4.3 inheritance Flashcards
what is an allele
a different form of a gene
homozygous
when two of the same alleles make up the genotype
heterozygous
when two different alleles make up the genotype
genotype
the genetic make up (ie. your alleles)
chromosome
genes and DNA are packaged into these in the nucleus of the cell
dominant
one allele is needed for the phenotype to be shown
recessive
both alleles are needed for the phenotype to be displayed
locus
position of a gene in a chromosome
F1 and F2
first and second generation
autosome
what the first 22 chromosomes are called
what is Mendel’s law of segregation
the characteristics of an organism are determined by factors (alleles) which occur in pairs. only one of a pair of factors (alleles) can be present in a single gamete
what is the law of segregation controlled by
(characteristics of an organism)
- by single genes
what are characteristics an example of
discontinuous variation
what is discontinuous variation
things that won’t change (skin colour, blood type)
how to write a punnet square
- suitable symbol for alleles
- parents genotype
3, gametes by each parent labelled and circled - use punnet square to show results
- Label F1 (F2 if necessary)
- indicate the ratio of the phenotype
what is a test cross used for
to determine whether an organism showing the dominant characteristics of a trait is homozygous
what does a test cross involve
crossing the organism with another that is homozygous recessive for the trait
what would show the parents must have been heterozygous in the test cross
if any of the offspring show the homozygous recessive trait in the phenotype, parent must have been heterozygous
what is co dominance
both alleles show up in the phenotype equally
what is incomplete dominance
-where the phenotype of the heterozygote is intermediate between the two parental phenotypes
what is mendels second law of inheritance called
the law of independent assortment
what is the law of independent assortment
‘each member of an allelic pair may combine randomly with either of another pair’
where there is variation due to independent assortment and crossing over (during meiosis)
what does the law of independent assortment apply to
dihybrid inheritance
independent assortment (meiosis)
homologous chromosomes line up at the equator randomly so equal chances of chromosomes going to either pole
each gene behave independently of each other so are unlinked
why did mendel use peas in his experiments
- easy to grow
- characteristics controlled by single gene
- characteristics controlled by genes on different chromosomes
what did mendels experiment conclude
- round seed shape was dominant and wrinkled was recessive
- yellow seed colour was dominant and green seed colour was recessive
what is dihybrid inheritance
the inheritance of two separate genes (that are unlinked)
when is inheritance down to linkage
when either medelian’s ratio’s are not shown
explain linkage
genes are on opposite chromosomes which segregate to oposite poles during meiosis
genes which are linked are on the same chromosome so cannot segregate during meiosis.
what can the chi-squared test be used for
to prove or disprove a null hypothesis
what is a null hypothesis
states that any deviation between observed and expected is due to chance (no significant difference)
when does the chi-squared test accept or reject
accept- no significant difference
reject - there is significant difference
what is a hermaphrodite
and examples
organism that has reproductive organs of both sexes
most angiosperms are hermaphrodite
also invertebrate hermaphrodites e.g. Mollusca and Annelida
what can hermaphrodites do
self-pollinate
other reproductive strategies
-monoceious plants
-dioecious plants
what is a dioecious plant
some plants are male other are female
e.g holly, willow, cannabit
what is a monocious plant
male and female om the same plant
e.g Maize
most vertebrates have separate males and females but what can the ratios be controlled by
Temperature
Sequential hermpahroditism
hierarchy
hermaphrodite (during deperate times)
ploidy level
chromosomes structure (humans)
what implications could having a different pair of chromosomes have on inheritance
if they’re not identical, no homologous portion on the Y chromosome
recessive alleles on this non-homologous portion of the X chromosome will appear more frequently in the male. no portion with dominant allele
what is sex linkage
where the phenotypic expression of the allele that is dependent on the sex of the individual and is directly tied to the sex chromosome
why is x linkage more common
the X chromosome is larger
part of it doesn’t have a homologous section on the Y chromosome, so only one allele of a gene will be present and will always be expressed
what part of the gene carry sex determining genes
non-homologous regions
examples of X- linked diseases
Haemophilia
Duchenne muscular dystrophy
red green colour blindness
what genes are described as sex linked
