Test Bank for Genetics
Essentials, 5th Edition
GENETICS- Chaviva Korb - ANSWERS
Lecture 1: - ANSWERS
- DNA is packaged as chromatin, chromosomes become visible during mitosis. - ANSWERS
Homologues- (maternal and paternal forms of same chromosome) - ANSWERS
Meiosis: G1, S, G2, M. Before replication phase, chromosomes have one chromatid and after
replication, chromosomes have 2 sister chromatids, held together at the centromere. Ends with 4
daughter cells with 1 chromatid each. - ANSWERS
Key differences from mitosis: Homologs pair (Prophase I), Sister centromeres act as a single
centromere (Metaphase I), Sister chromatids remain attached (Anaphase I), Meiosis I is a reduction
division- meaning start meiosis I with 46 units and end meiosis I with 23 units ( 2 chromatids in each
haploid daughter cells), Meiosis II is an equational division (identical to mitosis). - ANSWERS
Recombination occurs in Prophase I. Crossing over and recomb is Exchange of homologous segments
between non-sister chromatids. - ANSWERS
Homologues move apart during anaphase I-disjunction. 223 possible combinations of chromosomes.
In cytokinesis I, Cells divide into two haploid daughter cells. One cell receives most of the cytoplasm
and the other becomes the first polar body which doesn't go through Meiosis II. Meiosis I ends after
ovulation. Non dysjunction in meiosis I: can cause problems like Trisomy 21. - ANSWERS
-Brief interphase between first and second meiotic divisions. No S phase occurs in Meiosis II. -
ANSWERS
-In the second meiotic division a second polar body forms during oogenesis. In oogenesis, egg is
arrested in metaphase II until fertilization. - ANSWERS
,After Meiosis there is a Reduction of chromosome number 2nn. (diploid vs. haploid cells)- Notation
'n' has to do with amount of DNA (#of nucleotides), not the necessarily the # of chromosomes. -
ANSWERS
Lecture 2: Patterns of Inheritance - ANSWERS
Medelian Inheritance- determined by a single major gene. Based on independent assortment.
Dihybrid cross (2 genotypes) with ry, Ry, rY, Ry set up on both sidesphenotypic ratio of 9:3:3:1-
yellow round, green round, yellow wrinkled, green wrinkled, If get 9:3:3:1 you know that segregation
is independent. - ANSWERS
Multifactorial inheritance-multiple genetic and non-genetic factors involved - ANSWERS
Compound heterozygous: has two different mutant alleles for a character - ANSWERS
Linkage-2 genes physically near each other on a chromosome will not assort randomly in meiosis.
Tightly linked: will get 2 types of gametes ex. PL and pl. Unlinked: will get 4 types of gametes PL, Pl,
pL, pl. - ANSWERS
The frequency of recombination between two genes is proportional to the distance between the
genes. The closer the genes are on the chromosome the less likely crossing over occurs. Linkage
map: 1% recombination = 1 map unit = 1 centiMorgan (cM), Map distances are additive. - ANSWERS
The non-random association between alleles at two locations on a chromosome is called linkage
disequilibrium. If the frequency of chromosomes with AB=Ab=aB=ab then the genes are in
equilibrium. If frequency of 1 allele is seen more (A more than B for ex) then genes are in linkage
disequilibrium. - ANSWERS
Autosomal dominant inheritance Examples: - ANSWERS
Achondroplasia- FGFR3 mutations, Always full penetrance with achondroplasia (so normal parents
have a child with aplasia then it's a new mutation). Heterozygous b/c homozygotes usually die in
utero - ANSWERS
Neurofibromatosis- NF1 (neurofibromin) and NF2 (merlin) mutations. - ANSWERS
,Incomplete dominance- mixed phenotype. In cases of disease, Dominant disorders are more severe
in homozygotes then in heterozygotes (termed also "semidominant") Ie. Familial
Hypercholesterolemia. - ANSWERS
Co-dominance- phenotypic expression of two different alleles for a locus ie. Blood type. - ANSWERS
Autosomal recessive inheritance: Examples: Cystic fibrosis, Tay-Sachs disease, Sickle-cell disease -
ANSWERS
Pseudodominance: the inheritance of an autosomal recessive trait mimics an autosomal dominant
pattern - ANSWERS
Males are hemizygous with respect to X-linked genes. - ANSWERS
X-linked dominant inheritance: Affected females are twice as common as affected males but males
usually more severely affected or the disorder may be lethal in males (Rett syndrome). - ANSWERS
X-linked recessive: incidence is much higher in males and affected males do not usually transmit the
disorder unless mother is a carrier. Heterozygote females are usually unaffected, but some may
express the condition with variable severity as determined by the pattern of X inactivation. A
significant proportion of isolated cases are due to new mutation (Duchenne muscular dystrophy-
DMD). - ANSWERS
Pseudoautosomal inheritance- group of genes on the inactive x chromosome are NOT inactivated.
