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Genes & Evolution > Basic Genetics > Flashcards

Basic Genetics Flashcards

1
Q

Define Evolution

A

Evolution means change in the form and/or behaviour of organisms between generations

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2
Q

What is an allele?

A

A variant of a genetic character at a given locus on a chromosome

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3
Q

What is a gene?

A

A unit of heredity, carrying information for a single polypeptide of RNA

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4
Q

What is a gamete?

A

A mature sexual reproductive cell (the egg or sperm)

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5
Q

Briefly describe Mendel’s Law of Segregation

A
  • Somatic cells of a diploid organism contain 2 sets of chromosomes
  • Chromosomes in the reproductive cells segregate during meiosis
  • Produced gametes contain only one set of chromosomes, fusing of the two gametes results in a diploid zygote
  • The F1s produce two types of gametes
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6
Q

What equation would we use to answer the following question:

” What is the probability (P) of F2s inheriting two “S” or “s” alleles?”

A

Probability (P) = (number of times something happens) / (number of times it could happen)

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7
Q

What does the phenotypic ratio 3:1 (or 1:2:1) indicate?

A

It indicates a monogenic nature of the phenotype

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8
Q

Briefly describe Mendel’s 2nd Law (Independent Assortment)

A
  • Alleles of different genes assort independently
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9
Q

What is meant by the term “diploid”?

A

Refers to 2 copies of heritable units

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10
Q

What is meant by the term “haploid”?

A

Refers to 1 copy of heritable unit

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11
Q

What does “heterozygous” mean?

A

An individual with two different alleles of heritable unit (gene)

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12
Q

What does “homozygous” mean?

A

An individual with two of the same alleles of heritable unit (gene)

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13
Q

What does “hemizygous” mean?

A

An individual with one allele of heritable unit (gene)

  • Males are hemizygous for the X chromosome genes
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14
Q

Briefly describe recombinant phenotypes

A
  • Phenotypic combinations are not present in parents of F1s; they appear in the F2s
  • There is a bias towards the parental phenotype
  • Recombinant phenotypes appear because alleles of the two genes assort independently (providing the genes are not linked) during meiosis
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15
Q

What is “Linkage”?

A

Genes on the same chromosomes are linked.
- Closely linked genes DO NOT obey Mendel’s 2nd law & DO NOT assort independently

  • This characteristic can be used to determine location of genes in the genome and their “order” on the chromosome
  • Linkage also helps to understand the phenotypic resemblance between close relatives
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16
Q

Briefly describe how “crossing-over” and “recombination” are linked

A
  • During meiosis, homologous chromosomes pair up
  • At this time, crossing-over can occur between chromatids of different homologs
  • This creates 2 “recombinant” chromatids with genetic material from different homologs
  • The closer linked two genes are, the less likely the recombination will occur between them.
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17
Q

How can we calculate the recombination frequency?

A

Recombination frequency (RF) = Number of recombinants (R) / (Number of recombinants + Parental type (P))

RF = R/(R+P)

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18
Q

What is a genetic map?

A

A genetic map shows the position and order of genes along each chromosome and can help in understanding the function of the genes

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19
Q

What are the two main meanings of the term “haplotype”?

A
  • A group of genes inherited together from a single; two linked genes, within 2 existing alleles each, give 4 haplotypes
  • Can also refer to the inheritance of a cluster of Single Nucleotide Polymorphisms (SNPs); Haplotype is made up of a particular combination of alleles at nearby SNPs
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20
Q

What controls skin/coat colour in mammals?

A

Melanocytes

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21
Q

Briefly describe melanin synthesis

A
  • Tyrosynase (Tyr) catalyses several reactions converting tyrosine to precursors of melanin
  • Inactivation of Tyr due to mutation precludes production of melanin precursors; if homozygous, it results in albino phenotype
  • Tyrosynase related protein 1 (Typ1), encoded by Trp gene, participates in coversin of a lighter to darker eumelanin.
  • Homozygous carriers of mutant Trp11 turn brown.
  • The gene is relevant across various mammalian species - Trp1 mutation is underlying cause of melanesian blonde hair in humans
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22
Q

The Himalayan allele (C^h) is heat-sensitive. What does this mean?

A

No melanin is present in warmer areas of the body

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23
Q

What is the result of no melanocytes being present in certain areas?

A

Piebald patterning (spots)

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24
Q

Briefly describe the agouti allele

A

Wild type, produces agouti phenotype by introducing a band of yellow colour resulting from deposition of phaeomelanin (yellow pigment) on the dark shaft of hair

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25
Q

Briefly describe incomplete dominance

A
  • Many alleles are not completely dominant or recessive; their effects blend together or mix such alleles often referred to as ADDITIVE alleles
  • Examples include the colours of snap-dragon flowers (red//pink//white)
  • The inheritance still follows Mendel’s laws
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26
Q

Briefly describe co-dominance

A

Co-dominant alleles are ones whose effects can both be seen together in the phenotype

  • A good example is the human ABO blood group system
  • This has 3 alleles, I^A, I^B, I^O; their presence in a person’s blood can be detected using specific antibodies
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27
Q

What is epistasis?

