6.1.2 Patterns of Inheritance Flashcards
What influences the phenotype?
- its genotype
- its environment
What are some mutagens that can increase the rate of mutation?
What characteristics do mutations that occur during gamete formation have?
- persistent: they can be transmitted through many generations without change
- random: they are not directed by a need on the part of the organism in which they occur
What are some types of chromosome mutations during meiosis?
- deletion: part of a chromosome, containing genes and regulatory sequences, is lost
- inversion: a section of a chromosome mat break off, turn through 180 degrees and join again
- genes may still be present, but too far away from their regulatory nucleotide to be properly expressed
- translocation: a piece of one chromosome breaks off and then becomes attached to another
- duplication: a piece of a chromosome may be duplicated
- non-disjunction: one pair of chromosomes or chromatids fail to separate, leaving one gamete with an extra chromosome e.g. Down syndrome
What is aneuploidy?
- the chromosome number is not an exact multiple of the haploid number for the organisms
- sometimes chromosomes or chromatids fail to separate during meiosis
What is polyploidy?
- if a diploid gamete is fertilised by a haploid gamete, the resulting zygote will be triploid (has three sets of chromosomes)
- the fusion of two diploid gametes can make a tetraploid zygote
- many cultivated plants are polyploid
What may genetic variation result from?
- meiosis producing genetically different gametes due to:
- allele shuffling during crossing over in prophase 1
- independent assortment of chromosomes during metaphase/anaphase 1
- independent assortment of chromatids during metaphase/anaphase 2
What does haploid mean?
- contain only one of each pair of homologous chromosomes
- contain one allele for every gene
How does random fusion of gametes create more genetic diversity?
- any male gamete can potentially combine with any female gamete from an organism of the same species
- the random fertilisation of gametes, that are already genetically unique, produces extensive genetic diversity among the resulting offspring
What is some phenotypic variation caused solely by the environment?
- dialect
- losing a digit or limb
- scars
What is an example of variation caused by the environment interacting with genes?
- if plants are kept in dim light after germination, or if the soil contains insufficient magnesium, then leaves do not develop enough chlorophyll and are yellow
- this plant is chlorotic, suffering from chlorosis
- it cannot photosynthesise
- they have the genotype for making chlorophyll, but environmental factors are preventing the expression of these genes
What does heterozygous mean?
- not true breeding
- having different alleles at a particular gene locus on a pair of homologous chromosomes
What does homozygous mean?
- true breeding
- having identical alleles at a particular gene locus on a pair of homologous chromosomes
What does monogenic mean?
- determined by a single gene
How can we ascertain the genotypes of phenotypically similar individuals?
- using the test cross
- the organisms exhibiting the dominant phenotypes but of unknown genotype is crossed with one showing the recessive phenotype
- if any of the offspring have the recessive phenotype, the dominant is heterozygous
Show an example of a dihybrid cross
What is the ratio of phenotypes for dihybrid inheritance?
- 9:3:3:1
How can a gene have multiple alleles?
- when three or more alleles at a specific gene locus are known, the gene has multiple alleles
What is codominance?
- where both alleles present in the genotype of a heterozygous individual contribute to the individual’s phenotype
What is an example of multiple alleles?
- human ABO blood groups
What determines the four blood groups on humans?
- determined by three alleles of a single gene on chromosome 9
- the gene encodes an isoagglutinogen, I, on the surface of erythrocytes
- IA and IB are codominant
What are the coat colours in rabbits?
- agouti: wild type, each hair has a grey base, a yellow band and a black tip
- albino
- chinchilla: silvery grey, lack of yellow band
- himalayan: white but with black feet, ears, nose, tail
What are the rabbit coat colours determined by and what is the dominance hierarchy?
- they are determined by one gene that has four alleles
- Agouti, C, is dominant to all other alleles
- Chinchilla, Cch , is dominant to Himalayan Ch
- Albino, c, is recessive to all other alleles
What are the autosomes?
- the other 22 pairs of chromosomes that are not the sex chromosomes
- they are fully homologous (match for length and contain same genes at same loci)
How can XX and XY sex chromosomes pair up during meiosis if they are not fully homologous?
