genetics Flashcards
what is the genotype
genetic composition of an organism
sets limits within which individual characteristics can vary
vary may be due to environment
any change in genotype is a mutation and they can be inherited
what is the phenotype
genotype and environment
determined by interaction between genotype and environment
any change due to phenotype only is a modification and not inherited
what did Gregor Mendel do
study heredity
selected features of the garden pea
peas were easy to grow and had a short cycle
isolated pea plants that were pure breeding (both dominant alleles)
chose traits with contrasting features
how else can you describe first genertion
first filial generation = F1
what is self cross
crossing plants of the same generation
what did mendel find
tall : dwarf
3 : 1
what was the first mendelian law
characteristics of an organism are determined by internal factors (alleles) which occur in pairs. only one of each pair of factors (alleles) can be represented in a single gamete
what is monohybrid inheritance
inheritance involving a single characteristic determined by one gene
define gene
basic unit of heredity, codes for a specific trait
define loci
specific location of a gene on a chromosome
define genome
total heredity endowment of DNA of a cell or organism
define somatic
all body cells except reproductive cells
define gametes
reproductive cells (i.e. sperm and eggs)
define chromosomes
elongated cell structure composed of DNA and protein - carry DNA in cells
define diploid
chromsome type is represented by 2 homologous chromosomes
define haploid
chromsome type is represented by one chromosome
define homologous
same size and shape carrying same type of genes
define chromatid
one of 2 duplicated chromosomes connected at the centromere
define centromere
region of chromosome where microtubules attach
define dominant
alleles that are always expressed
define genotype
genetic make up made from alleles
define heredity
study of inheritance
define phenotype
features resulting from expression of genes
define allele
different forms of the sea gene
define homozygous
2 alleles of a pair are the same
define recessive
alleles only expressed if both alleles are present
define heterozygous
two alleles of a pair are different
what is a test cross
organism with a dominant characteristic could be homozygous or heterozygous. appearance (phenotype) is identical in both cases
accurate genotyping may be achieved by crossing organisms of unknown genotype with once which is known
what is codominance
both alleles in the heterozygote are expressed individually. a heterozygote will have a combination of both alleles. e.g. having a genotype for blood type Ia Ib then both alleles are expressed so their blood type is AB
what is incomplete dominance
phenotype is intermediate between the 2 parental phenotypes rather than their both being expressed e.g. red flowers crossed with white flowers have an F1 or pink flowers. neither the allele for red or white have complete dominance. they have the symbols R/Cr and W/Cw for white. when the F1 interbreed the ration of colours in the F2 gen. is (red:2 pink:1 white)
what is dihybrid inheritance
simultaneous inheritance of 2 unlinked genes (genes on different chromosomes) each controlled by a different chromosome at a different locus.
mendel studied dihybrid inheritance by crossing plants from 2 pure breeding strains - one tall with purple flowers and one dwarf with white flowers
what are the mendelian conclusions
2 pairs of characteristics behave independently of each other. led him to formulate his 2nd law of inheritance
what is the 2nd law of inheritance
either one of a pair of contrasted characteristics may combine with either of another pair
what is the expected mendelian ratio
expected ratio in the offspring of a monohybrid cross is 3:1 and a dihybrid cross is 9:3:3:1
any discrepancy found is due to statistical error
what is chi squared test used for
tests in the numbers of different phenotypes are closed enough to the predicted values to support the genetic explanation of how they arose. if the numbers are not close enough they must have arisen for another reason
what is the null hypothesis
a statement that there is no difference between the observed and expected results of a cross. a null hypothesis state that the observed results are due to mendelian cross and any deviation is due to chance
no difference between the observed and expected number of phenotypes
can either accept or reject the null hypothesis
what is the degrees of freedom
number of classes (categories) -1
predicted results are significant means that the predictions made are not true and there must be a different explanation
when are results significant
if the observed value is greater than or equal to the critical value. our results is significant
if the observed value is less than the critical value our result is not significant
(opposite way around for probability explanation)
what are linked genes
genes that appear on the same chromosome which prevents independent assortment of the genes
can still get a variation of genes through crossing over
recombinant chromosomes are less frequent
what is the main reason for going against the mendelian ratio
relying on chiasma formation not independent assortment
what is linkage
alleles of different genes cannot assort independently if the genes are on the same chromosome
tend to be inherited together i.e. they do not segregate in accordance with mender’s law of independent assortment
what occurs between meiosis and linkage
during meiosis linked genes are inherited together because they pass into the gamut, and hence the offspring together.
