10.2 Inheritance Flashcards
What does the law of independent assortment say about alleles?
Pairs of alleles are inherited independently of one another if their gene loci are on separate chromosomes
If gene loci are on separate chromosomes called?
Unlinked genes
What causes unlinked genes?
The random orientation of homologous pairs during metaphase I of meiosis
What does the independent segregation of unlinked genes cause a greater number of?
Of potential gamete combinations and a greater variety of possible phenotypes
What does a greater variety of possible phenotypes cause?
More complex inheritance patterns
What does a dihybrid cross determine?
The genotypic and phenotypic combinations of offspring for two particular genes that are unlinked
Why can there be up to four different gamete combinations?
Because there are two genes each with two alleles
What is the best way to work out potential gamete combinations in a dihybrid cross?
The FOIL method
What are the five steps to complete a dihybrid cross?
- designate characters to represent the alleles
- Write down the genotype and phenotype of the parents
- Write down all potential gamete combinations for both parents
- Use a Punnett square to work out potential genotypes of offspring
- Write out the phenotype ratios of potential offspring
What does a capital letter represent?
A dominant allele
What does a lower case letter represent?
A recessive allele
What should go first when writing down the genotype/phenotype?
The capitals first (AaBb not ABab)
When writing genotypes/ phenotypes what should you pair?
Alleles from the same gene
When using a Punnett square what should you only include?
The different gamete combinations for each parent
What do phenotypic ratios reflect?
The mathematical probabilities only and may not necessarily reflect actual offspring ratios
What is a linkage group?
A group of genes whose loci are on the same chromosome and don’t independently assort
How will linked genes be inherited?
Together
What do linked genes fail to follow?
Normal mendelian inheritance for a dihybrid cross
What will linked genes be more closely aligned to and why?
A monohybrid cross as the two genes are inherited as a single unit
How may linked genes become separated?
Via recombination
How are linked genes represented?
As vertical pairs
What did Thomas Hunt Morgan do?
Provided a key contribution to our current understanding of gene linkage by discovering non-mendelian ratios in fruit flies
What did Morgans breeding experiment with fruit flies show?
That linked genes were not independently assorted
When Morgan worked with red eyed wild types and white eyed mutants what did he discover?
A clear sex bias in phenotypic distribution
How did Morgan describe this distribution and explain why it happened:
All female offspring of a red-eyed male were red-eyed, whereas all male offspring of a white-eyed female were also white-eyed?
As sex limited inheritance and it was caused by the gene for eye colour was found on the sex chromosome
What did Morgan observe that the amount of crossing over between linked genes differed dependent on?
The combination of traits
What is crossover frequency a product of?
The distance between two genes on a chromosome
Describe the distance between genes with a higher crossover frequency?
Further apart
Describe the distance between genes with a lower crossover frequency?
Closer together
What are recombinants of linked genes?
Those combinations of genes not found in the parents
What causes recombinants to happen?
Crossing over of genetic material during prophase I of meiosis
What happens if linked genes become separated by chiasma?
There will be an exchange of alleles between the non sister chromatids
What do recombinants create?
New allele combinations that are different to those of the parent
Why is the frequency of recombinant phenotypes within a population is typically be lower than that of non-recombinant phenotypes?
Because crossing over is a random process
Do chiasmata form at the different locations in each meiotic division?
Yes
What is the relative frequency of recombinant phenotypes dependent on?
The distance between linked genes
Why will the recombination frequency between two linked genes be greater when the genes are further apart on the chromosome?
Because there are more possible locations where a chiasma could form between the genes
How can recombinant phenotypes be identified?
By performing a test cross
What can you tell by looking at the frequency of potential phenotypes?
Whether two genes are linked or unlinked
What possibility do offspring with unlinked genes have of inheriting any potential phenotypic combination?
Equal
When will offspring with linked genes express the phenotypic combinations present in either parent?
If crossing over occurs
Do unlinked recombinant phenotypes occur more or less frequently than linked parental phenotypes?
Less frequently
What are chi squared tests?
A statistical measure that are used to determine whether the difference between an observed and expected frequency distribution is statistically significant
If observed frequencies do not conform to those expected for an unlinked dihybrid cross, what does it suggest?
Genes are linked and not independently assorted
or
The inheritance of the traits are not random but are maybe affected by natural selection
What are the five steps of performing a chi squared test?
Identify hypothesis (null vs alternative)
Construct a table of frequencies (observed versus expected)
Apply the chi squared formula
Determine the degree of freedom
Identify the p value
What is a null hypothesis?
There is no significant difference between observed and expected frequencies
What is an alternative hypothesis?
There is a significant difference between observed and expected frequencies
How do you calculate expected frequencies?
By determining the expected ratios and then multiplying against the observed total
How do you calculate degrees of freedom?
df = (m – 1) (n – 1)
Where: m = number of rows ; n = number of columns
What is the chi squared formula?
x2 = ∑ (O-E)2 / E
What are the two types of variation in phenotypes?
Discrete or continuous
What are monogenic traits?
Characteristics controlled by a single gene loci
What are polygenic traits?
Characteristics controlled by more than one gene loci
What type of variation do monogenic traits exhibit?
Discrete variation
What type of variation do polygenic traits exhibit?
Continuous variation
What will an individuals phenotype be in a monogenic trait?
One of a number of distinct phenotypes
What will an individuals phenotype be in a polygenic trait?
Somewhere in a continuous spectrum of potential phenotypes
What does increasing the number of loci responsible for a particular trait do?
Increases the number of possible phenotypes
What does the distribution shape of a phenotype follow?
A bell shaped normal distribution curve
What is an example of a polygenic trait?
Maize grain colour
What is maize colour controlled by?
Three gene loci
What is the colour range of maize?
White to dark red
What does each maize allele have and what does it code for?
Two alleles which either code for red or white pigment
What is the most frequent combination of allele types in maize?
An equal number of both allele types
What does the overall patter of maize colour inheritance show?
Continuous variation
What are the two things that influence phenotypes?
Genotype and environmental factors
What is the added effect of environmental pressures functions?
Increases the variation seen for a particular trait
What are examples of a polygenic trait which is influenced by environmental factors?
Human height and skin colour
What controls human height?
Multiple genes resulting in a bell shaped spectrum of potential phenotypes
What environmental factors can influence an individual humans height?
Diet and health
What controls skin colour?
Multiple melanin producing genes
What environmental factors affect skin colour?
Sun exposure