- a small part of one matches a small part of the other
Briefly describe the human X chromosome
- contains over 1000 genes that are involved in determining many characteristics
- not concerned with sex determination
- most of them have no partner alleles on the Y chromosome
What happens if a female has one abnormal allele on one of her X chromosomes?
- she will probably have a functioning allele of the same gene on her other X chromosome
Why do males inherit diseases on X-linked genes?
- if he inherits an X chromosome with an abnormal allele for a particular gene, he will suffer from a genetic disease
- he will not have a functioning allele for that gene
- males are functionally haploid, or hemizygous for X-linked genes
- they cannot be heterozygous or homozygous for X-linked genes
Describe how haemophilia A is a sex-linked genetic disease
- one of the genes on the non-homologous region of the X chromosome codes for a blood-clotting protein called factor 8
- a mutated form of the allele codes for non-functioning factor 8
- a female with one abnormal and one functional allele produces enough factor 8 to allow her blood to clot normally, so she is a carrier for the disease
- if she passes the fault X chromosome to her son, he will suffer from haemophilia A
How is colour blindness a sex-linked disease?
Describe sex linkage in cats
- one of the genes, C, for coat colour is sex-linked
- it is on the non-homologous region of the X chromosome
- the allele CO produces ginger/orange fur
- the allele CB produces black fur
- these alleles are codominant, as cats with XCO XCB are tortoiseshell
- both orange and black alleles contribute to the phenotype, but the orange allele is only expressed in cells where the X chromosome bearing the black coat colour inactivated
- male cats cannot be tortoiseshell
Why do females not express twice the number of X-linked genes if they have two X chromosomes?
- a mechanism prevents this
- in every female cell nucleus, one X chromosome is inactivated
- this is random and happens during early embryonic development
What does codominant mean?
- where both alleles present in the genotype of a heterozygous individual contribute to the individual’s phenotype
Describe an example of codominance in animals
Describe codominance in the MN blood groups in humans
- the MN blood group system is controlled by a single gene with two alleles, GM and GN
- it codes for a particular protein on the surface of erythrocytes
- these alleles are codominant
Describe the codominance in sickle cell anaemia
- it is caused by a mutation in the gene that codes for the beta-globin chain of haemoglobin
- HbS is the mutant allele
- HbN is the normal allele
- in heterozygous people, at least half the haemoglobin in red blood cells in normal, and half is abnormal, but they do not suffer from sickle cell anaemia
- if the type of haemoglobin is the phenotype, then alleles are codominant
- but if sickle cell anaemia is the phenotype, then this disorder has a recessive inheritance pattern
Give some examples of codominance in plants
- some types of camellia have red flowers
- some have white flowers
- if they are crossed, the offspring will have red and white spotted flowers
What is autosomal linkage?
- gene loci present on the same autosome that are often inherited together
How are some linked genes always inherited as one unit?
- if linked genes are not affected by crossing over of non-sister chromatids during prophase 1 of meiosis, they are always inherited as one unit
Give an example of inheritance of autosomally linked genes with no crossing over
Show how autosomally linked genes may cross over
Give an example of autosomally linked genes with crossing over
What is epistasis?
- interaction of non-linked gene loci where one masks or suppresses the expression of the other
- the genes may work together antagonistically or in a complementary fashion
- they assort independently during gamete formation because the gene loci are not linked
- epistasis reduces the number of phenotypes produced in the F2 generation of dihybrid crosses and therefore it reduces genetic variation
In what two types of epistasis do genes work antagonistically?
- recessive epistasis
- dominant epistasis
Describe recessive epistasis
- the homozygous presence of a recessive allele at the first locus prevents the expression of another allele at the second locus
- the alleles at the first locus are epistatic to those at the second locus, which are hypostatic to those at the first
Give an example of recessive epistasis
- recessive epistasis in the inheritance of flower colour in Salvia
Describe dominant epistasis using feather colour in chickens
- there is an interaction between two gene loci, I/i and C/c
- the hypostatic gene, I/i, prevents the formation of colour, even if one C allele is present
- individuals carrying at least one dominant allele, I, have white feather, even if they also have one dominant allele for coloured feathers
- birds that are homozygous for the recessive allele, c, are also white, as this mutated allele does not causer pigment to be made
How can genes work in a complementary fashion?
