Chapter 3 - Epigenetics, Linkage, and Extranuclear Inheritance

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Questions and Answers

What term describes stable changes in gene expression that are passed from cell to cell without altering the DNA sequence?

Environmental mutation

Gene regulation

Genomic imprinting

Epigenetic regulation (correct)

Which of the following is NOT a rule of Mendelian inheritance?

Genes are passed unaltered from cell to cell

Crosses with more than two genes obey independent assortment

The genes obey Mendel's law of segregation

All genes are independent of each other (correct)

What does the term 'epi-' in epigenetics signify?

Altered

Beyond

Under

Over (correct)

What type of inheritance involves genetic material that is located outside of the nucleus?

Extranuclear inheritance Signup and view all the answers

Which process ensures that muscle cell genes are expressed at high levels during development?

Gene regulation Signup and view all the answers

Which of the following is an example of how environmental signals can influence gene expression?

Transient increases in RNA production Signup and view all the answers

What is the relationship between epigenetics and phenotypic variation in human cells?

Epigenetics alters gene expression, producing different phenotypes Signup and view all the answers

How do genes on the same chromosome typically behave in a genetic cross?

They may be linked and show various recombination frequencies Signup and view all the answers

What is the result of vernalization in flowering plants?

Epigenetic silencing of FLC gene is maintained until flowering Signup and view all the answers

Which of the following is true regarding X inactivation in female mammals?

Only one X chromosome remains active in all cells Signup and view all the answers

How is epigenetic inheritance described?

Some epigenetic changes are inherited while others are not Signup and view all the answers

What defines a mosaic individual in the context of Calico cats?

An individual with a mixture of cells expressing different alleles Signup and view all the answers

What is the primary cause of mitochondrial diseases in humans?

Mutations in mitochondrial genes inherited maternally Signup and view all the answers

How do linked genes behave during meiotic recombination?

They are inherited together more often than separate Signup and view all the answers

What is a Barr body?

An inactivated X chromosome in female somatic cells Signup and view all the answers

Which mechanism ensures equal expression levels of X-linked genes in males and females?

X inactivation in females Signup and view all the answers

In the case of Leber’s hereditary optic neuropathy (LHON), what is a significant feature of the inheritance pattern?

Affected individuals can show symptoms if mitochondrial mutation load is high Signup and view all the answers

What is the purpose of genetic mapping in the context of recombination frequencies?

To calculate distances between linked genes on chromosomes Signup and view all the answers

What does a recombination frequency of 1% correspond to in genetic mapping?

1 map unit (mu) or centiMorgan (cM) Signup and view all the answers

What can be concluded from the high frequency of nonrecombinant phenotypes in a dihybrid cross involving linked genes?

The genes are closely located on the same chromosome Signup and view all the answers

Which of the following describes extranuclear inheritance?

Transmission of genes located outside the nuclear genome Signup and view all the answers

Study Notes

Chapter 18: Epigenetics, Linkage, and Extranuclear Inheritance

Mendelian inheritance follows three rules:
- Genes are passed unchanged from cell to cell and across generations.
- Genes obey Mendel's law of segregation.
- Crosses with more than two genes follow Mendel's law of independent assortment.
This chapter examines deviations from Mendelian rules due to:
- Epigenetics
- Genetic material outside the nucleus
- Genes on the same chromosome

Epigenetics

Gene expression: DNA → RNA → Protein
DNA mutations can affect the phenotype by altering the RNA or protein produced.
Changes in gene expression can also affect the phenotype by changing the amount of RNA and protein produced.
Transient changes in gene expression due to environmental signals are considered gene regulation.
Epigenetic gene regulation involves stable changes in gene expression that are passed between cells but do not alter DNA sequence.

Example: Human Cell Types

All human cells contain the same DNA but exhibit different phenotypes.
For example, muscle cells express high levels of actin and myosin for contraction.
Epigenetic changes during development turn specific genes on or off, distinguishing cell types.

Example: Vernalization

Some flowering plants require cold temperatures (vernalization) to flower.
During winter, cold temperatures cause epigenetic silencing of the FLC gene, preventing flowering until warmer weather.
Epigenetic changes are reversed after flowering to promote flowering in the next season.

Epigenetic Inheritance

Some epigenetic changes can be inherited from parent to offspring through sperm or egg cells.
Most epigenetic changes are not inherited, like those induced by environmental factors (e.g., cigarette smoke).

X Inactivation

In female mammals, one X chromosome is randomly inactivated in all somatic cells during early embryonic development.
The inactivated X chromosome is condensed and visible as a Barr body.
This process ensures equal levels of expression for X-linked genes in males and females.

Calico Cats

Calico cats are heterozygous females (XX^BX^O) with fur color determined by genes on the X chromosome.
Inactivation of one X chromosome leads to a mosaic pattern of orange and black fur color in calico cats.

X Inactivation: Dosage Compensation

X inactivation is a dosage compensation mechanism.
Mammalian cells have molecular mechanisms for determining X chromosome number and inactivating all but one.

Extranuclear Inheritance

This involves transmission of genes located outside the cell nucleus.
Mitochondria and chloroplasts contain their own DNA (organelle genomes) and are typically inherited maternally.
Mutations in organelle genomes can lead to various rare diseases.

Example: Leaf Color

Mutations in the chloroplast genome can affect chlorophyll synthesis and leaf color (e.g., variegated patterns).
The inheritance pattern of leaf pigmentation in some plants doesn't follow Mendelian segregation; inheritance is solely maternal.

Mitochondrial Genomes

The mitochondrial genome contains genes for translation and oxidative phosphorylation (e.g., 37 genes in many mammals).
Mutations in mitochondrial genes can cause rare, maternally inherited diseases, primarily affecting organs requiring high ATP.
A male, if affected by a mitochondrial disease, can't pass it to his offspring.

Example: Leber's Hereditary Optic Neuropathy (LHON)

LHON is a maternally inherited form of vision loss caused by mutations in mitochondrial genes.
This reduces ATP production in optic nerve cells, leading to cell death.
Males inherit the disease from their mothers but can't pass it to their offspring.
Symptomology depends on the extent of the mutation.

Gene Linkage

When genes are located close together on the same chromosome, they tend to be inherited together as a unit; this is called linkage.
Linked genes are part of linkage groups, which do not follow the law of independent assortment.

Bateson & Punnett's Crosses

This 1911 study provided early evidence of gene linkage in sweet peas.
They observed that some phenotypes occurred more frequently than expected by independent assortment.

Morgan's Fruit Flies

Morgan's work with fruit flies demonstrated gene linkage and the concept of crossover configurations during meiosis.
Crossing over leads to new combinations of alleles in offspring.

Linkage Mapping

Recombination frequencies are used to map genes along chromosomes.
The closer the genes are located, the lower the recombination frequency.
Genetic maps provide the linear arrangement and relative distances of genes.

Example of Genetic Linkage Map

Maps were developed showing the locations and distances of genes (e.g., Drosophila traits).
Distance in map units corresponds with recombination frequencies in test crosses, where 1% recombination equals 1 map unit.

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Description

Explore the complexities of genetics in Chapter 18, which covers epigenetics, linkage, and extranuclear inheritance. Understand how gene expression and environmental factors can lead to variations that deviate from Mendelian inheritance. This chapter delves into how genetic material interacts beyond the nucleus and the implications for phenotype expression.