Genes and Genetic Diseases Flashcards
Genetics is the study of
biologic inheritance
the substance that gives the nucleus a granular appearance
chromatin
chromatin is observable in ___________ cells
non dividing
just before the cell divides, the chromatin condenses to form discrete, dark-staining organelles, which are called
chromosomes
With the rediscovery of Mendel’s important breeding experiments at the turn of the twentieth century, it soon became apparent that the chromosomes contained _____, the basic units of inheritance
genes
The primary constituent of chromatin is
deoxyribonucleic acid (DNA)
Genes are composed of
sequences of DNA
By serving as the blueprints of proteins in the body, genes ultimately
influence all aspects of body structure and function
Estimates suggest that there are approximately ____________ genes.
20,000 to 25,000
An error in one of these genes often leads to
a recognizable genetic disease
To date, more than _____ genetic traits and diseases have be identified and cataloged.
20,000
As infectious diseases continue to be more effectively controlled, the proportion of beds in pediatric hospitals occupied by children with genetic diseases has
risen
many common diseases that primarily affect adults, such as hypertension, coronary heart disease, diabetes, and cancer, are now known to
have important genetic components
“gene therapy” is
the utilization of normal genes to correct genetic disease
each cell nucleus contains an identical __________ __ ___________
complement of chromosomes
DNA is a double _____
helix
DNA is made up of these 3 basic components
five-carbon monosaccharide deoxyribose(sugar); a phosphate molecule; and four types of nitrogenous bases
Two of the bases, ________ and _______ are single carbon-nitrogen rings called ___________
cytosine and thymine
PYRIMIDINES
The other two double carbon-nitrogen ring bases are called _______. Their names are _______ and _______.
PURINES
adenine and guanine
the four bases are commonly represented by their first letter —-
ACTG
the sides of the dna ladder consist of
deoxyribose and phosphate molecules (united by strong phosphodiester bonds)
the rungs of the ladder are formed from
the nitrogenous bases
the base (rung) projecting from one side is bound to the base projecting from the other by a weak ________ bond
hydrogen
which nitrogenous bases pair with eachother?
adenine—thymine
guanine—cytosine
Each DNA subunit—consisting of one deoxyribose molecule, one phosphate group, and one base—is called
a nucleotide
dna directs the synthesis of all the body’s
proteins
proteins are composed of one or more
polypeptides (intermediate protein compounds)
polypeptides in turn consist of
sequences of amino acids
the body contains __ different types of amino acids. they are specified by the _ ___________ _____
20
4 nitrogenous bases
To specify (code for) 20 different amino acids with only 4 bases, different combinations of bases, occurring in groups of 3, are used. These triplets of bases are known as
codons
the genetic code is ‘universal’. what does this mean?
ALL living organisms use precisely the same DNA codes to specify proteins EXCEPT FOR MITOCHONDRIA - the cytoplasmic organelles in which cellular respiration takes place - they have their own extra nuclear DNA
each side of the DNA molecule consists of alternating
sugar and phosphate groups
the sequence of these nitrogenous pairs constitutes a genetic code that determines
the structure and function of a cell
The consistent pairing of adenine with thymine and of guanine with cytosine is known as _____________ ____ _______ and is the key to ________ ___________
complementary base pairing
a mutation is
any inherited alteration of genetic material
when one base pair replaces another, this type of mutation is called a
base pair substitution
base pair mutations may cause _______ or be ______ substitutions that do not change amino acids.
disease or be subtle
base pair mutations that DO NOT CHANGE THE AMINO ACID SEQUENCE and HAVE NO CONSEQUENCE are called
silent mutations
base pair mutations that do alter amino acids consist of two basic types:
missense and nonsense mutations
missense mutations produce a change in
a single amino acid
nonsense mutations produce
a stop codon in the messenger RNA (mRNA)
frameshift mutations result from
the ADDITION or DELETION of a number of bases THAT IS NOT A MULTIPLE OF THREE
a frameshift mutation alters
all of the codons downstream from the site of insertion of deletion
how many base pairs in a codon?
3
why are frameshift mutations called so?
they change the entire “reading frame”
why do “in-frame” insertions or deletions tend to have less severe disease consequences?
a multiple of three bases is inserted or lost
what are mutagens?
