (zygote)
Explains appearance of recessive traits in
subsequent generations

Height and seed color


Genes for different traits are inherited
independently of each other
Exception  genes on the same chromosome



Predict the ratio of all possible results for a
certain genetic cross
Not what will happen, but what could happen
Exact ratios are not seen in nature due to
chance
TY
Ty
tY
ty
TY
TTYY
TTYy
TtYY
TtYy
Ty
TTYy
TTyy
TtYy
Ttyy
tY
TtYY
TtYy
ttYY
ttYy
ty
TtYy
Ttyy
ttYy
ttyy
Type of cell division in which daughter
cells receive only half the # of
chromosomes of the parent cell

Chromosomes occur in pairs

1 allele is on each of the paired chromosomes
Diploid
Cell with 2 of
each kind of
chromosome
(2n)
Body cells
(somatic)
Monoploid
Cell with 1 of
each kind of
chromosome
(n) also called
haploid
Gametes 
sperm and egg


Each of a pair has genes for the same traits
They may carry different alleles


Associated with sexual reproduction  2
parents
Allows offspring to have the same number of
chromosomes as parents

No doubling of chromosome number
Meiosis I and II
2 separate divisions


Replication of the chromosomes
Same as in mitosis


Each pair of homologous chromosomes comes
together to form a tetrad
This is known as synapsis

Crossing-over may
occur at this point


Exchange of genetic material
between nonsister
chromatids
Results in genetic variation or
mutation
Completely random and
unpredictable

Tetrads line up at cell equator  metaphase
plate


Homologous chromosomes separate and move
to opposite poles  disjunction
Critical step  without disjunction, gametes
would have abnormal numbers of
chromosomes



Cytokinesis forms 2 daughter cells
Each cell has only 1 chromosome from each
homologous pair
Each chromosome is still doubled  another
division is required

Identical to mitosis

Meiosis Overview


Crossing-over results in genetic recombination
 gene shuffling
Almost endless number of different possible
chromosomes


You are not the exact blend of your parents
Explains Mendel’s results




Failure of homologous chromosomes to
separate
Both chromosomes move to the same pole
1 cell has an extra chromosome
1 cell is missing a chromosome


A gamete with and extra chromosome fuses
with a normal gamete
Zygote has 1 extra chromosome


47 instead of 46 in humans
Trisomy 21  Down syndrome


A gamete with a missing chromosome fuses
with a normal gamete
Zygote has 1 missing chromosome



45 instead of 46 in humans
Lethal most of the time
Turner syndrome  XO



Total lack of chromosomal separation
Lethal in animals
Frequent in plants


Larger and healthier fruits and flowers
Plant breeders induce polyploidy by using chemicals
that cause nondisjunction
Chapter 12


Graphic representation of genetic inheritance
A chart showing familial relationships and
patterns of trait inheritance
Sample pedigree
chart
Squares  male
Circles  female
Filled in 
afflicted
Blank  not
afflicted
Half filled in 
carrier
2 copies of allele are needed
(Most genetic disorders)




Formation and accumulation of mucus in lungs
and pancreas
Due to a defective protein
1 in 25 white Americans carry the allele
Resistance against tuberculosis



Buildup of lipids in brain cells (lethal)
Missing the gene coding for an enzyme
Amish and eastern European Jews





Accumulation of phenylalanine (amino acid) in
brain cells
Causes mental retardation
Missing the enzyme needed to break down the
amino acid
Standard test for all newborn infants
PKU mothers  can damage unborn child





Abnormally shaped red blood cells
Mutation in hemoglobin gene
Most afflicted don’t survive childhood
Carriers are more resistant to malaria
Africans




1 allele is needed
Tongue rolling
Hitchhiker’s thumb
Huntington’s chorea



Degeneration of brain cells  lethal
Onset of symptoms at 30-50 years of age
50% chance of passing on the allele


Dominant allele is only partially expressed
when recessive allele is present
Genetic blending



Both alleles are expressed equally
Cattle  red coat and white coat codominant
Hybrid  roan (mixture of red and white
hairs)


More than 2 possible alleles within a species
Human blood type  3 possible alleles

Sex chromosomes  only unmatched pair




All other are autosomes  22 pairs
Female  XX
Male  XY
It is the male that determines the sex of the
offspring






Controlled by genes on the X chromosome
Males are more likely to be afflicted
Females may be carriers
Color blindness
Hemophilia
Male pattern baldness *



Trait controlled by 2 or more genes
Not expressed in 2 contrasting forms, but
varies between the 2 extremes
Human height


Genes located on the same chromosome
Inherited together


External  temperature, light, nutrition,
infectious agents
Internal  hormones, age, sex

3 possible alleles



IA and IB are codominant
i is recessive
6 possible genotypes




IAIA or IAi  type A
IBIB or IBi  type B
IAIB  type AB
ii  type O

Red-green color blindness


Most common
Hemophilia

Commonly missing clotting factor VIII


Eye color, skin color, height
Almost all human traits


Autosomes  Down syndrome
Sex chromosomes
Turner syndrome  XO
 Klinefelter syndrome  XXY
 OY  lethal


Comparing specimen chromosomes to normal
chromosomes


Sample of free-floating fetal cells
Make a karyotype

Compare your genes to known disease markers