Genetics Flashcards
genetics I genetics II genetics III genetics IV genetics V
how did mendel discover the basic principles of heredity
he bread garden peas
what is a heritable feature that varies among individuals called
a character
what are the different variations of a character called
a trait
what are advantages of using peas to study heredity
there are many varieties
they have a short generation time
large number of offspring from each mating
in order to prevent the peas from self pollinating what did mendel do
he removed the stamens before they produced pollen and then extracted the pollen and dusted it onto a carpel of the desired plant he wanted to cross
what is true breeding
organisms that produce offspring of the same variety over many generations of self pollination
e.g. a plant with purple flowers is true breeding if over many generations of self pollination all plants only have purple flowers
what are the true breeding parents referred to as
the P generation
what is the result of breeding 2 true breeding varieties
production of a hybrid
what are the hybrid offspring called
F1 generation
what is produced when two F1s cross pollinate or an F1 self pollinates
F2 generation
what is the law of segregation
mendels first law - 2 alleles in a pair segregate into different gametes during gamete formation
what is the law of independent assortment
mendels second law - each pair of alleles segregates independently of the other pair during gamete formation
alternative versions of genes (alleles account for variations in ……………………..
inherited characteristics
for each character an organism inherits 2 ………….. of a gene, one from each parent
alleles
if the two alleles at a locus differ then one, ………….. allele, determines the organisms appearance and the other, the ………….. allele has no noticeable affect on the organisms appearance
dominant
recessive
the allele for which trait in pea colour is dominant
purple
two ……….. for a gene segregate during gamete formation and end up in a different gamete
alleles
if an organism has identical alleles for a particular character then that allele is present in some/all gametes
all
explain why the purple colour in pea plants is dominant
the allele for purple plant colour instructs the production of n enzyme that helps synthesise purple pigment but the allele for white plant colour results in the production of no enzyme. the presence of one purple allele results in sufficient pigment for purple flowers
homozygotes/heterozygotes are true breeding
homozygotes
given a purple coloured pea plant can we tell if it is homozygous or heterozygous by looking at it
no - because both genotypes Pp and PP give the same phenotype
how would we identify if a purple pea plant is homo/heterozygous
cross it with a white coloured true breeding plant (pp)
- if all the offspring flowers are purple then the purple pea must have been homozygous (PP)
- if both purple and white phenotypes appear in the offspring (Pp and pp) then the purple plant must have been heterozygous (Pp)
what is a testcross
breeding an organisms of unknown genotype with a recessive homozygote
what is a monohybrid
heterozygous for one particular character being followed in the cross (produced by homozygous parents)
what is a monohybrid cross
a cross between 2 organisms that are heterozygous for the character being followed
what is a dihybrid
an organism that is heterozygous with respect to 2 genes of interest (produced by parents that are homozygous for both traits)
what is a dihybrid cross
a cross between 2 organisms that are each heterozygous for both characters being followed
which law are dihybrid experiments the basis for
the law of independent assortment - 2 or more genes assort independently - each pair of alleles segregates independently of any other pair of alleles during gamete formation
the law of independent assortment only applies to genes on the same/different chromosomes
different chromosomes - chromosomes that are not homologous - or alternatively genes that are very far apart on the same chromosome
an event that is certain to occur has a probability of ……….
