7A: Inheritance Flashcards
Define gene
A sequence of bases on a DNA molecule that codes fr a protein which results in a characteristic e.g eye colour
Define allele
A different version of a gene.
- There can be many different alleles on a single gene, but in diploid organisms (e.g humans) only 2 are carried.
- The order of bases on each allele is slightly different –> they code for different versions of the same characteristic
Define genotype
The genetic constitution of an organism - the alleles it has
Define phenotype
The expression of the genetic constitution and its interaction with the environment
Define dominant
An allele whose characteristic appears in the phenotype even when there’s only one copy. Shown by a capital letter
Define recessive
An allele whose characteristic only appears in the phenotype if 2 copies are present. Shown by a lower case letter
Define codominant
When alleles are both expressed in the phenotype - neither one is recessive
Define locus
The fixed position of a gene on a chromosome. Alleles of a gene are found at the same locus on each chromosome in a pair
Define homozygote
An organism that carries 2 copies of the same allele, e.g BB or bb
Define heterozygote
An organism that carries two different alleles, e.g Bb
Define carrier
A person carrying an allele which is not expressed in the phenotype but can be passed to the offspring
What are diploid organisms?
They carry 2 sets of chromosomes
Define gametes
Sex cells
Define monohybrid inheritance
The inheritance of a characteristic controlled by a single gene
In the F1 generation of a monohybrid cross, what ratio of phenotypes would you expect to see in the offspring?
In F1 of a monohybrid cross, a homozygous dominant organism is bred with a homozygous recessive. This will produce offspring that are all heterozygous dominant
In the F2 generation of a monohybrid cross, what ratio of phenotypes would you expect to see in the offspring?
2 heterozygotes (F1) are crossed. In F2, you’d expect a ratio of 3:1 dominant to recessive phenotypes
Define dihybrid inheritance
When 2 different genes, on different chromosomes are inherited together
In the F2 generation of a dihybrid cross, what would be the expected phenotypic ratio? (providing no linkage/ epistasis is involved)
The expected ratio would be 9 : 3 : 3 : 1
Both dominant : Dominant 1st recessive 2nd : Recessive 1st Dominant 2nd: both recessive
In the F1 generation of a dihybrid cross, what ratio of phenotypes would you expect to see in the offspring?
In F1 of a dihybrid cross, a homozygous dominant organism is bred with a homozygous recessive. This will produce offspring that are all heterozygous dominant
What is codominance?
When both alleles are expressed in the phenotype, neither one is recessive
In the F1 generation of a codominant cross, what ratio of phenotypes would you expect to see in the offspring?
In F1 of a codominant cross, a homozygous dominant organism is bred with a homozygous recessive. This will produce offspring that are all heterozygous dominant
In the F2 generation of a codominant cross, what ratio of phenotypes would you expect to see in the offspring?
The expected ratio would be 1 : 2 :1
homozygous for 1 allele : heterozygous for the other allele
What is an example of a gene having multiple alleles?
In humans, there are 3 alleles for blood group (ABO)
What are the alleles for blood group in humans?
Io = Group O
IA = Group A
IB = Group B
IB IA = Group AB
Describe the alleles for blood group
Io = recessive
IA and IB are codominant
If someone has the alleles ‘IA IB’ what blood group will they be?
Group AB
Describe the offspring of a cross between a heterozygote of blood group A and a heterozygote of blood group B
Any offspring could have 1 of the 4 blood types. IA IB = AB IA Io = A IB Io = B Io Io = O
What would a person’s genotype be if they were Heterozygous blood group A?
IA Io
What chromosomes do females have?
XX
What chromosomes do men have?
XY
Describe what ‘sex linkage’ means
When an allele is located on a sex chromosome
Give an example of sex linkage, what chromosome is this disorder carried on?
Colour blindness - carried on the X chromosome
What is meant by autosomal gene linkage?
Genes on the same autosome are said to be linked - because they’ll stay together during independent segregation in meiosis 1 and their alleles will be passed on to their offspring together
What process may stop autosomally linked genes from being passed on together?
Crossing over
The what effect does the closeness of two genes on the autosome have on their chances of being inherited together?
The closer they are together the more closely they are said to be linked - because crossing over is less likely to split them up
What is an autosome?
Any chromosome that is NOT a sex chromosome
Describe epistasis
When one gene masks the expression of another gene
If two genes are autosomally linked, you won’t get the expected phenotypic ratio. What does this allow you to do?
You can use the predicted phenotypic ratio to identify autosomal linkage
What is the chi squared test used for?
To see if the results of an experiment support a theory.
What is a null hypothesis?
The null hypothesis is always that there is no significant difference between the observed and expected results (if there is a small difference that it is due to chance, not your theory being wrong)
In a chi-squared test, what do these values stand for?
O =
E =
Σ =
O = Observed results E = Expected results Σ = Sum of...
What is the critical value in a chi-squared test?
The value that you compare your calculated (chi-squared) value to.
In a chi-squared test, in what circumstance do you reject the null hypothesis?
If your calculated value is larger than or equal to the critical value (this means there IS a significant difference between O and E)
In a chi-squared test, in what circumstance do you accept the null hypothesis?
If your calculated value is smaller than the critical value (this means there is NOT a significant difference between O and E)
How do you calculate degrees of freedom?
No. of classes - 1
