ch 4: Genes and genetic ds's

Question 1

Study of Genomics/Inheritance

Answer 1

*Traits are passed through genetic composition
*Genes, not chromosomes, (codons/unit of heredity)-don’t have enough info to pass traits
*study of the structure of the genome representing genes of all species.

Question 2

DNA=Protein

Answer 2

Chromosomes contain genes.
Genes are the basic unit of inheritance and are
composed of DNA. (sequences to make single protein)
DNA subunit or nucleotide contains
* one pentose sugar (deoxyribose).
* one phosphate group.
* one nitrogenous base.
Cytosine (C), thymine (T), adenine (A), guanine (G)
RNA differs by T=Uricel
DNA has a double helix structure.

Question 3

DNA-Structure

Answer 3

Protein- 1+ chain of polypeptide/amino acids (20)
DNA in the nucleus codes for future proteins
Double helix: 1 strand has no purpose but to “complement” the other strand.
Meaningful strand vs. complement
*after replication 2 identical molecules of DNA, “unzips” and re-pairs

Question 4

DNA as genetic code

Answer 4

DNA provides the code for all body proteins.
Proteins are composed of one or more polypeptides.
Polypeptides are composed of amino acids; there are twenty (20) amino acids:
* The sequence of three bases (codons) direct the
production of amino acids.
* Termination and nonsense codons stop the production of protein.

Question 5

Replication

Answer 5

The DNA strand is untwisted and unzipped.
* Single strand acts as a template.
DNA polymerase pairs the complementary bases.
* Adenine-thymine; cytosine-guanine
DNA polymerase adds new nucleotides and “proofs” the new protein; if not correct, the incorrect nucleotide is excised and replaced.

Question 6

Q1:
Which information is correct
regarding DNA polymerase?
DNA polymerase functions to
1. signal the end of a gene.
2. pull apart a portion of a DNA strand.
3. add the correct nucleotides to a DNA strand.
4. provide a template for the sequence of mRNA
nucleotides

Answer 6

ANS: 3
* This enzyme functions to add correct
nucleotides to the DNA strand, to edit
incorrect nucleotides, and enhance
the accuracy of DNA replication.
* 1. Termination or nonsense codons
signal the end of a gene.
* 2. RNA polymerase binds to a
promoter site on DNA and pulls apart
a portion of the DNA strand.
* 4. One of the DNA strands exposed by
the action of RNA polymerase
provides a template for the sequence
of mRNA nucleotides

Question 7

Mutation (1): Mutagens: Are agents, such as radiation and chemicals, that increase the frequency of mutations.

Answer 7

Is any inherited alteration of genetic material.
* Chromosome aberrations in number or structure
* Base pair substitution or missense mutation
One base pair is substituted for another; may result in changes in amino acid sequence.
May or may not cause disease or problems.
* Frameshift mutation: Involves the insertion or deletion of one or more base pairs to the DNA molecule.

Question 8

4 Types of genetic mutation

Answer 8

1. Point mutations: ie: beta-chain in HgB and sickle cell pts.
2. Early Stop mRNA: cell realizes somethings wrong and destroys itself, so it’s not translated
3. Frameshift: Insertion/Deletion of 1 or 2 base pairs causing shifts of entire code up/down by 1-2–which shifts proteins coding and the remaining gene becomes defective.
4. Trinucleotide repetition: same 3 sequences (usually quanine or cystocine), repeated over and over. ie: Fragile X Syndrome

Question 9

From genes to Proteins

Answer 9

DNA is formed in the nucleus;
protein is formed in the cytoplasm.
**Transcription and translation (important to Replication): DNA code is transported from the nucleus to the cytoplasm, and protein is subsequently formed.
Ribonucleic acid (RNA) mediates both processes.
* RNA is a single strand.
* Uracil rather than thymine is one of the four bases; all the rest are the same as DNA.

Question 10

Genetic cell division

Answer 10

1. Duplication of DNA
2. Risomal RNA moves along the mRNA
3. Then tRNA moves it into position

**Only mRNA goes into the nucleus, the other 2 work in the cytoplasm

Question 11

Transcription: occurs inside the nucleus in Eurcaryotic cells

Answer 11

*RNA is synthesized from the DNA template via RNA polymerase.
*binds to the promoter site on DNA.
* RNA seeks out ribosome in cytoplasm (assemby/production line of protein synth.)

