10.2 Inheritance Flashcards
What does it mean if alleles are unlinked?
According to the law of independent assortment, pairs of alleles are inherited independently of one another if their gene loci are on separate chromosomes – these genes are said to be unlinked
Why are some alleles unlinked?
This is due to the random orientation of homologous pairs during metaphase I of meiosis
What does independent segregation lead to in terms of variation?
The independent segregation of unlinked genes results in a greater number of potential gamete combinations, as well as a greater variety of possible phenotypes
What does independent segregation lead to in terms of inheritance patterns?
This also results in more complex inheritance patterns (e.g. monohybrid versus dihybrid crosses)
What does a dihybrid cross determine?
A dihybrid cross determines the genotypic and phenotypic combinations of offspring for two particular genes that are unlinked
How many gamete combinations can there be in a dihybrid cross?
Because there are two genes, each with two alleles, there can be up to four different gamete combinations
What is a linked group?
A linkage group is a group of genes whose loci are on the same chromosome and hence don’t independently assort
What do linked genes not exhibit?
Linked genes will tend to be inherited together and hence don’t follow normal Mendelian inheritance for a dihybrid cross
What will be the phenotypic ratio for linked genes?
Instead the phenotypic ratio will be more closely aligned to a monohybrid cross as the two genes are inherited as a single unit
When may linked genes become separated?
Linked genes may become separated via recombination (due to crossing over during synapsis in meiosis I)
Who provided insight into gene linkage?
Thomas Hunt Morgan provided a key contribution to our current understanding of gene linkage by discovering non-Mendelian ratios in Drosophila melanogaster (fruit flies)
What did Thomas Hunt Morgan show in terms of linked genes?
His breeding experiments involving fruit flies clearly demonstrated that linked genes were not independently assorted
What did Thomas Hunt Morgan show in terms of sex linkage?
When cross-breeding red-eyed wild types with white-eyed mutants, he discovered a clear sex bias in phenotypic distribution
What were the phenotypes of the fruit flies which suggested sex linkage?
All female offspring of a red-eyed male were red-eyed, whereas all male offspring of a white-eyed female were also white-eyed
How did Morgan explain his findings related to sex linkage?
Morgan described this distribution as ‘sex-limited’ inheritance and inferred it was caused by the gene for eye colour being located on a sex chromosome (i.e. X-linked)
What other features did morgan find that did not abide by mendelian ratios?
Morgan went on to identify a number of different traits in fruit flies that did not conform to Mendelian ratios
Certain phenotypic combinations occurred in much lower frequencies than was to be expected
What two explanations did Morgan present for the offspring not exhibiting Mendelian ratios?
The alleles for these traits were located on a shared chromosome (gene linkage) and hence did not independently assort
Linked alleles could be uncoupled via recombination (crossing over) to create alternative phenotypic combinations, but these new phenotypes would occur at a much lower frequency
What did Morgan also observe in terms of crossing over?
Morgan also observed that the amount of crossing over between linked genes differed depending on the combination of traits
What could Morgan conclude about crossing over?
This led to the idea that crossover frequency may be a product of the distance between two genes on a chromosome – genes with a higher crossover frequency are further apart, whereas genes with a lower crossover frequency are closer together
What did Morgan develop with his findings?
Morgan used this concept to develop the first gene linkage maps that showed the relative positions of genes on a chromosome
What can be seen from Morgan’s gene linkage map?
From the above diagrams it can be seen that:
Long aristae and red eyes are more likely to be separated via recombination (high crossover frequency)
Long aristae and long legs are less likely to be separated via recombination (low crossover frequency)
This indicates that the aristae and leg genes are located closer together, whereas the eye gene is more distant
What are recombinants of linked genes?
Recombinants of linked genes are those combinations of genes not found in the parents
What creates recombinants?
Recombinants occur as a result of crossing over of genetic material during prophase I of meiosis
What occurs during prophase I which leads to recombinants?
If linked genes become separated by a chiasma, there will be an exchange of alleles between the non-sister chromatids
!This creates new allele combinations that are different to those of the parent
Will the frequency of non/recombinants be higher in a population?
