Chapter 5_Non-Mendelian Inheritance

Question 1

Maternal effect

Answer 1

• An inheritance pattern for certain nuclear genes (genes located on chromosomes that are found in the cell nucleus) in which the genotype of the mother directly determines the phenotype of her offspring.
• Surprisingly, the genotypes of the father and offspring themselves do not affect the phenotype of the offspring.

Question 2

How is the phenomenon maternal effect explained in simplest terms?

Answer 2

Accumulation of gene products that the mother provides to her developing eggs.

Question 3

Reciprocal Cross

Answer 3

Genotypes of the parents are flipped and then crossed.

Question 4

Explain the maternal effect on a molecular and cellular level.

Answer 4

• Can be explained by the process of oogenesis in female animals.
• As an animal egg matures, many surrounding maternal cells called nurse cells provide the egg with nutrients and other materials.
• Depending on meiosis, in a heterozygous female, its offspring may receive the D or d allele, but not buth.
• However, the surrounding nurse cells produce both D and d gene products (mRNA and proteins).
• These gene products are then transported to the egg.
• These gene products persist for a significant time after the egg has been fertilized and begins its embryonic development.

Question 5

Sum up how nurse cells affect the maternal effect.

Answer 5

The gene products of the nurse cells, which reflect the genotype of the mother, influence the early developmental stages of the embryo.

Question 6

Now apply this nurse cell shit to the actual developmental of shell pattern in snails.

Answer 6

• A female snail that is DD transmits only the D gene products to the egg. During the early stages of embryonic development, these gene products cause the egg cleavage to occur in a way that promotes a right-handed body plan.
• A heterozygous female transmits both D and d gene products. Because the D allele is dominant, the maternal effect also causes a right-handed body plan.
• A dd mother contributes only d gene products that promote a left-handed body plan, even if the egg is fertilized by a sperm carrying a D allele. The sperm’s genotype is irrelevant, because the expression of the sperm’s gene would occur too late.

Question 7

Explain how what the origin of dextral and sinistral coiling can be traced to.

Answer 7

Can be traced to the orientation of the mitotic spindle at the two to four cell stage of embryonic development. The dextral and sinistral snails develop as mirror images of each other.

Question 8

Researchers have found that maternal effect genes encode proteins that are important in the early steps of embryogenesis. The accumulation of maternal gene products in the egg allows embryogenesis to…

Answer 8

…proceed quickly after fertilization.

Question 9

• Maternal effect genes often play a role in…

- Therefore, defective alleles in maternal effect genes…

Answer 9

…cell division, cleavage pattern, and body axis orientation.
…tend to have a dramatic effect on the phenotype of the individual, altering major features of morphology, often with dire consequences.

Question 10

Epigenetic inheritance

Answer 10

A pattern in which a modification occurs to a nuclear gene or chromosome that alters gene expression, but is not permanent over the course of many generations.

Question 11

Epigenetic inheritance patterns are the result of…

Answer 11

…DNA and chromosomal modifications that occur during oogenesis, spermatogenesis, or early stages of embryogenesis.

Question 12

Can epigenetic changes permanently affect phenotype of an individual?

Answer 12

Yes, once they are initiated during early stages, epigenetic changes alter the expression of particular genes in a way that may be fixed during an individual’s lifetime.

Question 13

Do epigenetic changes change DNA sequence?

Answer 13

No, and they are not permanent over the course of many generations. For example, a gene may undergo an epigenetic change that inactivates it for the lifetime of an individual. However, when this individual makes gametes, the gene may become activated and remain operative during the lifetime of an offspring who inherits the active gene.

Question 14

Dosage compensation

Answer 14

The level of expression of many genes on the sex chromosomes (such as the X chromosome) is similar in both sexes even though males and females have a different complement of sex chromosomes.

Question 15

X inactivation

Answer 15

Female mammals equalize the expression of X-linked genes by turning off one of their two X chromosomes. This is how most female mammals apply dosage compensation.

Question 16

How do males accomplish dosage compensation if they have an x-linked gene?

Answer 16

Doubling the expression of most X-linked genes.

Question 17

Barr body

Answer 17

Highly condensed X chromsome.

Question 18

The Lyon Hypothesis

Answer 18

The mechanism of X inactivation.

Question 19

X-inactivation center (Xic)

Answer 19

A short region on the X chromosome that is known to play a critical role in the genetic control of inactivation at the molecular level.

Question 20

What is a lethal condition of a human female embryo?

Answer 20

Having two active X chromosomes.

Question 21

Xist vs. Tsix gene

Answer 21

The Xist gene is found in the inactivated X chromosome and is still active. It coats the X chromosome and inactivates it. Tsix prevents X inactivation. It inhibits the transcription of the Xist gene. It is expressed on the active X chromosome.

Question 22

X chromosomal controlling element (Xce)

Answer 22

Also affects the choice of the X chromosome to be inactivated. It makes the X inactivation skewed (nonrandom) because an X chromosome that carries a strong Xce is more likely to remain active.

Question 23

Describe the three phases of X inactivation.

Answer 23

• Initiation: 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 towarde both ends until the entire chromosome is inactivated. The Xist gene encodes an RNA that coats the X chromosome and recruits proteins that promote its compaction into a Barr body.
• Maintenance: Occurs from embryonic development through adult life. The inactivated X chromosome is maintained as such during subsequent cell divisions.

Question 24

Genomic Imprinting

Answer 24

An analogous situation in which a segment of DNA is marked, and that mark is retained and recognized throughout the life of the organism inheriting the marked DNA.

