Genetics: Linkage, Recombination, and Gene Mapping in Eukaryotes (CH7)

Principle of segregation

Alleles separate during meiosis.

Independent assortment

Alleles at one locus sort independently from alleles at another locus.

Recombination

Alleles sort into new combinations.

Dihybrid cross

A genetic cross between individuals that differ in two traits.

Expected ratio in dihybrid cross

9:3:3:1 ratio.

Nonindependent assortment

Occurs when two genes are linked.

Complete linkage

Leads to nonrecombinant gametes and nonrecombinant progeny.

Crossing over

Leads to recombinant gametes and recombinant progeny.

Recombinant gametes

Gametes that result from crossing over between linked genes.

Testcross

A cross between a homozygote and a heterozygote.

Frequency of recombinant gametes

Is half the frequency of crossing over.

Crossover frequency

Crossover occurs in about 50% of meiosis.

Largest possible recombination frequency

0.5.

Homozygous progeny

Progeny that are homozygous for a trait.

Heterozygous progeny

Progeny that are heterozygous for a trait.

Chromatids

The two identical halves of a replicated chromosome.

Linked genes

Genes that are located close together on the same chromosome and tend to be inherited together.

Mendel's laws

Principles of inheritance that describe how traits are passed from parents to offspring.

Genotype AaBb

The genotype of F1 progeny from a cross between AAbb and aaBB.

Nonrecombinant progeny

Progeny that exhibit the parental combinations of traits.

Progeny phenotypic ratio

1AB:1ab

Progeny phenotypic ratio (if genes A and B are completely linked)

1AB:1Ab:1aB:1ab

Maximum percentage of recombinant gametes (if genes A and B are not linked)

50%

Recombination frequency formula

Recombination frequency = (Number of recombinant progeny/Total number of progeny) × 100%

Coupling configuration

Wild-type alleles are found on one chromosome; mutant alleles are found on the other chromosome.

Repulsion configuration

Wild-type allele and mutant allele are found on the same chromosome.

Testcross progeny example

AaBb x aabb 🡪 10 AaBb, 40 aaBb, 40 Aabb, and 10 aabb.

Recombination frequency from progeny numbers

0

Gene order for three linked genes

A B C

Recombination frequency between genes A and B

10.2%

Double-crossover progeny total

4 and 6

Total number of progeny from the cross

1000

Progeny numbers from cross AB/ab×ab/ab

72 AB/ab, 68 ab/ab, 17 Ab/ab, and 21 aB/ab

Maximum percentage of recombinant gametes (if a single crossover occurs)

100%

Progeny phenotypic ratio (if genes A and B are not linked)

9AB:3Ab:3aB:1ab

Walter Sutton's chromosome theory of inheritance

Genes are physically located on chromosomes.

Nettie Stevens and Edmund Wilson's research

Sex was associated with a specific chromosome in insects.

Calvin Bridges's research

Nondisjunction of X chromosomes was related to the inheritance of eye color in Drosophila.

Harriet Creighton and Barbara McClintock's contribution

Intrachromosomal recombination was the result of physical exchange between chromosomes.

Evidence for genes on chromosomes

Barbara McClintock and Harriet Creighton provided evidence that genes are located on chromosomes.

Organism studied by McClintock and Creighton

Corn.

Supported theory by McClintock and Creighton

Chromosomal theory of inheritance.

Gene mapping with recombination frequencies

Genetics maps determined by recombinant frequency.

Map unit

CentiMorgans.

Genetic map vs physical map

Genetic maps are based on rates of recombination; physical maps are based on physical distances.

Most likely order of linked genes R, S, and T

S T R.

Requirement for topoisomerase II in crossing over

Supports the theory that crossing over is due to stress.

Testing for Independent Assortment

McClintock and Creighton provided evidence that genes are located on chromosomes.

Distance between R and S

22 m.u.

Distance between S and T

8 m.u.

Distance between R and T

14 m.u.

Concept Check 3

How does a genetic map differ from a physical map?

Answer to Concept Check 3

Genetic maps are based on rates of recombination. Physical maps are based on physical distances.

