module 6.1.2: patterns of inheritance Flashcards
what are the 2 types of phenotypic variation
discontinuous and continuous
what is discontinuous variation
genetic variation producing discrete (discontinuous) phenotypes, so two or more non-overlapping categories
- traits tend to be monogenic, so different alleles have very different effects on the phenotype.
- can be represented using a bar charts unaffected by the environment
what is continuous variation
genetic variation that produces phenotypic variation where quantitative traits vary by very small amounts between one group and the next
- characteristics tend to be polygenic
the greater the number of gene loci…..
the more continuous the variation
how can the continuous variation data be represented
represented on a histogram
what is continuous variation influenced by
the environment
- eg. if plants are kept in dim light after germination, or if the soil contains insufficient magnesium, then the leaves of the plant do not develop enough chlorophyll and are yellow-white. the plant produced is chlorotic and unable to photosynthesise. it has the genotype for making chlorophyll, but environmental factors have prevented the expression of these genes
what is the main role of meiosis
the production of haploid gametes as cells produced by meiosis have half the number of chromosomes
how is genetic variation achieved during meiosis
crossing over
independent assortment
what happens during crossing over and when does it happen
where pairs of homologous chromosomes line up and exchange some of their genetic material
prophase 1
what happens during the independent assortment of chromosomes and when does it happen
the production of different combinations of alleles in daughter cells due to the random alignment of homologous pairs along the equator of the spindle
what is an allele
a version of a gene
what is a locus
the specific position of a gene on a chromosome, the two alleles of a gene are found at the same loci on the chromosome pairs
what is a phenotype
visible characteristics of an organism
what is genotype
genetic makeup of an organism
what is a dominant allele
only a single allele is required for the characteristic to be expressed, that allele is always expressed in the phenotype
what is a recessive allele
the characteristic is only expressed if there is no dominant allele present
what is homozygous
two identical alleles
what is heterozygous
two different alleles
what is codominance
both alleles contribute to the phenotype
how is closeness of two linked genes on a chromosome linked to the number of recombinant offspring
more closely linked genes means less separation and so less recombinant offspring
what is a karyotype
display of every pair of homologous chromosomes within a cell, organized according to size and shape
what is a phenotype
physical characteristics of an organism
types of factors that contribute to phenotypic variation
environmental
genetic
environmental factor that contributes to phenotypic variation in animals
diet
examples of phenotypic variations caused by environmental conditions in plants
etiolation
chlorosis
etiolation
when a plant has elongated stems and has a pale colour due to a lack of chlorophyll because of insufficient light in the environment
chlorosis
when the leaves look pale or yellow because the cells aren’t producing enough chlorophyll
environmental factors that lead to chlorosis
lack of light leading to chlorophyll production decreasing to conserve resources, mineral deficiencies of iron or magnesium, viral infections
how is genetic variation created within a species
sexual reproduction
how can sexual reproduction lead to genetic variation within a species
meiosis, random fusion of gametes at fertilisation
f1 generation
generation produced by mating dominant and recessive homozygous individuals, all offspring are heterozygous
f2 generation
the offspring of mating two heterozygous individuals
phenotypic ratio in an F2 generation in monogenic inheritance
3:1
codominance
when two equally dominant alleles occur for a gene as the genotype is heterozygotic so are both expressed in the phenotype
how is codominance represented
capitals with a letter index
phenotypic ratio in an F2 generation in codominance
1:2:1 (homozygous, heterozygous, homozygous)
multiple alleles
when a gene has more than two versions
example of a characteristic caused by multiple alleles
blood group
characteristics of the multiple alleles involved in determination of blood group
IA and IB are codominant, IO is recessive, IA and IB are dominant to IO
X linkage
when a person assigned male at birth only has one copy of gene on the X chromosome so conditions caused by recessive alleles are more common in people assigned male at birth
why is X linkage a thing
the Y chromosome is smaller than the X chromosome so there are fewer genes on it
how to represent X linkage
X to an index, Y
phenotypic ratio when a carrier female mates with a normal male and the characteristic is X linked
half of all AMABs will have disorder, half of AFAB will be carriers
phenotypic ratio for dihybrid inheritance
9:3:3:1
why may an actual phenotypic ratio be different from the theoretical one
fertilisation is random so small sample can be skewed by a few chance events, genes being studied are both on the same chromosome in linkage
linkage
genes are on the same chromosome
how are linked genes inherited
as one unit, no independent assortment unless they are separated by chiasmata
the closer genes are on a chromosome…
the less likely they are to be separated during crossing over
why may a ratio for linkage not be the expected
crossing over
equation for recombination frequency
Recombination frequency = Number of recombinant offspring/ Total number of offspring
what does a recombination frequency of 50% indicate
no linkage
what recombination frequency suggests linkage
less than 50%
how to use recombination frequencies to map a chromosome
recombination frequency of 1% is 1 map unit on a chromosome
epistasis
interaction of genes at different loci
examples of gene epistasis
