Chapter 4 Flashcards, Genes and Genetic Diseases
What are genes?
Genes are the basic units of inheritance, composed of DNA and located on chromosomes.
What are the four types of nitrogenous bases in DNA?
The four types of nitrogenous bases in DNA are A, C, G, and T.
What is the physical structure of DNA?
The physical structure of DNA is a double helix.
How do DNA bases relate to proteins?
DNA bases code for amino acids, which make up proteins, specified by triplet codons of nitrogenous bases.
What is DNA replication based on?
DNA replication is based on complementary base pairing.
What is the role of DNA polymerase?
DNA polymerase is the primary enzyme involved in replication, adding bases to the new DNA strand and performing proofreading functions.
What is a mutation?
A mutation is an inherited alteration of genetic material (i.e., DNA).
What are mutagens?
Substances that cause mutations are called mutagens.
What is the mutation rate in humans?
The mutation rate in humans varies from locus to locus and ranges from 10−4 to 10−7 per gene per generation.
What are the two basic processes involving RNA?
Transcription and translation are the two basic processes in which proteins are specified by DNA.
How is RNA chemically different from DNA?
RNA is single stranded and has uracil rather than thymine as one of its four nitrogenous bases.
What is transcription?
Transcription is the process by which DNA specifies a sequence of mRNA.
What happens to RNA sequences before mRNA leaves the nucleus?
Much of the RNA sequence is spliced from the mRNA before it leaves the nucleus.
What are introns and exons?
Introns are excised sequences, while exons are the sequences that remain to code for proteins.
What are microRNAs (miRNAs)?
MicroRNAs are small RNA sequences, 17 to 27 nucleotides in length, that bind to specific mRNA sequences and down-regulate their expression.
What is long noncoding RNA (lncRNA)?
Long noncoding RNA (lncRNA) is a nontranslated RNA greater than 200 nucleotides in length, with at least 10,000 lncRNAs in the genome.
What do transcription factors do?
Transcription factors bind to DNA sequences called transcription factor binding sites to regulate the timing of transcription.
What is translation?
Translation is the process by which RNA directs the synthesis of polypeptides.
Where does translation take place?
Translation takes place in the ribosomes.
What is the role of tRNA during translation?
During translation, mRNA interacts with tRNA, which has an attachment site for a specific amino acid.
What types of cells do humans have?
Humans consist of diploid somatic cells (body cells) and haploid gametes (sperm and egg cells).
How many pairs of chromosomes do humans have?
Humans have 23 pairs of chromosomes: 22 pairs are autosomes and 1 pair are sex chromosomes.
What are the sex chromosomes for females and males?
Females have two homologous X chromosomes; males have an X and a Y chromosome.
What is a karyotype?
A karyotype is an ordered display of chromosomes arranged according to length and the location of the centromere.
What can be used to make chromosome bands more visible?
Various types of stains can be used to make chromosome bands more visible.
What is the prevalence of major diagnosable chromosome abnormalities?
About 1 in 150 live births has a major diagnosable chromosome abnormality.
What are the leading known causes of intellectual disability and miscarriage?
Chromosome abnormalities are the leading known cause of intellectual disability and miscarriage.
What is polyploidy?
Polyploidy is a condition in which a euploid cell has some multiple of the normal number of chromosomes.
What are triploidy and tetraploidy?
Triploidy is having three copies of each chromosome; tetraploidy is having four copies of each chromosome. Both conditions are lethal.
What are aneuploid cells?
Somatic cells that do not have a multiple of 23 chromosomes are aneuploid.
What causes aneuploidy?
Aneuploidy is usually the result of nondisjunction.
What is trisomy?
Trisomy is a type of aneuploidy in which one chromosome is present in three copies in somatic cells.
What is partial trisomy?
A partial trisomy is one in which only part of a chromosome is present in three copies.
What is monosomy?
Monosomy is a type of aneuploidy in which one chromosome is present in only one copy in somatic cells.
How do monosomies compare to trisomies in terms of physical defects?
In general, monosomies cause more severe physical defects than do trisomies.
What is Down syndrome?
Down syndrome is a trisomy of chromosome 21 and affects 1 in 800 live births.
