Non-Mendelian Inheritance
I.
QUANTITATIVE TRAITS
A. Background
1. Inheritance of phenotypes with easily distinguishable phenotypes is referred to
as qualitative traits
a) For example, red flowers or white flowers
2. However, other traits have a wide range of phenotypes, with no easily
distinguishable divisions, and are referred to as quantitative traits
a) Examples of quantitative traits in humans include height, weight, skin color,
and IQ
b) Quantitative traits are polygenic
(1) Controlled by several genes
B. Quantitative traits can be predicted using a modification of a trihybrid cross (if three
genes are involved), tetrahybrid cross (if four genes are involved), etc
1. You just have to remember that all the alleles for all genes are going to effect the
one trait you are examining
2. Let's take a look at flower color in a species that ranges from white, to red, with
many intermediate shades
a) This trait, flower color, is determined by three genes and two alleles (r =
recessive, no pigment; R = dominant, produces red pigment)
(1) We will cross a white flower with a red flower -- r1r1r2r2r3r3 X R1R1R2R2R3R3
(a) The genotype of the white flower indicates that it has two recessive alleles
for each gene
(b) The red flower has two dominant alleles for each gene
(c) The white flower must donate a recessive allele for each gene to its
gamete
(d) The red flower must donate a dominant allele for each gene.
R1R2R3
r1r2r3
R1r1R2r2R3r3
(2) The offspring is heterozygous at each gene
b) One characteristic of quantitative traits is that if parents of extreme
phenotypes are crossed, the resulting offspring will have an intermediate
phenotype
3. If we performed a cross between two of the offspring, we would obtain the
following genotypes
R1R2R3
R1R2r3
R1r2R3
r1R2R3
R1r2r3
r1R2r3
r1r2R3
r1r2r3
R1R2R3
R1R1R2R2R3R3
R1R1R2R2R3r3
R1R1R2r2R3R3
R1r1R2R2R3R3
R1R1R2r2R3r3
R1r1R2R2R3r3
R1r1R2r2R3R3
R1r1R2r2R3r3
R1R2r3
R1R1R2R2R3r3
R1R1R2R2r3r3
R1R1R2r2R3r3
R1r1R2R2R3r3
R1R1R2r2r3r3
R1r1R2R2r3r3
R1r1R2r2r3r3
R1r1R2r2r3r3
R1r2R3
R1R2r2R3R3
R1R1R2r2R3r3
R1R1r2r2R3R3
R1r1R2r2R3R3
R1R1r2r2R3r3
R1r1R2r2R3r3
R1r1r2r2R3R3
R1r1r2r2R3r3
r1R2R3
R1r1R2R2R3R3
R1r1R2R2R3r3
R1r1R2r2R3R3
r1r1R2R2R3R3
R1r1R2r2R3r3
r1r1R2R2R3r3
r1r1R2r2R3R3
r1r1R2r2R3r3
R1r2r3
R1R1R2r2R3r3
R1R1R2r2r3r3
R1R1r2r2R3r3
R1r1R2r2R3r3
R1R1r2r2r3r3
R1r1R2r2r3r3
R1r1r2r2R3r3
R1r1r2r2r3r3
r1R2r3
R1r1R2R2R3r3
R1r1R2r2r3r3
R1r1R2r2R3r3
r1r1R2R2R3r3
R1r1R2r2r3r3
r1r1R2R2r3r3
r1r1R2r2R3r3
r1r1R2r2r3r3
r1r2R3
R1r1R2r2R3R3
R1r1R2r2R3r3
R1r1r2r2R3R3
r1r1R2r2R3R3
R1r1r2r2R3r3
r1r1R2r2R3r3
r1r1r2r2R3R3
r1r1r2r2R3r3
r1r2r3
R1r1R2r2R3r3
R1r1R2r2r3r3
R1r1r2r2R3r3
r1r1R2r2R3r3
R1r1r2r2r3r3
r1r1R2r2r3r3
r1r1r2r2R3r3
r1r1r2r2r3r3
a) The phenotypic ratio is 1 / 64 (dark red -- 6 dominant alleles); 6 / 64 (red -- 5
dominant alleles); 15 / 64 (light red -- 4 dominant alleles); 20 / 64 (pink -- 3
dominant alleles); 15 / 64 (light pink -- 2 dominant alleles); 6 / 64 (very light
pink -- 1 dominant alleles); and 1 / 64 (white -- 0 dominant alleles)
b) The relationship of alleles in this case is referred to as additive
4. Another characteristic of polygenic traits is that if you cross two individuals
with intermediate characteristics, the offspring will have a range of
characteristics, with a few individuals and the extremes
C. The number of different phenotypes for additive alleles is the number of alleles an
individual has (in this example 6) plus one
1. If you want to determine the number of genes involved, you can use the formula
1/4n = ratio of F2 individuals expressing either extreme phenotype
2. The number of distinct phenotypes = 2n+1
a) Environmental effects can blur distinctions
II.MATERNAL
EFFECTS
A. Background
1. In some cases, the mother's genotype may determine the offspring's phenotype
a) The father's and offspring's genotypes are irrelevant