- epistasis is more often explained in terms of the genes working to code for two enzymes that work in succession, catalysing sequential steps of a metabolic pathway
Using the information, draw the reaction pathway of coat colour in mice
Describe how flower colour uses complementary epistasis
Describe how combs of domestic chickens uses complementary epistasis
When can you use the chi-squared test?
- the data are in categories and are not continuous
- we have a strong biological theory to use to predict expected values
- sample size is large
- the data are only raw counts
- there are no zero scores in the raw count data
How do you write the null hypothesis for chi-squared tests?
- there is no statistically significant difference between the observed and expected data
- any difference is due to chance
How do you apply the chi-squared test?
What is discontinuous variation?
- genetic variation producing discrete phenotypes
- two or more non-overlapping categories
- e.g. male or female, earlobes, ABO blood groups
Why are characteristics that exhibit discontinuous variation usually monogenic?
- they are usually determined by the alleles of a single gene locus
- however, sometimes the alleles of two genes interact to govern a single characteristic
How much of an extent do alleles and gene loci affect characteristics in discontinuous variation?
- different alleles at a single gene locus have large effects on the phenotype
- different gene loci have quite different effects on the characteristic
What is continuous variation and give examples of it
- variation that produces phenotypic variation where the quantative traits vcary by small amounts between one geoup and the next
- foot size, finger length, height, skin colour, heart rate in humans
- cob length in maize plants
- leaf length in many plants
- tail length in mice
- red kernel colour in wheat
Why are characteristics with continuous variation polygenic?
- many genes are involved in determining such characteristics
- the alleles of each gene may contribute a small amount to the phenotype
- therefore alleles have an additive effect on the phenotype
- as a result, the phenotypic categories vary in a quantitative way
- the greater the number of gene loci contributing to the determination of the characteristic, the more continuous the variation
Does the environment affect the expression of monogenic or polygenic characteristics more greatly?
- the environment affects the expressions of polygenes/polygenic characteristics more greatly
Describe what natural selection is
- mutations and migration introduce new alleles inito populations
- some individuals within a population will be better adapted than others to the environment, due to different in their genotypes and phenotypes
- these individuals are more likely to survive and reproduce, passing on the advantageous alleles
- over time, allele frequencies within the population will change
- this is natural selection
- natural selection may also maintain constancy of a species, as well as leading to new species
What are the three main types of natural selection?
- stabilisinig selection
- directional selection
- disruptive selection
What is stabilising selection?
- this normally occurs when the organisms’ environment remains unchanged
- it favours intermediate phenotypes
- e.g. in humans, babies of birth mass 3.5kg are more likely to survive
- their offspring inherit alleles from them, leading to this mean birth mass
What is directional selection?
- if the environement changes, such as by becoming colder, there may now be an advantage to being larger
- so a new larger mass becomes the ideal and will be selected for
- if more larger iindviduals survive and reproduce, they will more likely to pass genes and alleles for larger size to their offspring
- over several generations, there is a gradual shift in the optimum value for the trait
How is directional selection used by breeders?
- plant and animal breeders use directional selection to produce desirable traits
When does directional selection occur in nature?
- within a population, period of directional selection may alternate with period of stabilising selection
What is disruptive selection?
What is genetic drift?
- if a population descends from a small number of parents, the gene pool will lack genetic variation
- some alleles resulting from mutation confer neither an advantage not disadvantage on the individual so there will be no selection pressure acting upon them
- chance events may drastically alter the allele frequency
- imagine if you have a small population descended from one set of heterozygous parents
- the alleles are A and a
- if a catastrophic event occurs, leading to the death of many of an already small population, one of the alleles may disappear from this population
- when the population recovers and increases, it will have less genetic diversity than before and may lack particular alleles
- the alleles in question did not disappear due to selection pressure, but due to genetic drift
What two occurrences may lead to genetic drift?
