1.1. Non-Mendelian inheritance
MATERNAL INHERITANCE (CYTOPLASMIC INHERITANCE) – Although
substantial part of genetic information is encoded by nuclear DNA, some structural genes
are located in the circular mitochondrial (mt) genome and in case of plants, also in
chloroplast genome. The inheritance of traits encoded by mitochondrial (mitochondrial
inheritance) and chloroplast (chloroplast inheritance) genomes represent the examples
of non-Mendelian (non-mendelistic) inheritance (pattern of inheritance in which traits
do not segregate in accordance with Mendel’s laws). In contrast to Mendelian inheritance,
in case of maternal inheritance the genetic information originates always from female
gamete during sexual reproduction. In this case, the offspring inherit a gene exclusively
from their mothers; hence, this type of inheritance is called maternal. Cytoplasmic
inheritance was discovered by Carl Erich Correns, German botanist and geneticist, who
performed experiments with the four o'clock flower (Mirabilis jalapa). He is notable
primarily for his rediscovery of Gregor Mendel's findings (simultaneously but
independently of the botanists Erich Tschermak and Hugo de Vries).
MITOCHONDRIAL INHERITANCE – Many serious hereditary genetic diseases, such as
Leber's hereditary optic neuropath or Kearns-Sayre syndrome (also known as
Oculocraniosomatic neuromuscular disease with ragged red fibers) are linked to mtDNA
mutations. Individuals affected by a disease showing mitochondrial inheritance can be both
males and females but such a disease is inherited only in maternal lineage. A child inherits
mtDNA exclusively from mother. Typical pedigree showing inheritance of a disease
caused by mutation in mtDNA is shown bellow.
♂
♀
Fig. 1: Mitochondrial type of inheritance (Leber's hereditary optic neuropathy causing partial blindness in midage adults). Affected individuals are marked in black, unaffected ones in white. Mutation in mtDNA can be
inherited only from mothers, thus none of the children of a man affacted by a disease showing mitochondrial
inheritance pattern will be affected.
CHLOROPLAST INHERITANCE. Maternal inheritance pattern can be found in
the inheritance of variegated (green and white mottled) leaf color. The leaf color depends
on which parent had which trait. Leaf color in F1 generation is given by the color of leaves of
the female plant, because pollen grains produced by male plants contain hardly any cytoplasm
and thus they contain no chloroplasts carrying genetic information. If the female plant is
green, the progeny will be green. If the female plant is yellow (white), the progeny will be
also yellow (white). Variegated plants have green and white mottled leaves due to non-equal
distribution of chloroplasts during mitosis that causes yellow/white spots on the leaves. Nonequal distribution of plastids occurs also in meiosis, thus female plants produce different types
of gamets. Thus, F1 generation is not uniform and diverges from mendelian inheritance
pattern.
MATERNAL EFFECT relates to the development of an oocyte in mother's organism.
In case of the maternal effect, the phenotype is based on genetic information encoded by
nuclear gene. During gene expression of these genes the mRNA and proteins are produced
and they are transferred into developing oocyte prior to fertilization. These proteins
directly influence the embryo during its early development. Thus, an organism shows
the phenotype expected from the genotype of the mother, irrespective of its own genotype.
Example of this inheritance is coiling of the shell of snail (Lymnaea perena)
Model task:
The coiling of the shell of snail (Lymnaea peregra) is given by two different alleles of
a nuclear gene. Dominant allele D is completely dominant over d and determines right-handed
(R) coiling of shell, while recessive allele d determines left-handed (L) coiling of shell.
Phenotype of the offspring (regardless it’s genotype) depends on the genotype of its
mother (regardless her phenotype).
Principle: protein (product of maternal gene) influencing coiling of the shell is present in the
oocyte prior to fertilization. It influences the orientation of mitotic spindle during the first
mitosis after fertilization, thus influencing the final shell coiling (left-handed or right-handed)
in the offspring.
R
♀ DD
P:
×
♂
L
dd
P generation: crossing of female with right-handed
shell and male with left-handed shell.
F1:
phenotype
genotype
F1 generation: uniform offspring with genotype Dd,
phenotypically right-handed shell (determined by
mother’s genotype DD).
R
Dd
self-fertilisation
F2:
R
DD
R
Dd
R
Dd
R
dd
F2 generation: offspring with three different genotypes
(DD, Dd, dd), however phenotypically uniform (righthanded) due to effect of maternal genotype (Dd).
self-fertilisation
F3:
R
R
R
L
F3: mothers with genotypes DD and Dd produce righthanded offspring; mother with genotype dd produces
left-handed offspring. Genotype ratio is 1:2:1,
phenotype ratio is 3:1.
Fig. 2: Maternal effect on the inheritance of shell coiling in snail (Lymnaea peregra). Shell coiling is influenced
by a pair of alleles: D – dominant determining right-handed shell, d – recessive determining left-handed shell
(R=right-handed, L=left-handed).
___________________________________________________________________________
TASKS
Task 1: A man affected with mitochondrial optic neuropathy married a healthy
woman. What is the probability that their child will be affected with this disease?
Task 2: Recessive mutation in chloroplast DNA causes variegation of leaves in
Mirabilis jalapa. What plants can be expected in the offspring of variegated
maternal plant and green paternal plant?
Task 3: What will be the leaf color of the offspring resulting from pollination of
green plant with the pollen of variegated plant?
Task 4: What will be genotypes and phenotypes in F1, F2 and F3 generations,
when P generation is represented by female snail with left-handed shell and male
with right-handed shell.
Task 5:
What was the genotype and phenotype of parents of an offspring
with left-handed shell? What was the phenotype of this offspring?
Task 6: The pedigree bellow shows the inheritance of Kearns-Sayre syndrome,
a hereditary genetic disease caused by mtDNA mutation. It is myopathy
characterized by isolated involvement of the muscles controlling eye-lid and eye
movement and by progressive ophthalmoplegia. Mark the affected individuals in
the pedigree with black color.
I
1
2
II
1
2
3
4
5
6
7
8
9
10
III
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15