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10.2 Inheritance
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- 1Unlinked genes segregate independently as a result of meiosis According to the law of independent assortmentpairs of alleles are inherited independently of one another if their gene loci are on separate chromosomes – these genes are said to be unlinked
This is due to the random orientation of homologous pairs during metaphase I of meiosis
The independent segregation of unlinked genes results in a greater number of potential gamete combinations, as well as a greater variety of possible phenotypes
This also results in more complex inheritance patterns (e.g. monohybrid versus dihybrid crosses) - 2Completion and analysis of Punnett squares for dihybrid traitsA dihybrid cross determines the genotypic and phenotypic combinations of offspring for two particular genes that are unlinked
Because there are two genes, each with two alleles, there can be up to four different gamete combinations
The easiest way to work out potential gamete combinations in a dihybrid cross is to use the FOIL method:
FOIL = First / Outside / Inside / Last - 3How to Complete a Dihybrid CrossThe inheritance of dihybrid traits can be calculated according to the following steps:
Step 1: Designate characters to represent the alleles
Capital letter for dominant allele, lower case letter for recessive allele
Step 2: Write down the genotype and phenotype of the parents (P generation)
Always pair alleles from the same gene and always write capitals first (e.g. AaBb, not ABab)
Step 3: Write down all potential gamete combinations for both parents
Use the FOIL method (or the claw) to identify all possible combinations
Step 4: Use a Punnett square to work out potential genotypes of offspring
Only include the different gamete combinations for each parent (e.g. AaBB has two combinations = AB and aB)
Step 5: Write out the phenotype ratios of potential offspring
Phenotypic ratios reflect mathematical probabilities only and may not necessarily reflect actual offspring ratios - 4Gene loci are said to be linked if on the same chromosomeA linkage group is a group of genes whose loci are on the same chromosome and hence don’t independently assort
Linked genes will tend to be inherited together and hence don’t follow normal Mendelian inheritance for a dihybrid cross
Instead the phenotypic ratio will be more closely aligned to a monohybrid cross as the two genes are inherited as a single unit
Linked genes may become separated via recombination (due to crossing over during synapsis in meiosis I)
REMEMBER ANOTATED VERTICALLY LIKE DIVISON - 5Morgan’s discovery of non-Mendelian ratios in Drosophila Sex linkageThomas Hunt Morgan provided a key contribution to our current understanding of gene linkage by discovering non-Mendelian ratios in Drosophila melanogaster (fruit flies)
His breeding experiments involving fruit flies clearly demonstrated that linked genes were not independently assorted
Sex Linkage
When cross-breeding red-eyed wild types with white-eyed mutants, he discovered a clear sex bias in phenotypic distribution
All female offspring of a red-eyed male were red-eyed, whereas all male offspring of a white-eyed female were also white-eyed
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) - 6Gene LinkageMorgan 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
Based on this data, Morgan made two key proposals:
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
Morgan also observed that the amount of crossing over between linked genes differed depending on the combination of traits
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
Morgan used this concept to develop the first gene linkage maps that showed the relative positions of genes on a chromosome - 7Identification of recombinants in crosses involving two linked genesRecombinants of linked genes are those combinations of genes not found in the parents
Recombinants occur as a result of crossing over of genetic material during prophase I of meiosis
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
The frequency of recombinant phenotypes within a population will typically be lower than that of non-recombinant phenotypes
This is because crossing over is a random process and chiasmata do not form at the same locations with every meiotic division
The relative frequency of recombinant phenotypes will be dependent on the distance between linked genes
Recombination frequency between two linked genes will be greater when the genes are further apart on the chromosome
This is because there are more possible locations where a chiasma could form between the genes
Recombinant phenotypes can be identified by performing a test cross (crossing with a homozygous recessive for both traits) - 8Chi Squared Testhttp://ib.bioninja.com.au/higher-level/topic-10-genetics-and-evolu/102-inheritance/chi-squared-test.html
- 9Variation can be discrete or continuous The phenotypes of polygenic characteristics tend to show continuous variationVariation in phenotypes for a particular characteristic can be either discrete (discontinuous) or continuous
- 10Monogenic traits(characteristics controlled by a single gene loci) tend to exhibit discrete variation, with individuals expressing one of a number of distinct phenotypes
- 11Polygenic 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
- 12In the case of polygenic inheritance:Increasing the number of loci responsible for a particular trait increases the number of possible phenotypes
This results in a phenotypic distribution that follows a Gaussian (bell-shaped) normal distribution curve - 13Maize Grain ColourAn example of a polygenic trait is grain colour in maize (wheat), which is controlled by three gene loci
Grain colour can range from white to dark red, depending on the amount of pigment that is expressed
Each gene has two alleles, which either code for red pigment or white pigment
The most frequent combinations have an equal number of the two allele types
Conversely, combinations of one extreme or the other are relatively rare
The overall pattern of inheritance shows continuous variation - 14Polygenic traits such as human height may also be influenced by environmental factorsPhenotypic characteristics are not solely determined by genotype, but are also influenced by environmental factors
The added effect of environmental pressures functions to increase the variation seen for a particular trait
One example of a polygenic trait that is influenced by environmental factors is human height
Human height is controlled by multiple genes (polygenic), resulting in a bell-shaped spectrum of potential phenotypes
Environmental factors such as diet and health (disease) can further influence an individual human’s height
Another example of a polygenic trait that is influenced by environmental factors is human skin colour
Skin colour is controlled by multiple melanin producing genes, but is also affected by factors such as sun exposure
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- High School Diploma Biology (10) Genetics & Evolution) Karteikarten am 10.2 Inheritance, erstellt von Emilia Sullivan am 30/04/2017.
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