Genetics: Pedigree Analysis and Gene Mapping Flashcards

Vocabulary flashcards generated directly from genetics lecture notes covering pedigree inheritance modes, twin studies, linked genes, recombination frequencies, mapping, three-point testcrosses, and interference.

Pedigree

A diagram showing the inheritance of a trait through multiple generations of a family.

Proband

The individual through whom a pedigree is first identified or investigated.

Autosomal dominant inheritance

Inheritance pattern in which one dominant allele on an autosome is sufficient to produce the phenotype.

Autosomal recessive inheritance

Inheritance pattern in which two recessive alleles on an autosome are normally required to produce the phenotype.

Autosomal dominant pedigree

Affected individuals usually appear in every generation; males and females are affected similarly and can transmit the trait to either sex.

Autosomal recessive pedigree

Affected individuals may be born to unaffected carrier parents, and the trait can skip generations.

X-linked recessive pedigree

Usually more males are affected; affected males often inherit the allele from carrier mothers; father-to-son transmission does not occur.

X-linked dominant pedigree

Affected fathers transmit the trait to all daughters but no sons; affected heterozygous mothers can transmit it to either sex.

Y-linked inheritance

Trait passes from an affected father to all of his sons and no daughters.

Mitochondrial inheritance

Affected mothers can transmit mitochondrial traits to their children, whereas affected fathers normally do not transmit them.

Consanguinity

Mating between genetically related individuals.

Pedigree carrier

Unaffected individual who possesses and can transmit a recessive disease-causing allele.

Autosomal recessive carrier

A heterozygous individual who carries a recessive allele but normally does not express the recessive phenotype.

Twin study

Comparison of traits between twins to estimate genetic and environmental contributions to a phenotype.

Monozygotic twins

Twins produced from one fertilized egg; genetically very similar.

Dizygotic twins

Twins produced from two separate fertilized eggs; genetically similar to ordinary siblings.

Concordance

Occurrence of the same trait in both members of a pair, such as twins.

Concordance rate

Percentage of twin pairs in which both twins express a particular trait.

Genetic contribution in twin studies

Higher concordance in monozygotic than dizygotic twins supports a genetic contribution to a trait.

Environmental contribution in twin studies

Monozygotic twin concordance below $100\%$ indicates that factors beyond genotype can influence the phenotype.

Multifactorial trait

Trait influenced by multiple genes as well as environmental factors.

Threshold characteristic

Multifactorial characteristic that is expressed only when an underlying liability exceeds a threshold.

Recurrence risk

Probability that a genetic condition will occur again within a family.

Genetic counseling

Process of providing information about inheritance, genetic conditions, and genetic risks.

Linkage

Tendency of genes located on the same chromosome to be inherited together.

Linked genes

Genes located on the same chromosome that do not necessarily assort independently.

Complete linkage

Inheritance in which no crossing over occurs between two genes, producing only parental combinations.

Incomplete linkage

Linkage in which crossing over sometimes occurs, producing both parental and recombinant combinations.

Unlinked genes

Genes that assort independently, usually because they are on different chromosomes or very far apart on the same chromosome.

Parental gametes

Gametes containing the same allele combinations found on the parental chromosomes.

Nonrecombinant gametes

Another term for parental gametes; produced without a crossover between the loci being studied.

Recombinant gametes

Gametes containing new combinations of alleles produced by crossing over.

Parental progeny

Offspring resulting from parental or nonrecombinant gametes.

Recombinant progeny

Offspring resulting from recombinant gametes.

Crossing over

Exchange of genetic material between nonsister chromatids of homologous chromosomes during meiosis.

Nonsister chromatids

Chromatids belonging to homologous chromosomes rather than the same replicated chromosome.

Chiasma

Physical point at which homologous chromatids appear connected after crossing over.

Recombination

Formation of new combinations of alleles compared with those present in the parents.

Recombination frequency

Percentage of recombinant offspring produced in a genetic cross.

Recombination frequency formula

$$\text{Recombination frequency} = \left(\frac{\text{number of recombinant progeny}}{\text{total progeny}}\right) \times 100$$

Genetic map

Map showing the relative positions of genes based on recombination frequencies.

Genetic distance

Relative distance between genes estimated from recombination frequency.

Map unit

Unit of genetic distance corresponding approximately to $1\%$ recombination for relatively short distances.

Centimorgan (cM)

Unit of genetic map distance; $1\,\text{cM}$ corresponds approximately to $1\%$ recombination for short distances.

Maximum observable recombination frequency

$50\%$; genes showing about $50\%$ recombination behave as though they assort independently.

Physical distance

Actual amount of DNA separating two loci, usually measured in base pairs.

