a level biology > Inheritance > Flashcards

Inheritance Flashcards

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1
Q

Allele

A

Versions of the same gene

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2
Q

Genotype

A

Genetic makeup of an organism

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3
Q

Phenotype

A

Observable characteristics of an organism

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4
Q

Dominant allele

A
  • version of a gene that will always be expressed if present
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5
Q

Recessive allele

A
  • version of a gene that will only be expressed if 2 copies of this allele are present
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6
Q

Homozygous

A

2 identical alleles for a characteristic in an organism

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7
Q

Heterozygous

A

2 different alleles for a characteristic

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8
Q

What type of letter is used to denote a dominant allele

A

Capital letter

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9
Q

What type of letter is used to denote a recessive allele

A

Lowercase letter

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10
Q

how does variation occur

A
  • mutation
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11
Q

mutations

A
  • changes to the genetic code
  • that are random and constantly occurring
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12
Q

importance of variation

A
  • natural selection
  • = evolution
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13
Q

types of variation

A
  • environmental
  • genetic
  • often both
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14
Q

chlorosis

A
  • plant leaves are pale/yellow
  • as cells do not produce normal amount of chlorophyll
  • plant cannot do photosynthesis
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15
Q

environmental factors causing chlorosis

A
  • lack of light
  • mineral deficiencies
  • virus infections
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16
Q

lack of light

A
  • plants will turn off chlorophyll production in absence of light
  • to conserve resources
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17
Q

mineral deficiencies

A
  • e.g lack of iron/magnesium
  • plant cannot make chlorophyll
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18
Q

virus infections

A
  • viruses interfere with metabolism of cells
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19
Q

animals body mass - determined

A
  • genetic and environmental
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20
Q

environmental factors towards animal body mass

A
  • obesity
  • being underweight due to environmental factors
  • amount of food consumed
  • amount of exercise
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21
Q

when can obesity be genetic

A
  • obese mouse
  • causing pattern of fat deposition in its body to change
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22
Q

how is genetic variation created

A
  • by the versions of genes you inherit from your parents
  • as most genes have different alleles
  • alleles an organism inherits are the characteristics they will display
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23
Q