*DNA specifies a sequence of mRNA. (attaching to ribosomal RNA through ribosome.
*At the same time, amino acids are carried into ribosomes by tRNA, *Creating polypeptide chains

*Transcription continues until the termination sequence is reached. (tRNA is immature)

*Gene splicing occurs. (Immature RNA modified to remove Introns, then reassembled and sent to cytoplasm)
Introns and exons (carry info for protein synthesis)
*mRNA then moves out of the nucleus and into the cytoplasm.

Main players: STOP codon’s-marking the protein at which process will end.

Question 12

Transcripton (2)

Answer 12

*There are 20 amino acids and 20 tRNA’s
Assemble proteins
Fold and transport
*Folding: molecular chaperones
Oversee protein behavior to make sure its not unfolded, if it does:
a. Denaturation: unfolding and attaching to another protein (ie: egg whites turning white)
b. Disease: ie Alzh. or heat/chem exposure

Question 13

Translation

Answer 13

the process by which RNA directs the synthesis of a polypeptide via the interaction with transfer RNA (tRNA).

tRNA contains a sequence of nucleotides (anticodon) complementary to the triad of nucleotides on the mRNA strand (codon).

Ribosome is the site of protein synthesis.
* Ribosome helps mRNA and tRNA make polypeptides.
* When ribosome arrives at a termination signal on the mRNA sequence, translation and polypeptide formation cease.

Question 14

At what site does protein synthesis occur?
The site of protein synthesis is the
1. codon.
2. intron.
3. ribosome.
4. anticodon

Answer 14

ANS: 3
* The ribosome is the site of actual
protein synthesis.
1. The codon is a set of three adjacent
nucleotides or a triplet that constitutes
the genetic code for a particular amino
acid that is to be added to a polypeptide
chain in the synthesis of a protein.
* 2. The intron is an RNA sequence that
has been removed by enzymatic action
prior to translation.
* 4. The anticodon is a set of three
adjacent nucleotides that undergo base
pairing with the appropriate codon in
the mRNA

Question 15

Chromosomes

Answer 15

Somatic cells
* Contain 46 chromosomes (23 pairs).
* One member from the mother; one from the father
* Diploid cells

Gametes
* Sperm and egg cells
* Contain 23 chromosomes.
* Haploid cells
* One member of each chromosome pair

Meiosis
* Formation of haploid cells from diploid cells

Question 16

Meiosis process (formation of haploid and diploid cells)-leads to production of GAMETES

Answer 16

1. Chromo. duplicate: 1 set goes to 1st dtr cell, 2nd set goes to 2nd cell.
2. When lining up, pairs will exchange ends, forming Gametes.

**Crossing over: when chromosomes line up, exchange of genetic info, if parent 1 has X and parent 2 has the gene that turns X off. Same genetics but a different form of genes

**Linkage: 2 genes standing close together, the chrome will become linked, it will be inseparable and have difficulty in the crossing over stage–genes will be inherited together (not the same as Ring chromosome)

Question 17

Chromosomes (cont)

Answer 17

Autosomes
* Are the first 22 of the 23 pairs of chromosomes in males and females.
* The two members are virtually identical and are thus said to be homologous.

Sex chromosomes
* Make up the remaining pair of chromosomes.
* In females, it is a homologous pair (XX).
* In males, it is a non-homologous pair (XY).

Karyotype (an individuals complete set of chromosomes.)
* The length and centromere location determine the ordered display of chromosomes.

Question 18

Chromosomal Aberrations-most common is miscarriage

Answer 18

Euploid cells (normal-stable & correct # of chrome.
* Have a multiple of the normal number of chromosomes. 46 or 23 pairs
* Haploid and diploid cells are euploid forms.

Polyploid cells: An euploid cell has more than double the diploid number.
* Triploidy: Is a zygote that has three copies of each chromosome.
* Tetraploidy: Has four copies of each chromosome (92 total).
Triploid and tetraploid fetuses do not survive
or are stillborn or spontaneously aborted.

Question 19

Chromo Aberrations: Aneuploidy

Answer 19

Aneuploidy: A somatic cell that does not contain a multiple of 23 chromosomes.
*Trisomy (trisomic): Is a cell that contains three copies of one chromosome.
Infants can survive with trisomy of certain chromosomes. 21=Downs
* Monosomy: Is the presence of only one copy of any chromosome. (missing)
Is often fatal.

Aneuploidy of sex chromosomes
* Usually presents less serious consequences than autosomes.
* Y chromosome usually causes no problems since it contains little genetic material.
* For the X chromosome, inactivation of extra
chromosomes largely diminishes their effect.