Genetic distance vs physical distance

Genetic distance is based on recombination, while physical distance measures the actual DNA separating loci.

Coupling

Cis arrangement in which two alleles are located together on one homolog and their alternatives are together on the other, such as AB/ab.

Cis configuration

Another term for coupling; AB/ab.

Repulsion

Trans arrangement in which each homolog carries one allele from each pair, such as Ab/aB.

Trans configuration

Another term for repulsion; Ab/aB.

Testcross for linkage

Cross between an individual heterozygous at the genes being studied and an individual homozygous recessive at those genes.

Linked-gene testcross

Parental progeny are more common than recombinant progeny when the genes are linked.

Independent assortment testcross

Produces approximately a $1:1:1:1$ ratio for a two-gene heterozygote testcross.

Two-point testcross

Testcross involving two loci used to estimate recombination frequency and genetic distance.

Three-point testcross

Testcross involving three loci used to determine gene order and distances between genes.

Purpose of a three-point testcross

Determines gene order and map distances between three linked genes and can reveal double crossovers.

Three-point testcross parental classes

The two most numerous offspring classes are usually the parental or nonrecombinant classes.

Three-point testcross double-crossover classes

The two least numerous offspring classes are usually the double-crossover classes.

Finding the middle gene

Compare parental classes with double-crossover classes; the allele whose position changes is the middle gene.

Single crossover

A crossover occurring in one interval between linked genes.

Double crossover

Two crossover events occurring within the chromosome region being examined.

DCO

Double crossover.

Calculating an interval in a three-point cross

Add the single crossovers in that interval and the double crossovers, divide by total progeny, and multiply by 100.

Why include DCOs in map distance

A double crossover contains a crossover in each interval, so DCO offspring must be counted when calculating both interval distances.

Expected double crossovers

Product of the recombination frequencies of the two intervals multiplied by the total number of offspring.

Observed double crossovers

Actual number of double-crossover offspring found in the experiment.

Coefficient of coincidence

Ratio of observed double crossovers to expected double crossovers.

Coefficient of coincidence formula

$$\text{CoC} = \frac{\text{observed DCO}}{\text{expected DCO}}$$

Interference

Measure of the degree to which one crossover affects the probability of another crossover occurring nearby.

Interference formula

$$\text{Interference} = 1 - \text{coefficient of coincidence}$$

Positive interference

Fewer double crossovers occur than expected.

Zero interference

Observed and expected double crossovers are equal; one crossover has no detectable effect on another.

Negative interference

More double crossovers occur than expected.

Why recombination frequency underestimates long distances

Multiple crossovers can restore parental allele combinations and therefore go undetected when only recombinant phenotypes are counted.

Why RF cannot exceed 50%

At large distances, multiple crossovers make parental and recombinant combinations occur at approximately equal frequencies.

Gene order

Order of genes along a chromosome.

Gene mapping

Use of recombination frequencies to determine relative gene positions and distances.

Linkage group

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

Chromosome theory of inheritance

Genes are located on chromosomes, and chromosome behavior during meiosis explains patterns of inheritance.

Morgan

Thomas Hunt Morgan demonstrated the relationship between genes and chromosomes through experiments with Drosophila.

Drosophila melanogaster

Fruit fly widely used as a model organism in genetics.

Hemizygous lethal allele

An allele that causes death when it is the only copy present, such as some X-linked lethal alleles in males.

Effect of lethal alleles on linkage data

Lethal genotypes may remove expected progeny classes and must be considered when interpreting recombination data.

37.1% recombination

A recombination frequency of $37.1\%$ corresponds to approximately $37.1\,\text{cM}$ for the flour-beetle problem from HW #5.

R-W2-L2 gene order

Gene order from HW #5 because $\text{R-W2} = 17\,\text{cM}$, $\text{W2-L2} = 18\,\text{cM}$, and $17 + 18 = 35\,\text{cM}$, matching $\text{R-L2}$.

Pedigree analysis

Using family relationships and phenotypes to determine the most likely inheritance pattern and possible genotypes.

Affected individual

Filled symbol in a standard pedigree.

Unaffected individual

Unfilled symbol in a standard pedigree.

Pedigree male

Square in a standard pedigree.

Pedigree female

Circle in a standard pedigree.

Pedigree mating line

Horizontal line connecting two individuals.

Pedigree offspring line

Vertical line connecting parents to their offspring.

Dominant pedigree clue

An affected individual usually has an affected parent when penetrance is complete and the allele is not newly mutated.

Recessive pedigree clue

Two unaffected heterozygous parents can produce an affected child.

X-linked recessive clue

More males are often affected and there is no father-to-son transmission.

Autosomal inheritance clue

Male-to-male transmission is possible because the gene is not restricted to the X chromosome.