4.3 inheritance Flashcards
what is a gene?
a sequence of DNA on a chromosome normally coding for a specific polypeptide, which occupies a specific position or locus
genes normally exist as two or more alleles.
what is an allele?
a different form of the same gene, coding for a specific polypeptide
what is the genotype of an organism?
- the genetic make-up
- i.e the actual alleles it possesses
what is the phenotype of an organism?
- it represents the characteristics of an organism
- the expression of an organism’s genetic make up combined with its interaction with the environment
when both alleles are the same, the organism is said to be (heterozygous/homozygous) for that gene?
homozygous
- e.g RR or rr
when both alleles are different, the organism is said to be (heterozygous/homozygous) for that gene?
heterozygous
- Rr
dominant allele definition
alleles that are always expressed
recessive allele definition
alleles that are only expressed if 2 copies are present
what are a few rules to follow to present a genetic cross between two organisms?
- choose a single letter to represent each characteristic e.g R
- use upper case letters to represent dominant features (R), lower case for recessive (r), and state what they represent
- clearly a punnett square to calculate crossing
- state the phenotype and ratios of offspring. first generation is represented by F1, the second by F2
what does monohybrid inheritance involve?
- the inheritance of a single gene
why did Mendel conduct his experiment with peas?
- they were easy to grow
- they showed clear differences in phenotypes e.g tall and dwarf plants, those with purple of white flowers, yellow or green seeds
- they produced a large number of seeds making the results reliable
monohybrid inheritance example:
- peas with purple flowers x white flowers
- (parents): PP x pp
- (gametes): P x p
- F1: all purple Pp
- F1 x F1
- (parents) Pp x Pp
- (gametes) P, p x P, p
- 1 PP (purple), 2 Pp (purple), 1 pp (white)
when is a test cross/backcross performed? (in monohybrid inheritance)
to show if a dominant characteristic is determined by one or two dominant alleles (heterozygous or homozygous dominant)
- i.e PP or Pp
what does a test cross involve?
- crossing the organism with the unknown genotype with the homozygous recessive
- e.g cross either PP or Pp with pp
- if the F1 generation are all purple then the purple plant was pure-bred/homozygous, but if there were 1 purple plant to 1 white plant then the plant was not pure-bred i.e heterozygous
what does co-dominance mean?
- both alleles involved are dominant and therefore both are expressed equally
- e.g ABO blood groups: A and B are co-dominant
- IA IA = blood group A
- IB IB = blood group B
- IA IB = blood group AB
(when showing co-dominance, it is easier to use a letter to represent the gene e.g I and use subscripts to show the alleles as you have to use different letters)
whats the difference between co-dominance and incomplete dominance?
- co-dominance - both alleles are expressed equally so both characteristics are seen
- incomplete dominance - has an intermediate phenotype result
what is dihybrid inheritance?
the simultaneous inheritance of two unlinked genes (genes on different chromosomes)
- (the determination of a trait by the inheritance of two genes)
- mendel carried out experiments with pea plants involving two different characteristics at the same time
- e.g plants that produced yellow or green seeds AND wrinkled or round seeds
- mendel noticed that the colour of the seed was inherited independently from the seed texture (wrinkled or round)
dihybrid inheritance example:
- pure bred (homozygous) yellow wrinkled peas x green smooth peas
- key: Y yellow, y green, R round, r wrinkled
- yellow round seeds x green wrinkled
- (genotype): YYRR x yyrr
- F1 genotype: YyRr
- F1 phenotype: yellow round peas
- F1 generation were self pollinated
- (phenotype): yellow round x yellow round
- (genotype): YyRr x YyRr
- F2: phenotype ratio:
9 yellow round : 3 yellow wrinkled : 3 green round : 1 green wrinkled
when does autosomal linkage occur?
