Genetics

Question 1

gene

Answer 1

determinants of a particular characteristic

- can be heterozygous or homozygous

Question 2

allele

Answer 2

variant form of a gene

Question 3

genotype

Answer 3

genetic constitution summing all of the alleles

Question 4

phenotype

Answer 4

observed form of a character of an organism

Question 5

Mendel’s Experiments

Answer 5

• proposed rules to predict types of progeny and proportions of each type knowing information about the parents
• single gene model with equal segregation of alleles

Question 6

DNA during cell division

Answer 6

• meiosis separates homologous chromosomes
• sister chromatids separate and migrate to opposite poles (mitosis)
• meiosis gives cells with single versions of each allele
• each gamete has one version of an allele

Question 7

Dihybrid Ratios

Answer 7

• Mendel looked at two genes segregating in the pea hybrids

- independent assortment of alleles shows each possible fertilisation event is independent of each other

Question 8

Wild-type and mutant alleles

Answer 8

wild-type: more frequent allele

if there is no wild type allele the gene would’ve been polymorphic

Question 9

Sex-linked inheritance

Answer 9

• females have homomorphic pair (X,X)
• males have heteromorphic pair (X,Y)
  Sex linked traits more often found in males and are carried on the X chromosome

Question 10

Lethal alleles

Answer 10

• disrupt 3:1 ratio
  eg. agouti mice:
• YY allele is lethal, but Yy is not
• therefore 2:1 ratio (Yy:yy) is produced indicating a lethal allele is present

Question 11

Environmental influence on phenotypes

Answer 11

• Phenylketonuria (PKU) - rare recessive allele
  homozygotes have severe mental retardation
• Codes for Phenylalanine Hydroxylase
• Affected individuals can’t convert Phe to Tyr
• Phe accumulates in cerebrospinal fluid.
• Changing diet (environment) of PKU individuals at birth (to low protein) allows them to be virtually normal (wild type)

Question 12

Departures from Mendelian Ratios

Answer 12

• if more than one gene affects the same characteristic, novel phenotypes and unusual ratios will appear
• this is still the Mendelian dihybrid ratio, but with differing phenotypes:
  9 doubly dominant phenotype
  3 dominant first recessive second gene
  3 recessive first dominant second gene
  1 doubly recessive

Question 13

Genotype Masking

Answer 13

recessive epistasis: the expression of one gene pair is dependent on another gene pair

eg. laboradors
- recessive ee genotype (yellow) masks expression of any other genotypes (B)

Question 14

Gene Linkage + Crossing Over

Answer 14

• genes on the same chromosome are usually inherited together
• causes differing allele ratios
• crossing over is the breaking and rejoining of homologous chromosomes in the same position to generate recombinants

Question 15

Genetic Maps

Answer 15

• The phenomenon of recombination or genetic exchange allows for the construction of genetic maps
• Genes are placed on the map as a result of linkage to other genes whose position is already known
• The map distance is the unit of recombination - 1% of recombination frequency or 1cM

Question 16

DNA based markers for genetic mapping

Answer 16

• Since there are so few genetic markers in humans, use DNA sequence variation to detect molecular alleles
• Forms the basis of the genetic map of the human genome
• This was a necessary pre-requisite to whole genome sequencing since the genetic markers acted as landmarks for the sequencing

Question 17

Restriction Fragment Length Polymorphism

Answer 17

1. digest DNA with restriction enzymes
2. fragments separated by electrophoresis and visualised with Southern blotting
3. hybridization with DNA probe determines lengths of complementary fragments

• RFLPs occur when the length of a detected fragment varies between individuals indicting non identical sequence homologues

Question 18

Cystic Fibrosis Gene Cloning ***

Answer 18

• DNA based markers have allowed for the identification of disease genes such as the cystic fibrosis transmembrane conductance regulator (CFTR)
• Finding several closely linked markers eventually allowed the gene to be identified by positional cloning
• identification based on proximity to known markers
• presence of site: small fragment because of cut and vice versa
• used KM19 locus that was a restriction site and is very close to CFTR gene

Question 19

Positional Cloning ***

Answer 19

Forms overlapping clones and identifies genes by its position in the genome

• take probe (identifies difference between 2 genotypes) and identify clone place of DNA by hybridizing to genomic libraries
• form overlapping clones to walk along genes

Question 20

Linkage Disequilibrium

Answer 20

• nonrandom association between 2 loci
• groups of alleles of closely linked genes remain stick together = disequilibrium
• haplotype: tightly linked allele groups
• if you know the allele of one locus in a haplotype you can predict other alleles in this locus

Question 21

Gene Association Studies

Answer 21

• compare SNPs of case studies against controls
• look for SNP alleles that associate with gene of interest
• variants will be more present in those with the trait
• this defines a region of the genome that may contain a candidate gene
• identifies associations between loci and traits