b) Phenotype is controlled by maternal gene products acting on the zygote /
embryo
B. One example of this maternal effect is shell coiling
1. Shells may either coil to the right (D - dominant) or coil to the left (d - recessive)
2. If a female (DD) is crossed with a male (dd), all the offspring (Dd) would have
right-coiled shells
3. If a female (dd) is crossed with a male (DD), all the offspring (Dd) would have
left-coiled shells)
4. If the F1 were crossed, all offspring (DD, Dd, dd) would have right-coiled shells
a) Since none of the mothers had the dd genotype
b) If the F2 were crossed, only the females with the dd genotype would produce
offspring with the recessive (left-coiled shell) trait, regardless of the father's
genotype
III.GENE
IMPRINTING
A. General
1. In gene printing, an allele may have different effects on the offspring depending
upon whether it was inherited maternally or paternally
a) This means different individuals that are heterozygous may have different
phenotypes depending upon which allele they inherited from their mother
and which they inherited from their father
(1) For some genes, the allele inherited from the mother is always inactivated,
and for other genes the allele inherited from the father is always inactivated
(2) For example, a type of tumor of the inner ear (glomus tumors) is inherited by a
recessive allele from the father
(a) If an offspring is DD it will be normal
(b) If it is dd, it will have the tumor
(c) If it is Dd, it will have a tumor if the father donated the recessive allele, or
normal if the mother donated the recessive allele
2. Imprinting is not permanent
a) Thus an allele that is inactivated in one generation may be the active allele in
the next
3. Mechanism
a) Different sexes methylate DNA differently, thus affecting their expression
b) These methyl groups
B. Prader-Willi syndrome and Angelman syndrome
1. Symptoms
a) Prader-Willi syndrome
(1) Mental retardation
(2) Extremely obese
b) Angelman syndrome
(1) Mental retardation
(2) Jerky, erratic, "happy puppet" type behaviour
2. Cause
a) A defective gene on chromosome 15
(1) If inherited maternally, the offspring will have Prader-Willi syndrome
(2) If inherited paternally, the offspring will have Angelman Syndrome
C. Fragile X syndrome
1. The defective X chromosome must be passed maternally
IV.EXTRANUCLEAR
INHERITANCE
A. Background
1. Mitochondria are found in most eukaryotes, while chloroplasts are found in
photosynthetic eukaryotes
a) These organelles are interesting in that they contain their own genomes and
replicate within the cell
(1) The genome is a single covalently closed circular DNA molecule
(2) 13,000 - 18,000 base pairs in animals and 300,000 - 500,000 base pairs in
plants
(3) Humans mitochondria genomes contain 37 genes
(a) 13 of the 80 needed for ATP synthesis (67 are nuclear) and 24 involved in
gene regulation and replication
(b) 16, 569 base pairs
(c) 5 - 20 genomes per mitochondria
(d) 100 (skin) to 1000 (liver) to 10 million (oocyte) mitochondria per cell
(e) 500 - 200 million genomes per cell
b) Generally, these organelles are inherited from the female egg, and are thus
passed maternally
2. The terms homozygous and heterozygous do not apply here, since these
organelles are haploid
a) However, an individual cell may have many organelles, some with different
alleles then others
(1) Homoplasmic
(a) Organelles with only one allele
(2) Heteroplasmic
(a) Different alleles in organelles
(i) Same organelle
(ii) Different organelles in a cell
(iii) Different organelles in different cells of various tissues
B. Chloroplasts
1. Characteristics
a) 130,000 to 150,000 base pairs
b) 110 genes, 50 of which are involved in photosynthesis
c) 2 - 40 chloroplasts per cell
d) 20 - 80 genomes per chloroplast
e) 40 - 3200 genomes per cell