gene regulation
hypostatic
gene that is affected by another gene
epistatic
gene that affects the expression of another gene
recessive epistasis
homozygous presence of a recessive allele prevents expression of another allele at a second locus
phenotypic ratio for recessive epistasis
9:3:4
dominant epistasis
presence of dominant allele at one gene locus masks the expression of alleles at a second locus
phenotypic ratio for dominant epistasis
12:3:1
complementary epistasis
homozygous recessive genotypes at either locus masks the expression of the dominant allele at the other locus
phenotypic ratio for complementary epistasis
9:7
role of chi squared test
to determine the significance of the difference between observed and expected results
degrees of freedom for chi squared test
n-1
factors that lead to continuous variation
polygenetic
environmental
factors that lead to discontinuous variation
few genes
allele frequency
relative frequency of a particular allele in a population
gene pool
sum total of all the genes in a population at a given time
letter that represents the frequency of a dominant allele in the Hardy-Weinberg equation
p
letter that represents the frequency of a recessive allele in the Hardy-Weinberg equation
q
equations for Hardy-Weinberg
p + q = 1, p^2 + 2pq + q^2 = 1
Hardy-Weinberg Principle
in a stable population with no disturbing factors, the allele frequencies will remain constant from one generation to the next and there will be no evolution
assumptions for the Hardy-Weinberg Principle
no mutations, no migration, equal fertility of each phenotype, each phenotype is as preferable as each other, large population required
p^2
frequency of homozygous dominant genotype in the population
2pq
frequency of heterozygous genotype in the population
q^2
frequency of homozygous recessive genotype in the population
role of the Hardy-Weinberg Principle
to calculate allele frequencies in a population
allele
version of a gene with a unique base sequence
recessive
an allele that won’t be expressed in the phenotype if there are any dominant alleles present
which type of variation has an additive effect
continuous
factors that can affect the evolution of a species
stabilising selection, directional selection, genetic drift, genetic bottleneck, founder effect
stabilising selection
when the mean value is selected for and the extreme values are selected against
effect of stabilising selection on the normal distribution curve
gets narrower
directional selection
when the organisms with extreme phenotypes are selected for as a result of a change in the environment
example of directional selection
peppered moths in the industrial revolution
genetic drift
changes in allele frequency due to the random nature of mutations
which populations will genetic drift have a larger effect in
small populations
why will genetic drift have a larger effect in small populations
smaller gene pool
genetic bottleneck
large decrease in population size that lasts for at least one generation
founder effect
when a new colony is established by a few individuals, leading to the creation of a small population
what is the founder effect an example of
genetic drift
types of speciation
allopatric
sympatric
allopatric speciation
when some members of a population are separated from the larger population by a physical barrier and are geographically isolated, leading to the evolution of a new species
how does allopatric speciation lead to the evolution of a new species
selection pressures different in different environments, different alleles considered advantageous, different alleles selected for
example of allopatric speciation
Darwin’s finches
sympatric speciation
when organisms are isolated by reproductive mechanisms but live in the same habitat, leading to the evolution of a new species
examples of mechanisms of reproductive isolation
ecological isolation, behavioural isolation, mechanical isolation, gametic isolation, temporal isolation
artificial selection
organisms with alleles that are advantageous to the breeder are selected for so its frequency increases
example of selective breeding in plants
hybridising okra with hibiscus to confer resistance to yellow vein mosaic disease
examples of selective breeding in animals
Belyaev’s Foxes, breeding of dachshunds for small size and short legs so they could follow prey into burrows (I don’t trust any dog whose stomach touches the floor)
example of maintaining resources of genetic material
seed banks
importance of maintaining resources of genetic material
used in selective breeding
use of genetic resources from seed banks
outbreeding to reduce the occurrence of homozygous recessive conditions due to inbreeding
ethical considerations surrounding the use of artificial selection
can lead to health problems and homozygous recessive conditions
examples of health problems occurring as a result of the use of artificial selection
big dogs have hip and heart problems, skull of the King Charles spaniel is too small to accommodate the brain
monogenic inheritance
when a phenotype or trait is controlled by a single gene
- eg. cystic fibrosis where the individuals with doubly recessive genotype are affected
dihybrid cross
inheritance of two genes simultaneously. the two genes are inherited independently of each other and so each gamete has one allele for each of the two gene loci. this means that during fertilisation any one of an allele pair can combine with any one of another allele pair. the probability of any two traits being inherited together if they are not linked is the product of the individual probabilities
autosomal linkage and when does it occur
genes which are located on the same chromosome (which is not a sex chromosome) and tend to be inherited together in the offspring. occurs as the chromosome, not the gene, is the unit of transmission during meiosis, so linked genes are very unlikely to be separated by independent assortment. if linked genes are not affected by crossing over of non-sister chromatids during prophase 1 then they are always inherited as one unit
