Genetics Chapter 5: Non-Mendelian Inheritance

Extranuclear Inheritance AKA cytoplasmic inheritance

Inheritance patterns involving genetic material outside the nucleus

• Mitochondria and chloroplasts are the most important due to genetic material within organelles found in cytoplasm, located in nucleoid

T/F: Chloroplasts tend to have more nucleoids per organelle than mitochondria

TRUE

Maternal effect

inheritance pattern for certain nuclear genes in which the genotype of the female parent directly determines the phenotype of the offspring

• the genotypes of the male parent and offspring themselves do not affect the phenotype of the offspring

• due to the accumulation of gene products that the female parent provides to developing eggs

Discovery of Maternal Effect Genes

• First discovered in 1920s by AE Boycott studying water snails

• Right-handed: Dextral

• Left-handed: Sinistral

• Dextral is more common and dominant

The phenotype of the offspring depended solely on the genotype of the mother, NOT her phenotype

• DD or Dd mothers produce dextral offspring

• dd mothers produce sinistral offspring

• The genotypes of the father and offspring do not affect the phenotype of the offspring

This non-Mendelian inheritance pattern can be explained by the process of oogenesis

Epigenetic Inheritance

pattern in which a modification occurs to a nuclear gene or chromosome that alters gene expression

• The expression is not permanently changed over the course of many generations b/c the DNA sequence does not change

What are epigenetic changes caused by?

Epigenetic changes are caused by DNA and chromosomal modifications

• Occurs during oogenesis, spermatogenesis or early embryonic development

Dosage compensation

The purpose of dosage compensation is to offset differences in the number of active sex chromosomes

Mechanism of X-chromosome Inactivation

During X chromosome inactivation, the DNA becomes highly compacted

• Most genes on the inactivated X cannot be expressed

When this inactivated X is replicated during cell division:

• Both copies remain highly compacted and inactive

• X inactivation is passed along to all future somatic cells

Nucleation

Occurs during embryonic development. The number of X-inactivation centers (Xics) is counted and one of the X chromosomes remains active and the other is targeted for inactivation.

Spreading

Occurs during embryonic development. It begins at the Xic and progresses toward both ends until the entire chromosomes is inactivated and becomes a Barr body.

Maintenance

Occurs from embryonic development through adult life. The inactivated X chromosomes is maintained as such during subsequent cell divisions.

Genomic Imprinting

phenomenon in which a segment of DNA is marked and the effect is maintained throughout the life of the organism inheriting the marked DNA

• Depending on how the genes are “marked”, the offspring expresses either the maternally-inherited or the paternally-inherited allele

• Not both

• Termed monoallelic expression

Stages of imprinting

1. Establishment of the imprint during gametogenesis

• imprinting occurs during gametogenesis in the lgf2 gene, which exists in the lgf2 allele from the male and the Igf2- allele from the female. This imprinting occurs so that only the paternal allele is expressed.

1. Maintenance of the imprint during embryogenesis and in the adult somatic cells

• After fertilization, the imprint pattern is maintained throughout development. In this example, the maternal Igf2- allele will not be expressed in somatic cells.

1. Erasure and reestablishment of the imprint in the germ cells

• In the germ-line cells, the Imprint is erased. The female mouse produces eggs in which the gene is silenced. The male produces sperm in which the gene can be transcribed into mRNA.