Who is more likely to have a child with Down syndrome?
Down syndrome is much more likely to occur in the offspring of women older than 35 years of age.
How do sex chromosome aneuploidies compare to autosome aneuploidies?
Most aneuploidies of the sex chromosomes have less severe consequences than those of the autosomes.
What are the most commonly observed sex chromosome aneuploidies?
The most commonly observed sex chromosome aneuploidies are 47,XXX; 45,X (Turner syndrome); 47,XXY (Klinefelter syndrome); and 47,XYY.
What are some abnormalities of chromosome structure?
Abnormalities of chromosome structure include deletions, duplications, inversions, and translocations.
What are Mendelian traits?
Mendelian traits are caused by single genes, each occupying a position, or locus, on a chromosome.
What are alleles?
Alleles are different forms of genes located at the same locus on the chromosome.
What does an individual have at any given locus in a somatic cell?
An individual has two genes, one from each parent, and may be homozygous or heterozygous for a locus.
What is an individual’s genotype and phenotype?
An individual’s genotype is their genetic makeup, while the phenotype reflects the interaction of genotype and environment.
What happens at a heterozygous locus?
At a heterozygous locus, a dominant gene’s effects mask those of a recessive gene.
When is a recessive gene expressed?
The recessive gene is expressed only when it is present in two copies.
What modes of inheritance do genetic diseases caused by single genes usually follow?
Autosomal dominant, autosomal recessive, or X-linked recessive.
What is an important tool in the analysis of modes of inheritance?
Pedigree charts.
What do recurrence risks specify?
The probability that future offspring will inherit a genetic disease.
What is the recurrence risk for autosomal dominant diseases?
Usually 50%.
How can germline mosaicism alter recurrence risks?
Unaffected parents can produce multiple affected offspring due to a mutation in the germline.
Are skipped generations seen in classic autosomal dominant pedigrees?
No.
Are males and females equally likely to exhibit autosomal dominant diseases?
Yes.
What is incomplete penetrance?
A gene that is not always expressed phenotypically.
Is penetrance age-dependent?
Yes, as seen in Huntington disease and familial breast cancer.
What is variable expressivity?
A characteristic of many genetic diseases.
What is the recurrence risk for autosomal recessive diseases?
25% when both parents are heterozygous carriers.
Are males and females equally likely to be affected by autosomal recessive diseases?
Yes.
What is the relationship between consanguinity and autosomal recessive diseases?
Consanguinity is sometimes present and becomes more prevalent with rarer recessive diseases.
What is available for an increasing number of autosomal recessive diseases?
Carrier detection tests.
How does the frequency of genetic diseases change in the offspring of first-cousin matings?
It approximately doubles.
What happens to one of the two X chromosomes in each normal female somatic cell?
One is inactivated early in embryogenesis.
What is X inactivation?
Random, fixed, and incomplete inactivation of one X chromosome.
How is gender determined embryonically?
By the presence of the SRY gene on the Y chromosome.
What are X-linked genes?
Genes located on the X chromosome.
Why are X-linked recessive diseases seen more often in males?
Males need only one copy of the gene to express the disease.
Can fathers pass X-linked genes to their sons?
No.
Are skipped generations often seen in X-linked recessive disease pedigrees?
Yes, due to transmission through carrier females.
What do recurrence risks for X-linked recessive diseases depend on?
The carrier and affected status of the mother and father.
What is a sex-limited trait?
A trait that occurs in only one of the sexes.
What is a sex-influenced trait?
A trait that occurs more often in one sex than in the other.
What occurs during meiosis I that can cause recombinations of alleles?
Crossing over occurs and can cause recombinations of alleles located on the same chromosome.
How can the frequency of recombinations be used in genetics?
The frequency of recombinations can be used to infer the map distance between loci on the same chromosome.
What is the role of a marker locus in genetic disease prediction?
A marker locus, when closely linked to a disease-gene locus, can be used to predict whether an individual will develop a genetic disease.
What will facilitate gene identification, diagnosis, and disease treatment?
The complete human genome sequence will facilitate gene identification, diagnosis, and disease treatment.