The frequency of recombinant phenotypes within a population will typically be lower than that of non-recombinant phenotypes
Why will non-recombinant phenotypes be more rare?
This is because crossing over is a random process and chiasmata do not form at the same locations with every meiotic division
What will the relative frequency of recombinant phenotypes be dependent on?
The relative frequency of recombinant phenotypes will be dependent on the distance between linked genes
When will recombination frequency be greater?
Recombination frequency between two linked genes will be greater when the genes are further apart on the chromosome
Why is recombination frequency greater when genes are further apart?
This is because there are more possible locations where a chiasma could form between the genes
How can recombinant phenotypes be identified?
Recombinant phenotypes can be identified by performing a test cross (crossing with a homozygous recessive for both traits)
How is it possible to determine whether genes are liked or unlinked?
It is possible to infer whether two genes are linked or unlinked by looking at the frequency distribution of potential phenotypes
How is it possible to tell that offspring have unlinked genes?
Offspring with unlinked genes have an equal possibility of inheriting any potential phenotypic combination
Why do offspring with unlinked genes have an equal possibility?
This is due to the random segregation of alleles via independent assortment
How is it possible to tell that offspring have linked genes? (there is one exception)
Offspring with linked genes will only express the phenotypic combinations present in either parent unless crossing over occurs
Which phenotype appears less often in offspring with linked genes?
Consequently, the ‘unlinked’ recombinant phenotypes occur less frequently than the ‘linked’ parental phenotypes
What are chi-squared tests used for?
Chi-squared tests are a statistical measure that are used to determine whether the difference between an observed and expected frequency distribution is statistically significant
What does it suggest if observed frequencies do not conform to those expected for an unlinked dihybrid cross?
Genes are linked and hence not independently assorted
The inheritance of the traits are not random, but are potentially being affected by natural selection
How may phenotypes vary? (2 ways?
Variation in phenotypes for a particular characteristic can be either discrete (discontinuous) or continuous
What are monogenic traits?
Monogenic traits (characteristics controlled by a single gene loci) tend to exhibit discrete variation, with individuals expressing one of a number of distinct phenotypes
What are polygenic traits?
Polygenic traits (characteristics controlled by more than two gene loci) tend to exhibit continuous variation, with an individual’s phenotype existing somewhere along a continuous spectrum of potential phenotypes
For polygenic inheritance, what does increasing the number of loci do?
Increasing the number of loci responsible for a particular trait increases the number of possible phenotypes
What curve does the phenotypic distribution show for polygenic traits?
This results in a phenotypic distribution that follows a Gaussian (bell-shaped) normal distribution curve
What is an example of a polygenic trait?
An example of a polygenic trait is grain colour in maize (wheat), which is controlled by three gene loci
How may grain colour vary and why?
Grain colour can range from white to dark red, depending on the amount of pigment that is expressed
What does each gene have in maize?
Each gene has two alleles, which either code for red pigment or white pigment
What do the most frequent combinations have in terms of alleles for maize?
The most frequent combinations have an equal number of the two allele types
What is rare in maize?
Conversely, combinations of one extreme or the other are relatively rare
What pattern does polygenic genes in maize show?
The overall pattern of inheritance shows continuous variation
What can phenotypic variations also be determined by, apart from the genotype?
Phenotypic characteristics are not solely determined by genotype, but are also influenced by environmental factors
What is the role of environmental factors in determining phenotype?
The added effect of environmental pressures functions to increase the variation seen for a particular trait
What is an example of a polygenic trait affected by environmental factors?
One example of a polygenic trait that is influenced by environmental factors is human height
What is human height controlled by?
Human height is controlled by multiple genes (polygenic), resulting in a bell-shaped spectrum of potential phenotypes
Apart from the genotype, what environmental factors can affect human height?
Environmental factors such as diet and health (disease) can further influence an individual human’s height
Apart from height, what other trait is polygenic and affected by environmental factors?
human skin colour
What is human skin colour controlled by?
Skin colour is controlled by multiple melanin producing genes, but is also affected by factors such as sun exposure