genes that are located on the sex chromosomes
what does the study of sex linked genes involved
examining both the sex of the offspring and the genetic trait of interest
Y linked diseases
are rare and debateable
it is argued that there is little room on the Y chromosome for anything other than genes controlling the testes formation and function
what is haemophillia
x-linked disease where blood clots slowly leading to internal bleeding (lethal if not treated)
explain the development of haemophilia
recessive allele with altered DNA nucleotides, they do not code for required protein for blood clotting
Only linked to X chromosomes (none on Y)
treatment for haemophiliacs
protein can be extracted from donated blood and given to them
explain why males always inherit haemophilia from their mother (3 marks)
- males can only obtain Y chromosome from father
-males X chromosome must come from mother
-allele that doesn’t code for clotting protein is linked to X chromosome
explain colour blindness
allele for colour vision carried on X chromosome, males will only have one allele for colour vision
there is a defective, recessive allele of the colour vision gene that can lead to colour blindness (especially red or green)
what is duchenne muscular dystrophy
duchenne is the most common and severe form of MD- about 100 boys are born with it each year
X-linked recessive allele of dystrophin gene
Dystrophin is a component of a glycoprotein stabilising cell membrane of muscle fibres
leading to a loss of muscle mass and weakness
what is a mutation
a change in the amount, arrangement or structure of the DNA in an organism, they are random
2 types of mutations
gene mutations e.g sickle cell
chromosome mutation
e.g downs
what does a mutagen do
increase the chance of mutation
examples of mutagens
ionising radiation
x rays
polucyclic hydrocarbons in cigarette smoke
chemicals e.g benzene
when is a mutation expressed in
haploid / diploid organisms
haploid= mutation is expressed
diploid= mutations only expressed if dominant but this is rare or on the X-chromosome
different ways which mutations can occur
gene or point mutation
chromosome mutation
aneuploidy
polypoidy
explain gene or point mutation
changes in base sequence occurs if DNA is not copied correctly in S phase before cell division, this can lead to a different amino acid being added so a different protein is then translated
what is duplication
the same gene twice
what is inversion
where DNA bases exchange positions
what are the effects of mutation
(silent)
changes in base doesn’t change the amino acid it codes for. mutation does not have a significant effect on the phenotype,
it might also be in an intron
or could be amino acid with similar chemical nature so won’t be of much effect
what happens if a mutation is on an active site of enzyme
it would have a significant effect
what is Duchenne muscular dystrophy
the most common and severe form of MD, about 100 boys are born with it each year, includes a loss of muscle mass and progressive muscle weakness
what is DMD caused by
a sex linked recessive allele of a gene that codes for the protein dystrophin, dystrophin is a component of a glycoprotein that stabilises the cell membranes of muscle fibres
when are sex linked recessive alleles expressed
only expressed in females if both X chromosomes carry the allele
are always expressed in the male because the Y chromosome does not have a homologous locus for the gene
what is sickle cell an example of
point mutation
explain how sickle cell anaemia is caused
by a single mutation in the gene coding for haemoglobin
HBB gene is on chromosome 2
this mutation involves a substitution of A for a T (valine instead of glutanic acid)
this changes 1 amino acid (changes properties)
what happens to people with sickle cell anaemia when oxygen levels are too low
red cells deform and block capillaries
what does the HbSHbS allele do
reduces the ability of RBC to carry oxygen which can lead to death
what is an example of a chromosome mutation
down syndrome (Trisomy 21)
how common is downsyndrome
1/800 births
what is downsyndrome a consequence of
non-disjunction in humans
what is non-disjunction (in downsyndrome)
- where chromosomes fail to separate at anaphase 1 of meiosis
occurs for chromosome 21 resulting in a gamete containing an extra chromosome 21, this is then fertilised by a normal gamete
(47 gametes)
what happens if non-disjuntion occurs on a different chromosomes
will normally result in miscarriages of the foetus
why can the offspring survive with a chromosomal mutation (down syndrome)
because the 21st chromosome is relatively small carrying only around 200 to 300 genes out of an estimated total of 20000-25000 for the human genome so the offspring can survive
what is translocation downs
still have 46 chromosomes
fragments of chromosome 21 attaches to chromosome 14
what are mutations a source of
genetic variation which can result in evolution through natural selection
when do most mutations occur
