Genetics Exam 2 Study Guide
Chapter 6
Pedigree: Family tree that outline the inheritance of one or more characteristics.
Proband: Person from whom the pedigree is initiated.
Autosomal Recessive Trait
o Appear equally in males and females.
o Affected individuals often born to unaffected parents.
o Tend to skip generations.
o More likely to appear among progeny of related parents.
Autosomal Dominant Trait
o Appear equally in males and females.
o Appears in every generation.
o Unaffected persons do not transmit the trait.
o Affected persons have at least one affected parent.
X-Linked Recessive Trait
o An affected male does not pass the trait to his sons but can pass the allele to a daughter, who is
unaffected and passes it to sons who are.
o Can skip a generation because affected sons are usually born to unaffected mothers.
o Appear more frequently in males.
o Rule out X-Linked Recessive if any affected daughters are born to affected mothers and
unaffected fathers.
X-Linked Dominant Trait
o Appears in both males and females; often more females.
o Do not skip generations.
o Affected males pass the trait on to all their daughters and none of their sons.
o Affected females (if heterozygous) pass the trait on to about half of their sons and about half of
their daughters.
o All affected males have an affected mother.
o Rule out X-Linked Dominant if any daughters of an affected father DON’T have disease.
o Suspect if only daughters of an affected father are affected.
Y-linked Trait
o Appear only in males.
o All male offspring of an affected male are affected.
Dominant mutations are usually lethal when homozygous.
o Assume the individuals carrying dominant mutations are heterozygotes.
Chapter 7
Linkage: Genes with loci physically located on the same chromosome.
Crossing over
o Occurs during Prophase I during Meiosis I
o Exchange of material between non-sister chromatids of a homologous pair.
o Can break linkage between genes, altering phenotypic ratios of the offspring.
o Crossing over involves 2 of the 4 chromatids, resulting in 2 recombinant and 2 nonrecombinant
chromosomes.
o If crossing over occurs in some but not all meiotic cells, the majority of offspring will be
nonrecombinant and resemble the parents.
, o Occurs randomly along the chromosome.
o Genes farther apart are more likely to crossover than those close together.
o The more recombinant offspring, the more times crossing over must have occurred between
linked genes.
Recombination: process resulting in new allele combinations on a chromosome.
Nonrecombinant: original (parental) combinations.
Recombinant: new combinations.
Recombination Frequency
o Distance between to genes (number of recombinant offspring out of total offspring).
o 1% recombination = 1 map unit = 1 centiMorgan (cM)
o Recombination frequencies between two genes cannot exceed 50%.
How to Measure Frequency of Crossing Over Between Linked Genes
o Testcross: individual 1 heterozygous for alleles of both genes crossed with individual 2
homozygous recessive.
o Independently Assort
Nonrecombinant and recombinant progeny are observed in a 1:1:1:1 ratio.
Genes NOT linked.
o Complete Linkage
Alleles of two genes linked on the same chromosome are so close together, they are
NEVER separated by crossing over (rare in nature).
All of the progeny are nonrecombinant with the phenotypes of both parents in a 1:1
ratio.
o Linked, but Some Crossing Over
Crossing over occurs in some meiotic cycles but not all.
Recombination frequency = (# of recombinant / total # of progeny) x 100.
More nonrecombinant progeny than recombinant.
Double Crossover
o A single crossover will switch the alleles on homologous chromosomes, but a second crossover
will reverse the effects of the first, restoring the original parental combination of alleles and
producing only nonrecombinant genotypes in the gametes, although parts of the chromosomes
have recombined.
o Recombinant chromosomes resulting from the double crossover have only the middle gene
altered.
o Probability of a double crossover is the product of the probabilities of each single crossover, so
double crossover progeny are the rarest class of offspring.
Chapter 8
Aneuploidy: too few or too many individual chromosomes.
Polyploidy: change in number of whole chromosome sets.
Tandem duplication: duplicated region adjacent to original segment.
Displaced duplication: duplicated region is located some distance away from original segment on same
or different chromosome.
Reverse duplication: duplicated region inverted.
If heterozygous for duplication, loops form to align genes during synapsis of homologs in prophase I.
Deletion: chromosome segment is missing.
o Homozygous deletions are often lethal.
o If centromere region deleted, chromosome will be “lost”.
o If heterozygous for deletion, synapsis requires looping as with a heterozygous duplication.