Diseases associated with these genes are inherited similar to autosomal inheritance. - ANSWERS
Same amount of X-linked gene products between males and females achieved through dosage
compensation. Lyon Hypothesis states that the inactive X is NOT randomly chosen in each cell Ex. A
structurally abnormal X is preferentially inactivated. Inactivation is NOT complete- some genes can
escape inactivation (ie. Those with a functional homolog on the Y). Inactivation is NOT permanent-
reversed in development of germ cells (not passed on to gametes). - ANSWERS
The key player is the X-linked gene XIST→ X (inactive) specific transcript. XIST is transcribed to
produce a non-coding RNA that "coats" the X-chromosome and inactivates it. XIST is only expressed
from the inactive X. The histones on the coated X undergo methylation which causes the
chromosome to condense (heterochromatin), forming a Barr body. - ANSWERS
, -Some genes do not have Y homologue and do not undergo inactivation (e.g. steroid sulfatase gene)
- ANSWERS
-Random/skewed inactivation may result in affected/totally healthy heterozygotes. - ANSWERS
Variable expression of X-inactivation: On both extremes, a heterozygous female with recessive x-
linked disease could manifest the disease. In a case with a dominant X-linked trait, in which almost
all of a females X chromosomes with mutation is inactive, might not manifest this disease. Identical
twins could even have diff phenotypes due to skewed X inactivation. - ANSWERS
Mosaicism- X chromosome inactivation occurs randomly and inactivation pattern is passed to cell
progeny. Result: functional mosaicism in which female is a mosaic with respect to expression of
genes on X chrom. Ex. Calico Cat, B - dominant orange, b - recessive gene black, Genes for white:
autosomal. - ANSWERS
Genetic Heterogeneity can be the result of Locus Heterogeneity , Allelic Heterogeneity, and Modifier
Loci. - ANSWERS
Locus Heterogeneity- a single disorder, trait, or pattern of traits caused by mutations in genes at
different chromosomal loci. Ex. retinitis pigmentosa has autosomal dominant, autosomal recessive,
and X-linked origins. However, only one mutant locus is needed for the phenotype to manifest -
ANSWERS
Allelic heterogeneity - Many genetic loci possess more than one mutant allele. In the CFTR gene,
nearly 1400 mutations found. Some mutations cause classical CF + pancreatic insufficiency +
congenital absence of vas deferens, others cause lung disease with normal pancreatic fxn. Other
cause only male sterility. - ANSWERS
Modifier gene: A gene that affects the phenotypic expression of another gene. Specific alleles of one
or more genes (modifier genes) can sometimes dramatically modify the clinical severity of the
phenotype produced by mutations in a disease-causing gene.This source of clinical heterogeneity is
often referred to as genetic background. Ex. Twins with the same mutation in cystic fibrosis but one
is severely sick and one only moderately sick- explained by modifier genes. - ANSWERS
Candidate loci acting as modifiers for CFTR mutations:TGFB1 (cytokine transforming growth factor b
and MLB2 (Mannose-binding lectin). Different alleles of these genes modify the severity of CF -
ANSWERS
Essentials, 5th Edition
GENETICS- Chaviva Korb - ANSWERS
Lecture 1: - ANSWERS
- DNA is packaged as chromatin, chromosomes become visible during mitosis. - ANSWERS
Homologues- (maternal and paternal forms of same chromosome) - ANSWERS
Meiosis: G1, S, G2, M. Before replication phase, chromosomes have one chromatid and after
replication, chromosomes have 2 sister chromatids, held together at the centromere. Ends with 4
daughter cells with 1 chromatid each. - ANSWERS
Key differences from mitosis: Homologs pair (Prophase I), Sister centromeres act as a single
centromere (Metaphase I), Sister chromatids remain attached (Anaphase I), Meiosis I is a reduction
division- meaning start meiosis I with 46 units and end meiosis I with 23 units ( 2 chromatids in each
haploid daughter cells), Meiosis II is an equational division (identical to mitosis). - ANSWERS
Recombination occurs in Prophase I. Crossing over and recomb is Exchange of homologous segments
between non-sister chromatids. - ANSWERS
Homologues move apart during anaphase I-disjunction. 223 possible combinations of chromosomes.