2. Some chloroplast lack an allele for chlorophyll production and will therefore be
white
a) A homoplasmic plant will not survive
b) A heteroplasmic plant may be variegated
c) Genotype of pollen is irrelevant
(1) A green pistil mated with white pollen will result in green offspring
(2) A white pistil mated with a green pollen will result in white offspring
(3) A variegated flower mated with green pollen will result in white, green, and
variegated offspring
C. Transmission
1. Heteroplasmic females may produce progeny with different genotypes and
phenotypes depending upon segregation of organelles during gametogenesis
a) This is due to a "sampling error" in egg production
(1) Many mitochondria disintegrate and a small sample then repopulates the
developing oocyte
b) This can confuse patterns of inheritance, since some diseases are not
manifested unless a certain percentage of organelles possess the mutant
allele
(1) For example, myoclonic epilepsy and ragged-red fibers (MERFF) is a disease
that shows symptoms only if 65% or more of the mitochondria have the mutant
allele
(a) Many symptoms due to decreased ATP production
(i) Neural and muscle cells die and tissues disintegrate
(b) Someone could be a carrier, with 64% or less or organelles with the
disease allele
(2) Luft disease
(a) Excessive sweating and general weakness
(b) Lever optic atrophy
(i) Degeneration of the optic nerve leading to blindness around 20 - 24
years of age
(a) Age of onset varies with degree of heteroplasmy
(ii) Optic nerve has high energy demand
(a) Heart also effected
D. Infectious agents
1. Organelle independent
a) Only a small amount of cytoplasm needed
2. Examples
a) Algae
(1) Chorella vulgaris (a single-celled green algae) is inherited via the cytoplasm of
hydra eggs, sponges, and sea anemones
b) Bacteria
(1) Kappa particles (Caedobacter taeniospiralis) in Paramecium
(2) During conjugation, if there is no cytoplasmic exchange they will retain their
phenotype
(3) With cytoplasmic exchange, both cells will be killer cells
c) Viruses
(1) Germline viruses of both males or females and infection after conception
(females)
d) Viroids
(1) Hepatitis D in animals, but otherwise, mostly in plants
e) Prions
(1) Cause neural disorders in animals
V.
MODIFIER GENES
A. Enhancers
1. Increases the severity of another mutation
B. Suppressors
1. Decreases the severity of another mutation
2. May be intergenic or extragenic
C. These can be worked as a dihybrid cross
1. If the modifier is dominant you would get a 15:1 ratio
2. If the modifier is recessive you would get a 13:3 ratio
VI.PLEIOTROPIC
EFFECTS
A. An allele has effects on many different traits
1. For example, an allele that causes a white seed coat may also cause a white
flower
2. Always correlated
VII.PARTIAL
PENETRANCE
A. Only some individuals with a mutant genotype shows the trait
1. Environmental susceptibility and gene interactions are possible causes
2. The percentage that show the trait is the penetrance of the mutation
a) There are no set phenotypic ratios
VIII.ESPRESSIVITY
A. Degree of mutant phenotypes differ
B. Variable espressivity
1. Example, the shape of an eyespot on butterfly wings
C. Constant espressivity
1. All bristles light
IX.EPISTASIS
A. Interaction of non-allelic gene products
1. Referred to as hypostatic if it masks another allele
a) The epistatic allele can be dominant or recessive
(1) If dominant, should get a 12:3:1 ratio
(2) If recessive, should get a 9:3:4 ratio
X.
OTHERS
A. In addition to the above, other types of inheritance can be observed
1. In some cases, mitochondrial gene products and nuclear gene products interact
with each other, making analysis difficult
2. Also, cytoplasmic symbionts can be transmitted to offspring
3. Some viruses integrate their genome into the host genomic material and can be
passed to offspring with the nuclear material