1
an event that is certain not to occur has a probability of ..……
0
what is complete dominance
when the dominant allele is completely dominant - the heterozygote and the dominant homozygote are indistinguishable
what is incomplete dominance
neither allele is completely dominant - F1 hybrids have a phenotype somewhere between those of the 2 parent varieties
e.g. red flower x white flower = pink flower
what is codominance
the 2 alleles each affect the phenotype in 2 separate distinguishable ways (in a normal heterozygote only one allele would be seen in the phenotype but in codominance both are apparent)
what is pleiotropy
the ability of a single gene to have multiple effects on the phenotype
what is epistasis
a type of gene interaction in which the phenotypic expression of one gene (at one locus) alters that of another independently inherited gene (at a different locus)
what are quantitative characters
vary in populations along gradations along a continuum
what type of inheritance is usually indicated by quantitative characters
polygenic inheritance e.g. height results from polygenic inheritance - over 180 genes that effect height
what is polygenic inheritance
an additive effect of 2 or more genes on a single phenotypic character (converse of pleiotropy where a single gene affects multiple characters but here multiple genes affect one character)
generally, the phenotypic range is broadest for what kind of characters
polygenic
what is a multifactorial character
many factors, both genetic and environmental influence phenotype
what are carriers
people that are heterozygous for a recessive disorder and do not themselves have the disease but they could pass it to their offspring
what happens to people with cystic fibrosis
chloride transport channels are defective or absent and this causes an uptake of water into cells
this makes the mucus that coats certain cells thicker and stickier than normal leading to pleiotropic effects such as chronic bronchitis and recurrent bacterial infections
what is sickle cell disease caused by
substitution of a single amino acid in the haemoglobin of red blood cells
is sickle cell disease dominant or recessive
recessive - because two alleles are required to experience full blown sickle disease
explain why heterozygotes for sickle cell disease experience symptoms
the normal allele is incompletely dominant to the sickle cell allele
what is the advantage of being heterozygous for sickle cell anaemia
reduces severity of malaria attacks especially among young children because it results in lower parasite densities in the blood - this represents heterozygous advantage
a lethal recessive allele is only lethal when ……………
homozygous
a lethal dominant allele can/cannot be passed to future generations
cannot - individual die before they can mature and reproduce
what is amniocentesis
a needle is inserted in to the uterus to collect some amniotic fluid
some genetic disorders can be detected from the presence of certain molecules in the amniotic fluid
the cells can be cultured to obtain sufficient numbers for karyotyping
what is chronic villus sampling
a narrow tube is inserted into the uterus through the cervix and suctions out a small sample of tissue from the placenta
the karyotype can be carried out immediately because the cells are proliferating rapidly enough
apart from amniocentesis and CVS what else can be done to test the DNA of the fetus
blood testing of the mother as it contains the DNA of the fetus
what is PKU
when those with the disease cannot properly metabolize phenylalanine and I can accumulate with its by product in the blood causing mental retardation
if detected early a special diet can be used to prevent this from occurring
how many chromosomes does the fruit fly have
4 pairs - 3 pairs of autosomes and 1 pair of sex chromosomes
what is the mutant eye colour in fruit flies
white
which eye colour is dominant in fruit flies
red
on which sex chromosome is the gene for white eyes in fruit flies
the X chromosome
why can X and Y chromosomes behave like homologs even though they are different lengths
short segments at either end of the Y chromosomes are the only regions that are homologous with regions of the X.
this allows male X and Y chromosomes to pair and behave like homologs in meiosis in the testes
after meiosis each egg receives one …. chromosome
X
after meiosis half of the sperm receive an ……. chromosome and the other half receive a ……….. chromosome
X and Y
on fertilization of an egg, a sperm containing an X chromosome will produce a …………. zygote whereas a sperm containing a Y chromosome will produce a ……….. zygote
female
male
what is the SRY
a gene on the Y chromosome - the sex determining region of Y
it is required for the development of testes
what happens to the embryo in the absence of SRY
the gonads develop into ovaries
a gene located on either sex chromosome is called what
a sex linked gene
the genes located on the X chromosome are called what
X linked genes
the genes located on the Y chromosome are called what
Y linked genes
the development of female gonads requires which gene located on chromosome 1 (autosome)
WNT4
what does the gene WNt4 code for
a protein that promotes ovary development
the X/Y chromosome has more genes unrelated to sex than the X/Y chromosome
X has more than Y
fathers pass X-linked alleles to all of their ………… but none of their ………….
