6.1.2- Patterns of Inheritance Flashcards
what is a gene?
a section of DNA that codes for a protein
what is an allele?
different forms of the same gene
what is a genotype?
the alleles an organism has
what is a phenotype?
the observable characteristics of an organism
what is the locus?
the position of a gene on a chromosome
what is a dominant allele?
the allele which exerts an affect on the phenotype of the heterozygote.
what is a recessive allele?
the allele which has no effect on the phenotype of the heterozygote
what does homozygous mean?
two identical alleles for a charcteristic
what does heterozygous mean?
two different alleles for a characteristic
what is variation?
the presence of differences between individuals, it can be continuous or discontinuous
what is continuous variation?
when there is a range of values between two extremes
eg- height
what is discontinuous variation?
when there are two or more distinct categories with no intermediates
eg- tongue rolling
what is monohydrid inheritance?
the inheritance of a single pair of alleles of a single gene
what are used to allow us to see the inheritance of genes and their alleles?
genetic diagrams, in particular a genetic cross
what are the 6 key steps when completing a genetic cross?
1- state the phenotype of both parents
2- state the genotype of both parents
3- state the gametes of both parents (circle the letter to show that the cell has gone through meiosis)
4- use a punnet square to show the random fusion of the gametes
5- state the proportion of each phenotype produced within offspring
6- state the corresponding phenotype for each genotype.
what can a test cross be used for?
to find out the genotype of a normal organism
what are the four blood groups and what alleles make these up?
blood group A= IAIA or IAIO, has A antigens on rbc
blood group B= IBIB or IBIO, has B antigens on rbc
blood group AB= IAIB, has A and B antigens on rbc
blood group O= IOIO, no antigens
what does codominant mean?
inheritance of alleles which both contribute to the phenotype if they both occur
eg- A and B are codominant and so when together, create the blood group AB
what number chromosome in humans is known as the sex chromosome and so genetically determines the sex of an individual?
the 23rd chromosome
when are characteristics sex linked?
if the gene that codes for it is found on one of the sex chromosomes
where are most of the sex-linked genes found?
on the X chromosome (found in both males and females)
how many alleles for sex-linked genes do females carry and why?
2, because they are XX
how many alleles for sex linked genes do males carry and why?
one (hemizygous), because they are XY
what are sex-linked genetic disorders caused by?
caused by recessive alleles
what are three examples of sex-linked genetic disorders?
-haemophilia
-Duchenne muscular dystrophy
-red-green colour blindness
why are males more likely to suffer from X-linked genetic disorders?
-if females carry a normal dominant allele on one X chromosome while the other X chromosome has a defective recessive allele, then they will be a carrier and not be affected
-however, as males only have one X chromosome, if the defective recessive gene is on this, it will affect the phenotype because it is the only allele of that gene present, and so they will suffer
what is a dihybrid cross?
It follows the inheritance of two genes located on different pairs of homologous chromosomes
what is unlinked dihybrid?
-the two gene loci are unlinked (carried on different chromosomes)
-the genes do not interact (they do not influence each others expression)
what is the common ratio used for unlinked dihydrid?
9 : 3 : 3 : 1
what is linkage dihybrid?
-genes for different characteristics that are present on the same chromosome are linked and inherited together
what does linkage do in terms of phenotypes?
it reduces the number of phenotypes resulting from a given genetic cross
what is the common ratio for linkage dihybrid?
3 : 1
what happens if the genes are linked and crossing over occurs?
the phenotypic ratio of the offspring will not follow a normal ratio for linked genes or unlinked genes
what are the offspring called that are produced due to crossing over and have different combinations of alleles than either parents?
recombinant offspring
if genes are very close together, will crossing over occur?
no, it crossing over is less likely to occur during mitosis
what is the equation for recombinant frequency?
recombinant frequency = number of recombinant offspring / total number of offspring
what is the recombinant frequency used for?
it is a measure of the amount of crossing over that has happened in meiosis, giving an indication of the relative distance separating the gene loci on a chromosome
what is the equation for Chi-Squared?
x2 = sum of (observed-expected^2) / expected
how do you work out the degrees of freedom?
number of groups/phenotypes - 1
what do you say if the x2 is smaller than the critical value?
-no significant difference
-accept the null hypothesis
-greater than a 5% probability that the results occurred by chance
what do you say if the x2 is larger than the critical value?
-significant difference
-reject null hypothesis
-less than a 5% probability the results occurred by chance
what is epistasis?
the interaction of different gene loci so that one gene locus masks or suppresses the expression of another gene locus
what is a hypo static gene?
the gene that is affected by another gene
what is a epistaxis gene?
a gene that affects the expression of another gene
what is recessive epistasis?
when the homozygous recessive allele of one gene prevents the expression of alleles of a second gene
what is the expected ratio for recessive epistasis?
