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10.2 U 1 Gene loci are said to be linked if on the same chromosome.
10.2 U 1 Gene loci are said to be linked if on the same chromosome.
Be able to:
Define autosome and sex chromosome.
Describe what makes genes “linked.”
Linkage refers to genes that are located on the same chromosome. Linked genes tend to be inherited together and the extent of crossing over depends on how close together they are on the chromosome. Linkage generally reduces the variety of offspring that can be produced. In genetic crosses, linkage is indicated when a greater proportion of the offspring resulting from a cross are of the parental type (than would be expected if the alleles were assorting independently). If the genes in question had been on separate chromosomes, there would have been more genetic variation in the gametes and therefore in the offspring.
10.2 U 2 Unlinked genes segregate independently as a result of meiosis.
10.2 U 2 Unlinked genes segregate independently as a result of meiosis.
Be able to:
State the difference between independent assortment of genes and segregation of alleles.
Describe segregation of alleles and independent assortment of unlinked genes in meiosis.
Unlinked genes are on different chromosomes. At Metaphase 1, each pair of homologous chromosomes line up at the equator of the spherical cell. For each homologous pair, the chromosomes assort to one side or the other. The alignment of any pair of homologous chromosomes is independent of all the other pairs. Thus, the mixture of unlinked genes in gametes is random.
10.2 U 3 Variation can be discrete or continuous.
10.2 U 3 Variation can be discrete or continuous.
Discontinuous Variation:
This is where individuals fall into a number of distinct classes or categories, and is based on features that cannot be measured across a complete range. You either have the characteristic or you don't. Blood groups are a good example: you are either one blood group or another - you can't be in between. Such data is called discrete (or categorical) data. Chi-squared statistical calculations work well in this case.
Monogenic traits (characteristics controlled by a single gene loci) tend to exhibit discrete variation, with individuals expressing one of a number of distinct phenotypes
Continuous Variation:
There is a complete range of measurements from one extreme to the other. Height is an example of continuous variation - individuals can have a complete range of heights, for example, 1.6, 1.61, 1.62, 1.625 etc metres high.
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
10.2 U 4 The phenotypes of polygenic characteristics tend to show continuous variation
10.2 U 4 The phenotypes of polygenic characteristics tend to show continuous variation
Be able to:
Explain polygenetic inheritance using an example of a two gene cross with codominant alleles.
State that a normal distribution of variation is often the result of polygenic inheritance.
State example human characteristics that are associated with polygenic inheritance.
Continuous variation is the combined effect of many genes (known as polygenic inheritance) and is often significantly affected by environmental influences. Milk yield in cows, for example, is determined not only by their genetic make-up but is also significantly affected by environmental factors such as pasture quality and diet, weather, and the comfort of their surroundings.
Polygenic traits are controlled by two or more than two genes (usually by many different genes) at different loci on different chromosomes. These genes are described as polygenes. Examples of human polygenic inheritance are height, skin colour and weight. Polygenes allow a wide range of physical traits. For instance, height is regulated by several genes so that there will be a wide range of heights in a population.
Maize Grain Colour
An 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
10.2 U 5 Chi-squared tests are used to determine whether the difference between an observed and expected frequency distribution is statistically significant
10.2 U 5 Chi-squared tests are used to determine whether the difference between an observed and expected frequency distribution is statistically significant
Be able to:
State the two possible hypotheses of a statistical test.
Calculate the chi square value to determine the significance of differences between the observed and expected results of a genetic cross.
Determine the degrees of freedom and critical value for the chi-square test.
Draw a conclusion of significance by comparing the calculated and critical chi-square values.
Click here for a great resource on chi square by Bozeman Science
A chi-square test is a statistical test that can be used to determine whether observed frequencies are significantly different from expected frequencies. These statistical tests enable us to compare observed and expected frequencies empirically and to decide if the results we see are statistically significant. Statistical significance in this case implies that the differences are not due to chance alone, but instead may be caused by other factors at work.
What it basically means is the sum of the (observed minus the expected) squared, divided by the expected. This value is then compared to a critical value from a chi-squared table (similar to a t-table) to determine if the numbers we see are due to random factors. This is useful in determining if the results from a dihybrid genetic cross are due to independent assortment.
Degree of freedom - the number of independent values or quantities that can be assigned to a statistical distribution.
To find the degree of freedom = n - 1, where n is the number of data points
A critical value is the number you compare your own result to (critical values would be provided in a test).
A confidence interval does not quantify variability. A 95% confidence interval is a range of values that you can be 95% certain contains the true mean of the population.
10.2 A1 Morgan’s discovery of non-Mendelian ratios in Drosophila.
10.2 A1 Morgan’s discovery of non-Mendelian ratios in Drosophila.
Be able to:
Determine possible allele combinations in gametes for crosses involving two genes.
Use correct notation to depict a dihybrid cross between two unlinked genes.
Construct a Punnett square to show the possible genotype and phenotype outcomes in a dihybrid cross.
Male fly with white eyes (flies usually have red eyes) was crossed with a female fly with red eyes (a wild type). The offspring of the first generation were red eyed
Males and females of the first generation were crossed with each other
The white-eye trait reappeared in the expected 3:1 Mendelian ratio for a recessive trait. However only the males had white eyes
This suggested that the white-eye trait is carried on the X chromosome
Crossing white-eyed males and red-eyed females from the second generation produced equal numbers of offspring with each eye colour
Males have white eyes when they inherit the mutant gene on the X chromosome from their mother. Females only show the trait if they inherit mutant genes on both X chromosomes. This became known as "sex linkage
10.2 A 2 Completion and analysis of Punnett squares for dihybrid traits.
10.2 A 2 Completion and analysis of Punnett squares for dihybrid traits.
Be able to:
Describe how Morgan discovered relationship between eye color and sex in Drosophila.