A

Phenotypic expression of one gene can be conditional on the allele of another gene.

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28
Q

Describe Siamese cats as an example of gene-environment interaction

A
  • Pigments produced because enzymes are active in cooler parts of the body
  • If you were to remove some dark fur and then keep the cat in a warm environment, the fur would grow back lighter
  • The proportion of individuals carrying the allele that actually show the phenotype is called the PENETRANCE
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29
Q

What is pedigree analysis?

A

When monogenic traits are followed in families

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30
Q

Briefly describe autosomal dominant inheritance

A
  • Every affected individual has an affected parent
  • ~50% of the offspring are affected
  • Both sexes affected
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31
Q

Briefly describe autosomal recessive inheritance

A
  • Both parents not affected
  • ~25% of the offspring affected
  • Phenotype occurs in both sexes
32
Q

When did sexual reproduction first appear?

A

~1.2 billion years ago

33
Q

What are the advantages of sexual reproduction?

A
  • Genetic diversity due to recombinations

- Complementation or hybrid vigor

34
Q

What does “monoecious” mean?

A

An individual that can produce both male and female gametes

35
Q

What does “dioecious” mean?

A

Individuals are either male or female

36
Q

Briefly describe sex determination in bees; state whether they are monoeicious or dioecious

A
  • Dioecious
  • Unfertilised haploid eggs produce males (drones)
  • Fertilised haploid eggs produce females (workers, queen)
  • Same in ants and many other invertebrates
  • Halplodiploid sex determination
37
Q

Briefly describe reproduction in grasshoppers; state whether they are monoeicious or dioecious

A
  • Dioecious
  • All diploid, males have one X chromosome & females have two X chromsomes
  • XO sex determination
38
Q

Briefly describe reproduction in humans & fruit flies; state whether they are monoeicious or dioecious

A
  • Dioecious
  • Haploid sperm, contain a X or a Y chromosome; eggs contain a X chromosome
  • Females are XX
  • Males are XY
  • XY sex determination
39
Q

Briefly describe sex determination in birds & butterflies

A
  • Males are ZZ
  • Females are ZW
  • ZW sex determination
40
Q

What are the different sex determination systems in dioecious organisms?

A
  • Haplodiploid
  • XO
  • XY
  • ZW
41
Q

Describe reproduction in bees

A
  • Bees do not have sex chromosomes
  • To be female, the bee has to be heterozygous for complementary sex determiner (CSD) gene
  • CSD has 19 different alleles so a diploid bee is likely to be heterozygous
  • Rare homozygous diploid bees develop into sterile males and are killed by workers
  • Intensive selection by bee-keepers can reduce genetic diversity and increase chances of CSD homozygosity
42
Q

What is the CSD gene?

A

Complementary sex determiner

43
Q

SRY is a located on the Y chromosome & DAX1 is located on the X chromosome. How do these interact with each other in terms of sex determination?

A

DAX1 protein prevents the development of testis and allows the development of ovaries;
SRY protein overcomes the effect DAX1 and allows testis to develop

  • Primary sexual characteristics
44
Q

What are secondary sexual characteristics controlled by?

A

Hormones, not by X & Y directly.

45
Q

Briefly describe sex determination in Drosophila

A
  • Sex is determined by the ratio of the number of X-chromosomes relative to the autosomes
  • The key genes are 4 transcription factors; sis-a; sis-b; sis-c & run, a.k.a “X-linked numerator” genes (all on the X chromosome) & dpn, autosomal “denominator” gene (2R)
  • Ratio of these genes determines whether individual develops male or female
  • XO is a sterile male
  • XXY is a fertile female
46
Q

Briefly describe sex-linked inheritance

A
  • There are very few genes on the Y chromosome but, lots on the X chromosome. Their inheritance show sex-linkage in pedigrees
  • The outcome of a cross depends on which parental genotype is from the father and which from the mother
  • The reciprocal crosses do NOT produce the same results
47
Q

What is the cause of haemophilia?

A

A hereditary disorder caused by a defective gene on the X-chromosome

48
Q

From which parent do we inherit mitochondrial DNA?

A

Mother (maternal inheritance)

49
Q

Why are bacteria good model systems for mutation?

A

Bacteria are: unicellular; they have a single, haploid chromosome; they replicate by binary fission* & simple gene structure

*Meaning the daughter cells are identical to parent; clonal & no genetic variation from growth cycle

50
Q

What is a “mutant”?

A

An altered organism

51
Q

What is a “mutation”?

A

An altered genome

52
Q

What is “mutagenesis”?

A

The process of alteration

53
Q

Briefly describe mutants in bacteria

A

Mutations can affect bacteria in a number of ways:

  • Inability to use a substrate (i.e. sugar)
  • Making them auxotroph; needing another amino acid or vitamin to survive
  • Resistance; to antibiotics; phage or toxic chemicals
54
Q

How do we select and detect mutants?