they are agents increase the frequency of mutations
radiation, mustard gas, formaldehyde etc
mutations are rare events. The rate of spontaneous mutations in humans is
10^-4 to 10^-7 per gene per generation
dna sequences that have particularly high mutation rates are called
mutational hot spots
DNA is formed and replicated in the
cell nucleus
protein synthesis takes place in the
ribosome
DNA code is transported from
nucleus to cytoplasm
protein is formed through two basic processes:
transcription and translation
transcription and translation are mediated by
ribonucleic acid (RNA)
a karyotype or karyogram is
an ordered display of chromosomes
A cell that does not contain a multiple of 23 chromosomes is
an aneuploid cell
Aneuploidy is usually the result of
nondisjunction
among live births, how many have a form of sex chromosome aneuploidy
about 1 in 500 males and 1 in 900 females
trisomy x effects
1 in 1000 newborn females
describe trisomy x
no overt physical abnormalities, although sterility, menstrual irregularity, or mental retardation is sometimes seen
in general, the more x chromosomes a female has the more
retardation tends to be present, along with various physical defects in those with 5 or more
sterile means
not able to produce children
Genetic diseases caused by single genes usually follow
autosomal dominant, autosomal recessive, or X-linked recessive modes of inheritance.
Pedigree charts are important tools in the
analysis of modes of inheritance
Recurrence risks specify
the probability that future offspring will inherit a genetic disease.
For single-gene diseases, recurrence risks remain
the same for each offspring, regardless of the number of affected or unaffected offspring.
The recurrence risk for autosomal dominant diseases is usually
50%
Germline mosaicism can alter recurrence risks for genetic diseases because
unaffected parents can produce multiple affected offspring
This situation in germline mosaicism occurs because
the germline of one parent is affected by a mutation but the parent’s somatic cells are unaffected.
Skipped generations are not seen in
classic autosomal dominant pedigrees
Males and females are equally likely to exhibit _________ _______ _______ and to pass them on to their offspring
autosomal dominant diseases.
Many genetic diseases have a _______ age of onset.
delayed
A gene that is not always expressed phenotypically is said to have
incomplete penetrance
incomplete penetrance means
A gene that is not always expressed phenotypically is said to have
Variable expressivity is
a characteristic of many genetic diseases
Genomic imprinting, which is associated with ________, results in
______________________________________________________________________
methylation
differing expression of a disease gene, depending on which parent transmitted the gene.
Epigenetics involves changes, such as
the METHYLATION of DNA bases, that do not alter the DNA sequence but can ALTER THE EXPRESSION OF GENES
Most commonly, parents of children with autosomal recessive diseases are
BOTH HETEROZYGOUS CARRIERS of the disease gene.
The recurrence risk for autosomal recessive diseases is
25%
When it comes to autosomal recessive diseases, males and females are
equally likely to be affected
Consanguinity is sometimes present in
families with autosomal recessive diseases
Consanguinity also becomes more prevalent with
rarer recessive diseases
Carrier detection tests for an increasing number of _________ _________ ________ are available.
autosomal recessive diseases
The frequency of genetic diseases approximately _______ in the offspring of first-cousin matings
DOUBLES
In each normal female somatic cell, one of the two X chromosomes is
inactivated early in embryogenesis
X inactivation is
random, fixed, and incomplete
X inactivation also may involve
METHYLATION
Gender is determined embryonically by the presence of
the SRY gene on the Y chromosome
Embryos that have a Y chromosome (and thus the SRY gene) become
males
Embryos that lack the Y chromosome become
females
When the Y chromosome lacks the SRY gene
an XY female can be produced
an X chromosome that contains the SRY gene can produce
an XX male
X-linked genes are those that are located on the
X chromosome
Nearly all known X-linked diseases are caused by
X-linked recessive genes
Males are __________ for genes on the X chromosome
hemizygous
X-linked recessive diseases are seen much more often in which sex? And why?
Males because they need only one copy of the gene to express the disease.
Fathers cannot pass ________ genes to their sons
X-linked
Skipped generations often are seen in X-linked recessive disease pedigrees because
the gene can be transmitted through carrier females
Recurrence risks for X-linked recessive diseases depend on
the carrier and affected status of the mother and father
A sex-limited trait is one that
occurs only in one sex (gender).
A sex-influenced trait is one that occurs
more often in one sex than in the other.
A marker locus, when closely linked to a disease-gene locus, can be used to
predict whether an individual will develop a genetic disease
A more complete gene map will facilitate
marker studies, gene cloning, studies of gene function and interaction, and gene therapy.
Traits that result from the combined effects of several loci are
polygenic
Multifactoral means
When environmental factors also influence the trait
Many multifactorial traits have a __________ __ _________ that once its crossed has been crossed, the disease may be expressed.
threshold of liability
What is used to estimate recurrence risks for multifactorial diseases?
Empirical risks - based on direct observation of large numbers of families
Recurrence risks for multifactorial diseases become higher if
more than one family member is affected or if the expression of the disease in the proband is more severe.
Recurrence risks for multifactorial diseases ________ _______ for more remote relatives.
decrease rapidly
The frequency of recombinations can be used to
infer the map distance between loci on the same chromosome
During meiosis I, _________ occurs and can cause ____________________________________________________
crossover
recombinations of alleles located on the same chromosome.