9:3:4
what is dominant epistasis?
when the dominant allele of one gene prevents the expression of alleles of a second gene
what is the expected ratio of dominant epistasis?
12:3:1
what does each part of the 9 : 3 : 3 : 1 ratio mean?
-9 A-B-
-3 A-bb
-3 aaB-
-1 aabb
what does each number in the ratio of 9:3:3:1 mean in terms of phenotype?
-9:3= all genotypes with A give the same phenotype
-3:3:1= all genotypes without both A and B give same phenotype
-3:1= all phenotypes with aa give same phenotype
what is a gene pool?
the total number of alleles in a particular population at a specific time
what is allele frequency?
the number of times an allele occurs in population
what is the Hardy-Weinberg Principle?
the idea that the frequency of alleles will stay constant from generation to generation providing certain points?
what are the 5 points that must be controlled according to the Hardy-Weinberg Principle?
-no mutations
-no selection for or against certain alleles
-the population is large and genetically isolated
-mating occurs at random
-no movement into or out of the gene pool
what does the Hardy-Weinberg Principle calculate?
it calculates the allele frequency for dominant and recessive alleles of any phenotypes controlled by two alleles of a single gene
what does the gene pool always equal according to the Hardy-Weinberg Principle?
1
what is the first equation to do with the Hardy-Weinberg Principle?
p + q = 1
what is the second equation to do with the Hardy-Weinberg Principle?
p^2 + 2pq + q^2 = 1
what do each letter stand for within the 1st equation for the Hardy-Weinberg Principle?
p= dominant allele frequency
q= recessive allele frequency
what do each letter stand for within the 2nd equation for the Hardy-Weinberg Principle?
p^2= homozygous dominant frequency
q^2= homozygous recessive frequency
2pq= heterozygous frequency
what are the 4 factors that affect evolution?
-random mutation
-sexual selection
-gene flow
-population size
what are selection pressures?
the environmental factors that affect the chance of survival of an organism
what are the 3 types of selection?
-stabilising selection
-directional selection
-disruptive selection
what is stabilising selection?
a natural selection that keeps allele frequencies relatively constant over generations
- the mean is selected for and the extremes are selected against
- the characteristics of a population are preserved
what happens to the graph during stabilising selection?
the curve becomes narrower and higher, but the mean does not change
what is directional selection?
the natural selection that produces a gradual change in allele frequencies over several generations
-extremes are favoured
-changes of the mean characteristics of a population
what happens to the graph during directional selection?
the curve remains the same but shifts to either the left or right
what is disruptive selection?
the mean changes and the extremes increase
what happens to the graph during disruptive selection?
it no longer is a bell shape but becomes two bells/S shaped
what is genetic drift?
when chance affects which individuals in a population survive, breed and pass on their alleles
It is the random change in the allele frequency of a population, which causes large changes in allele frequency in small population
when does genetic drift occur?
when some alleles pass to the next generation and some disappear by chance because not all individuals reproduce
what happens to the genetic variation when genetic drift occurs and why?
genetic variation reduces because some phenotypes become rarer and others become more common
what is genetic/population bottleneck?
when there is large reductions in population size which last for at least one generation, and so the gene pool and genetic diversity greatly reduces.
what is the founder effect?
when small populations arise due to the establishment of new colonies by a few isolated individuals.
what is an example of the founder effect?
the amish community
what are the key points of large populations?
-many different genes/alleles
-high genetic diversity
-have capacity to adapt to changes
what are the key points of small populations?
-few different genes/alleles
-low genetic diversity
-cannot adapt to change as easily
what are density-dependent factors?
factors that are dependant on population size
what are the examples of density-dependent factors?
-competition
-predation
-parasitism
-communicable disease
what are density-independent factors?
the factors that affect populations of all sizes in the same way
what are the density-independent factors?
-natural disasters
-deforestation
-climate change
what is speciation?
the formation of new species through the process of evolution
what are the two types of speciation?
-allopatric
-sympatric
what is allopatric speciation?
-different country, occurs due to geographical isolation
- occurs when populations of a species become separated from each other by geographical barriers
what is sympatric speciation?
-same country, reproductively isolated
-A group of the same species could be living in the same place but in order for speciation to take place there must exist two populations within that group and no gene flow occurs between them
what are the two types of separation to do this sympatric speciation and what do these mean?
-Ecological separation: Populations are separated because they live in different environments within the same area
-Behavioural separation: Populations are separated because they have different behaviours