Question 25

Monoallelic expression

Answer 25

Depending on how the genes are marked, the offspring expresses only one of the two alleles.

Question 26

What are the three stages of genomic imprinting?

Answer 26

• The establishment of the imprint during gametogenesis
• The maintenance of the imprint during embryogensis and in adult somatic cells
• The erasure and reestablishment of the imprint in the germ cells.

Question 27

DNA methylation

Answer 27

The attachment of a methyl group onto a cytosine base

Question 28

Imprinting Control Region (ICR)

Answer 28

Located near the imprinted gene, is involved in genomic imprinting.

Question 29

What is the differentially methylated domain? (DMD)

Answer 29

A portion of the DNA in the imprinted gene. Depending on the particular gene, the DMD is methylated in the egg or the sperm, but not both.

Question 30

Does methylation in imprinted genes inhibit or express gene expression?

Answer 30

For most, it inhibits gene expression.

Question 31

Describe the imprinted relationship of H19 and Igf2

Answer 31

• When the ICR is unmethylated, a protein called CTC-binding factor (cytosine thymine cytosine) binds to the ICR. This prevents the binding of activator proteins to the Igf2 gene, thereby shutting off this gene. In contrast, it permits activator proteins to turn on the H19 gene.
• When the ICR is highly methylated, CTC binding factor is unable to bind to the ICR. This permits activator protein o turn on the Igf2 gene. The DNA methylation also causes the repression of the H19 gene so it is not transcribed.

Question 32

When a maternal chromosome is normal but the paternal chromosome is imprinted, describe which offspring will have methylation or none.

Answer 32

• In female cells, imprinting is erased during early oogenesis.
• In the male, the imprinting is also erased during early spermatogenesis, but then de novo (new) methylation occurs in both ICRs. Therefore, the male will transmit a methylated gene to its offspring.

Question 33

What are some human diseases that are linked to imprinting.

Answer 33

• Prader Willi Syndrome (PWS): Characterized by reduced motor function, obesity, and small hands and feet.
• Angelman Syndrome (AS): Thin and hyperactive, have unusual seizures and repetitive symmetrical muscle movements, and exhibit mental deficiencies.
  Both involve a small deletion in human chromosome 15. If the deletion is inherited from the mother, it leads to Angelman syndrome; if inherited from the father, it leads to Prader Willi syndrome.

Question 34

Extranuclear (cytoplasmic) inheritance

Answer 34

Non Mendelian inheritance patterns that involve genes not located in the cell nucleus. The mitochondria and chloroplasts are the main examples.

Question 35

Mitochondrial DNA (mtDNA)

Answer 35

• Consists of a circular DNA molecule that is only 17,000 bp in length. (less than 1% of a typical bacterial chromosome)
• Human mtDNA carries relatively few genes.
• Carries genes that encode ribosomal RNA and transfer RNA. These are necessary for the synthesis of the 13 polypeptides that are encoded by the mtDNA.
• These 13 polypeptides are subunits of proteins that function in a process known as oxidative phosphorylation, in which mitochondria use oxygen and synthesize ATP.

Question 36

Chloroplast DNA (cpDNA)

Answer 36

• Tend to be larger than mitochondrial genomes, and they have a correspondingly greater number of genes. (100,000 to 200,000 bp, so about 10 times larger than mitochondrial genome of animal cells).
• These genes encode ribosomal RNAs, transfer RNAs, and many proteins required for photosynthesis.

Question 37

Maternal Inheritance

Answer 37

A type of extranuclear inheritance. The item in question is only inherited through the cytoplasm of the egg. The mother is the one who contributes the cytoplasm (which is from the egg) to the new offspring.

Question 38

Heteroplasmy

Answer 38

A cell may contain both types of chloroplasts.

Question 39

Whya re most extranuclear inheritance genes inherited through the mother cell?

Answer 39

The maternal cell is much larger, and contributes most of the cell, including the cytoplasm. The paternal cell at most contributes the nucleus.

Question 40

Paternal leakage

Answer 40

Paternal parent may occasionally provide mitochondria via the sperm.

Question 41

Mitochondrial diseases can occur in two ways.

Answer 41

• Diseases are transmitted from mother to offspring.
• May occur in somatic cells and accumulate as a person ages. Mitochondria are particularly susceptible to DNA damage. When more oxygen is consumed than is actually used to make ATP, mitochondria tend to produce free radicals that damage DNA. Unlike nuclear DNA, mitochondrial DNA has very limited repair abilities and almost no protective ability against free radical damage.

Question 42

Explain why heteroplasmy is an important factor in mitochondrial disease.

Answer 42

Within a single cell, some mitochondria may carry a disease causing mutation whereas others may not. As cells divide, mutant and normal randomly segregate into the resulting daughter cells. To cause disease that affects a particular cell or tissue, the ratio of mutant to normal mitochondria must exceed a certain threshold value before disease symptoms are observed.

Question 43

Endosymbiosis

Answer 43

A symbiotic relationship in which the symbiont actually lives inside the host.

Question 44

Endosymbiosis Theory

Answer 44

Chloroplasts and mitochondria were descended from an endosymbiotic relationship between cyanobacteria and eukaryotic cells.

Question 45

Why would have endosymbiosis in eukaryotic organisms a long time ago been useful?

Answer 45

• Chloroplasts were derived from cyanobacteria, a bacterial species that is capable of photosyntehsis. The ability to carry out photosynthesis enabled algal and plant cells to use the energy from sunlight.
• Mitochondria are thought to have been derived from a different type of bacteria known as gram negative nonsulfur purple bacteria. They were enabled to synthesize greater amounts of ATP in eukaryotic cells.