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SP MCAT Biology > Genetics > Flashcards

Genetics Flashcards

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1
Q

Genes

A
  • DNA sequences that code for heritable traits that can be passed from one generation to the next
  • determine physical and chemical characteristics of every human
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2
Q

Alleles

A
  • alternative forms of each gene

- cell holds two of these – one from mom and one from dad

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3
Q

Genotype

A

genetic combination possessed by an individual (ex. TT, tt, Tt)

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4
Q

Phenotype

A

the manifestation of a given genotype as an observable trait

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5
Q

Homologues

A
  • two copies of each chromosome (ex. XX)

- expressed by each human except for males who have XY

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6
Q

Chromosome

A
  • genes are organized into these
  • ensure that genetic material is passed easily to each daughter cell during meiosis and mitosis
  • each one of these is part of a homologous pair
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7
Q

How many alleles does each person inherit for each gene?

A

two alleles for all genes except for male sex chromosomes (XY)

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8
Q

Dominant Allele

A
  • only one copy of an allele is needed to express a given phenotype
  • usually expressed with a capital letter
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9
Q

Recessive Allele

A
  • two copies of the allele are needed to express a given phenotype
  • usually represented with a lowercase letter
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10
Q

Hemizygous Genotype

A

describes a situation in which only one allele is present for a given gene, as is the case for parts of the X chromosome in men

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11
Q

Complete Dominance

A
  • when only one dominant and one recessive allele exist for a given gene
  • presence of one dominant allele will mask the presence of the recessive allele
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12
Q

Codominance

A
  • when more than one dominant allele exists for a given gene
  • Ex. person with one allele for A blood antigen and one allele for B blood antigen will express both antigens simultaneously
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13
Q

Incomplete Dominance

A
  • when a heterozygote expresses a phenotype that is intermediate between the two homozygous genotypes
  • Ex. when a red flower is crossed with a white flower and it results in pink flowers
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14
Q

Penetrance

A
  • the frequency that a genotype will show up in a phenotype
  • the proportion of the population with a given genotype who actually express the phenotype
  • considered a population parameter
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15
Q

Full Penetrance

A
  • occurs when 100% of individuals with a given genotype express the phenotype
  • Ex. if you have the gene for being smart then there is a 100% chance you will be smart
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16
Q

High Penetrance

A

occurs when most, but not all, of those with the allele show the given phenotype

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17
Q

Expressivity

A
  • to what degree a gene is expressed
  • the different manifestations of the same genotype across the population
  • defined as varying phenotypes despite identical genotypes
  • commonly considered at the individual level
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18
Q

Constant Expressivity

A

all individuals with a given genotype express the same phenotype

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19
Q

Variable Expressivity

A

individuals with the same genotype may have different phenotypes

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20
Q

Mendel’s First Law of Segregation

A
  • genes exist in alternative forms (alleles)
  • an organism has two alleles for each gene, one inherited from each parent
  • two alleles segregate during meiosis resulting in gametes that carry only one allele for any inherited trait
  • if two alleles of an organism are different, only one will be fully expressed and the other will be silent (codominance and incomplete dominance are exceptions to this)
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21
Q

What is the key cellular correlate of Mendele’s First Law?

A

separation of homologous chromosomes during anaphase 1 of meiosis

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22
Q

Mendel’s Second Law of Independent Assortment

A
  • states that the inheritance of one gene does not affect the inheritance of another gene
  • exhibited during prophase I of meiosis when homologous chromosomes pair up to form tetrads and swapping of genetic material between chromatids can occur (recombination) which ultimately allows the inheritance of one gene to be independent of others
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23
Q

____ and ____ ____ allow for greater genetic diversity in the offspring

A
  • segregation

- independent assortment

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24
Q

Gene Pool

A

all of the alleles that exist within a species

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25
Q

Mutation

A

change in the DNA sequence that results in a mutant allele

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26
Q

Wild-Type

A

alleles that are considered “normal” or “natural” and are ubiquitous in the population

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27
Q

Mutagens

A
  • substances that cause mutations

- all carcinogens are these

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28
Q

Transposons

A
  • can lead to mutations
  • elements that can insert and removed themselves from the genome
  • if it is inserted in the middle of a coding sequence, the mutation will disrupt the gene
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29
Q

What are the different nucleotide-level mutations that occur?

