Gene Mapping

Class 7

Parental types (independent assortment)

• 1:1 ratio

  • total is 4 types (1:1:1:1)

• gametes having the allele combination seen in the original female of the earliest generation

• gametes having the allele combination seen in the original male of the earliest generation

Recombinant types (independent assortment)

• 1:1 ratio

  • total is 4 types (1:1:1:1)

• gametes having the allele combination NOT seen in the original female or male of the earliest generation

• produced by a reshuffling of alleles during independent assortment

Linkage

• the number of F₂ progeny with parental genotypes exceed number of F₂ progeny with recombinant genotypes

• instead of independent assortment, particular alleles are connected most of the time

  • will NOT see 9:3:3:1 mandelian phenotypic ratio

• RF less than 50%

  • will never be more than 50%, though can have a distance greater than 50 m.u.

Map units (m.u.)/centimorgan (cM)

• used to gauge physical distance separating any two genes on the same chromosome

• an index of recombination probabilities assumed to reflect gene distances

• corresponds to 1 million base pairs in humans

Linkgage group

• genes chained together by linkage relationships

• number of groups equates to number of pairs of chromosomes in species

  • humans have 23

No crossovers (NCOs)

• NO recombinant type gametes

• ALL parental chromosomes

Double crossover (DCOs), 2-strand

• NO recombinant type gametes

• ALL parental type

Double crossover (DCOs), 3-strand

• HALF recombinant type gametes

• HALF parental type

• one of two ways this event can happen

Double crossover (DCOs), 3-strand

• HALF recombinant type gametes

• HALF parental type

• one of two ways this event can happen

Double crossover (DCOs), 4-strand

• ALL recombinant type gametes

• NO parental type

Single crossover (SCOs)

• HALF recombinant type gametes

• HALF parental type

Double crossover (DCOs), all types

• all four types are EQUALLY likely

• would happen more on smaller chromosomes (as opposed to larger ones)

• more rare than a single crossover event

  • rare events occurring simultaneously is more rare than the individual event occurring

• are not independent due to chromosomal interference

  • interference: making sure homologous chromosomes have at least one crossover

Crossing over event (COs)

• reciprocal exchange of parts between maternal and paternal chromosomes

• contributes to genetic diversity

• happens in meiosis I with homologous chromosomes

• helps with orientation at metaphase plate

  • without which, chromosomes would segregate improperly

• the closer genes are, the less likely crossing over will happen

• frequency increases as distance between genes increases

Recombination frequency (RF)

• is equal to map units

• less than 50% = linkage (< 0.5)

• is NEVER more than 50%

Two-point test cross

• used to calculate RF of two linked genes

  • useful for determining frequency of gametes produced

• dominant phenotype x recessive phenotype to estimate zygosity of dominate phenotype

• not great for judging accurate distances between genes

  • only involving two genes makes it difficult to know the order in which the genes are placed on the chromosomes

  • accuracy declines the smaller the distance between genes

Three-point test cross

• involves three genes

• a more accurate judge of distances between genes that a closer together

• accounts for double crossovers

Gene mapping (summary)

• help prediction of inheritance patterns of specific traits

• all about ordering genes and their distances from each other