- when two different genes are found on the same chromosome and therefore cannot segregate independently
- this applied to autosomes, chromosomes other than the sex chromosomes
- crossing over could occur if the two genes are not too close together, but this is a random event, so does not always occur on every chromosome pair
in chi squared tests, if your calculated value (is less than/exceeds) the critical value (at p=0.05), there would be a significant difference between the observed and expected results, so you can reject the null hypothesis, as any differences seen were not due to chance at p=0.05. e.g due to linkage
(this means that mendel’s laws DO NOT apply)
- exceeds
in chi squared tests, if your calculated value (is less than/exceeds) the critical value (at p=0.05), there is not a significant difference between the observed and expected results, so you must accept the null hypothesis, as any differences seen were due to chance at p=0.05. e.g random fertilisation
(this means that mendel’s laws DO apply)
less than the critical value
what does Σ mean?
sum of
what is the chi squared tests used for?
- to determine if the difference between the observed and expected numbers of offspring of different phenotypes is due to chance or a real effect
what is the chi squared test formula?
x^2 = Σ ((O-E)^2 / E)
how do you calculate chi squared?
- null hypothesis: there is no significant difference between the observed and expected numbers of offspring of the different phenotypes
- enter the observed numbers (O) into the table
- calculate the expected numbers (E) from the total observed (e.g if total 100 and expected ratio 1 in 4, 0.25 x 100 =25)
- take observed value away from its corresponding expected value, then square that value
- divide each (O-E)^2 by its expected value
- then add them all up to get x^2
- then use a chi-squared table to see if the calculated value exceeds the critical (table) value
- e.g CV at p=0.05 = 7.82 (degrees of freedom = n-1)
- bc the calculated value is less than the critical value at p=0.05, we can accept the null hypothesis, and say that any differences seen were seen was due to cjance.
how do you work out the degrees of freedom?
no. of classes - 1
how do you determine the critical value?
- use the column p=0.05
what are autosomes?
all the chromosomes apart from the sex chromosomes
- humans have 46 chromosomes arranged into 23 pairs
- 22 pairs match i.e contain the same genes - are the autosomes
- the 23rd pair = sex chromosomes
- XX in females, XY is males
- these determine the sex
- in humans, there is a 50:50 chance that any child will be male or female
what is sex linkage?
- when a gene is carried by a sex chromosome so that a characteristic it encodes is seen predominately in one sex
- as males contain XY chromosome, the Y must come from his father, the X from his mother
- the Y chromosome is smaller than X, and there are some genes that are only carried on the X chromosome so males only receive one copy
what are some examples of a sex-linked disorder?
- haemophilia
- duchenne muscular dystrophy (DMD)
what is haemophilia?
- a recessive sex-linked, X chromosome disorder
- results in excessive bleeding and blood that is slow to clot
- the gene for clotting factor VIII, needed to clot the blood following injury, is only carried on the X chromosome
why is haemophilia more likely to occur in males rather than females?
- its a recessive sex-linked, X chromosome disorder
- females have two X chromosomes, while males have only one
- if a male receives the recessive haemophiliac allele (which codes for the altered faulty factor VIII causing haemophilia), he will suffer from the disease, as he lacks a second X chromosome that might carry a normal allele
- females are almost exclusively carriers of the disorder as they may inherit the defective allele from their mother or father, or as a result of a new mutation
- only under rare circumstances do females actually have haemophilia as they would need to inherit the faulty allele from both parents (homozygous recessive)
what is DMD?
- also an X-linked recessive condition
- but involves the gene which codes for dystrophin, a component of a glycoprotein that stabilises the cell membranes of muscle fibres
- the disease is characterised by progressive muscle loss and weakness
- like haemophilia, the defective allele is passed to children from their mothers (as boys receive the Y chromosome from their father)
what are pedigree diagrams?
- family trees that show the instances of a particular inherited condition within a family
a pedigree diagram shows the inheritance of haemophilia within a family:
- the diagram shows that as haemophilia only occurs in males in this family, it is likely to be sex-linked
- and as it is inherited through the mother it it likely to be carried on the X-chromosome
what is a mutation?
- a change in the volume, arrangement or structure of the DNA in an organism
- may affect a single gene or a whole chromosome
what are the two types of mutations?