A
  • point mutations (silent mutations, missense mutations, nonsense mutations)
  • frameshift mutations
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30
Q

Point Mutations

A
  • occur when one nucleotide in DNA (A, C, T, G) is swapped for another
  • includes silent, missense, and nonsense mutations
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31
Q

Silent Mutations

A
  • occur when the change in nucleotide has no effect on the final protein synthesized from the gene
  • most commonly occurs when the changed nucleotide is transcribed to be the third nucleotide in a codon because of the wobble position
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32
Q

Missense Mutations

A

occur when the change in nucleotides results in substituting one amino acid for another in the final protein

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33
Q

Nonsense Mutations

A

occur when the change in nucleotide results in substituting a stop codon for an amino acid in the final protein

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34
Q

Frameshift Mutations

A
  • occur when nucleotides are inserted into or deleted from the genome
  • happens because DNA is always read in 3 letter sequence codons, so when there is insertion or deletion of nucleotides the reading frame can shift and result in changes in the amino acid sequence or premature truncation of the protein
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35
Q

Chromosomal Mutations

A
  • larger scale mutations in which large segments of DNA are affected
  • includes the following mutations: deletion, duplication, inversion, insertion, translocation
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36
Q

Deletion Mutations

A
  • occur when a large segment of DNA is lost from a chromosome
  • small deletion mutations are considered frameshift mutations
37
Q

Duplication Mutations

A

occur when a segment of DNA is copied multiple times in the genome

38
Q

Inversion Mutations

A
  • occur when a segment of DNA is moved from one chromosome to another
  • small insertion mutations are considered frameshift mutations
39
Q

Translocation Mutations

A

occur when a segment of DNA from one chromosome is swapped with a segment of DNA from another chromosome

40
Q

Inborn Errors of Metabolism

A
  • class of deleterious mutations

- defects in genes required for metabolism

41
Q

Genetic Leakage

A
  • flow of genes between species

- in some cases individuals from different (but closely related species) can mate to produce hybrid offspring

42
Q

Genetic Drift

A
  • refers to changes in the composition of the gene pool due to chance
  • tends to be more pronounced in small populations
  • may result in loss of some alleles (including beneficial ones) and the fixation (rise to 100% frequency) of other alleles
43
Q

Founder Effect

A
  • more extreme case of genetic drift
  • occurs when a small population of species finds itself in reproductive isolation from other populations as a result of natural barriers, catastrophic events or other bottlenecks that drastically and suddenly reduce the size of the population available for breeding
44
Q

Inbreeding

A
  • may occur in populations due to the founder effect
  • occurs when there is mating between two genetically related individuals
  • encourages homozygosity which increases the prevalence of both homozygous dominant and recessive genotypes
45
Q

Inbreeding Depression

A

condition where there is loss of genetic variation leading to reduced fitness of the population

46
Q

Outbreeding/Outcrossing

A

introduction of unrelated individuals into a breeding group which could result in increased variation within a gene pool and increased fitness

47
Q

What can cause a reduction in genetic diversity?

A
  • genetic drift
  • founder effect
  • inbreeding
48
Q

Punnett Squares

A

diagrams that predict the relative genotypic and phenotypic frequencies that will result from the crossing of two individuals

49
Q

Monohybrid Cross

A

cross in which only one trait is being studied

50
Q

P Generation

A
  • parent generation

- refers to the individuals being crossed

51
Q

F Generation

A
  • filial generation

- refers to the offspring of the crossed individuals

52
Q

What is the resulting offspring from the crossing of two heterozygotes for a trait with complete dominance?