Recombination frequency

The percentage of offspring that exhibit a recombinant phenotype due to crossing over during meiosis.

Two-Point Testcross

A genetic mapping method used to determine the distance between two linked genes based on recombination frequency.

Three-Point Testcross

A more efficient mapping technique that allows for the determination of the order of three linked genes in a single set of progeny.

Double crossover progeny

Offspring resulting from two crossover events between linked genes, leading to a combination of alleles different from the parental types.

Gene order determination

The process of establishing the sequence of genes on a chromosome based on crossover data from progeny.

Middle locus

The gene that is located between two other genes in a three-point testcross.

Crossover progeny

Offspring that result from the exchange of genetic material between homologous chromosomes during meiosis.

Single crossover

A crossover event that occurs between two loci on the same chromosome, resulting in recombinant offspring.

Concept Check 4

An exercise to write the genotypes of all recombinant and nonrecombinant progeny expected from a three-point cross.

Concept Check 5

An exercise to identify the middle locus in a three-point testcross based on the genotypes of progeny.

Progeny genotype

The genetic constitution of the offspring resulting from a cross, which can be recombinant or nonrecombinant.

Steps in determining gene order

A systematic approach to establish the sequence of genes in a three-point cross.

Mapping distances

The measurement of the distance between genes on a chromosome, often expressed in centimorgans (cM) based on recombination frequency.

Recombinant phenotype

A phenotype that arises from the combination of alleles that differ from those of the parental generation due to recombination.

Figure 7.13

A diagram illustrating the process of determining gene order in a three-point testcross.

Figure 7.14

A diagram showing the location of crossovers in a three-point testcross.

A, B, C loci

The specific locations of genes being studied in a three-point testcross.

All classes of progeny

Refers to considering nonrecombinant, single-crossover, and double-crossover progeny when determining gene order.

nonrecombinant

Progeny that do not show recombination of alleles.

single-crossover

A type of genetic recombination where two chromatids exchange segments at one location.

double-crossover

A type of genetic recombination where two separate exchanges occur between chromatids.

coefficient of coincidence

The ratio of the number of observed double crossovers to the number of expected double crossovers.

interference

The phenomenon where the occurrence of one crossover event affects the likelihood of another crossover event occurring nearby.

negative interference

Indicates that more double crossovers took place than expected based on single-crossover frequencies.

positive interference

Indicates that fewer double crossovers took place than expected based on single-crossover frequencies.

mathematical mapping functions

Functions that relate recombination frequencies to actual physical distances between genes.

Poisson distribution

A statistical distribution that predicts the probability of multiple rare events.

mapping human genes

The process of determining the locations of genes on human chromosomes.

expected double crossovers

The predicted number of double crossover events based on recombination frequencies.

observed double crossovers

The actual number of double crossover events recorded in a genetic experiment.

crossover

The exchange of genetic material between homologous chromosomes during meiosis.

locus

The specific physical location of a gene on a chromosome.

gene order

The arrangement of genes on a chromosome, which can be determined through testcrosses.

effects of multiple crossovers

The impact that multiple crossover events have on the genetic mapping and recombination frequency.

crossovers

Events during meiosis where homologous chromosomes exchange segments.

total progeny

The total number of offspring produced in a genetic cross.

recombination rates

The frequency at which recombination occurs between genes.

Mapping with molecular markers

A technique used to identify specific locations of genes on chromosomes.

RFLPs

Restriction Fragment Length Polymorphisms, a method used in genetic mapping.

Genomewide association studies

Studies that look for associations between genetic variants and traits across the genome.

Haplotype

A combination of alleles at multiple loci that are transmitted together.

Linkage disequilibrium

The non-random association of alleles at different loci.

Somatic-cell hybridization

A method used to determine which chromosome contains a gene of interest.

Deletion mapping

A technique that reveals the chromosomal locations of genes by observing deletions.

Physical mapping through molecular analysis

Determining the physical locations of genes on chromosomes using molecular techniques.

In situ hybridization

A technique used to locate specific DNA sequences within a chromosome.

Fluorescence In Situ Hybridization (FISH)

A method that uses a single-stranded complementary DNA probe to identify specific DNA sequences.