- gene or point mutations
- chromosome mutations
what is gene/point mutation?
- e.g sickle cell anaemia
- a change to at least one nucleotide base in DNA or the arrangement of bases
- caused during DNA replication
- can be beneficial, damaging or neutral
what is chromosome mutation?
- a change in the structure or number of whole chromosomes
- affecting many genes
- they result in an entire chromosome being added or lost (called aneuploidy)
- e.g down’s syndrome
- or when one chromosome breaks off and attaches to another chromosome called translocation e.g translocation Down’s
are mutations spontaneous?
yes
- and they are random events
- occurring with equal probability anywhere in the genome of diploid organisms
- mutations do occur randomly
- but they do occur with a set frequency which varies between species
- mutagens increase the rate at which mutations occur
what are some things that increase mutation rates/examples of mutagens?
- ionising radiation
- x-rays
- polycyclic hydrocarbons in tobacco smoke
- some chemicals e.g benzene
are mutations more common in organisms with short life cycles?
yes
- e.g in bacteria where the life cycle is as short as 20 minutes
- because more frequent meiosis, so show a greater rate of mutation
when do most mutations occur?
- during crossing over in prophase I
- and non-disjunction in anaphase I and anaphase II of meiosis
mutations that occur in somatic (body) cells are (non-heritable/heritable)
non-heritable
what do point mutations occur as a result of?
- addition or subtraction: where a base is added/deleted - both result in a frame shift, where the reading frame moves one place and usually results in a non-functional protein as the amino acid sequence changes significantly
- substitution: where one base is ‘swapped’ for another, which may result in a change of codon, and hence amino acid
why do many point mutations have no effect?
because the change:
- is silent i.e the base changes but the amino acid for which the codon codes does not
- may be in a non-coding region or intron
- may be in a recessive allele and so is not expressed
- alters the amino acid but may not result in a change to the functioning of the protein
what is an example of a point mutation disease?
- sickle cell anaemia
what is sickle cell anaemia?
- genetic disorder caused by a substitution mutation on chromosome 11
- results in abnormal haemoglobin which distorts red blood cells
.
- a mutation involves a substitution of adenine for thymine which results in the change of one amino acid to valine, causing red cells to deform and block capillaries under low partial pressures of oxygen
- the alleles for normal and affected haemoglobin are co-dominant, so people who have one copy of the faulty allele do have symptoms, but not as severe as sufferers
- but show increased resistance to malaria which is an example of a heterozygote advantage
- common in Afro-Caribbean, middle eastern, eastern mediterranean and asian populations that evolved in malaria habitats
what is an example of a chromosome mutation disease?
- down’s syndrome
what is down’s syndrome?
- genetic disorder characterised by delayed development and learning disabilities
- due to non-disjunction, an affected individual possesses three copies of chromosome 21
.
- chromosomal disorder
- occurring in around 1 in 800 births
- results when a person inherits all or part of an extra copy of chromosome 21
- this occurs most commonly following non-disjunction of chromosome 21 during anaphase 1 or 2 of meiosis, where both copies of chromosome 21 enter the gamete
- when this is then fertilised by a normal gamete, three copies of chromosome 21 result
- the oocyte containing no copies of chromosome 21 fails to develop further
- the risk of down’s syndrome is related to the mother’s age:
• 18yrs old = 1 in 2100 births
• 30yrs old = 1 in 1000 births
• 40yrs old = 1 in 100 births - there is no treatment, but it can be diagnosed by prenatal tests
what does trisomy mean?
three copies of a chromosome
what is a mutagen that causes cancer known as?
a carcinogen
what does cancer result from?
- mutagens in protooncogenes producing oncogenes that cause the rate of cell division to increase by resulting in the production of excess growth factor, or by mutated receptor proteins that do not require growth factor to initiate cell division
what do tumour suppressor genes do?
- slow cell division
what can a mutated suppressor gene also cause?
- the rate of cell division to increase because the gene is inactivated
what do oncogenes do?
increases the rate of cell division
what is variation within a population due to?