A

In:

  • Non-selective media; all cells grow so we test individual clones (replica plating)
  • Selective media; antibiotic present; absence of amino acid
  • Indicator media; MacConkey agar (Lac^+/-)
55
Q

How do we look at mutants in higher organisms?

A

In:

  • Appearance; i.e. colour, shape or size
  • Behaviour; inborn errors of metabolism
56
Q

What are different types of point mutations?

A
  • Nucleotide substitutions (SNPs)

- Frameshift mutations

57
Q

What are the different types of “Larger-Scale” mutations?

A
  • Deletions; removes gene(s); may change phenotype
  • Insertions; adds gene(s); may change phenotype
  • Rearrangements; re-orders gene(s); may not change phenotype
58
Q

What is a “forward mutation”?

A

Wild type phenotype –> mutant phenotype

Wild type sequence –> mutant sequence

59
Q

What is a “reversion”?

A

Mutant phenotype –> Wild type phenotype

Reverse (back) mutation:
Mutant sequence –> Wild type sequence

Suppressor mutation:
Mutant sequence –> More mutations elsewhere

60
Q

What equation do we use to measure mutation rate?

A

Ratio = (number of mutants/number of wild types)

61
Q

What do we use mutation rates for?

A
  • Population genetics
  • Evolutionary studies
  • Measuring effects of mutagens
62
Q

What is a “transition mutation”?

A
  • Purine Purine

- Pyrimidine Pyrimidine

63
Q

What is a “transversion mutation”?

A
  • Purine Pyrimidine
64
Q

Briefly describe spontaneous mutations

A
  • Mutation is a random event
  • Mutations occur independently of a selective (dis)advantage to host
  • Each GENE mutates at a characteristic rate; probability of mutation in a particular gene
  • Each TYPE of mutation occurs at a characteristic rate
65
Q

What are “base analogues”?

A
  • Molecule similar to one of the 4 DNA bases; can ONLY be incorporated into DNA at replication; can pair with a normal base
  • Analogue occasionally mis-pairs with other bases; nucleotide change (mutation) will then occur during DNA replication
66
Q

5-bromouracil is a base analogue. Describe.

A
  • Analogue of thymine so pairs with adenine

- Conformational change of a 5-BU leads to pairing with guanine

67
Q

Briefly describe DNA modifying chemicals

A
  • React with normal DNA bases to change their base pairing
  • Active on replicating AND non-replicating DNA
  • Different agents are more or less specific in their alternations; NITROUS ACID: A-T –> G-C transitions; ALKYLATING AGENTS: A-T G-C transitions
68
Q

Briefly describe intercalating chemicals

A
  • Planar, ringed molecules the size of a base pair: e.g. acridine, ethidium bromide
  • Intercalate into dsDNA between base pairs
  • At DNA replication, get nucleotide added (or deleted) in daughter strand
  • Frameshift mutations in coding sequence
69
Q

Briefly describe the effect of ultraviolet radiation on DNA

A
  • UV energy absorbed by base
  • Chemical modification of base
  • Adjacent pyrimidines covalently bond; Pyrimidine dimers
  • DNA helix distorted; Replication & transcription blocked
70
Q

Briefly describe the effect of ionising radiation on DNA

A

4 types

  • Free radicals formed
  • React with & damage DNA: nucleotide substitutions (easy to repair); ssDNA breaks (easy to repair); dsDNA breaks (hard to repair)
  • Dose mutation rate
71
Q

Why are bacteria particularly prone to DNA damage?

A
  • Haploid; all mutations dominant

- Unicellular; whole organism affected

72
Q

When is DNA repair most effective?

A

Repair most effective before replication

73
Q

Briefly describe apurinic gap repair

A
  • Most common spontaneous degradation of DNA; hydrolysis of sugar-purine bond–> loss of purine
  • Repair by AP endonuclease; removes damaged base, ss gap filled by polymerase
  • If no repair, adenine is inserted at replication
74
Q

Briefly describe mismatch repair

A
  • Mismatch repair gene: mut
  • Mismatched base pair detected
  • Nearby ss cut & excision of ssDNA past mismatch; Daughter strand preferably excised
  • DNA polymerase repairs gaps
75
Q

Briefly describe photoreactivation repair

A
  • Various enzymes with specific properties

e. g.
- Cleavage of pyrimidine dimers (T^T)
- Enzyme binds to dimer
- Energy of blue light cleaves bond

76
Q

Briefly describe excision repair

A
  • Multi-enzyme system

- Repairs stretch of damaged nucleotides; damaged ssDNA excised; gap repair by polymerase

77
Q

Briefly describe post-replication repair

A
  • Polymerase can’t replicate across damaged nucleotides; it leaves gaps
  • Gap filled by strand exchange from the other dsDNA
  • Secondary gap repaired by polymerase

Genes & Evolution (8 decks)

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