Chapter 6
Pedigree: Family tree that outline the inheritance of one or more characteristics.
Proband: Person from whom the pedigree is initiated.
Autosomal Recessive Trait
o Appear equally in males and females.
o Affected individuals often born to unaffected parents.
o Tend to skip generations.
o More likely to appear among progeny of related parents.
Autosomal Dominant Trait
o Appear equally in males and females.
o Appears in every generation.
o Unaffected persons do not transmit the trait.
o Affected persons have at least one affected parent.
X-Linked Recessive Trait
o An affected male does not pass the trait to his sons but can pass the allele to a daughter, who is
unaffected and passes it to sons who are.
o Can skip a generation because affected sons are usually born to unaffected mothers.
o Appear more frequently in males.
o Rule out X-Linked Recessive if any affected daughters are born to affected mothers and
unaffected fathers.
X-Linked Dominant Trait
o Appears in both males and females; often more females.
o Do not skip generations.
o Affected males pass the trait on to all their daughters and none of their sons.
o Affected females (if heterozygous) pass the trait on to about half of their sons and about half of
their daughters.
o All affected males have an affected mother.
o Rule out X-Linked Dominant if any daughters of an affected father DON’T have disease.
o Suspect if only daughters of an affected father are affected.
Y-linked Trait
o Appear only in males.
o All male offspring of an affected male are affected.
Dominant mutations are usually lethal when homozygous.
o Assume the individuals carrying dominant mutations are heterozygotes.
Chapter 7
Linkage: Genes with loci physically located on the same chromosome.
Crossing over
o Occurs during Prophase I during Meiosis I
o Exchange of material between non-sister chromatids of a homologous pair.
o Can break linkage between genes, altering phenotypic ratios of the offspring.
o Crossing over involves 2 of the 4 chromatids, resulting in 2 recombinant and 2 nonrecombinant
chromosomes.
o If crossing over occurs in some but not all meiotic cells, the majority of offspring will be
nonrecombinant and resemble the parents.
, o Occurs randomly along the chromosome.
o Genes farther apart are more likely to crossover than those close together.
o The more recombinant offspring, the more times crossing over must have occurred between
linked genes.
Recombination: process resulting in new allele combinations on a chromosome.
Nonrecombinant: original (parental) combinations.
Recombinant: new combinations.
Recombination Frequency
o Distance between to genes (number of recombinant offspring out of total offspring).
o 1% recombination = 1 map unit = 1 centiMorgan (cM)
o Recombination frequencies between two genes cannot exceed 50%.
How to Measure Frequency of Crossing Over Between Linked Genes
o Testcross: individual 1 heterozygous for alleles of both genes crossed with individual 2
homozygous recessive.
o Independently Assort
Nonrecombinant and recombinant progeny are observed in a 1:1:1:1 ratio.
Genes NOT linked.
o Complete Linkage
Alleles of two genes linked on the same chromosome are so close together, they are
NEVER separated by crossing over (rare in nature).
All of the progeny are nonrecombinant with the phenotypes of both parents in a 1:1
ratio.
o Linked, but Some Crossing Over
Crossing over occurs in some meiotic cycles but not all.
Recombination frequency = (# of recombinant / total # of progeny) x 100.
More nonrecombinant progeny than recombinant.
Double Crossover
o A single crossover will switch the alleles on homologous chromosomes, but a second crossover
will reverse the effects of the first, restoring the original parental combination of alleles and
producing only nonrecombinant genotypes in the gametes, although parts of the chromosomes
have recombined.
o Recombinant chromosomes resulting from the double crossover have only the middle gene
altered.
o Probability of a double crossover is the product of the probabilities of each single crossover, so
double crossover progeny are the rarest class of offspring.
Chapter 8
Aneuploidy: too few or too many individual chromosomes.
Polyploidy: change in number of whole chromosome sets.
Tandem duplication: duplicated region adjacent to original segment.
Displaced duplication: duplicated region is located some distance away from original segment on same
or different chromosome.
Reverse duplication: duplicated region inverted.
If heterozygous for duplication, loops form to align genes during synapsis of homologs in prophase I.
Deletion: chromosome segment is missing.
o Homozygous deletions are often lethal.
o If centromere region deleted, chromosome will be “lost”.
o If heterozygous for deletion, synapsis requires looping as with a heterozygous duplication.