during meiosis at least one chiasma forms between 2 homologous chromosomes, however it may not form between a particular pair of genes. the further apart the genes are the more likely it is that crossing over will results in recombinants
what is human sex determination
in humans there are 23 pairs of chromosomes. of these, 22 pairs are identical in both sexes. the 23rd pair is different in the male from the females
what are autosomes
22 identical pairs
what are heterosomes
sex chromosomes
what is sex linkage
genes that are located on one or the other of the sex chromosomes are said to be sex linked. the genes determine body characters
sex linked genes may be on the X or the Y chromosome
the X chromosomes carries many sex linked genes
X linkage
- x chromsome is much larger than the Y chromosome, most sex-linked genes are located on the X chromosome
- x chromosome carries genes for characteristics such as the ability to see particular colours and the ability to clot blood efficiently
when will recessive alleles express more
in males are they are unpaired so no dominant allele is present.
important feature of X linkage
a male cannot pass on an X linkage characteristic to his sons as they must receive his Y chromosome. on the other hand all his daughters must receive allele from him
normal compared to haemophilia
1) bleeding
2) vessels constrict
3) platelet plug
4) fibrin clot
1) bleeding starts
2) vessels constrict
3) incomplete platelet plug, continued bleeding
4) incomplete or delayed formations of fibrin clots, continued bleeding
what is haemophilia
condition in which the blood does not clot properly
often results in excessive bleeding both internally and externally
lacks of one or more clotting factor which play an important role in the chain reaction of blood clots
what is haemophilia A in males
- sex linked characteristics caused by a excessive allele carried on the x chromosome
- females have a pair of allele for the genes that control the production of factor VIII but males only have one.
- if a male inherits one allele for haemophilia A he has the diseases because he cannot mask it with another allele
what is haemophilia A in females
only has the disease if she has inherited 2 recessive alleles, one from each parents
- heterozygous females are carriers of the disease, so a 50% chance of a carrier mother transmitting the recessive alleles to any one
- very very rare for females to have haemophilia as are normally miscarried
what are mutations
is a change in the heredity material of an individual
what mutations may occur
- changes to the base sequence of DNA
- change to a large section of a chromosome
- may involve chromosomes to be missing or extra
what are the two ways of describing mutations
spontaneous = as they may happen without an apparent cause random = as they appear to happen with equal probability anywhere in the genome of diploid organism
which mutations can be inherited
only mutations in gametes
what affects the mutation rates
occurs in dna replication so in general organism with short life cycles and frequent meiosis show a greater rate of mutations
how can you increase the rate of mutations
- ionising radiation = UV light 260nm is particularly mutagenic as DNA absorbs it most efficiently. an incorrect nucleotide will be inserted
- mutagenic chemicals = eg cigarette smoke and acridine
in what 4 ways do mutations happen
- gene or point mutation
- chromosome mutation
- aneuploidy
- polyploidy
what is gene (point) mutation
DNA not copied accurately in S phase
what is chromosome mutation
may get damaged and break
what is aneuploidy
whole chromosome lost or added
what is polyploidy
number of chromosomes may double
what are the four different ways of point mutations occurring
addition
duplication
subtraction
inversion
what is addition
a base is added. if this happens in 3 places on extra amino acids is added
what is duplication
same base is incorporated twice
what is subtraction
a base is deleted. if this happens in 3 places, the polypeptide has one fewer amino acids
what is inversion
adjacent bases on the same DNA strand
describe sickle cell anaemia
substitution point mutation
original strand = CTC (glutamate) (large and hydrophilic)
substitutes A for T
produces CAC which is valine (small and hydrophobic)
cell membrane collapses and becomes sickle cell shaped
are the alleles for sickle cell codominant
yes
if someone has alleles for normal and sufferer then they will suffer less severe versions on the symptoms.