Linked genes that have undergone recombination can be distinguished from unlinked genes via a test cross because the frequency of the recombinant genotypes will always be less than would occur for unlinked genes (crossing over does not happen every time)
For example:
Heterozygous test cross of unlinked genes results in 1 : 1 : 1 : 1 phenotypic ratio
Heterozygous test cross of linked genes results in 1 : 1 : 0.1 : 01 phenotypic ratio (uncommon phenotypes are recombinants)
A dihybrid cross is a cross between two individuals that shows the inheritance of two different genes at the same time; usually involving unlinked autosomal genes.
Note: The following example contains two unlinked genes, which means the genes are on different chromosomes. This means they follow Mendel’s law of independent assortment.
Dihybrid Cross Example
Since almost all examples look at Mendel’s pea plants, for this example we will look at two traits in cats; hair length and color.
If the question stated that short hair (S) is dominant over long hair (s) and black fur (B) is dominant over white fur (b), what would be the genotypic and phenotypic ratio of a cross between a short hair black cat that is homologous for these traits and a long hair white cat?
First, write out the cross: SSBB x ssbb
Next, write out the possible gametes: SB, SB, sb and sb or simply just SB and sb
10.2 A 3 Polygenic traits such as human height may also be influenced by environmental factors.
10.2 A 3 Polygenic traits such as human height may also be influenced by environmental factors.
Be able to:
Outline two example environmental factors that can influence phenotypes.
Compare continuous to discrete variation.
When one gene controls the expression of a trait, the number of phenotypes that are expressed is limited to the dominant phenotype or recessive phenotype. If there is co-dominance, this adds another possible phenotype that can be expressed.
With polygenic inheritance when two or more genes control the expression of a phenotype many possible phenotypes can exist. As the amount of genes that control one trait increase, the number of phenotypes increases to a point where it is impossible to determine the genotype by just observing the phenotype. Each additional gene has an additive affect, increasing the phenotypes. This is called continuous variation.
For example, people’s skin color varies dramatically around the world, between people of different races and within the same race. The multiple genes affect the intensity of the pigments in the skin. Another example is human height, which varies from person to person within the same race, and varies between different races. Height shows continuous variation.
If you graphed the frequency of the occurrence of different phenotypic variations in a population, there should be a normal distribution. Continuing with human height, there will be some really tall people and some really short people, but the majority of people will be average height (normal distribution). As the number of genes that control a certain trait increases, the closer the distribution of the phenotypes represents a normal distribution.
10.2 S 1 Calculation of the predicted genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes.
10.2 S 1 Calculation of the predicted genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes.
Be able to:
Determine the predicted genotype and phenotype ratios of F1 and F2 offspring of dihybrid crosses. (see A2 above)
10.2 S 2 Identification of recombinants in crosses involving two linked genes.
10.2 S 2 Identification of recombinants in crosses involving two linked genes.
Be able to:
Use correct notation to show alleles of linked genes.
Construct a Punnett square to show the possible genotype and phenotype outcomes in a dihybrid cross involving linked genes.
Explain how crossing over between linked genes can lead to genetic recombinants.
In a cross between two true breeding or homozygous parents the following would be the genotypes of the cross: AABB x aabb.
A linkage group is a group of genes whose loci are on the same chromosome and therefore do not follow the law of independent assortment
Linked genes will tend to be inherited together - the only way to separate them is through recombination (via crossing over during synapsis)
When two genes are linked, they do not follow the expected phenotypic ratio for a dihybrid cross between heterozygous parents
Instead the phenotypic ratio will follow that of a monohybrid cross as the two genes are inherited together
This means that offspring will tend to produce the parental phenotypes
Recombinant phenotypes will only be evident if crossing over occurs in prophase I and would thus be expected to appear in low numbers (if at all)
An example of a cross between two linked genes is the mating of a grey bodied, normal wing fruit fly with a black bodied, vestigial wing mutant
Recombinants of linked genes are those combinations of genes not found in parents.
For example, in a test cross of a heterozygous fruit fly (grey bodied, normal wings) with a homozygous recessive mutant (black bodied, vestigial wings), the recombinants would be the grey bodied, vestigial winged offsprings and the black bodied, normal winged offspring.
10.2 S 3 Use of a chi-squared test on data from dihybrid crosses.
10.2 S 3 Use of a chi-squared test on data from dihybrid crosses.
Be able to:
Calculate a chi-square value to compare observed and expected results of a dihybrid genetic cross.
Using the df and critical chi-square value, determine if there is a significant difference between observed and expected results of a dihybrid cross.
For an example, use Mendel’s results from his pea plant crosses. When he did a dihybrid cross between two heterozygotes RrYy x RrYy, the expected phenotypic ratio due to independent assortment would be 9:3:3:1. Look at the chart below to see his actual results.
Possible hypotheses
Ho (null hypothesis) the results are due to independent assortment
Ha (alternative hypothesis) the alleles do not assort independently and the results are due to gene linkage.
As you can see from the table above the critical value at the 0.05 level of significance is 7.815.
What the p-value of 0.05 or 5% indicates is the probability of getting the results you did (or more extreme results) given that the null hypothesis is true.
If the calculated value is above or equal to 7.815 then we reject the Ho and accept the HA that the alleles in question are linked. However, since our chi-squared value of 0.47 is significantly less than this value (it has a p-value of 0.90 or 90%) we accept the Ho that the results that Mendel saw were due to independently assortment of the alleles.
**NOTE: Alleles are usually shown side by side in dihybrid crosses, for example, TtBb. In representing crosses involving linkage, it is more common to show them as vertical pairs, for example:
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