Nondisjuction-usually the cause of aneuploidy.
Failure of homologous chromo. or sister chromatids to separate normally during meiosis or mitosis.

Question 20

Autosomal Aneuploidy-can occur in any chromosome.
(female eggs can become damaged even while dormant d/t cellular proteins and nondisjunction)

Answer 20

Trisomy
* Chromosomes 13, 18, and 21 can survive; most others do not.
21: Down’s 1:800 live births, incidence incr w/ maternal age >45y (5% chance)
M: varying cognitive ability, low nasal bridge, epicanthal folds, protruding tongue, poor muscle control/tone
DS: incr risk non/congenital heart ds, resp infections, GI tract d/o, incr risk for Leukemia
13/18: present w/ more signif./severe complaints
16: very common–not viable

Partial trisomy
* Only an extra portion of a chromosome is present in each cell.
* Is not as severe as trisomies.

Chromosomal mosaics: trisomies that occur in only some cells of the body.
* Body has two or more different cell lines, each of which has a different karyotype.
* early in mitotic non-disjunction
*can occur in 1embryo cell, but not the other
*NOT as SEVERE b/c portion of chrome. is still present

• Two or more offspring have an autosomal dominant
  disease when the family has no history of the disease.
• Parent carries the mutation in his or her germline but does not actually express the autosomal dominant
  disease but transmits it to his or her offspring.

Question 21

Sex Chromosome Aneuploidy (similar to autosomal, but w/less severe presentation)
Trisomy X

Answer 21

Sex Chromosome Aneuploidy
*Occurs 1 in 400 males and 1 in 650 females.

Trisomy X is one of the most common aneuploidy.
* Females have three X chromosomes.
* Occurs 1 in 1000 female births.
* Symptoms are variable and include sterility, menstrual, irregularity, and/or cognitive deficits.
* Symptoms worsen with each additional X chromosome.

If incr Y-insigificant conseq. d/t small amt of genetic material and approx. location on gene
If Incr X’s-more sev.:
w/ no X, 2Y’s-no chance of existence (zygote dies)

Question 22

**Sex Chrom. Aneuploidy: Turner Syndrome

Answer 22

Turner syndrome
* Females have only one X chromosome, Denoted as karyotype 45,X.
* Occurs 1:2500 female births

Characteristics include
** absence of ovaries (sterile).
** short stature.
* webbing of the neck.
* widely spaced nipples.
* high number of aborted fetuses.
edema
Coarction of aorta (narrowing, incr force needed)
**gonadal streaks: underdev. overaries; susceptible to NEOPLASM
*** X chromosome that is usually inherited from the mother.
Tx:
* Teenagers receive estrogen.

Question 23

**Sex Chrom. Aneuploidy: Klinefelter Syndrome

Answer 23

Klinefelter syndrome
* Individuals with at least one Y and two X chromosomes. (47XXY Karotype)

Characteristics include:
* male appearance.
* femalelike breasts (gynecomastia).
* small testes. (sterile)
* sparse body hair.
* 1 in 1000 male births.
* Long limbs
* Some individuals can be XXXY and XXXXY; will have male appearance; abnormalities will increase with each X; can also have an extra Y chromosome.
* Disorder increases with the mother’s age
**Can show up in next generation

@Chrom. can be lost or interchanged or duplicated; Altered d/t small amt of genetic material involved

Question 24

Q3: A female has one X chromosome. Which
diagnosis will the nurse observe documented on the chart?
1. Trisomy X syndrome
2. Klinefelter syndrome
3. Fragile X syndrome
4. Turner syndrome

Answer 24

ANS: 4 Turner
*Another sex chromosome aneuploidy is the
presence of a single X chromosome and no
homologous X or Y chromosome, resulting in
a total of 45 chromosomes. The karyotype is
designated 45,X, and it causes a set of
symptoms known as Turner syndrome.

*1. Instead of two X chromosomes, these
females have three X chromosomes in each
cell.
2. Individuals with at least two X chromosomes and a Y chromosome in each cell (47,XXY karyotype) have a disorder known as Klinefelter syndrome.
*3. Fragile X syndrome is usually caused by
an elevated number (more than about 200)
of repeated DNA sequences in the first exon
of the fragile X gene.

Question 25

Abnormalities of Chrom. Structure- Breakage
Effects may or may not have serious consequences.