what are chromosome mutations
changes in the structure or number of chromosomes in cells
what are the changes in structure
in prophase 1 of meiosis, homologous chromosomes pair and exchange material at chiasmata. mutations arise when a chromosome does not rejoin accurately at the corresponding points. the gametes therefore end up with different genes
what are the changes in number of chromosomes
most likely to occur during meiosis, when chromosomes separate at anaphase 1 or chromatids separating at anaphase 2.
a faulty spindle can result in chromosomes not being shared equally. this is non-disjunction. the faulty division means one of the daughter cells receives 2 copies of a chromosome while the other gets none.
example of changes in no. of chromosomes
e.g. down’s syndrome
what is down’s syndrome
- one in 1000 births
- chromosome number 21 is affected
- either has no chromosome 21 or 2
- one with no chromosome 21 does not produce a viable embryo
- the others contains 3 no. 21’s in total - is called trisomy 21 and produce’s down’s syndrome
what is translocation down’s
- a fragment of one chromosome has attached to another
- 5% of sufferers have 46 chromosomes
- during meiosis in a gamete that produced them, a fragment of chromosome 21 attached to chromosome 14
- it produces an embryo with 2 normal copies of chromosome 21 and an additional attached to chromosome 14
define euploid
cells with complete sets of chromosomes
define aneuploid
small number of extra or too few chromosomes as a result of non-disjunction
define polyploidy
several sets of chromosomes
how does the number of chromosomes change
1) defect in the spindle at meiosis may result in all chromosomes @ anaphase 1 or chromatids @ anaphase 2 moving to the same pole of a cell
2) makes gametes with 2 of each chromosome instead of one
3) when a diploid gamete is fertilised by a normal haploid gamete, a triploid gamete with 3 sets of chromosomes forms
4) not able to produce gametes so will be infertile
most plants produce asexually so triploidy does not prevent reproduction
what happens if 2 diploid gametes fuse
a tetraploid (4n) is produced
what is endomitosis
replicate of chromosomes not followed by cytokinesis - 4 sets of chromosomes are incorporated into the new nuclear envelope and successive rounds of mitosis continue to produce tetraploid cells
what happens when two triploids fuse
forms hexaploid (6n) they are fertile as they can make homologous pairs
what is polyploidy in plants
more common in plants
associated with beneficial traits such as vigour and disease resistance
can reproduce asexually and hermaphrodite so do not use chromosomes to determine their sex
what is a carcinogen
chemicals which are capable of causing mutations in DNA which leads to cancer
what is cancer
uncontrollable cell division
usually there is a mutation in one of the genes that control the cell cycle
what is the tumour suppressor gene
a mutation in these genes will lead to uncontrolled cell division and a tumour
they regulate mitosis and prevent cells dividing too quickly
mutation will use it to lose the regulatory function
then go through continual, repeated mitosis
abnormalities in the gene TP53 which codes for p53 protein have been identified in more than half of all human cancers
what are oncogenes
porto-oncogenes code fon the proteins required for cell division- a mutation will lead to formation of oncogenes and too much protein and corresponding increase in cell division
mutation may switch on gene permanently. when photo-oncogenes mutate such that it causes chromosomes to rearrange - permanently activates proto-oncogenes
there may be an extra copy of proto-oncogenes so produces too much
what are epigenetic
environment can alter the transcription of genes and therefore their expression without altering the genes themselves
give two examples
methylation of bases
histone modification
describe methylation of bases
DNA can have methyl groups added to the bases, this makes them less likely to be transcribed
describe histone modification
histone proteins can be modified (groups added to the amino acids)
following translation histone proteins are the proteins around which DNA coils in the nucleus to form chromosomes
when histones are unmodified the DNA winds tightly and genes are less accessible for transcription (switched off)
DNA winds less tightly around modified histones, genes are more accessible and more likely to be transcribed
