Genetics Flashcards
What is an allele?
A certain version of a gene
Define Mendel’s Law of Segregation
Alleles segregate (at meiosis) into the gametes
Define Mendel’s Law of Independent Assortment
The segregation of each pair of alleles is independent
Define genotype
The DNA sequence (alleles) at a particular locus
Define homozygous genotype
2 identical alleles at a given locus
Define heterozygous genotype
2 different alleles at a given locus
Define hemizygous genotype
Refers mostly to males (XY), who have just a single copy of each X-chromosomal gene
Define phenotype
The observed/measured trait
Define dominance
A phenotype that is expressed (observed) in the heterozygous state
Define recessive
A phenotype that is expressed only in homozygotes or hemizygotes
Define semi-dominant
When the heterozygous phenotype is intermediate between the two homozygous phenotypes
Define penetrance
Some with a mutation can still be unaffected; ex. 80% penetrance indicates that 80% with the mutation are affected; Used with dominant and recessive mutations
Define expressivity
Severity of phenotype in those who Are affected
Define pleiotropy
Genes that have multiple effects in body; polysystemic versus monosystemic
What are three threats to Mendelian inheritance?
Penetrance, expressivity, & pleiotropy
Define environmental factors
Factors that are not genetic that affect the expression of Mendelian diseases
Define modifier genes
Genetic factors that influence a phenotype
Define stochastic effects
Stochastic (random) effects can influence the expression of phenotypes
Define phenocopy
Same phenotype as a genetic condition due to non-genetic factors
Define proband
The affected member through whom a family with a genetic disorder is brought to attention
Define consultand
The person who brings the family to attention (affected or unaffected)
Define locus
Segment of DNA occupying a particular position/location on a chromosome
Define compound heterozygote
Two recessive mutant alleles, but they’re different
What is the difference between autosomal and sex-linked?
Autosomal affects males and females equally. Sex-linked means the mutation is on X chromosome
Define allelic heterogeneity
A single disorder, trait, or pattern of traits caused by different mutations within a gene
Define locus heterogeneity
A single disorder, trait, or pattern of traits caused by mutations in genes at different chromosomal loci
Define phenotypic heterogeneity
When different mutations in same gene cause different phenotypes
What are “threats” to Mendelian genetics?
Factors or characteristics of mutations that discourage phenotypes from following Mendelian rules or expectations.
What is the Mendel’s First Law of Inheritance?
The Principle of Segregation: A gene pair separates, sending one allele to one gamete and the other to another gamete
What is Mendel’s Second Law of Inheritance?
The Principle of Independent Assortment: Genes for different traits assort independently of one another in the formation of gametes
How many pairs of chromosomes make up the human genome?
23 pairs (one of which is a sex pair of chromosomes)
What is one chromosome made up of?
One single continuous double-helixed DNA strand
What is a typical gene? A retroposed genes?
A typical gene is intron-containing, while a retroposed gene is intronless
About what is the size range of chromosomes?
~50,000,000 bp - 250,000,0000 bp
What is a genetic map vs. a physical map in genomics?
Both maps are a collection of genetic markers and gene loci, but genetic map distances are based on the genetic linkage information measured in Centi-Morgans, while physical maps use actual physical distances measured in base pairs.
What is the Human Genome Sequence a record of?
Human evolutionary history
What is the average propensity of single nucleotide polymorphisms (SNP) in humans?
1 SNP every 1000 bp between any two randomly chose unrelated human genomes
What are the three gene-poor human chromosomes?
13, 18, 21 (viable)
Why are chromosomal banding patterns possible?
There is clustering (non-random distribution) of GC-rich and AT-rich regions
What are the two methods for sequencing the human genome?
- Make clone libraries of genome, map them out, then sequence overlapping clones
- Shotgun sequencing: break apart entire genome, sequence, then assemble it
Does genome sequencing sequence euchromatic or heterochromatic regions?
Euchromatic regions (accessible - heterochromatic regions usually just contain copies of genes)
What are DNA tandem repeats? aka “satellite DNAs”
Short sequences of DNA (2-5bps) that are repeated numerous times in a head-tail manner. Used as the basis for cytogenetic banding. Hotspots for human-specific evolutionary changes.
What are DNA alpha-satellite repeats?
A 171 bp repeat unit found near centromeric regions of all human chromosomes; may be important to chromosome segregation in mitosis & meiosis
What is non-allelic homologous recombination (NAHR)?
NAHR is a form of homologous recombination that occurs between two lengths of DNA that have high sequence similarity, but are not alleles.
What is retrotransposition?
The insertion of DNA sequences mediated by an RNA intermediate. In humans, carried out through reverse-transcriptase.
How do we measure changing number of copies?
Array CGH (comparative genomic hybridization)
What is a ‘gene family’ composed of?
Composed of genes with high sequence similarity that may carry out similar but distinct functions.
How are gene families the major mechanism behind evolutionary change?
When a gene duplicates it frees up one copy to vary while the other can continue carrying out its critical function
For what percentage of genes do we not know specific molecular function?
41%
What is the primary type of genome structural variation?
Copy number variations
What do duplications of the DUF1220 indicate?
Macrocephaly & autism
What do deletions of the DUF1220 indicate?
Microcephaly & schizophrenia
What is indicated by ‘gene-rich’ and ‘gene-poor?’
A gene-rich region of DNA has multiple genes within in. A gene-poor region… self-explanatory.
What is the missing heritability problem?
Studies that look for genetic factors in complex diseases only truly discover a small percentage of factors. Many genetic contributions are yet to be discovered and sequenced.
Chromosomal abnormalities occuring during meiosis can result in what kinds of mutations?
germ line mutations
Chromosomal abnormalities occuring during mitosis can result in what kinds of mutations?
somatic mutations, germ line mutations - results in mosaicism
Define mosaicism
Cells with more than one chromosomal composition - not all cells are identical when they should be
What are the basic steps of meiosis and what is the haploid state at each step?
2N cell undergoes DNA replication -> 4N -> Homologs pair and separate -> 2N -> another round of segregation to result in 4 haploid (N) gamete cells
How are X and Y chromosomes paired up for meiosis when they are not identical homologs?
There are regions called pseudoautosomal regions that share homology so that they can be properly aligned.
Define epigenetics
Expression of genes from characteristics other than DNA sequences
What on the DNA can be methylated?
CpG sites; sites where a cytosine and guanine are separated by only one phosphate, cyosine is 5’ to guanine; cytosine methylated at 5’ position
Describe Waddington’s Landscape as Epigenetic Visualization
A ball (ex. pleuripotent stem cell) sits at top of hill. It waits for signals so that it can decide which route to roll down for different gene expression.
Non-disjunction during Meiosis I results in what?
100% abnormal gametes
Non-disjunction during Meiosis II results in what?
50% abnormal gametes
How are chromosomes visualized? (striped)
Giemsa Staining; GC do not stain (Giemsa-light) and AT stain (Giemsa-dark)
What are the three chromosomal structural classifications?
Metacentric, submetacentric, acocentric
In what order are karyotypes listed?
Number of chromosomes; gender; findings
Define mosaicism
Two or more chromosome complements present within a single individual; commonly caused by nondisjunction in an early post-zygotic mitotic division
What is polyploid mosaicism?
Diploid/Triploid