Answer 25

.Chromosome breakage
*If a chromosome break occurs, then the break is usually repaired with no damage.
* Breaks can stay or can heal in a way that alters the structure of the chromosome.
* Can occur spontaneously.
* Agents of chromosome breakage include Ionizing radiation, chemicals, and viruses

Question 26

**Abnormalities of Chrom. Structure: *Deletions, Duplications, and *Inversions

Answer 26

Deletions
* Chromosome breakage or loss of part of DNA
*Ex: Cri du chat syndrome or “cry of the cat” (5P deletion) C: duplication of short arm of chrome. + deletion.
M: High-pitched cry at birth, Low birth weight, mentally challenged, and microcephaly
*when gamete w/ deletion is present, decrease probability of uniting w/ another gamete w/ deletion.
usually unite w/ “normal” gamete

Duplications
* Excess genetic material
* Usually have less serious consequences.

Inversion (Flips & is reversed end to end)
* Chromosomal rearrangement in which a chromosome segment is inverted: ABCDEFG becomes ABEDCFG.
* Usually affects offspring.

Question 27

Abnorms of Chrom. Structure
*Translocation and *Ring Chromosome

Answer 27

Translocation: the interchange of genetic material between nonhomologous chromosomes.

2 types: Robersonian and Reciprocal

*** Robertsonian: Long arms of two nonhomologous chromosomes fuse at the centromere, forming a single chromosome; (short arm lost completely in 2nd div)
*is common in Down syndrome.
13, 14, 15, 21, 22 (short arms too small w/ no ess. genetic material)
Presents: normal
* Reciprocal: Breaks take place in two different chromosomes, and the material is exchanged.

Ring Chromosome: when telomeres of each arm have been deleted and broken ends fuse together in a ring.
*Triggers series of breakage, fusions, and bridge-events, which continues breakage down the line.
*Ring is NOT dangerous, but following series of breakage is
*P: diverse symptoms, FTT (mild-sev)
C : iodinized radiation, chem exp to affected chrome.

Question 28

Abnorms of Chrom. Structure
*Fragile Sites-Fragile X Syndrome

Answer 28

Fragile sites
* Chromosomes develop breaks and gaps when the cells are cultured in a folate-deficient medium. (see on microscope)
* Most have no apparent relationship to disease.

**Fragile X Syndrome can occur here
long arm of the X chromosome; has an elevated number of repeated DNA sequences.
O: 1:400 male and 1:8000 female
* Is associated with being mentally challenged;
*is second in occurrence to Down syndrome.
if female inherits, trait may be expressed or passed on
if male, it WILL be expressed

Question 29

Elements of Formal Genetics (1)

Answer 29

Genetic inheritance
* Mechanisms by which an individual’s set of paired chromosomes produces traits.
* Explains the patterns of inheritance for traits and diseases that appear in families.

Mendelian traits (KNOW PUNET SQUARE)
* Are inherited traits primarily attributed to single genes.
* Gregor mendel: Garden pea experiment, w/ notion of hybridization
Dominant vs Recessive Traits
AA=A; AB=A, BB=B
*Harder to express ressesvie traits-2 must be present, also may lead to disease process

Question 30

Punet Sqaure’s

Answer 30

if 2 dom traits = new cell w/ dom traits exp.
if 1 dom/1 ress= new cell w/ dom traits exp
if 2 ress= ress trait expressed

Question 31

Elements of Formal Genetics (2)
Locus, Allele, Polymorphic

Answer 31

*Locus: Is the location occupied by a gene on a chromosome.

*Allele: Is one of several different forms of a gene at a locus. (2 alleles for ea gene)
* One member of a gene from the mother; one member of a gene from the father
* *Homozygous: When genes are identical
* *Heterozygous: When genes are different

Polymorphism or polymorphic
* Is a locus that has two or more alleles that occur with appreciable frequency.

Question 32

Elements of Formal Genetics (3)
Genotype vs Phenotype
Ex: PKU-phelylketonuria

Answer 32

*Genotype: composition of genes at a given locus. (genetic material for trait)

*Phenotype: outward appearance of an individual, results from genotype and the environment. (expression of trait)
Complimentary vs Collaborative: types of genes depend on another, not separately

EX: Blood type A (phenotype) is genetically AA or AO (genotype)

Example: Infant with phenylketonuria (PKU) has the PKU genotype.
PKU-Enzyme defect of PAH protein on genotype phenylalanine
tx: dietary changes
* If left untreated, the infant will have cognitive impairments, which is the PKU phenotype.
* If treated, the infant will still have the PKU genotype but can have a normal phenotype.