daughters - they receive an X chromosome from their mother and their father
sons - a boys X chromosome always comes from his mother and the Y chromosome has to come from the father
TRUE/FALSE mother can pass X linked alleles to both their sons and daughters
TRUE the mother passes an X chromosome to both her daughters and sons
if an x linked trait is due to a recessive allele when will a female express the phenotype
only if she is homozygous for the allele
will a male receiving an x linked recessive allele form his mother express the trait
yes because he only has one X chromosome so there isn’t anything to mask it
what is Duchenne muscular dystrophy
an X linked recessive disease characterized by the progressive weakening of muscles and loss of coordination
it is caused by the absence of a key muscle protein called dystrophin - the gene for which has a specific locus on the X chromosome
what is haemophilia
an X linked recessive disorder defined by the absence of one or more of the proteins required for blood clotting
do females produce twice the amount of the x linked proteins compared to males
no - one of their X chromosomes is almost fully inactivated during early embryonic development so males and females have the same effective dose of most x linked genes
what happens to the inactivated X chromosome in females
it condenses into a compact object called a Barr body which lies along the inside of the nuclear envelope
what happens to Barr body chromosomes in the ovaries
they are reactivated in the cells that give rise to eggs, resulting in every female gamete having an active X chromosome after meiosis
which X chromosome in females that is inactivated to the Barr body is random and occurs independently of each embryonic cell present at the time of inactivation
as a consequence females consist of a mosaic of 2 types of cells - describe each one
- those with the active X derived from the father
- those with the active X derived from the mother
TRUE/FALSE if a female is heterozygous for an X linked trait half of the cells will express one allele while the other half express another allele
TRUE
describe a case in which mosaicism can be observed in females
X linked mutation that prevents the development of sweat glands
a woman that is heterozygous for the trait will have patches of normal skin and patches of skin lacking sweat glands
what does inactivation of an X chromosome involve
modification of DNA and histones, including attachment of methyl groups to DNA nucleotides
- a region on both X chromosomes associate briefly with each other in each cell a an early stage of embryonic development
- the XIST gene becomes active only on the chromosome that will become the Barr body
- multiple copies of the RNA product of this gene are attached to the X chromosome of the activated gene almost covering it which initiates X inactivation
what are linked genes
genes located close enough together on a chromosome that they tend to be inherited together
what are vestigial wings
wings that are much smaller than normal
which 2 traits of fruit flies are linked
body colour and wing size - means thy are located close together on the same chromosome and are usually inherited together
what is the difference between linked genes and sex linked genes
linked genes - genes on the same chromosome that are inherited together
sex linked genes - single gene on a sex chromosome
what is genetic recombination
the production of offspring with combinations of traits that differ from those found in either p generation parent
what 3 processes generate variation in sexual reproduction
independent assortment
crossing over
random fertilization
what are F2 generation called if their phenotype matches one of the parents in the P generation
parental types
what are F2 generation called if their phenotype does not match either of the parents in the P generation
recombinant types
if there is a 50% frequency of recombination for a cross of 2 characters are the genes linked or not
they are not linked - they are located on different chromosomes - recombination only occurs between unlinked genes - independent assortment of the 2 unlinked genes at the metaphase plate causes recombination
what does crossing over account for
the recombination of linked genes
what happens in crossing over
a protein orchestrates the exchange of corresponding segments of one maternal and one paternal chromatid (they are non-sister)
what is a genetic map
an ordered list of the genetic loci along a particular chromosome
the further apart the two genes, the higher/lower the probability of a cross over between them and therefore a higher/lower recombination frequency (% recombinant offspring)
higher
higher
the greater the distance between 2 genes the more/less points there are between them where crossing over can occur
more
what is a linkage map
a genetic map based on recombination frequencies
can genes be on the same chromosome but be unlinked
yes - if they are far enough apart they can behave like unlinked genes and assort independently
what is non-disjunction
members of a pair of homologous chromosomes do not segregate correctly during meiosis I or sister chromatids fail to separate in meiosis II
as a result one gamete receives 2 of the same type of chromosome and another gamete receives no copy (the other chromosomes are distributed normally
what happens if either of the abnormal gametes (as a result of nondisjunction) fertilize a normal gamete
the zygote will have an abnormal number of chromosomes - known as aneuploidy
if a zygote has 2n-1 chromosomes what is it called
monosomic for the particular chromosome
if a zygote has 2n+1 chromosomes what is it called
trisomic for the particular chromosome
what happens if non disjunction occurs in mitosis