how many alleles are generally inherited

A

2 = 1 from each parent

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24
Q

modifications

A

any changes the environment makes to a persons phenotype

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25
types of variation
- continuous - discontinuous
26
continuous variation
- characteristic that can take any value within a range - genetic and environmental - controlled by multiple genes
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polygenes
controlled by multiple genes
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examples of continuous variation
- leaf surface area - animal mass - skin colour
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discontinuous variation
- characteristic that can only appear in specific values - mainly genetic - 1 or 2 genes involved
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example of discontinuous variation
- blood group - albinism - round and wrinkled pea shape
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monogenetic inheritance
inheritance of a single gene
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who is famous for laws by which characteristics are inherited
Gregor Mendel
33
Mendel - work
- pea plants - green and yellow - crossed green and yellow pea plants - found green was dominant - true breeding - studied what happened when 2 true breeding individuals are crossed
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true breeding
organisms that contain homozygous alleles for a particular gene
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codominance
- occurs when 2 different alleles occur for a gene - both of which are equally dominant - so both alleles for the gene are expressed in the phenotype
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example of codominance
- colour of snapdragon flowers - equally dominant alleles exist - 1 codes for red flowers the other white flowers
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what colour flowers can snapdragon flowers produce
- red = plant is homozygous for red pigment - white = plant is homozygous for white pigment - pink = plant is heterozygous
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how are alleles presented in codominance
- a letter is chosen to represent the gene - as one is no more dominant than the other - different alleles are represented using a second letter in superscript
39
multiple alleles
- genes which have more than 2 versions - however only 2 can be present in an individual - as an organism only carries 2 versions of the gene
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give an example of multiple alleles
blood group
41
immunoglobulin gene
- codes for production of different antigens present on the surface of red blood cells
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what are the 3 different alleles for the antigen present on the surface of red blood cells
- 1a = produces antigen A - 1b = produces antigen B - 10 = produces neither antigen
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different alleles of antigens on surface of rbcs - relationships
- 1a and 1b are codominant - 1o is recessive to both - different combinations of these alleles result in the 4 blood groups
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4 blood groups
- blood group A - blood group B - blood group AB - blood group O
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which alleles code for blood group A
- IaIa - IaIo
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which alleles code for blood group B
- IbIb - IbIo
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which alleles code for blood group AB
- IaIb
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which alleles code for blood group O
IoIo
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how is sex determined
whether the sperm fertilises an egg containing an X or Y chromosome
50
sex linked
some characteristics are determined by genes carried on the sex chromosomes
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how do sex linked characteristics work
- Y chromosome is smaller than X - there are genes the X chromosome has that males only have 1 copy of - so any characteristic caused by a recessive allele on the section of the X chromosome which is missing in the Y chromosome - occurs more frequently in males - as females have a dominant allele present in their cells
52
haemophilia
- sex linked genetic disorder - patients have blood which clots slowly - due to absence of a protein blood clotting factor - so injury can result in prolonged bleeding
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what happens if a male inherits a recessive allele that codes for haemophilia
- they cannot have a corresponding dominant allele on their Y chromosome - so develop the condition - most haemophilia sufferers are male
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what happens to females who are heterozygous for the haemophilia coding gene
- carriers
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carriers
- do not suffer from the disorder themselves - however may pass the disorder onto their children
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what is haemophilia an example of
sex linked genetic disorder
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faulty allele
allele coding for the disorder
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dihybrid inheritance
inheritance of 2 genes
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why does the actual ratio of offspring produced differ from expected
- fertilisation of gametes is random so in a small sample few chance events can skew the ratio - genes being studied are both on the same chromosome - if no crossing over occurs the alleles for the 2 characteristics will always be inherited together
60
linked genes
- genes being studied are on the same chromosome - inherited as 1 unit - no independent assortment in meiosis - unless alleles separated by chiasmata
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dihybrid cross
- shows inheritance of 2 genes - that may be located on 2 different pairs of homologous chromosomes - each of these genes may have 2+ alleles
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how are dihybrid crosses different to monohybrid crosses
4 alleles shown at each stage instead of 2
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phenotypic ratios
- the ratios of phenotypes you would expect to see in offspring produced from dihybrid cross - larger the sample = closer numbers will be to expected ratio
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why do ratios expected from dihybrid crosses often differ significantly from what is observed
- due to linkage
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linkage
genes located on same chromosome
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autosomal linkage
genes that are linked are found on 1 of the other pairs of chromosomes
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recombinant offspring
have different combinations of alleles than either parent
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the closer the genes are on a chromosome, the less likely ...
- separation in crossing over - fewer recombinant offspring produced
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recombinant frequency
amount of crossing over that has happened in meiosis
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recombinant frequency - calculation
number of recombinant offspring / total number of offspring
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what does recombinant frequency of 50% indicate
- no linkage - genes on separate chromosomes
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recombinant frequency of less than 50%
- gene linkage - independent assortment hindered
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what is the degree of crossing over determined by
- how close genes are on a chromosome - closer they are = less chance of separation
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what can recombination frequencies be used for
- mapping genes on chromosome
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chi squared
- statistical test - measures size of the difference between results you actually get - and results you expected to get - helps to determine whether differences in observed and expected values are significant or not
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what hypothesis does chi squared usually test
null
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null hypothesis
- no significant difference between what we expect and what we observe - any differences are due to chance
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large chi squared values
- significant difference between observed and expected results - probability these results are due to chance is low
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degrees of freedom
number of comparisons being made
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chi squared significant difference
- greater or equal to critical value
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epistasis
- interaction of genes at different loci - gene regulation is a form of epistasis
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gene regulation
- regulatory genes control activity of structural genes - e.g lac operon
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epistatic gene
- gene that affects the expression of another gene
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hypostatic gene
- gene that is affected by another gene
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recessive epistasis
- presence of 2 recessive alleles at a gene locus - led to a lack of an enzyme
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how can an epistatic gene influence the activity of other genes
- due to presence of dominant or recessive alleles
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dominant epistasis
- if a dominant allele results in a gene having an effect on another gene
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example of dominant epistasis
- an epistatic gene coded for an enzyme that modified one of the precursor molecules in the pathway - the next enzyme in the pathway would then lack a suitable substrate molecule - so pigment would not be produced - all genes in the sequence would be masked
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evolution
- the change in inherited characteristics of a group of organisms over time
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why does evolution occur
- due to changes in the frequency of different alleles within a population
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factors affecting evolution which lead to changes in the frequency of alleles within a population