In cytokinesis I, Cells divide into two haploid daughter cells. One cell receives most of the cytoplasm
and the other becomes the first polar body which doesn't go through Meiosis II. Meiosis I ends after
ovulation. Non dysjunction in meiosis I: can cause problems like Trisomy 21. - ANSWERS
-Brief interphase between first and second meiotic divisions. No S phase occurs in Meiosis II. -
ANSWERS
-In the second meiotic division a second polar body forms during oogenesis. In oogenesis, egg is
arrested in metaphase II until fertilization. - ANSWERS
,After Meiosis there is a Reduction of chromosome number 2nn. (diploid vs. haploid cells)- Notation
'n' has to do with amount of DNA (#of nucleotides), not the necessarily the # of chromosomes. -
ANSWERS
Lecture 2: Patterns of Inheritance - ANSWERS
Medelian Inheritance- determined by a single major gene. Based on independent assortment.
Dihybrid cross (2 genotypes) with ry, Ry, rY, Ry set up on both sidesphenotypic ratio of 9:3:3:1-
yellow round, green round, yellow wrinkled, green wrinkled, If get 9:3:3:1 you know that segregation
is independent. - ANSWERS
Multifactorial inheritance-multiple genetic and non-genetic factors involved - ANSWERS
Compound heterozygous: has two different mutant alleles for a character - ANSWERS
Linkage-2 genes physically near each other on a chromosome will not assort randomly in meiosis.
Tightly linked: will get 2 types of gametes ex. PL and pl. Unlinked: will get 4 types of gametes PL, Pl,
pL, pl. - ANSWERS
The frequency of recombination between two genes is proportional to the distance between the
genes. The closer the genes are on the chromosome the less likely crossing over occurs. Linkage
map: 1% recombination = 1 map unit = 1 centiMorgan (cM), Map distances are additive. - ANSWERS
The non-random association between alleles at two locations on a chromosome is called linkage
disequilibrium. If the frequency of chromosomes with AB=Ab=aB=ab then the genes are in
equilibrium. If frequency of 1 allele is seen more (A more than B for ex) then genes are in linkage
disequilibrium. - ANSWERS
Autosomal dominant inheritance Examples: - ANSWERS
Achondroplasia- FGFR3 mutations, Always full penetrance with achondroplasia (so normal parents
have a child with aplasia then it's a new mutation). Heterozygous b/c homozygotes usually die in
utero - ANSWERS
Neurofibromatosis- NF1 (neurofibromin) and NF2 (merlin) mutations. - ANSWERS
,Incomplete dominance- mixed phenotype. In cases of disease, Dominant disorders are more severe
in homozygotes then in heterozygotes (termed also "semidominant") Ie. Familial
Hypercholesterolemia. - ANSWERS
Co-dominance- phenotypic expression of two different alleles for a locus ie. Blood type. - ANSWERS
Autosomal recessive inheritance: Examples: Cystic fibrosis, Tay-Sachs disease, Sickle-cell disease -
ANSWERS
Pseudodominance: the inheritance of an autosomal recessive trait mimics an autosomal dominant
pattern - ANSWERS
Males are hemizygous with respect to X-linked genes. - ANSWERS
X-linked dominant inheritance: Affected females are twice as common as affected males but males
usually more severely affected or the disorder may be lethal in males (Rett syndrome). - ANSWERS
X-linked recessive: incidence is much higher in males and affected males do not usually transmit the
disorder unless mother is a carrier. Heterozygote females are usually unaffected, but some may
express the condition with variable severity as determined by the pattern of X inactivation. A
significant proportion of isolated cases are due to new mutation (Duchenne muscular dystrophy-
DMD). - ANSWERS
Pseudoautosomal inheritance- group of genes on the inactive x chromosome are NOT inactivated.