A

1:2:1 ratio of genotypes and a 3:1 ratio of phenotypes

53
Q

Test Cross

A
  • used to determine an unknown genotype
  • in this type of cross the organism with an unknown genotype is crossed with an organism known to be homozygous recessive
  • if all the offspring are of the dominant phenotype then the unknown genotype is likely to be homozygous dominant
  • if there is a 1:1 distribution of dominant to recessive phenotypes then the unknown genotype is likely to heterozygous
54
Q

Dihybrid Cross

A
  • extended punnett square that accounts for inheritance of two different genes
  • punnett square is 4x4
55
Q

What is the resulting offspring from the crossing of dihybrid parents that are heterozygous for both traits?

A

9:3:3:1 genotypic ratio and a 3:1 phenotypic ratio for each trait

56
Q

Sex-Linked (X-Linked) Traits

A
  • unless otherwise stated assume that sex-linked traits are recessive
  • because females are XX they may be homozygous or heterozygous for a condition carried on the X-chromosome
  • males (XY) are hemizygous for many genes carried on the X-chromosome, and more commonly have sex-linked traits since having only one recessive allele is sufficient for expression of the recessive phenotype
57
Q

What is true of the offspring of a man with a sex-linked trait?

A
  • he will have daughters that are all either carriers of the trait or who express the trait (if his partner also has an affected allele)
  • he will never pass down a sex-linked trait to his son (this is because the egg carries the X chromosome, and it is the sperm that determines the sex of the child)
58
Q

Linkage

A
  • due to independent assortment, genes on different chromosomes are randomized, but genes on the same chromosome can’t be randomized this way
  • genes on same chromosome are linked to some extent
  • crossing over reduces linkage but is only efficient when the genes are physically apart from each other on the chromosome – when genes are further apart on chromosome then crossing over makes them less linked
  • physically closer the genes are on the chromosome, the more linked they are
59
Q

Recombination

A
  • the process that introduces genetic diversity into gametes during meiosis
  • two processes that makeup recombination: independent assortment and crossing over (occurs during prophase I at the chiasma which is made possible via the pairing of homologous chromosomes called the tetrad)
60
Q

Recombination Frequency

A
  • likelihood that two alleles are separated from each other during crossing over
  • roughly proportional to the distance between the genes on the chromosome
61
Q

Genetic Map

A
  • created by analyzing recombination frequencies

- represents the relative distance between genes on a chromosome

62
Q

What does one map unit/centimorgan correspond to?

A

a 1% chance of recombination occurring between the two genes

63
Q

Allele Frequency

A
  • tells us how often an allele appears in a population
  • does not indicate whether organism is homozygous or heterozygous; only tells the representation of the allele across all chromosomes in a population
64
Q

What 5 criteria must be met in a population with a stable gene pool (gene frequencies are not changing) where evolution is not occurring?

A
  1. population is very large (no genetic drift)
  2. no mutations that affect the gene pool
  3. random mating (no sexual selection)
  4. no migration of individuals into or out of the population (no gene flow)
  5. genes in the population are all equally successful at reproducing (no natural selection)
65
Q

Hardy-Weinberg Equilibrium

A
  • population can be considered to be in this state when all 5 criteria above are met
  • allele frequencies do not change if population is in this state
66
Q

Hardy-Weinberg Equation that tells frequency of alleles in the population:

A

p+q=1

where p is the frequency of the dominant allele and p is the frequency of the recessive allele

67
Q

Hardy-Weinberg Equation that tells frequency of genotypes and phenotypes in a population:

A

p^2 + 2pq + q^2 = 1
where p^2 is the frequency of the homozygous dominant allele, 2pq is the frequency of the heterozygous phenotype, and q^2 is the frequency of the homozygous recessive genotype

68
Q

What is the phenotypic ratio seen in the offspring of cross: Bb x Bb?

A

3 dominant : 1 recessive

69
Q

What is the phenotypic ratio seen in the offspring of cross: Aa x aa?

