Genetics

Question 1

What is a karyotype?

Answer 1

The number and appearance of all the chromosomes in a cell. Usually presented as a picture of the mitotic (wound up into x-shape) chromosomes.

Question 2

How big does a mutation have to be in order to be seen on a karyotype?

Answer 2

At least 4 mb (megabases)

Question 3

What is an ideogram?

Answer 3

A diagram of the banding pattern of a chromosome

Question 4

What are the parts of a chromosome?

Answer 4

Long arm (q)
Short arm (p for petite)
Centromere

Question 5

How many pairs of autosomes and how many sex chromosomes do humans have?

Answer 5

22 pairs of autosomes and two sex chromosomes

Question 6

How do the medical and general usage of the word “gene” differ?

Answer 6

General: any functional unit of a chromosome
Medical: strictly the protein-coding sequences

Question 7

What is a locus?

Answer 7

The exact physical location of a gene on a chromosome, its address.
Includes the chromosome number, long or short arm (p or q) and the number giving its position.

Question 8

What are alleles?

Answer 8

Variants of a gene that are present in the population. They’re the different options for that gene.

For example, in sickle cell disease you have a normal allele and the mutated allele that produces the disease, so there are two alleles for that gene in the population.

Question 9

What is the difference between genotype and phenotype?

Answer 9

Genotype: which alleles you have for a particular trait

Phenotype: the observable trait

Example: I have Factor V Leiden. For the gene Factor V, I have a heterozygous genotype (one dominant allele, one recessive). My phenotype is a propensity to develop blood clots.

Question 10

What is a SNP?

Answer 10

Single Nucleotide Polymorphism
A single nucleotide difference between two homologous sequences of DNA.

Example: I have Factor V Leiden, a mutation in the gene for clotting factor V. The difference between the mutated gene and the normal gene is a SNP.

Question 11

List the ways autosomal dominant and autosomal recessive modes of inheritance differ.

Answer 11

Autosomal means the locus is on an autosome

Dominant: Only one copy of the abnormal allele is needed in order for the trait to be expressed (heterozygous genotype produces the phenotype)
Every generation is affected.

Recessive: Two copies of the abnormal allele needed for expression (homozygous recessive genotype produces the phenotype)
Every OTHER generation is affected.

Question 12

What are the five points of control for gene expression?

memorize for boards

Answer 12

Chromatin stage (unwinding the DNA)
Transcriptional stage
Translational stage
Post translational control into cytoplasm
Post translational modification

Question 13

Which parent do you inherit mitochondrial DNA from?

Answer 13

Mom

Question 14

What are the genetic principles of pleiotropy, aneuploidy, and anticipation?

Answer 14

Pleiotropy: A single disease-causing mutation affects multiple organ systems. Common feature of genetic diseases. Ex: Marfan syndrome, makes fibrillin stretchy.

Aneuploidy: Having an abnormal number of chromosomes (either extra copies or missing copies). All autosomal aneuploidies are inconsistent with life except trisomy 21 (Down syndrome)

Anticipation: the propensity of later generations with a genetic disease to develop the disease earlier in life than prior generations. This is due to extra copies of the gene (repeats) inserted in the chromosome.

Question 15

For X-linked recessive inheritance, who is affected and who is a carrier?

Answer 15

In X-linked inheritance, the gene of interest is located on the the X chromosome.

Women carry the disease but usually do not express it, because the copy with the mutation is silenced.

Men express the disease in all cases because there is no second X chromosome to mask the mutated one.

Additionally, all of affected males’ daughters will carry the mutation.

Question 16

Why are females more likely to get an X-linked dominant disease than males?

Answer 16

Females are twice as likely to get the disease because they have two X chromosomes–twice as many chances as males.

Question 17

What are the major features of Turner syndrome, and why is it called a mosaic disease?

Answer 17

X monosomy
Features: short stature, ovarian dysgenesis, neurocognitive problems
Mosaic: different populations of cells have different numbers of X chromosomes (one, two, or three). Think of the coloring of a calico cat.

Question 18

Why is fragile X syndrome called “fragile”? What are its major features?

Answer 18

Due to an unstable CGG repeat on the X chromosome which makes it breakable.

Broad forehead, elongated face, large ears
Crossed eyes, hyperextensible joints
self-abuse, indented chest, hypotonia, flat feet

Question 19

What is the difference between imprinting of the X chromosome and imprinting of autosomes?

Answer 19

Imprinting ensures that only one of the inherited alleles (the non-imprinted one) is ever expressed in the individual, and it’s dependent on the parent it comes from.
Imprinting on the X chromosome results in total silencing of an entire chromosome.
Imprinting on autosomes results in silencing of individual genes.
Silencing means that the gene is permanently turned off and will not be transcribed.

Question 20

Prader-Wili and Angelman syndrome are examples of this epigenetic disease process.

Answer 20

Microdeletions, specifically on chromosome 15.

These are examples of imprinting: Prader-Willi results from microdeletion on the paternal chromosome, Angelman syndrome from microdeletion on the maternal chromosome.

Question 21

Why does DNA naturally wrap around histones, and what epigenetic changes can occur to histone tails to encourage winding/unwinding?

Answer 21

DNA is negatively charged, histones are positively charged.
Histone methylation: may up- or downregulate transcription
Histone acetylation: upregulates transcription
DNA methylation: downregulates transcription

Question 22

Why should we consider sex chromosome aneuploidies as traits, rather than diseases?

Answer 22

They don’t result in dysfunction.

Question 23

What is a promoter region, and why are they generally not methylated?

Answer 23

The promoter is a region upstream of the gene where the transcription complex binds. They aren’t methylated because it results in gene silencing.

Question 24

What is a non-mutagen epigenetic carcinogen?

Answer 24

Compounds that result in increased incidence of tumors, though their effects result in epigenetic changes rather than genetic mutations.

Question 25

How do HAT-HDAC drugs work?

Answer 25

HAT and HDAC are chromatin remodelers that normally work together to control gene transcription.
Deregulated HAT and HDAC plays a role in the development of cancers, so inhibitors of these enzymes can work as anticancer agents.

Question 26

Are all genes possible proto-oncogenes?

Answer 26

No. Proto-oncogenes are typically involved in accelerating the cell cycle, so that when they become mutated they promote out of control replication.

Question 27

How do oncogenes and tumor suppressors differ? How many alleles need to be mutated in order to develop cancer?

Answer 27

Oncogenes normally stimulate the cell cycle. Only one mutated allele of the oncogene is needed to develop cancer, because the mutations result in gain of function, or more production of protein. (one-hit hypothesis)

Tumor suppressors inhibit the cell cycle or promote apoptosis and undergo loss-of-function mutations, creating no active proteins. Both alleles must be mutated in order to develop cancer. (two-hit hypothesis)