Unit 4 (Week 14 Epigenetics, Linkage, and Extranuclear Inheritance) Flashcards
Mendelian inheritance patterns follow three general rules which are? (3)
- Except in the case of rare mutations, genes are passed unaltered from cell to cell, and from generation to generation.
- The genes obey Mendel’s law of segregation.
- For crosses involving two or more genes, the genes obey Mendel’s law of independent assortment.
Select all that apply
What are the three general rules for genes that follow a Mendelian pattern of inheritance?
Multiple select question.
The genes interact in an additive way to produce a phenotype.
For crosses involving two or more genes, the genes obey Mendel’s law of independent assortment.
The genes obey Mendel’s law of segregation.
Genes are generally passed unaltered from cell to cell, and from generation to generation.
For crosses involving two or more genes, the genes obey Mendel’s law of independent assortment.
The genes obey Mendel’s law of segregation.
Genes are generally passed unaltered from cell to cell, and from generation to generation.
Who coined the term epigenetics in 1941?
Conrad Waddington. Prefix, epi, means over, which suggests that some types of changes in gene expression are at a level that goes beyond changes in DNA sequences.
How do geneticists distinguish an epigenetic effect from other types of gene regulation?
An epigenetic effect begins with an initial event that causes a change in gene expression.
For example, DNA methylation may inhibit transcription. However, for this to be an epigenetic effect, the change must be passed from cell to cell and must not involve a change in the sequence of DNA.
What is the key feature of an epigenetic effect?
The long-term maintenance of a change in genetic expression.
As an example, let’s consider muscle cells in humans. Some genes in the human genome should be expressed in muscle cells, such as the genes that encode the proteins called actin and myosin, which are required for muscle contraction.
In embryonic muscle cells, these genes are subjected to epigenetic changes that promote their expression through adulthood. Alternatively, in other cell types, these same genes are inhibited by epigenetic changes such as DNA methylation. As the embryo grows and eventually becomes an adult, these epigenetic changes are passed from cell to cell so that adult muscle cells express actin and myosin genes at very high levels, whereas many other cell types do not.
T/F Some epigenetic changes, such as those involving the expression or inhibition of actin and myosin genes, are relatively permanent during the life of an individual.
True.
However, other epigenetic changes may be reversible during the life of an individual or may be reversible from one generation to the next.
What is the process in which certain species of plants require an exposure to cold temperatures in order to flower?
Vernalization
After vernalization, plants do not necessarily initiate flowering, but they acquire the ability to do so. Most commonly, plants are vernalized by exposure to cold winter temperatures and then flower the following spring or summer.
Vernalization involves epigenetic changes to genes that play a role in flowering. These epigenetic changes are induced by cold winter temperatures, and they are maintained during the flowering season. However, the epigenetic changes are reversed after the flowering season is over.
What is the study of mechanisms that lead to changes in gene expression that can be passed from cell to cell and are reversible, but do not involve a change in sequence of DNA?
Epigenetics.
This type of change may also be called EPIMUTATION.
This is a heritable change in gene expression that does not alter the sequence of DNA.
What is an epigenetic change that is passes from parent to offspring? An example is genomic imprinting.
Epigenetic inheritance
What is a case of epigenetic changes that are not inherited from parents?
For example, a person may be exposed to an environmental agent in cigarette smoke that causes an epigenetic change in a lung cell that is subsequently transmitted from cell to cell and promotes lung cancer. Such a change would not be transmitted to offspring.
What are the MOST common types of molecular changes that underlie epigenetic effects on gene expression?
- DNA methylation
- Chromatin remodeling
- Covalent histone modification
- Localization of histone variants
These types of changes are also involved in transient (nonepigenetic) gene regulation that is not transmitted from cell to cell.
The term which describes changes in gene expression that are not related to variations of the DNA sequence, yet are transmissible and are reversible, is __________
epigenetics or epigenetic
True or false: An epigenetic effect causes a change in gene expression without altering the sequence of DNA.
True false question.
True
False
True
Which of the following statements best describes epigenetic changes during the life of an individual?
Multiple choice question.
Some epigenetic changes are permanent while others are reversible.
All epigenetic changes are permanent and irreversible.
All epigenetic changes are transient and reversible.