A

1 dominant : 1 recessive

70
Q

What is the phenotypic ratio seen in the offspring of cross: DdEe x ddEE?

A

1 dominant (for D)/dominant (for E) : 1 recessive (for D)/dominat (for E)

71
Q

What is the phenotypic ratio seen in the offspring of cross: XqX x XY?

A

Female: all unaffected
Male: 1 unaffected and 1 affected

72
Q

What is the phenotypic ratio seen in the offspring of cross: XrX x XrY?

A

Both male and female: 1 unaffected and 1 affected

73
Q

What can be said about a population if cross between two parents results in offspring with the exact same allele frequencies?

A

when allele frequencies are unchanged compared to the parent generation, the population can be said to be in Hardy-Weinberg Equilibrium

74
Q

Polygenic

A
  • occurs when a trait is determined by several genes with several alleles
  • examples: height. hair color, eye color, skin color
75
Q

Epistasis

A
  • occurs when there is dominance of a completely different gene over another gene – not between different alleles of the same gene
  • expression of one gene prevents expression of another gene (one gene’s allele masks the phenotype of another gene’s allele)
  • Ex. incomplete dominance for “hair shape gene” (SS= straight hair, SC=wavy hair, CC=curly hair) but if you have the gene XbaldY then you will have no hair no matter what hair shape gene you express
  • Ex. albino individuals (still have genes for pigmentation but albino gene completely dominants)
76
Q

Human Genome

A
  • 46 total chromosomes
  • 22 pairs of autosomes, 1 pair of sex chromosomes
  • one locus per gene (but can have multiple genes per locus)
  • 4 RNA bases
  • 64 codons
  • 1 amino acid per codon (but multiple codons for most amino acids)
77
Q

Leakage

A
  • gene flow from one species to another
  • Ex. brown bear x poilar bear = polar bear with brown spots – the genes between the brown bear and polar bear “meshed” to produce the polar bear with brown spots
78
Q

Recessive Epistasis

A
  • recessive allele of one gene masks the action of another

- Ex. have eye color cross of blue eyes (Aa) with green eyes (Bb) where “a” masks Bb so results in blue eye color

79
Q

What is the F2 generation ratio of recessive epistasis?

A

9 : 3 : 4

dominant phenotype : blended phenotype : less dominant phenotype

80
Q

Dominant Epistasis

A
  • dominant allele of one gene will mask the action of another
  • Ex. have eye color cross of blue eyes (Aa) with green eyes (Bb) where “A” masks Bb so results in blue eye color
81
Q

What is the F2 generation ratio of dominant epistasis?

A

12 : 3 : 1

dominant phenotype : blended phenotype : less dominant phenotype

82
Q

Males and Females have the ___ amount of genes

A

SAME!

-this is because one of the female’s x chromosomes’ is inactivated (barr body)

83
Q

Can an inactivated X chromosome be reactivated?

A

No

84
Q

Sex linked = _-linked

A

x-linked

CAN’T HAVE Y-LINKED

85
Q

X-Linked Recessive Pedigrees:

A

THINK: XaXa

  • male offspring are more affected than female offspring
  • if a male is affected, female offspring will get the affected chromosome (will be carriers) because females get one X from mom and the other X from dad
86
Q

X-Linked Dominant Pedigree:

A

THINK: XAXa or XAXA

  • good chance daughters are affected and very good chance sons are affected
  • if father is affected (XAY) then all of the daughters will be affected even if mom is unaffected, but none of the sons will be affected (assuming mother is not affected)
87
Q

For an X-linked dominant trait, what does the mom have to be for all offspring to have the trait?

A

homozygous dominant

88
Q

In which inheritance are males affected more than females: x-linked dominant or x-linked recessive?

A

x-linked recessive

89
Q

Inactivation Center

A

-region on each x chromosome that determines the fate of the chromosome

SP MCAT Biology (21 decks)

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