Diseases associated with these genes are inherited similar to autosomal inheritance. - ANSWERS
Same amount of X-linked gene products between males and females achieved through dosage
compensation. Lyon Hypothesis states that the inactive X is NOT randomly chosen in each cell Ex. A
structurally abnormal X is preferentially inactivated. Inactivation is NOT complete- some genes can
escape inactivation (ie. Those with a functional homolog on the Y). Inactivation is NOT permanent-
reversed in development of germ cells (not passed on to gametes). - ANSWERS
The key player is the X-linked gene XIST→ X (inactive) specific transcript. XIST is transcribed to
produce a non-coding RNA that "coats" the X-chromosome and inactivates it. XIST is only expressed
from the inactive X. The histones on the coated X undergo methylation which causes the
chromosome to condense (heterochromatin), forming a Barr body. - ANSWERS
, -Some genes do not have Y homologue and do not undergo inactivation (e.g. steroid sulfatase gene)
- ANSWERS
-Random/skewed inactivation may result in affected/totally healthy heterozygotes. - ANSWERS
Variable expression of X-inactivation: On both extremes, a heterozygous female with recessive x-
linked disease could manifest the disease. In a case with a dominant X-linked trait, in which almost
all of a females X chromosomes with mutation is inactive, might not manifest this disease. Identical
twins could even have diff phenotypes due to skewed X inactivation. - ANSWERS
Mosaicism- X chromosome inactivation occurs randomly and inactivation pattern is passed to cell
progeny. Result: functional mosaicism in which female is a mosaic with respect to expression of
genes on X chrom. Ex. Calico Cat, B - dominant orange, b - recessive gene black, Genes for white:
autosomal. - ANSWERS
Genetic Heterogeneity can be the result of Locus Heterogeneity , Allelic Heterogeneity, and Modifier
Loci. - ANSWERS
Locus Heterogeneity- a single disorder, trait, or pattern of traits caused by mutations in genes at
different chromosomal loci. Ex. retinitis pigmentosa has autosomal dominant, autosomal recessive,
and X-linked origins. However, only one mutant locus is needed for the phenotype to manifest -
ANSWERS
Allelic heterogeneity - Many genetic loci possess more than one mutant allele. In the CFTR gene,
nearly 1400 mutations found. Some mutations cause classical CF + pancreatic insufficiency +
congenital absence of vas deferens, others cause lung disease with normal pancreatic fxn. Other
cause only male sterility. - ANSWERS
Modifier gene: A gene that affects the phenotypic expression of another gene. Specific alleles of one
or more genes (modifier genes) can sometimes dramatically modify the clinical severity of the
phenotype produced by mutations in a disease-causing gene.This source of clinical heterogeneity is
often referred to as genetic background. Ex. Twins with the same mutation in cystic fibrosis but one
is severely sick and one only moderately sick- explained by modifier genes. - ANSWERS
Candidate loci acting as modifiers for CFTR mutations:TGFB1 (cytokine transforming growth factor b
and MLB2 (Mannose-binding lectin). Different alleles of these genes modify the severity of CF -
ANSWERS
