concept 18 - BIO10008

what is the ratio observed in a dihybrid cross when each of the loci has one phenotype which is completely dominant over the other?

9:3:3:1

what is the ratio observed in a test cross involving a dihybird when each of the loci has one phenotype which is completely dominant over the other?

1:1:1:1

how can we observe linkage in dihybird cross? A test cross involving a dihybrid?

linkage in dihybrid cross can be seen if the parental combination is more frequent than the recombinant

when ratio of the dihybird/test cross deviate from its expected independently assorted ratio

what determines if two genes are linked or not?

they are linked if they are inherited together, and the genes are found on the same chromosome. where parental frequency is higher than recombinant

why is it that two genes can be on the same chromosome but appear to be genetically unlinked

they are far apart on the same chromosome above 50mu and have equal frequency of parental and recombinant gene occurrence

frequency for independent assortment

how does crossing over give recombinant outcomes

crossing over occurs in meiosis 1 in prophase 1 where, the chromosome first decondense and pair up with a homologous chromosome then forms a chiasma which allows one sister chromatid to recombine with the other. that crossed over sister chromatid is the recombinant outcomes.

what is the parental arrangement of alleles?

the parental arrangement of alleles is the combination the parents had, same chromosome that appeared on the parent passed onto the offspring

what is recombinant arrangement of alleles

this is caused from crossing over of two parent chromatids and is genetically different

why is parental arrangement of alleles observed more frequent for linked loci

they are inherited as a unit as they are closer together and less likely to be segregated

why is there no difference in the frequency of parental and recombinant alleles when genes are on different chromosomes

they have equal frequency as independent assortment is random for genetic diversity

how can genes be on the same chromosome but not genetically linked

due to being far apart on the chromosome

how do we determine the map distance between two loci

no. of recombinant/total *100 = map distance/recombinant factor

more than 50% means independent assortment on different chromosomes

why is a test cross more useful in linkage analysis than a dihybrid cross

it allows the relationship of which allele from the heterozygous F1 offspring passes on, it only generates homozygous recessive gametes thus the phenotypes observed in the progeny directly relate to their genotypes and allows us to distinguish those who receive the parental and recombinant alleles

it simplifies the interpretation of offspring genotypes and phenotype

cis arrangement of alleles

wildtypes/dominant on one chromosome and mutants/recessive on the other side

trans arrangement of alleles

wildtypes and mutants on one side of chromosome and dominant and recessive on one side of chromosome

how the difference in observed progeny phenotypes can allow you to distinguish between cis and trans arrangement of alleles in a test cross

cis: w+p+ and wp > w+p and wp+ OR AB and ab> aB and Ab

trans: w+p and wp+ > w+p+ and wp OR aB and Ab > AB and ab