Patterns of inheritance Module 6 Flashcards
What is continuous variation?
When the individuals within a population vary within a range eg.height
What’s discontinuous variation?
When there are 2 or more distinct categories, each individual only falls into one of these catergories
eg. blood group
How can variation be influenced by your genes?
Individuals of the same species have the same genes but different versions of them called alleles - making up it’s phenotype
Sexual reproduction leads to variation in genotypes within a species, as meiosis makes gametes with a unique assortment of alleles through crossing over and the independent assortment of chromosomes
The random fusion of gametes during fertilisation also increases the gentic variation in the offspring
This all leads to phenotypic variation
What are inherited characteristics that show continuous variation are usually influenced by?
Many genes so are known as polygenic
Inherited characteristics that show discontinuous variation are usually influenced by?
One gene so are monogenic
2 examples of environment affecting variation?
Etiolation- when plants go abnormally long and spindly because they aren’t getting enough light
Chlorosis - this is when plants don’t produce enough chlorophyll and turn yellow due to a lack of magnesium
Definition of gene?
A sequence of bases of a DNA molecule that codes for a protein which results in a characteristic
Definition of an allele?
A different version of a gene, humans have 2 alleles of each gene, one from each parent
Definition of a genotype?
The alleles an organism has, eg BB, Bb or bb
Definition of a phenotype?
The characteristics the alleles produce
Definition of a dominant allele?
An allele whose characteristic appears in the phenotype even when there’s only one copy, it’s shown by having a capital letter
Definition of a recessive allele?
An allele whose characteristics only appear in the phenotype if 2 copies are present. Shown by a lower case letter
Definition of codominant?
Alleles that are both expressed in the phenotype - neither one is recessive
Definition of a 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
Definition of a homozygote?
An organism that carries 2 copies of the same allele eg. BB or bb
Definition of a heterozygote?
An organism that carries 2 different alleles eg. Bb
Definition of a carrier?
A person carrying an allele which is not expressed in the phenotype, but can be passed on to the offspring
Body cells of individuals have?
2 alleles of each gene
Gametes have?
Only one allele of each gene
How would you produce the monogenic cross for the alleles N normals wings and n little wings
The parents are NN and nn
Cross the parents so NN x nn
Write out each ones gametes alleles so they are going to be N N n n
Then put them in a punnet square to find the possible genotypes of the F1 offspring, going to Nn, Nn, Nn, Nn
Now found the gamete allels again, going to be N n N n
Punnet square it again, leaving you with the genotypes NN, Nn, Nn ,nn the F2 offspring
So phenotypic ratio will be 3:1 of normal wings and little wings
RATIO WILL ALWAYS BE 3:1
Describe how to do the punnet square for sickle cell amenia when the alleles are codominant so neither one is recessive for 2 heterozygous parents (H(s)H(n)?
These parents will have been produced from crossing H(s)H(s) with H(n)H(n)
H(n)H(n) so homozygous for normal don’t have the disease so have normal haemoglobin
H(s)H(s) so homozygous for sickle anaemia so all their blood cells are sickle shaped
Some people have H(s)H(n) so have sickle cell trait, so have some normal haemoglobin and some sickle cell shaped - as the alleles are co dominant as both expressed in the phenotype
Do the cross by doing H(n)H(s) x H(n)H(s)
Find the gametes which will be H(n), H(s), H(n), H(s)
Cross them in a punnet square to get H(n)H(n) , H(n)H(s), H(n)H(s), H(s)H(s)
So 25% chance will be unaffected, 50% chance will have mixed traits and 25% chance will have sickle cell amenia
Describe how to do inheritance when genes have multiple alleles such in blood groups and cross the parents with the genotypes I(a)I(o) and I(b)I(o)?
I(o) is the allele for blood group O
I(a) is the allele for blood group A
I(b) is the allele for blood group B
Allele I(O) is recessive and I(a) and I(b) are codominant
So will be crossing I(a)I(o) x I(b)I(o)
Gametes will be I(a), I(o), I(b), I(o)
Now when you cross the alleles you will get I(a)I(b), I(a)I(o), I(b)I(o), and I(o)I(o)
So the phenotypes will be AB, A, B and O as the O is recessive to the A and the B and the A and B are codominant
What’s dihybrid inheritance?
