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Nonmendelian inheritance

Maternal Effect.

-Refers to inheritance patterns for certain nuclear genes.
-The geneotype of the mother directly determines the offsprings phenotype.

Nuclear Genes?

Genes located on chromosomes that are found in the cell nucleus.

First example of a maternal effect gene?

-Arthur Boycott.
- morphological features of the water snail ( Lymnaea peregra.)
- The internal organs can be arranged in either a right-handed(DEXTRAL) or left-handed (SINISTRAL) direction.

-Dextral is dominant.

Reciprocal cross?F1 and F2 generation of snails.

-A sinister female (dd) is crossed to a dextral male ( DD) all offspring were sinistral. (F1 generation)

-F2 generation- 1DD: 2Dd: 1dd ( 3:1) predicted.... except all were sinistral.

Female gametes receive....

Gene products from the mother that affect early developmental stages of the embryo.

The non-mendelian inheritance pattern of maternal effect genes can be explained by

the process of oogenesis in female animals.

( As an oocyte matures, many surrounding maternal cells ( nurse cells) provide the egg with nutrients. )

Mechanism of maternal effect in snail coiling

1. Transfer of gene products from nurse cells to egg.
2. Nurse cells are heterozygous. (Both D and d are activated in the cell to produce both genes (mRNA or proteins)
3. Products are transported to cytoplasm of egg.
4. the direction of coiling is determined by the differences in the cleavage planes during early development.

Maternal effect genes play a role in?

1. Cell division.
2. Cleavage pattern
3. body axis orientation.

The origin of dextral and sinistral coiling can be traced to

the orientation of the mitotic spindle.

Epigenetic inheritance?

Modifications to nuclear genes or chromosomes that alter gene expression, but are NOT due to DNA sequence changes.

- Not permanent over generations.

When do Epigenetic modifications occur?

-Early stages of embryogenesis.

Examples of Epigenetic inheritance?

-Dosage compensation
-Genomic Imprinting.

Dosage Compensation?

-Phenomenon in which level of expression of many genes on sex chromosomes is similar in both sexes, even though males and females have different sex chromosome configurations.
(eye color mutation in drosophila)


Turning off one of their two x chromosomes.

Placental? Marsupial? Drosophila? Caenorhabditis Elegans? Dosage compensation.

1. One chromosome gets inactivated.
2. Paternal x-chromosome inactivated
3. Increase transcription of males.
4. both (x) decreased in hermaphrodites is decreased by 50%

Barr body?

Inactivated X. More compact.

X-inactivation process is also known as the ?

Lyon hypothesis

Example of x chromosome inactivation?

- Fur pattern in cats.
- Orange patches due to inactivated x-chrom that carries a black allele.

- Only heterozygous female cats can be calico.

Normal female has how many barr bodies? ( xx)?


A normal male has how many Barr bodies?


A female with Turner syndrome has how many barr bodies? ( X0 )


A female with triple x syndrome? barr bodies? ( XXX)


A male with klinefelter syndrome has how many barr bodies? ( XXY)?


X-inactivation in mammals depends on ?

The x-inactivation center (Xic)
- The Xist gene.

Xist gene?

- Active gene
- produces an RNA molecule that doesn't encode proteins.
- instead it coats the x- chromosome and inactivates it.
-afterwards other proteins are compacted into a barr body.

Tsix gene?

-Preventing x-inactivation.
-inhibits the transcription of the Xist gene.


-X chromosomal controlling element.
- affects the choice of the x-chromosome to be inactivated.
-an x-chromosome that carries a strong Xce is more likely to remain active.

Function of the Xic during x chromosome inactivation?

( One x targeted)
(expression of xist )
( repeated cell divisions)

Genomic imprinting?

A segment of DNA is marked and that mark is retained throughout the life of organism inheriting the marked dna.
-non-mendalian inheritance because the offspring will express only one of the two alleles. ( monoallelic expression)

Examples of genomic imprinting?

-Igf2 gene in mice.
-encodes a growth hormone
-paternal allele transcriped not maternal.
-can cause dwarfism.

Genomic imprinting also occurs during

-The imprint is erased.
-The female produces eggs in which the gene is silenced.
-The male produces sperm in which the gene can be transcribed into mRNA

Imprinting involves?

DNA methylation.
-Attachment of a methyl group to cytosine.
-DMR ( differentially methylated region)
-contains binding sites for proteins
-dominated in egg or sperm.. not both.

CTC binding factor has two effects

1. activator proteins are prevented from activating Igf2
2. activator proteins are allowed to activate H19

CTC binding factore cannot bind to ICR

1. Activator proteins are allowed to activate Igf2
2. Methylation represses the H19 gene.

What is Igf2? H19?

- Paternal ( Insulin like groth factor II)
- Maternal ( unknown)

Example of effect of imprinting in humans?

- Small deletion of a region of chromosome 15 can lead to two outcomes
- Angelman syndrome ( retardation, thin, poor muscle, very happppy)
-Prader-willi sndrome( small hands/feet, obesity, sexual immaturity)

Molecular cause of andelman and willi syndrome?

PWS (silenced in egg) and AS genes (silenced in sperm).

Extranuclear inheritance?

Cytoplasmic inheritance
-Inheritance of genes on the mitochondrial and chloroplast genomes.
-not located in cell nucleus.


Genetic material of mitochondria and chloroplasts are located.

Cholorplast genomes vs mitochondrial

chloroplasts are larger and have more genes.

Maternal inheritance?

occurs because the chloroplass are inherited only through the cytoplasm of the egg.


a cell may contain both types of chloroplasts.
-can result in variegated phenotypes.

Chloroplasts are inherited only

through cytoplasm of egg.

In S. cerevisiae, mutations which cause a segregational petite phenotype:

Are transmitted according to Mendelian rules

Neutral petite mutations:

Are an example of extranuclear inheritance

A female mouse has been identified in which the vast majority of active X chromosomes were inherited from her father. Which of the following statements may explain this

The Xce allele inherited from her father is particularly weak.

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