Module 6: Genes and Genetic Diseases

Question 1

Define: Gene

Answer 1

a sequence of DNA that encodes a protein

Question 2

Define: Codon

Answer 2

3 nucleotides that encode one amino acid

ex. AUG- start codon

UAA, UAG, UGA = stop codons

Question 3

Define: nucleotide

Answer 3

• molecules that contain a sugar ring (deoxyribose), an organ base (purine or pyrimidine), and a phosphate group (PO4)
• subunits of DNA, act like letters in a code
• 4 nucleotide bases in DNA:
  
  • A-adenine
  • G-guanine
  • T-thymine
  • C-cytosine

Question 4

Define: Chromosome

Answer 4

• a long DNA strand with some or all of the genetic material of an organism
• Humans have 23 TYPES of chromosomes
  
  • numbered 1-22, X and/or Y

Question 5

Somatic Cells and Chromosomes

Answer 5

Somatic Cells: majority of cells in the body

• contain 2 copies of the DNA in the nucleus
• They are diploid:
  
  • there are 2 of each type of chromosome
    
    • paired chromosomes are homologous chromosomes
      
      • encode the same genes but the information within them is slightly different depending on what is inherited
      • one inherited from mother and one from father
• Multiply via mitosis

Question 6

Homologous chromosomes

Answer 6

• Paired chromosomes, 2 of the same type of chromosome
• encode the same genes but the information within them is slightly different depending on what is inherited
  • one inherited from mother and one from father

Question 7

Reproductive Cells and Chromosomes

Answer 7

• form gametes (oocytes in females, spermatocytes in males)
• 1 copy of DNA in each mature gamete
  
  • haploid
    
    • 23 x 1 = 23 chromosomes
• multiply via meiosis I and meisosi II

Question 8

Define: Locus

Answer 8

• Location of a gene on a chromosome
  
  • ex. the gene for red hair is MCR1 gene and is located on chromosome 16

Question 9

Define: allele

Answer 9

• the specific form of the gene that is present on each chromosome
  
  • ex. MCR1 is a key gene for pigmentation, the two major pigmentations are eumelanin and pheomelanin
  • R = pheomelanin (produced brown or dark hair), r = eumelanin (red or light hair)
• homozygous alleles: two copies of the same gene RR or rr
• heterozygous alleles: different variations Rr

Question 10

Define: Genotype

Answer 10

the combination of genes carried within an individual’s chromosomes

Question 11

Define: Phenotype

Answer 11

the traits expressed by an individual based on their genotype

Question 12

Penetrance vs Age-dependent penetrance

Answer 12

• penetrance: % of individuals with a certain genotype that express the expected trait
• age-dependent: individuals with a certain genotype may not express the trait until later in life

Question 13

Mutagens: Definition, Causes, and Inheritance

Answer 13

• definition: agents that increase the frequency of mutations
• causes:
  
  • radiation: fragmentation of DNA
  • mutagenic chemicals: alter DNA due to their molecular structure, by mimicking ionizing radiation, or interfere with base pairing
• Inheritance:
  
  • mutations in somatic cells can cause disease but will not be inherited
  • mutations in reproductive cells CAN be inherited

Question 14

Define: Genetic Mutation

Answer 14

• alterations of genetic material

Question 15

Whole Chromosome Non-Disjunction

Answer 15

• sister chromatids fail to separate during mitosis or meiosis
• monosomy
• trisomy

Question 16

Partial Chromosome Breakage

Answer 16

• deletion
• duplication
• inversion–two breaks, reinsertion

Question 17

Chromosome Translocation

Answer 17

• interachange genetic material between non-homologous chromosomes

Question 18

Fragile Sites on Chromosomes

Answer 18

certain chromosomes are more prone to breakage and gaps

Question 19

Point Mutations

Answer 19

• Alteration of a single nucleotide within DNA
  
  • base pair substitution
  • base pair deletion
  • base pair addition
• redundancy of code allows for some margin of error without consequences
• HOWEVER, alterations can be profound
  
  • missense: change of amino acid
  • frameshift: change entire gene from mutation forward
  • nonsense: stop gene from being produced (stop codon)

Question 21

Missense Point Mutation

Answer 21

causes change of amino acid

Question 22

Frameshift Point Mutation

Answer 22

change entire gene from mutation forward due to an insertion or deletion

Question 23

Nonsense Point Mutation

Answer 23

stop gene from being produced (a stop codon)

Question 24

Autosomal Dominant and Example(s)

Answer 24

• a pattern of inheritance in which an affected invidiaul has one copy of a mutant gene and one normal gene on a pair of autosomal chromosomes
  
  • ex. polycystic kidney disease
  • ex. Neurofibromatosis (nodules on nerve endings in skin)

Question 25

Autosomal Recessive

Answer 25

• trait/phenotype is only expressed in homozygotes
  
  • ex. albinism
  • ex. PKU (phenylketonuria)
  • ex. sickle cell anemia

Question 26

X-linked Recessive

Answer 26

• traits whose alleles are carried only on the X chromosome and require 2 copies for expression unless they are unopposed (with a Y chromosome)
  
  • ex. Hemophilia A
  • ex. Duchenne Muscular Dystrophy
  • ex. Red/Green Color blindness

Question 27

Mitochondrial Inheritance

Answer 27

• Variable presentations:
  
  • in individuals
  • among family members
  • heritability patterns
• homoplasmic/ heteroplasmic (one type of mtDNA in an individual/ more than one type)
• ex. Leber Hereditary Optic Neuropathy (LHON)–> vision loss
• ex. myoclonic epilepsy with ragged-red fiber syndrome (MERRF)
• if a mother is afflicted her children will have it; if father has it but mother does not, their children will not have it
• 95% of mtDNA comes from OVA/mom

Question 28

Trinucleotide Repeat Disorder

Answer 28

• genetic mutations that are amplified in successive generations
  
  • each generation has worsened phenotypes
• ex. Fragile X syndrome (mental disability)
• ex. Friedrich’s Ataxia (progressive nervous system damage and movement problems)
• Huntington’s Chorea aka huntington’s disease (breakdown of nerve cells in the brain)

Question 29

Recurrence and Punnet Square

Answer 29

• the probability that a family member of an affected individual will have the same disease