The inheritance of 2 characteristics why are controlled by different genes
How would you do a dihybrid cross for pea plants when
R = round seed r= wrinkled seed Y = yellow seed y = green seed
So we will be crossing the parents with the genotypes RrYy x RrYy in the F1 offspring, the parents are RRYY and rryy from the original)
Giving us the gametes, RY, Ry, rY, ry,RY, Ry, rY, ry
Then we produce a 4 x 4 punnet square which will produce us the phenotypic ratio of 9 yellow and round seeds, 3 Round and green seeds, 3 wrinkled and yellow seeds and 1 wrinkled and yellow seed
For a monogenic cross, what parents are you crossing, what will be the phenotype in the F1 ratio, and the phenotypic ratio in the F2?
Parents crossing will be homozygous dominant x homozygous recessive
Phenotypic ratio in the F1 will be all heterozygous offspring eg Rr
Phenotypic ratio in F2 will be 3:1 dominant to recessive
For a dihybrid cross, what parents are you crossing, what will be the phenotype in the F1 ratio, and the phenotypic ratio in the F2?
Parents are homozygous dominant x homozygous recessive (eg RRYY x rryy)
Phenotypic ratio in offspring will all be heterozygous (eg RrYy)
Phenotypic ratio in F2 will be 9:3:3:1
Dominant both: dominant 1st recessive 2nd : dominant 2nd recessive 1st : both recessive
For a codominant cross, what parents are you crossing, what will be the phenotype in the F1 ratio, and the phenotypic ratio in the F2?
Parents are homozygous for one allele and homozygous for the other allele, eg H(n)H(n) x H(s)H(s)
Phenotypic ratio in the F1 will be all heterozygous H(n)H(s)
Phenotypic ratio in the F2 will be 1:2:1
Homozygous for 1 allele : heterozygous : homozygous for the other allele
The genetic information for gender is carried on?
2 sex chromosomes
What are the chromosomes for female and male?
XX = female XY = male
A characteristic is said to be sex linked when?
When the allele that codes for it is located on a sex chromosome
Why are most genes only carried on the X chromosome?
The Y chromosome is smaller than the X chromosome, so can’t carry as many genes
Why are males more likely to express recessive phenotypes?
They only have 1 X chromosome, so they often only have 1 allele for for sex linked genes, so they will express it even if it’s recessive
How would you draw a genetic diagram for colour blindness
X(N) = normal vision
X(n) = colour blind
Y = no effect as dosen’t have an allele for it
With a carrier female and an unaffacted male?
Crossing X(N)X(n) (female) with X(N)Y
Gametes will be
X(N), X(n), X(N), Y
Cross them to get the offspring
X(N)X(N), X(N)Y, X(n)X(N), X(n)Y
So produces an unaffected male, an unaffected female, a carrier female, and a colour blind male
What’s an autosome?
Any chromosome which isn’t a sex chromosome, so autosomal genes are located on autosomes
Why are genes on the same autosome said to be linked?
Because they are on the same autosome they will stay together during the independent asortment of chromosomes in meiosis 1, and their alleles will be passed on to the offspring together- the only reason they won’t be together is if crossing over splits them up first
The closer 2 genes are together on the autosome means?
The more closely they are said to be linked, as crossing over is less likely to split them up
What’s the expected dihybrid phenotypic ratio if the chromosomes are autosomally linked?
3:1
What does an epistatic gene do?
Masks the expression of another gene, so won’t result in the expected phenotypic ratio
What ratio will you get for a dihybrid cross involving a recessive epistatic allele - so if it’s recessive homozygous it will mask the affect of the other gene?
9:3:4
At least one dominant from each : The epistatic gene being dominant and the other gene being recessive : The effect of both episatic alles being recessive
What ratio will you get if the there’s a dominant epistatic allele?
12:3:1
How do to a Chi-Squared test in genetics?
State the null hypothesis, and the alternate hypothesis
Draw the observed table
Then create an expected table by using the ratio required for cross, eg monogenic = 3:1
Work out the X^2 (chi squared value) using the formula of: Sum of all ((observed - expected)^2)/E
Compare it to the critical value will always be at 0.95 probability, degrees of freedom = number of phenotypes -1
Compare the critical value and the test value, if test value is higher there is evidence to reject H0, if it’s lower then there is evidence to accept H0