Some epigenetic changes are permanent while others are reversible.
The molecular mechanisms of epigenetics are varied. The most common types are: DNA ________, chromatin ________, covalent ________ modification, and localization of histone variants.
Methylation, remodeling, and histone
Mendelian inheritance is characterized by three general rules: 1. Except in the case of rare _______, genes are passed unaltered from generation to generation; 2. Each gene obeys Mendel’s law of __________; and 3. During crosses, two or more genes obey Mendel’s law of independent __________-.
Mutations, segregation, assortment
What type of inheritance describes genomic imprinting?
Multiple choice question.
Mendelian
Extranuclear
Epigenetic
Mitochondrial
Epistatic
Epigenetic
Select all that apply
Which of the following statements are true regarding epigenetics?
Multiple select question.
Epigenetic changes may be transmitted to offspring.
Epigenetic changes are transmissible from cell to cell.
Variations of gene expression that result from DNA mutations
Variations are reversible from one generation to the next.
Variations of gene expression that are unrelated to DNA sequence variants
Epigenetic changes may be transmitted to offspring.
Epigenetic changes are transmissible from cell to cell.
Variations are reversible from one generation to the next.
Variations of gene expression that are unrelated to DNA sequence variants
Which of the following characterize an epigenetic effect?
Multiple select question.
The change must be passed from cell to cell
Leads to changes in the DNA sequence
Has no effect on gene expression
Causes a change in gene expression
The change must be passed from cell to cell
Causes a change in gene expression
[Information on Chromatin Modifications]
DNA methylation
Methyl groups may be attached to cytosine bases in DNA. When this occurs near promoters, transcription is usually repressed.
Chromatin remodeling
Nucleosomes may be moved to new locations or evicted. When such changes occur in the vicinity of promoters, the level of transcription may be altered. Also, larger-scale changes in chromatin structure may occur, such as those that happen during X-chromosome inactivation in female mammals, discussed later in this chapter.
Covalent histone modification
Specific amino acid side chains in the amino terminal tails of histones can be covalently modified. For example, they can be acetylated or phosphorylated. Such modifications may repress or activate transcription.
Localization of histone variants
Histone variants may become localized to specific locations, such as near the promoters of genes, and affect transcription.
None
[18.2 Epigenetics I: Genomic Imprinting]
What does the term imprinting implie?
A type of marking process that has memory.
For example, newly hatched birds identify marks on their parents, which allows them to distinguish their parents from other individuals.
What is the phenomenon in which a segment of DNA is imprinted, or marked, during egg or sperm formation in a way that affects gene expression throughout the life of the individual who inherits that DNA?
Genomic imprinting
However, depending on the gene, it may be marked by females during egg formation or by males during sperm formation, but not both. This marking process, which involves epigenetic modifications, affects whether or not the gene is expressed in the offspring.
What is dependent on how a particular gene is marked by one of the parents?
The offspring expresses either the maternal or the parental allele, but not both.
Does imprinting follow the Mendelian pattern of inheritance?
No, imprinted genes do not follow a Mendelian pattern of inheritance because imprinting causes the offspring to distinguish between maternal and paternal alleles.
Give some information on the first imprinted gene that was identified.
- Gene called Igf2, found in mice and other mammals.
- This gene encodes a growth hormone called insulin-like growth factor 2 (Igf2), which is needed for proper growth
- If a functional copy of this gene is NOT expressed, a mouse will be a dwarf.
[Remember] Mice are diploid meaning they have two copies of this gene, one from each parent.
How is a mutant allele, using Igf2 as an example, designated?
Igf2- (with a minus sign)
[Mice interaction with Lgf2- gene]
When a homozygous Igf2 Igf2 mouse and a homozygous Igf2−Igf2− mouse are crossed, the results are surprising.
If the female parent is homozygous for the mutant allele and the male parent is homozygous for the functional allele, all of the offspring grow to a normal size.
By contrast, if the female is homozygous for the functional allele and the male is homozygous for the mutant allele (right side), all of the offspring are dwarf.
Why is this surprising and give an explanation?
The mice have the same genotype yet they have different phenotypes!
In mice, the Igf2 gene is imprinted in such a way that only the paternal allele is expressed, which means it is transcribed into mRNA.
This breaks the laws of Mendelian inheritance.