Question 33

Elements of Formal Genetics (4)
Genetic Expression (lecture notes)

Answer 33

**Gene Expression (when visible trait): process of genes being turned “off/on” via induction or repression.
If off-protein won’t be completed…not exp

Intermediate Penitrance (if recessive gene expressed)
Polygenetic: recessive trait expressed d/t MANY coding genes or environment change
Multifactoral: when 1 gene “masks” another, the other will never be expressed

Question 34

Elements of Formal Genetics (5)
Dom, Rec, Carrier

Answer 34

Dominance and recessiveness
* *If two alleles are found together, then the allele that is observable is dominant and the one whose effects are hidden is recessive.
dominant allele = a capital letter
recessive allele = a lowercase letter.
* Alleles are either heterozygote or homozygote.
* Alleles can be codominant; that is, both alleles are expressed. (rec x2)

Carrier
* Has a disease allele but is phenotypically normal.
* Can pass disease to offspring.

EX: Sickle Cell S= no trait; s= +trait exp
Ss-no ds, but carrier
SS-neither ds/carrier
ss-DS

Question 35

What are 3-4 genetic DS categories (lecture notes)
Suggested that when looking at diseases categorize them into the following:

Answer 35

1. Mendelian d/o: single gene mutations
   a. autosomal dominant: abnormal protein development. Disease WILL BE present
   b. autosomal recessive: missing normal protein (1 normal allele + 1 defective) Diisease will NOT be present
   c. X-linked (dom/rec): defective gene only on X-chrom
   -men will/won’t carry
   - women won’t carry
2. Complex Multigenetic: multifactorial +environ
3. Changes in entire Chromosome
4. Grab bag/Misc.

Question 36

Transmission of Genetic DS

Answer 36

Mode of inheritance: Is the inherited pattern through
the generations of a family.

***Mendel’s two laws
** Principle of segregation
* Homologous genes separate from one another.
* Each cell carries only one of the homologous genes.
** Principle of independent assortment
* Hereditary transmission of one gene has no effect on the transmission of another.

Chromosome theory of inheritance
* Chromosomes follow Mendel’s two laws.

Pedigree
* Is the tool used to study specific genetic disorders within families.
* Begins with the proband.
* Propositus (male) or proposita (female)
Usually the first person in the family diagnosed or seen in a clinic

Question 37

Autosomal Dominant Inheritence DS (rare)

Answer 37

Occurs in fewer than 1 of 500 individuals.
Expressed = M/F
Passed on = M/F
No generational skipping

The union of a normal parent with an affected
heterozygous parent usually produces the affected
offspring.
An affected parent can pass either a disease gene or
a normal gene to his or her children; each event has
a probability of 0.5; on average, half will be
heterozygous and will express the disease and half
will be normal.

Recurrence risk: probability that a family member will have a genetic disease.
* When one parent is affected by an autosomal dominant disease and the other is normal, the occurrence and recurrence risks for each child are one half. and each birth is an independent event

*New mutation: Is when no history of an autosomal
dominant condition is present, but the child develops
the mutation.
* Parent’s subsequent offspring is not greater than that of the general population.
* Offspring of the affected child will have a recurrence risk of one half.

Question 38

**Penetrance

Answer 38

percentage of individuals with a specific genotype
who also express the expected phenotype.

Incomplete penetrance
* Individual who has the gene for a disease but does not express the disease
* Ex: Retinoblastoma (eye tumor in children) (90%)

Age-dependent penetrance
* Does not express a disease until a certain age is reached.
* Ex: Huntington disease

**Important to note: single gene ds can be modified by other genes…Incr/decr the severity of expression.
Modifications (ie: Race) can effect all ds processes and other genetic modifications.

Question 39

Expressivity: Rate of presentation of ds/trait

Answer 39

a variation in a phenotype associated with a
particular genotype.

Can be caused by modifier genes, environmental
factors, and mutations.

Ex: von Recklinghausen disease
* Is autosomal dominant.
* Expressivity varies from brown spots on the skin to
malignant tumors, scoliosis, gliomas, and neuromas.

Ex: Hemophelia A: mutation to just 1 amino acid @ Factor 8 gene

Variability: 2 people w/ same genetic sequences, 1 has severe presentation the other has mild-mod.