- both allelic variations
- and the environment
what do epigenetic modifications occur as a result of?
- environmental conditions
what do epigenetic modifications do?
cause changes to how genes are transcribed
- and therefore affect how that gene is expressed
visible differences seen in identical twins are largely due to what?
epigenetic modifications
what are epigenetic modifications caused by?
- DNA methylation
- histone modification
what is DNA methylation?
- the addition of a methyl group (CH3) to cytosine bases
- reducing transcription of the genes
-by preventing proteins binding to the DNA that are needed to start transcription
what is histone modification?
- histone modification post-translation
- resulting in changes to the way the histones interact with DNA by causing a looser arrangement of the nucleosomes
- this causes increased transcription because RNA polymerase and other transcription factors have easier access to the DNA
- histone modification can involve the addition of an acetyl or methyl group to lysine, or phosphate groups to serine and threonine
as stem cells differentiate, epigenetic changes result in the production of different proteins, e.g melanin within skin cells, and amylase within pancreatic cells
are epigenetic modifications heritable?
- some are heritable if changes occur in gametes which is referred to as genomic imprinting, and whole chromosomes can be inactivated
- e.g X chromosome inactivation which results in the patchwork of tortoiseshell cats
what is a locus?
- the fixed position on a DNA molecule occupied by a gene
what is the F1 generation?
F2?
- F1 = the first generation of offspring resulting from the cross of two individuals in the parental generation
- F2 = the second generation of offspring resulting from the cross of two individuals in the F1 generation
what is Mendel’s first law of inheritance?
- law of segregation
- alleles separate randomly into gametes
- each parent passes one allele to their offspring
what is Mendel’s second law of inheritance?
- law of independent assortment
- the alleles of genes assort independently of other genes during gamete formation
what does Mendelian inheritance assume?
it assumes that the genes involved are not linked
why are haemophilia and DMD more common in males?
- they are X-linked recessive conditions
- males only inherit one X chromosome so are more likely to express the gene in their phenotype
what is linkage?
- two or more genes positioned on the same autosome
- unlikely to be separated by crossing over during meiosis so often inherited together
what are the series of steps of how to perform a chi-squared test?
- make a null hypothesis
- use Mendelian ratios to calculate the expected numbers
- calculate chi-squared value using chi-squared equation
- calculate the degrees of freedom
- select an appropriate significant level (normally 0.05)
- find the critical value
- compare the chi-squared value with the critical value
- accept or reject the null hypothsis
if the chi-squared value is greater than or equal to the critical value, is the null hypothesis accepted or rejected?
rejected
what does it mean if the null hypothesis is rejected?
there is a significant difference between the observed and expected results
if the chi-squared value is less than the critical value, is the null hypothesis accepted or rejected?
accepted
what does it mean if the null hypothesis is accepted?
- the difference between the observed and expected results is not significant
- the results occurred due to chance
what is chromosome nondisjunction?
- failure of homologous chromosomes to separate in meiosis I or sister chromatids to separate in meiosis II
- results in gametes with one extra or less chromosome than normal
what is a mutagen?
- a chemical, biological or physical agent that increases the rate of gene mutations above normal level
what is a carcinogen?
- a type of mutagen that causes cancer
what is an oncogene?
- mutations of proto-oncogenes that are activated continuously, resulting in uncontrolled cell division
define epigenetics
the study of changes in gene expression that are not due to alterations in the nucleotide base sequence of DNA
homologous:
a pair of chromosome - one of which has come from the mother, the other from the father
- the pair of chromosomes can form a bivalent during prophase of meiosis I
what does it mean if the plants were bred true?
they were homozygous for that characteristic
what is the expected phenotype ratio for monohybrid inheritance (from a cross of two heterozygotes)
3:1
what is the expected phenotype ratio for dihybrid inheritance for a cross between homozygous RRYY and homozygous rryy?