if it takes place in early embryonic development then the aneuploidy condition is passed along by mitosis to a large number of cells and is more likely to have a substantial effect on the organism
what is polyploidy
when an organism has more than 2 complete chromosome sets
how can a triploid arise (cell that has 3 complete sets of chromosomes)
fertilization of an abnormal diploid egg produced by non-disjunction of all of its chromosomes
how can a tetraploidy arise (cell that contains 4 complete sets of chromosomes)
the failure of a 2n zygote to divide after replicating its chromosomes
mitotic divisions would then produce a 4n embryo
what are the four types of changes in chromosome structure
deletion
duplication
inversion
translocation
what happens when deletion occurs in a chromosome
a chromosomal fragment is lost which results in the chromosome missing certain genes
what happens when duplication occurs in a chromosome
a deleted section (from deletion on a chromosome) may become attached as an extra segment too a sister or non sister chromatid producing a duplication of that portion of the chromosome
what happens when inversion occurs in a chromosome
a chromosomal fragment reattaches to the original chromosome but in reverse orientation
what happens when translocation occurs in a chromosome
when a chromosomal fragment joins to a non homologous chromosome (reciprocal - receives fragment in return) (non-reciprocal - doesn’t receive fragment in return)
why are deletions a duplications especially likely to occur during meiosis
in crossing over, non-sister chromatids sometimes exchange unequal sized segments of DNA
the result is one chromosome with a deletion and one with a duplication
what is down syndrome the result of
an extra chromosome 21 - trisomy 21 - so that each body cell has 47 chromosomes
what are the symptoms of down syndrome
characteristic facial features
short stature
heart defects
developmental delays
what increases the frequency of down syndrome
maternal age
aneuploid conditions that involve sex chromsomes/autosomes upset the genetic balance less than those involving sex chromosomes/autosomes
sex chromosomes upset the balance less than autosomes
- possibly because the Y chromosome carries so few gens and because the extra X chromosomes become inactivated to Barr bodies
what is Klinefelters syndrome
XXY - when a male has an extra X chromosome - they have male sex organs but the testes are small and the man is sterile - they experience gynecomastia and other female characteristics
what is turner syndrome
monosomy X - female but sterile because their sex organs don’t mature - when provided with oestrogen replacement therapy secondary sex characteristics develop
what is cry of the cat
results from a deletion in chromosome 5 - severely intellectually disabled - cry sounds like a distressed cat - usually die in infancy/early childhood
what id CML (chronic myelogenous leukaemia)
occurs when reciprocal translocation occurs during mitosis of the cells that are precursors of white blood cells - it leads to cancer due to uncontrolled cell cycle progression
what is genomic imprinting
variation in the phenotype depending on whether the allele (the one active copy) is inherited from the male or female parent
(unlike sex-linked genes, most imprinted genes are on autosomes)
it occurs during gamete formation and results in the silencing or activation of a particular allele of certain genes
in many cases the imprint consists of methyl groups attached to cytosine nucleotides which can silence or activate the allele
result: only one allele is phenotypically expressed
most imprinted genes are critical for what
embryonic development
what is an imprinted gene
a gene whose expression has been affected by genomic imprinting so that only a single (active) allele functions
in plants a zygote receives all its plastids from the egg/sperm
egg - so DNA in plastids is from the maternal side
in most plants and animals does mitochondrial DNA of offspring come from the mother or father
mother
give examples of mitochondrial diseases
mitochondrial myopathy - weakness, muscle deterioration, intolerance of exercise
Leber’s hereditary optic neuropathy - sudden blindness in young people - defect in oxidative phosphorylation
how can we avoid inheritance of mitochondrial diseases
3 people IVF
- insert chromosomes of mother into healthy egg that has had its chromosomes removed
- fertilize the egg with the fathers sperm
- transplant into the womb of the prospective mother
in peas what is dominant - round or wrinkly
round
the wrinkly allele in peas has a mutation which prevents the production of which enzyme
starch branching enzyme
if your blood type is A/B/AB/O what carbohydrates are present on the red blood cell surface
carbohydrate A
carbohydrate B
carbohydrates A and B
no carbohydrates
males/females are heterogametic
males
X chromosomes include the many housekeeping genes whereas the Y chromosomes are mostly involved in what process
spermatogenesis
what is patau syndrome
trisomy 15
what is Edward syndrome
trisomy 18
what happens in sickle cell anaemia
- red blood cell adopts sickle shape under low oxygen conditions
- sickle red blood cells cannot deform like normal blood cells when going through narrow vessels
- blockages can lead to ischaemia
- defective red blood cells are removed by the spleen
- anaemia results
- spleen becomes damaged all caused by a single amino acid change in the beta subunit
why is being heterozygous for sickle cell anaemia an advantage
homozygous with sickle cell and heterozygous with sickle cell = protected from malaria
but heterozygous is favoured because the disease isn’t as serious
• the malaria parasite is carried by the mosquito and invades red blood cells
• invasion of the parasite causes sickle cells to sickle
• the parasite cannot grow so well in sickle cells
• sickle cells (and the parasite) are cleared more rapidly by the spleen