- after a genetic bottleneck
- as the result of the founder effect
Describe a genetic bottleneck
- when a population size shrinks and increases again, it is said to have gone through a genetic bottleneck
- after this, the genetic diversity within the population is reduced
- there may be a loss of some advantageous alleles or a disproportionate frequency of deleterious alleles
- this may put the population’s chance of long-term survival at risk
- sometimes, a population shrink to such a small size that fertility is affected, leading to species endangerment and then extinction
- however, if the ones that survive have a particular advantage, then a bottleneck could improve the gene pool whilst also shrinking genetic diversity
Describe the Founder effect
- if a new population is established by a very small number of individuals who originate from a larger, parent population, the new population is likely to exhibit a loss of genetic variation
- some groups of migrating humans, have resulted in a small gene pool
- e.g. Iceland, Easter Island, Amish in North America
What is a population?
- members of a species, living in the same place and at the same time, that can interbreed
What does population genetics study?
- it studies the variation in the alleles and genotypes within the gene pool and how their frequencies vary over time
What factors affect allele frequencies within populations?
- population size
- mutation rate
- migration
- the types of natural selection
- changes to the environment e.g. bottleneck
- isolation of a population from other populations of the same species (founder effect)
- non-random mating
- genetic drift
- gene flow
What is the Hardy-Weinberg principle and what does it assume?
- the population is large enough to make sampling error negligible
- mating within the population occurs at random
- there is no selective advantage for any genotype and hence no selection
- there is no mutation, migration or genetic drift
Describe the process of speciation
- for a species to evolve into two species, it must be split into two isolated populations
- if this happens, then any mutations that occur in one population are not transmitted by interbreeding to the other population
- in each location, there will be different selection pressure and each population will accumulate different allele frequencies
- hence each population can evolve along its own lines
- at times during the evolutionary process, the two populations will be different but still be able to interbreed
- this is called sub-species
- when there have been sufficient genetic, behavioural and physiological changes in the two populations so that they can no longer interbreed, they are then separate species
What are the two types of isolating mechanisms that lead to speciation?
- geographical isolation
- reproductive isolation
Describe geographical isolation
- if populations are separated and isolated from each other by geographical features such as lakes, rivers, oceans and mountains, these also act as barriers to gene flow between populations
- the isolated populations are subject to different selection pressures in two different environments, so undergo independent changes to the allele frequencies and/or chromosome arrangement within their gene pools
- these genetic changes may be the result of mutation, selection and genetic drift
- as a result of natural selection, each population becomes adapted to its environment
- this type of speciation is called allopatric speciation (in different countries)
Describe reproductive isolation
Biological and Behavioural:
- biological and behavioural changes within a species may lead to reproductive isolation of one population from another
- if a mutation leads to some organisms in a population changing their foraging behaviour and becoming at a different time of day, it enables them to exploit a new niche
- so, the members of the diurnal populations will be unlikely to mate with crepuscular or nocturnal populations
Genetic:
- a change in chromosome number may:
- prevent gamete fusion
- make zygotes less viable so that they will fail to develop
- lead to infertile hybrid offspring with an odd number of chromosomes, so that chromosome pairing during meiosis cannot occur
Mating:
- courtship behaviour e.g. time of year for mating or courtship rituals
- animal genitalia or plant flower structure
Speciation resulting from reproductive isolation is called sympatric (same country) speciation
What is artificial selection?
- selective breeding of organisms
- involves humans choosing the desired phenotypes and interbreeding those phenotypes individually
- therefore, selecting the genotypes that contribute to the gene pool of the next generation of these organisms
What kind of organisms are bred for artificial selection?
What is inbreeding depression?
- at each stage of selective breeding, individuals with the desirable characteristics and no or few undesirable characteristics are selected
- this leads to genetic diversity reduction in the gene pool
- if related individuals are crossed, inbreeding depression can result
- the chances of an individual inheriting two copies of a recessive harmful allele are increased
What is hybrid vigour?
Why does climate change mean a new type of wheat may have to be developed?
What are some examples of gene banks?
What are some ethical consideration of artificial selection?
Describe some health conditions certain dog breeds may be susceptible to