9:3:3:1
the most important step in answering questions on dihybrid inheritance is working out the combination of alleles in the gametes.
it’s important to remember:
- each pair of letters represents the allele for one gene on a pair of homologous chromosomes
- in the genotype of the parent plant, there are always two letters because the organism is diploid
- when gametes are formed, only one of each pair of letters passes into a gamete because gametes are haploid - so gametes only contain one letter for each characteristic
- if the genes are on different chromosomes, the letter for one gene that is found in a particular gamete is independent of the letter for another gene that is also found in the same gamete
e.g RrYy
- RY or Ry or rY or ry in gametes
when is a backcross/test cross used in dihybrid inheritance?
to determine if an organism with two dominant phenotypes is:
- homozygous for both characteristics
- homozygous for one and heterozygous for the other
- heterozygous for both characteristics
- is carried out by crossing the unknown organisms with one that is recessive for both characteristics
what is the expected ratio for a dihybrid cross between a heterozygous RrYy and homozygous rryy?
1:1:1:1
- if the alleles are on the same chromosomes but are further apart, crossing over in prophase I can result in different combinations of alleles
- this is called _____
- and results in ____
- incomplete linkage
- recombinant chromosomes
what should the null hypothesis state in a chi squared test?
- there is no statistically significant difference between the observed and the expected results
what does it mean if you accept the null hypothesis in the chi squared test?
- there is NO significant difference between the observed and expected values
- and the differences observed and expected values are due to chance alone
- e.g random fertilisation
what does it mean if you reject the null hypothesis in the chi squared test?
- there IS a significant difference between the observed and expected values
- and the differences between them are NOT due to chance alone
- e.g linkage
why can mutations be important?
- because they increase variation in a population
only mutations which occur in the formation of ___ can be inherited?
gametes
- many cells normally undergo a programmed form of death called apoptosis
- activated oncogenes can cause those cells to survive and increase in number instead
- most oncogenes requires a mutation to cause cancer
- proto-oncogenes are genes that if they mutate can become oncogenes
what effect does a mutation in an intron have?
no effect
- as introns are spliced out during the formation of functional mRNA
what effect does a silent mutation that changes the triplet code but not the amino acid have?
no effect on the protein
what effect does a missense mutation that changes the triplet code and amino acid have?
- as it changes the triplet code and amino acid incorporated into the protein
- this can change the way that a polypeptide chain folds and can affect its functionality
what is aneuploidy?
- the loss of gain of a single chromosome
what is polyploidy?
- it involves changes in the number of whole sets of chromosomes
(has played an important role in the evolution of many plants)
if there are uneven/odd numbers of chromosomes, bivalents cannot form, meiosis doesnt continue and no gametes are produced
- if a tumour suppressor gene is silenced there is no control over cell growth and division which can lead to cancer development
- over expression of an oncogene can lead to increased rates of cell division and again cause cancer
generally, the more methyl groups added to the promoter region of a gene, the (less/more) likely that the gene will be expressed?
less
- flies with ebony bodies and scarlet eyes were crossed with flies homozygous for grey bodies and red eyes
- all the F1 flies had grey bodies and red eyes
- when the F1 hybrid flies were crossed the following results were obtained:
• red eyes grey body 91 flied
• red eyes ebony body 3
• scarlet eyes grey body 2
• scarlet eyes ebony body 32 - the table shows that some of the offspring were far more common than expected and some phenotypes were very rare. explain both of these observations [2]
- (common phenotypes/red grey and scarlet ebony) are due to linkage
- (rare phenotypes/ red ebony and scarlet grey) due to crossing over/recombinants
- in another cross between two individuals with the genotype DdEe, where the gene D and E are the same chromosome, the offspring showed four different types of phenotype
- the phenotype of some of the offspring were far more common than expected and some phenotypes were very rare
- explain these observations [2]
- incomplete linkage
- genes (further/far) apart on same chromosome
- (crossing over/chiasmata) can occur
- four types of gametes produced (but not in equal numbers)
- small numbers of recombinants / large numbers parental types
- recombinants equal in numbers / parental equal in numbers
