Mendelian Inheritance Flashcards
monohybrid cross
cross between two parents that differ in a single characteristic
4 conclusions
- one character is encoded by two genetic factors
- two genetic factors separate when gametes are formed
- concept of dominant and recessive traits
- two alleles separate with equal probability into the gametes
true breeding strains
physical appearance of offspring in each successive generation is identical to previous one
reciprocal cross
two crosses are performed between a pair of individuals, but the sexes of the individuals contributing specific traits are reversed in the second cross. This approach is used to determine whether a particular trait is influenced by the sex of the parent or by sex-linked inheritance.
mendels experiment
(1. prevent self fertilisation)
2. cross true breeding strains of parents to produce F1 progeny
3. performed reciprocal crosses
4. allow F1 to self fertilise to produce F2
mendel’s findings
F1 progeny of a cross between plants with different traits did not breed true
in F2, the recessive trait reappeared
3:1 ratio in F2 for the traits
what mendel concluded from his findings
inheritance is not blending
F1 still has information for both phenotypes
alleles segregate 50:50
law of segregation
each individual diploid organism possesses two alleles for any characteristic and these two alleles segregate when gametes are formed where one goes into each gamete
backcross
F1 crossed with one parent
testcross
individual of unknown genotype crossed to known type
degrees of freedom
number of phenotypes - 1
incomplete dominance
phenotype of the heterozygous genotype is intermediate between those of the homozygous genotypes
appears like blending inheritance in F1 but parental phenotypes reappear in F2
1:2:1
dihybrid cross
genetic cross between two individuals that differ in two traits, each determined by two alleles. The purpose of a dihybrid cross is to examine how alleles for two different traits segregate and independently assort during meiosis, following Mendel’s Law of Independent Assortment.
mendels dihybrid cross
because of dominance, the seeds in F1 are genetically identical
F1 can produce 4 different types of gametes
9:3:3:1 because the alleles for the different traits are assorted into gametes independently of each other
independent assortment
nonhomologous chromosomes can orient in either of two ways that are equally likely
segregation ratios for two loci on the same chromosome
dependent on the rate of crossing over between the loci
no crossing over forms non recombinants
dihybrid cross for linked genes, no crossing over
only nonrecombinant progeny produced
linked genes segregate together and crossing over produces recombination between them
dihybrid cross for unlinked genes
half progeny are recombinant and half are not
due to independent assortment
estimating crossing over rate
recombination frequency= (number of recombinant progeny/total number of progeny) x100
used to find order and distance between loci on a chromosome (linkage map)
epistasis
effect of one locus on a phenotype depends on genotype at the second locus
gives alternative ratios to 9:3:3:1
penetrance
how many individuals with a genotype show a phenotype
expressivity
how variable is the degree/severity of the phenotype
incomplete penetrance
Individuals with a genotype corresponding to a trait do not actually show the phenotype, either because of environmental effects or because of interactions with other genes.
The trait is either expressed or it is not.
variable expressivity
A particular phenotype is expressed with a different degree of severity in different individuals.
The trait is always expressed, though the severity varies.
what to look out for in a dominant trait in a pedigree analysis
check affected individuals are equalling likely to be females or males
most matings that produce affected offspsring should have only one affected parent
among matings in which one parent is affected, half the offspring should be affected
affected individuals should appear in each successive generation
what to look out for a recessive trait in a pedigree analysis
again males=females
trait may skip one or more generations
individuals may have unaffected parents