If you crossed an Igf2 Igf2− male mouse to an Igf2 Igf2− female mouse, what would be the expected results?
Only the allele inherited from the father would be expressed in the offspring. Because he is heterozygous, half of the offspring would be normal size and half would be dwarf.
Why is the maternal gene encoding Igf2 not transcribed into mRNA?
DNA methylation, which is the attachment of methyl (—CH3) groups to bases of DNA, can alter gene transcription.
Researchers have discovered that DNA methylation is the marking process that occurs during the imprinting of certain genes, including the Igf2 gene.
For most genes, DNA methylation silences gene expression by inhibiting the initiation of transcription or by causing the chromatin in a region to become more compact. By contrast, for a few imprinted genes, DNA methylation may enhance gene expression by attracting activator proteins to the promoter or by preventing the binding of repressor proteins
True or false: As a result of genomic imprinting, individuals with the same genotype may have different phenotypes.
True false question.
True
False
True
True or false: Epigenetic changes that occur in an individual are permanent during its lifetime.
True false question.
True
False
False
The gene Igf2 encodes a(n) ______.
Multiple choice question.
growth hormone
transcription factor
enzyme
neurotransmitter
growth hormone
Select all that apply
Which of the following are common types of molecular changes that result in epigenetic effects on gene expression?
Multiple select question.
Covalent histone modification
DNA methylation
Localization of histone variants
DNA mutation
DNA translocation
Chromatin remodeling
Covalent histone modification
DNA methylation
Localization of histone variants
Chromatin remodeling
What does the expression of the Igf2 gene depend on in mice?
Multiple choice question.
Alleles inherited from both parents, with only the dominant gene expressed
Parental origin, with only the maternal gene expressed
Parental origin, with only the paternal gene expressed
Alleles inherited from both parents, with only the mutant gene expressed
Parental origin, with only the paternal gene expressed
Genomic imprinting is an example of a phenomenon called _________ inheritance.
epigenetic
Imprinted genes do not follow a Mendelian inheritance pattern because ______.
Multiple choice question.
the DNA sequence is altered in each generation
the alleles from the parents are expressed differently
individuals with imprinted genes are mosaics with different expression patterns in different somatic cells
the alleles from the parents are expressed differently
In mice, the Igf2 gene inherited from the ______ is not transcribed into mRNA because the DNA is ______.
Multiple choice question.
mother; methylated
father; methylated
mother; mutated
father; mutated
mother; methylated
Individuals may have the same genotype but very different phenotypes as a result of ______.
Multiple choice question.
polygenic inheritance
genetic linkage
genomic imprinting
epistasis
genomic imprinting
The marking process that occurs during genomic imprinting is the addition of ___________ groups to DNA.
methyl, CH3, or -CH3
One of the first imprinted genes to be identified was a gene in mice called Igf2. This gene encodes a growth hormone called ______-like growth ________ 2 (lgf2).
insulin, factor
The transcription of a gene can be altered by the attachment of _________ groups to the bases of DNA.
methyl, CH3, or -CH3
In mice, the Igf2 gene is imprinted in such a way that only the _________ allele is expressed.
paternal
Select all that apply
How does DNA methylation inhibit gene expression?
Multiple select question.
It attracts activator proteins to the promoter.
It causes mutations in the gene.
It prevents the initiation of transcription.
It prevents binding of repressor proteins.
It causes chromatin to become more compact.
It prevents the initiation of transcription.
It causes chromatin to become more compact.
Why don’t imprinted genes follow a Mendelian pattern of inheritance?
Multiple choice question.
Individuals may have the same genotype but different phenotypes because the offspring expresses only the maternal or paternal allele.
Imprinted genes are inherited only maternally because they are found on the mitochondrial genome.
The genes are linked on the same chromosome, violating Mendel’s law of independent assortment.
The genes do not separate during meiosis, violating Mendel’s law of segregation.
Individuals may have the same genotype but different phenotypes because the offspring expresses only the maternal or paternal allele.
How is the maternal version of the Igf2 gene silenced in mice?
Multiple choice question.
Methylation is erased from the gene, inhibiting transcription.
Methyl groups are attached to the gene, inhibiting transcription.
Compaction of one of the X chromosomes into a Barr body silences the gene.
