genetics Flashcards
what is the genotype
the genetic constitution of an organism
what is the phenotype
the expression of the genotype and its interaction with the environment
what are alleles
variants of a particular gene
what is a characteristic that has multiple alleles
eye colour e.g. blue, green, brown
what is the cause of multiple alleles
mutations in a gene that occur at different positions in the genes
what occurs in diploid cells
chromosomes form pairs called homologous chromosomes
how can a pair of alleles be homozygous/ heterozygous
alleles at a specific locus can be homologous if they’re the same type of allele
they are heterozygous if the alleles are different
how many copies of a particular allele in a genotype
there are 2 copies of a particular allele for the genotype as chromosomes form homologous pairs
one copy is from each parent
when is an allele dominant
an allele is dominant if it’s always expressed in the phenotype( of a heterozygous) so only one copy is needed for expression
when is an allele recessive
an allele is recessive if it’s not expressed in the phenotype of a heterozygous
so 2 copies have to present for expression
what are codominant alleles
both alleles are equally dominant and equally expressed in the phenotype e.g. blood group AB
what can monohybrid crosses be used for
they can be used to predict the outcome of crossing two different genotypes
e.g. Parental gentoype CC cc
Gametes: C C c c
what is the ratio when you cross heterozygous genotypes together in monohybrid crosses
3: 1
dominant to recessive
how can a monohybrid cross be used to identify unknown genotypes
e.gg. an unknown genotype is crossed with a homozygous recessive individual
If all the offspring have a dominant phenotype, the unknown genotype is homozygous dominant as they must have inherited the homozygous allele from the unknown genotype
if half of the offspring have recessive phenotype then the unknown genotype is heterozygous as some individuals have inherited the recessive allele form the unknown genotype
what do we use dihybrid crosses for
to work out the inheritance of 2 different genes together
e.g. parental alleles: RRYY rryy R= round r= wrinkles Y = green y = yellow
Gametes: RY RY ry ry
what is the ratio of characteristics for dihybrid crosses
9: 3: 3: 1
what is an autosome
autosomes are chromosomes that are not sex chromosomes
what happens when there are two or more genes located on the same autosome
when there is 2 or more genes located on the same autosome, they are less likely to be separated during crossing over so they are inherited together
this can impact the ratio of phenotypes
e.g. the GB alleles and the gb alleles in the GgBb parent may have been linked. This means the GgBg parent produced mostly GB and gb gametes and makes the GbBg and ggbb genotypes more common in the offspring.
This results in a higher proportion of the offspring would have their parent’s phenotypes, instead of the split even of phenotypes predicted
what are do the phenotypes of the offspring in autosomal linkage tend to look like
offspring are more likely to enable parental phenotype as the alleles are less likely to cross over during meiosis
when does sex linkage occur
when the gene is only on the X chromosome
therefore, males are more likely to have recessive conditions e.g. hemophilia because they only require one copy of the sex-linked recessive allele for expression ( they have 1 X and 1 Y ) instead of 2
for females, they need to carry two (x and x) and when males have been x linked recessive condition, they have inherited it form their mother
what is epistasis
it is the interaction between 2 genes where one gene masks the expression of the other gene in the phenotype
what is the suppressing gene called
it is called the epistatic gene
what is the suppressed gene called
the hypostatic gene
what can epistatic interactions be
dominant or recessive
what is a dominant epistatic interaction
the dominant allele ( 1 or 2 copies) of the epistatic gene masks the expression of hypostatic
what is a recessive epistatic interaction
two copies require to mask the expression of the hypostatic gene
what is chi-squared test
it is a statistical test that determines the probability of an unexpected result being due to chance or being significant
we use chi-squared if the data used is categorical and we are comparing the observed results with the expected results
when is the result of a chi - squared test significant
it is significant when the calculated value is more than the critical value
what is the gene pool
all the alleles pf all the genes within a population at one time
what is population
all the individuals of one species in one area at one time
what is allele frequency
the proportion of an allele within the gene pool
what are the assumptions of the Hardy - Weinberg equation
It assumes that there will be no change in the allele frequency between generations within a population, so it is not perfectly accurate:
the assumptions are:
no migration to introduce or remove alleles from the population
no mutation to create new alleles
no selection favouring particular alleles
mating is random (no inbreeding)
the population is large
the assumptions impair the accuracy of the equation
what is the equation useful in estimating
the allele frequency
what is the Hardy - Weinberg equation formula
p squared + 2pq + q squared where:
p+q = 1
-p = the frequencey of the dominant allele
-q = is the frequency of the recessive allele
-p squared = the frequency of the homozygous dominant genotype
2pq = the frequency of the heterozygous genotype
q squared = the frequency of the homozygous recessive genotype
what type of questions are genetic pedigree diagrams
- the condition s caused by recessive/ dominant/ sex-linked allele
Explain the evidence for this given in the diagram - How does the diagram prove the condition is most likely to be caused by a recessive/ dominant allele?
- what are the possible genotypes of individual x in the diagram
what are patterns to spot in genetic pedigree diagrams
1/ evidence that the condition is caused by a recessive allele
two parents who do not have the condition have a child who does have the condition
Therefore both parents must be carriers of the recessive allele, but due to the presence of a dominant allele, they do not have the condition themselves
2/ evidence the condition is caused by a dominant allele
two parents who do not have the condition have a child who does not have the condition therefore, both parents must be carriers of the recessive allele, but due to the presence of dominant alleles, they have the condition themselves
what is the best way to model genetic pedigree diagrams
Give evidence
then explain
how can we disprove sex linkage
if the condition is caused by a dominant allele, any affected father would pass on the condition to any daughter they have as the daughter is XX and one of the X is from the father’s XY
if this is not the case and the daughter is unaffected then it is not sexed linked
what happens when you do a monohybrid cross with two parents involving codominant alleles
you would expect to see a 1:2:1 ratio in the offspring
what are the phenotypic ratios for epistatic genes
recessive epistatic alleles:
-if you cross a homozygous recessive parent with a homozygous dominant parent you will produce a 9:3:4
dominant both: dominant epistatic: recessive other
dominant epistatic alleles:
crossing a homozygous recessive parent with a homozgous dominant parent will produce a 12:3:1 phenotypic ratio of dominant epistatic: recessive episatic: dominant other: recessive both in the F2 generation