during crossing over during prophase-1 of meiosis or as a result of non-disjunction during anaphase-1 or anaphase-II of meiosis
what are ploidy levels
changes in numbers of sets of chromosomes
explain endomitosis
division of chromosomes not followed by nuclear division that results in an increased number of chromosomes in the cell
what can mutations affect
protein synthesis and can change the phenotype of an organism, but some mutations have no effect on the phenotypes
what do gene/point mutations affect
a single base in a gene and chromosomal mutations affect many genes
what is a mutagen that causes cancer called
carcinogen
what can happen to protein-oncogenes
they can mutate to become oncogenes which are involved causing uncontrolled cell division to form a cancer
what is the role of the tumour suppressor gene
regulate cell division
what happens if a mutation affects a tumour suppressor gene
then cell division is no longer controlled
how is a tumour formed
if the mass of cells (from the tumour suppressor genes being mutated) isn’t detected and destroyed by the immune system then it will continue to divide
when do tumour suppressor genes cause cancer
when it’s turned off
what is a proto-oncogene
a gene that helps to control cell division, codes for a protein that contributes to cell division
when do proto-oncogenes cause cancer
when turned on
what is an oncogene
a mutated protein-oncogene
what is epigenetics
the study of changes to DNA after DNA replication
what is gene expression controlled by
factors other than the DNA sequence
what are variation in organisms usually caused by
- different alleles (a difference in nucleotide sequence)
- environmental evidence
what can also bring about epigenetic changes
the environment
what does DNA being modified post-replication mean
doesn’t change the DNA base sequence but changes the ability of a gene to be transcribed during protein synthesis
what can epigenetic changes be influenced by
-age
-environment
-lifestyle
-drugs
-during growth in pregnancy or childhood
what are the 2 ways to affect the transcription of genes without changing the nucleotide sequence
-methylation of cytosine
-histone modification
what is methylation of cytosine
-cytosine can have a methyl group added to it, it can still pair with Guanine but regions heavily methylated cannot transcribe DNA as well
what does the addition of methyl groups to bases do
reduces the ability of that gene being expressed
what is gene expression
is the process by which information from a gene is used in the synthesis of a functional product e.g. proteins or functional RNA
what is histone modification
DNA is coiled around proteins called histones
histones can be modified epigenetically and these changes alter interaction with DNA to make it more or less densely coiled
what does it mean when the histone coils more tightly
they can prevent gene expression
what does it mean when the histone coiled loosely
they can increase gene expression
where can different epigenetic modification occur
in cells of the same tissue and in different tissues resulting in different expression of the same gene in different parts of the same organism
explain what X-inactivation is
epigenetic changes can switch off a whole organism e.g. in females only 1x chromosome is used, the other is inactivated
an example of X-inactivation
fur colour is on the X chromosome and the alternate chromosomes are switched off in different groups of cells
what is a stem cell
an undifferentiated human cell that are able to undergo mitosis to develop to many specialised things
explain how DNA methylation can result in tumour growth
mRNA from regulator is not produced so regulator protein isn’t produced. suppressor gene is not expressed/switched on. so uncontrolled mitosis
if tumour suppressor genes are permanently switched off what would the consequences be
increased risk of cancer
passed on to next generation
what is polyploidy
some organisms have additional complete sets of chromosomes (e.g three sets of chromosomes=triploid 3n)
what does meiosis do
brings about variation in offspring produced by sexual reproduction
three ways meiosis brings about variety
-2 haploid gametes must fuse during feretilisation, each contains half the genetic information of the parent resulting in a unique new combination of genetic info
- random distribution and independent assortment of homologous chromosomes (random)
- crossing over during chiasmata formation during prophase I of meiosis, new genetic combinations and the separation of linked genes
why is variation so important
ensures some will have characteristics that allow survival even if the environment changes-
why is crossing over so important
if it occurs between two genes, this separates alleles that were previously linked and allows them to combine in new recombiant genotypes (important source of variation)
what are common phenotypes often a result of
linkage
what are rare phenotypes often a result of
crossing over