- mutation - sexual selection - gene flow - genetic drift - natural selection
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mutation (evolution)
- necessary for existence of different alleles in the first place - formation of new alleles = variation
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sexual selection (evolution)
- alleles coding for characteristics which improve mating success = increase in frequency
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gene flow (evolution)
- movement of alleles between populations - immigration and emigration can result in changes of allele frequency - within a population
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genetic drift (evolution)
- occurs in small population - change in allele frequency due to random nature of mutation - appearance of a new allele will have a greater impact in a smaller population than larger population
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why does genetic drift have a greater impact on smaller populations
- more alleles present in the gene pool of larger populations
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natural selection (evolution)
- individuals have characteristics that aid their chances of survival - so reproduction rate will increase in number - as will alleles coding for these characteristics - how environment changes lead to evolution
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population genetics
how allele frequencies within populations change over time
99
large populations - population genetics
- gene pool = lots of genetic diversity - due to many different genes and alleles
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genetic diversity
- leads to variation within a population
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selection pressures
- changes in environment - new diseases - prey - competitors - human influence - lead to evolution - populations adapts to change over time
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small populations - population genetics
- limited genetic diversity - cannot adapt to change as easily - more likely to become extinct - mew strain of pathogen could wipe whole population
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limiting factors
- factors which decrease the size of populations
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2 types of limiting factors for population size
- density dependent factors - density independent factors
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density dependent factors
- dependent on population size
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examples of density dependent factors
- competition - predation - parasitism - communicable disease
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density independent factors
- affect populations of all sizes the same way
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examples of density independent factors
- climate change - natural disasters - seasonal change - human activities
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population bottlenecks
- large reductions in population size - for at least 1 generation
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effect of population bottlenecks
- gene pool and genetic diversity decreased - effects seen in future generations - as it takes thousands of years for genetic diversity to develop in a population
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example - population bottlenecks
- Northern elephant seals - almost hunted to extinction - when hunting stopped only around 20 left - now have population of 30 000 - but much less genetic diversity than southern elephant seals who did not experience genetic bottleneck
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genetic bottleneck - positives
- beneficial mutation will have a much greater impact - will lead to quicker development of a new species
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founder effect
- extreme example of genetic drift - small populations have smaller gene pools than original population - display less genetic variation - if carried to new population frequency of alleles that were rare in original population will be much higher in the new, smaller population - = bigger impact in natural selection
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how does the founder effect arise
- small populations arise due to establishment of new colonies by a few isolated individuals
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normal distribution
- distribution of different variants will take form of a bell shaped curve
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stabilising selection
- norm or average is selected - extremes are selected against - resulting in reduction of frequency of alleles at extremes - increase frequency of average alleles - seen in normal distribution
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example of something which shows normal distribution and stabilising selection
- birth weight of babies - babies with average birth weight are most common - peaking graph - extremes in weight reduce survival chances - so members of survivals of very big or small babies are low
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directional selection
- change in environment - where normal phenotype is not the most advantageous - organisms which are less common and have more extreme phenotypes are selected positively - allele frequency shifts towards extreme frequencies
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directional selection - example
- peppered moths in industrial revolution - lots of smoke released from factories killing lichens growing on tree bark - soot made bark black - peppered moths were white and so could camouflage from birds due to lichen - when lichens died trees became black - so light coloured moths visible and dark were camouflaged - so allele frequency shifted due to natural selection - most peppered moths had darker colour
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disruptive selection
- extremes are selected for and the norm is selected against
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example of disruptive selection
- Darwins finches on Galapagos islands
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disruptive selection - feather colour in male lazuli buntings native to North America
- lots of competition between male birds to establish territories and attract females - dull, brown males seen as non threatening and bright - blue males seen as threatening to adult males and so are left alone - intermediate colour attacked by adult birds and fail to made or establish territory - extremes selected for - distribution of phenotypes shows 2 peaks
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speciation
- formation of a new species - through process of evolution
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speciation - features
- organism belonging to new species can no longer interbreed with organisms belonging to original species to produce fertile offspring
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events of speciation - overview
- members of population become isolated - alleles within groups undergo random mutations - accumulation of mutations and changes in allele frequencies lead to changes
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speciation - members of population become isolated
- no longer interbreed with rest of population - resulting in no gene flow between 2 groups
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speciation - alleles within groups undergo random mutations
- environment of each group may be different or change - different selection pressures - so different characteristics selected for and against
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speciation - accumulation of mutations and changes in allele frequencies lead to changes
- lead to changes in phenotype - over many generations - members of different populations become so different they can no longer interbreed - they are now reproductively isolated and are a different species
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types of speciation
- allopatric - sympatric
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allopatric speciation
- members of a population are separated from the group - by a physical barrier, e.g sea - so are geographically isolated
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what occurs in allopatric selection
- environments in different groups are often different - as will selection pressures resulting in different physical adaptations
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in allopatric selection what may happen if a small group of species is isolated
- founder effect - leading to genetic drift - enhancing differences in populations
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example of allopatric speciation
- Finches - Galapagos islands - small groups of finches from mainland have flown to/been stranded on different islands - formed new colonies
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how did finches evolve - allopatric speciation
- adapted to different environments, especially food sources present on island - example of adaptive radiation - finches can not breed with each other - new species have evolved with unique beaks adapted to type of food available
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adaptive radiation
rapid organism diversification takes place
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sympatric speciation
- occurs within populations that share the same habitat/live in same area - more common in plants
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when can sympatric speciation occur
- when members of 2 different species interbreed and form fertile offspring - hybrid is formed which has a different number of chromosomes to either parent - may no longer be able to interbreed with members of either parent populations - stops gene flow - reproductively isolates hybrid organisms
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other examples of sympatric speciation leading to reproductive isolation
- disruptive selection - mating preferences - behavioural differences - still living in same habitat - so gene flow will be reduced - interfering with speciation
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reproductive barriers
- barriers to successful interbreeding - postzygotic barriers - prezygotic barriers
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postzygotic barriers
- barriers after fertilisation occurred - hybridisation = may reduce viability / reproductive potential of offspring
141
prezygotic barriers
- barriers before fertilisation occurred - e.g habitat isolation
a level biology
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