The gene is found in mitochondria, which are only inherited from the father.
Methyl groups are attached to the gene, inhibiting transcription.
[18.3 Epigenetics II: X-Chromosome Inactivation]
What is a process that causes an X chromosome to become highly compacted and silences the genes that it carries?
X-chromosome inactivation (XCI)
Female mammals carry two X chromosomes in their cells, whereas males carry one X and one Y.
During embryonic development in female mammals, one of the X chromosomes undergoes an epigenetic change called X-chromosome inactivation (XCI). This process causes that X chromosome to become highly compacted, which silences the genes that it carries.
What were the two lines of evidence to indicate XCI?
First came from microscopic studies of mammalian cells.
In 1949, Canadian physicians Murray Barr and Ewart Bertram identified a highly condensed structure in the cells of female cats that was not found in the cells of male cats.
This structure was named a Barr body after one of its discoverers.
In 1960, Asian-American geneticist Susumu Ohno correctly proposed that a Barr body is a highly condensed X chromosome.
Lyon’s second line of evidence was the inheritance pattern of variegated coat colors in certain female mammals. A classic case is the calico cat, which has randomly distributed patches of black and orange fur.
What is a highly condensed X chromosome present in the cells of female mammals?
Barr body
How is the Barr body different from the other X chromosome in this cell?
The Barr body is much more compact than the other X chromosome in the cell. This compaction prevents most of the genes on the Barr body from being expressed.
How do we explain this patchwork phenotype in the calico cat?
According to Lyon’s hypothesis, the calico pattern is due to the permanent inactivation of one X chromosome in each cell that forms a patch of the cat’s skin.
The gene involved is an X-linked gene that occurs as an orange allele, XO, and a black allele, XB. A female cat heterozygous for this gene will be calico. (The cat’s white underside is due to a dominant allele of a different autosomal gene.)
At an early stage of embryonic development, one of the two X chromosomes is randomly inactivated in each of the cat’s somatic cells, including those that will give rise to the hair-producing skin cells. As the embryo grows and matures, the pattern of XCI is maintained during subsequent cell divisions.
Skin cells derived from a single embryonic cell in which the XB-carrying chromosome has been inactivated produce a patch of orange fur, because they express only the XO allele that is carried on the active chromosome. Alternatively, a group of skin cells in which the chromosome carrying XO has been inactivated express only the XB allele, producing a patch of black fur.
If female mammals are heterozygous for X-linked genes, approximately half of their somatic cells express one allele, whereas the rest of their somatic cells express the other allele. The result is an animal with randomly distributed patches of black and orange fur. These heterozygotes are called mosaics because they have somatic regions that are composed of two types of cells.
Addition of Information (Further explanation)
The calico pattern is due to random XCI in a female that is heterozygous for an X-linked gene with black (XB) and orange (XO) alleles. The cells at the top of this figure represent a small mass of cells making up the very early embryo. In these cells, both X chromosomes are active. At an early stage of embryonic development, one X chromosome is randomly inactivated in each cell. The initial inactivation pattern is an epigenetic change that is maintained in the descendants of each cell as the embryo matures into an adult. The pattern of orange and black fur in the adult cat reflects the pattern of XCI in the embryo.
For x-linked traits in humans, what will females who are heterozygous for recessive x-linked alleles usually show?
The dominant trait because the expression of the dominant allele in 50% of their cells is sufficient to produce the dominant phenotype.
How is it possible for a female who is heterozygous may show mild or even severe hemophilia symptoms when half of the cells should be expressing enough factor VIII for proper blood clotting?
X-chromosome inactivation in humans occurs when an embryo is 10 days old. At this stage, the liver contains only about a dozen cells. In most females who are heterozygous for the dominant and recessive (hemophilia-causing) alleles, roughly half of their liver cells express the dominant allele.
However, on rare occasions, all or most of the dozen embryonic liver cells may inactivate the X chromosome carrying the dominant functional allele. Following growth and development, such a female will have a very low level of factor VIII and as a result will show symptoms of hemophilia.
What is the phenomenon in which the expression of X-linked genes is equalized between males and females; in mammals, the inactivation of one X chromosome in the female reduces the number of expressed copies (doses) of X-linked genes from two to one?
Dosage compensation