Question 40

Autosomal Recessive Inheritance

Answer 40

Rare, but many individuals are carriers.
Expressed = M/F

Abnormal allele is recessive, and the person must be
homozygous to express the disease.
Both alleles abnorm, 2 parents carriers=1/4 offspring w/ 75% chance of ds?

NOT sex-linked
Trait usually appears in the children, not in the
parents. (Generational skipping)
Consanguinity (marriage b/w related Ind.)

Ex: Cystic fibrosis
* Gene forms chloride channels with defective transport, which leads to a salt imbalance that results in abnormally thick, dehydrated mucus. The lungs and pancreas are affected; the person does not survive past 40 years of age.

More freq penetrence early in life:
Expressivity: Varies
**If 1 rec gene can pair w/ 1 dom gene-ds could be irradiated (carrier tests available
Recurrence Risk: When both parents are heterozygous carriers of an autosomal recessive disease, the occurrence and recurrence risks for each child are 25%; one-quarter of the offspring are normal, and one-half are carriers.

Question 41

Consanguinity

Answer 41

Is known as inbreeding.
Is the mating of two related individuals.
Offspring are termed inbred.
Proportion of shared genes depends on the closeness
of the biologic relationship.
Dramatically increases the recurrence risk of
recessive disorders.
* Offspring of marriages of first cousins who are affected by genetic diseases is approximately double that of the general population.

Question 42

X-Linked Inheritence

Answer 42

disorder that involves X and Y chromosomes.
Y-linked disorders are uncommon because the Y chromosome contains relatively few genes.

Females: Have two X chromosomes ; can be homozygous for the disease, homozygous for normal, or heterozygous.

Males: Have one X chromosome; are always hemizygous; if inherits an X recessive gene, then he will express the disease because no normal allele is present to counteract the diseased allele;
males are affected more often with X
recessive conditions.

X-inactivation: process by which one X chromosome in the somatic cells of females is permanently inactivated.
* Barr bodies: Inactivated X chromosome
* Females have 1 inactive X chromosome.
* Males have no inactive X chromosomes.
* Is always one less than the number of X chromosomes in the cell.
* Occurs early in embryonic development.
* Can have incomplete inactivation.
* X-inactive specific transcript (XIST) gene which causes X-inactivation uses methylation.

Sex determination
* Begins during the sixth week of gestation.
* One copy of the Y chromosome is sufficient to initiate the process of gonadal differentiation that produces a male fetus.
* Number of X chromosomes does not alter this process.
* Sex-determining region on the Y chromosome (SRY) gene begins male gonadal development.
* Triggers other genes.
* Can cross over to the X chromosome; is an apparently normal XX karyotype but with a male phenotype.
* Can be deleted from the Y chromosome: XY female.

Sex-limited trait: can occur in only one of the sexes.

Sex-influenced trait: occurs significantly more often in one sex than in the other.

Evaluation of pedigrees
* Is sometimes difficult to predict.
* Uses computer programs and statistical techniques.

Question 43

X-linked Recessive

Answer 43

Sig. > M than F
Females must inherit two copies of the recessive allele (one from each parent) to express the disease, whereas males need only one copy (from the mother) to express the disease.
* Because a father can give a son only a Y chromosome, the trait is never transmitted from father to son. but to all dtrs (phenotypically carriers) to 1/2 sons who WILL be affected.
Trans: through series of female carriers, looking like generational skipping

Ex: Duchenne muscular dystrophy (DMD)
* Occurs 1 in 3500 males.
* Exhibits progressive muscular degeneration.
* Deletion of DMD gene causes dystrophin not to work properly; consequently, muscle cells do not survive.

Question 44

Q4: Which information indicates that
the nurse has a good understanding of X-linked
recessive inheritance?
1. The gene is passed from an affected father to all of his daughters.
2. The trait is observed significantly more often in
females than in males.
3. Males are said to be heterozygous for the X
chromosome.
4. A sex-limited trait is one that occurs significantly more often in one sex than in the other.

Answer 44

ANS: 1
The gene is passed from an affected father to
all his daughters, who, as phenotypically
normal carriers, transmit it to approximately
half their sons, who are affected.

2. The trait is seen much more often in males
   than in females because females must inherit
   two copies of the recessive allele (one from
   each parent) to express the disease, while
   males need only inherit one copy (from their
   mother) to express the disease.
3. Males, having only one X chromosome, are said
   to be hemizygous for genes on this chromosome.
4. A sex-influenced trait is one that occurs much
   more often in one sex than in the other. A sex-
   limited trait is one that can occur in only one of the
   sexes.