genetics - inheritance Flashcards
what is the genotype
all the genes an organism contains
what is the phenotype
the observable characteristics of an organism as a result of the genotype plus the effect of the environment
what is a gene
a section of chromosome carrying the code for a specific polypeptide
what is the locus
The position of a gene on a chromosome
what is an allele
different forms of the same gene
what is homozygous / pure breeding
having two identical alleles at a gene locus
what is heterozygous
having two different alleles at a gene locus
what is the dominant allele
an allele which only needs to be present once in the genotype to be shown in the phenotype
what is a recessive allele
an allele which must be present twice in the genotype to be shown in the phenotype
what are codominant alleles
alleles which contribute equally to the phenotype
what are multiple alleles
presence of more than two alleles at a given gene locus
what are sex-linked alleles
alleles which are found on either the X or Y chromosomes
what is monohybrid inheritance
inheritance pattern of a single gene
give an example of codominance
A good example of codominance can be seen in human blood types
The gene for blood types is represented in the genotype by I and the three alleles for human blood types are represented by A, B and O
Allele A results in blood type A (IAIA or IAIO) and allele B results in blood type B (IBIB or IBIO)
If both allele A and allele B are present in a heterozygous individual they will have blood type AB (IAIB)
Blood type O (IOIO) is recessive to both group A and group B alleles often 2:1:1 ratio
what is the standard mendelian monohybrid ratio
3:1
what did Mendel discover
Gregor Mendel (1822-1884) was a 19th century monk who studied the inheritance of traits in pea plants and is known as the Father of Genetics
that genes were separated and then came back together upon fertilisation
what can be used to predict monohybrid inheritance
Genetic diagrams are often used to present this information in a clear and precise manner so that predictions can be made
These diagrams include a characteristic table called a Punnett square
what is important to remember for genetic crosses in terms of offspring form different fertilisation events
The predicted genotypes that genetic diagrams produce are all based on chance
There is no way to predict which gametes will fuse so sometimes the observed or real-life results can differ from the predictions
so each event is new ratio
give an example of a genetic cross
One of the genes for the coat colour of horses has the following two alleles:
B, a dominant allele produces a black coat when present
b, a recessive allele produces a chestnut coat when present in a homozygous individual
In this example a heterozygous male is crossed with heterozygous female
Parental phenotype: black coat x black coat
Parental genotype: Bb Bb
Parental gametes: B or b B or b
then place in punnet square - make sure to put circles around everything
Predicted ratio of phenotypes in offspring – 3 black coat : 1 chestnut coat
Predicted ratio of genotypes in offspring – 1 BB : 2 Bb : 1 bb
multiple alleles genetic cross- standard notation
results in more than one phenotype
C then subcase dominant / recessive
where are the sex-linked genes located
sex chromosomes X and Y
what is homogametic sex
all gametes produced carry an X chromosome
what is heterogametic sex
can produce gametes with an X chromosome or a Y chromosome
how is sex linkage notated
Sex linkage is notated using a capital letter to represent the chromosome X or Y and a superscript letter to represent the allele
NOTHING ON THE Y
what are the genotypes for a female
3
.g. for a genetic trait caused by a recessive allele
XAXA = unaffected
XAXa = carrier
XaXa = affected
what are the genotypes for a male
2
XAY = unaffected
XaY = affected
Males can’t be carriers of x-linked traits, nor for them to pass such traits on to their sons; males only pass y chromosomes on to their sons
what do pedigree diagrams show
the incidence of inherited conditions in successive generations of a family - particularly useful in looking at the pattern of sex-linked genes
what are the rules for interpreting pedigree diagrams
-males are represented by a square
-female represented by a circle
-shaded symbol usually represents the presence of a characteristic
-open symbol indicates no characteristic
-dot in circle can be used to represent a female carrier - ie normal phenotype but heterozygous geno
what is important not to assume with pedigree diagrams
that they are not always showing a sexlinked characteristic - could be single hybrid
what is dihybrid inheritance
inheritance patterns of two characteristics/alleles which are determined by different genes found on different chromosomes
what to make sure of when writing the different genotypes of a dihybrid genetic cross
When writing the different genotypes write the two alleles for one gene, followed immediately by the two alleles for the other gene. Do not mix up the alleles from the different genes
If there was a gene with alleles Y and y and another gene with alleles G and g an example genotype for an individual would be YyGg
what is the usual standard ratio of a dihybrid cross
9:3:3:1
example of dihybrid cross
Horses have a single gene for coat colour that has two alleles:
B, a dominant allele produces a black coat
b, a recessive allele produces a chestnut coat
Horses also have single gene for eye colour
E, a dominant allele produces brown eyes
e, a recessive allele produces blue eyes
In this example a horse which is heterozygous for both genes has been crossed with a horse that is homozygous for one gene and heterozygous for the other
Parental phenotypes: black coat, brown eyes x chestnut coat, brown eyes
Parental genotypes: BbEe bbEe
Parental gametes: BE or Be or bE or be bE or be
what is autosomal linkage
where some genes located on the same chromosome stay together in the original parental combination - as not all genes assort independently during meiosis
how does autosomal linkage affect inheritance
Linkage between genes affects how parental alleles are passed onto offspring through the gametes
When writing linked genotypes it can be easier to keep the linked alleles within a bracket
For example an individual has the genotype FFGG however if there is linkage between the two genes then it would be written as (FG)(FG)
example of autosomal linkage
The genes for tail length and scale colour in a species of newt have displayed autosomal linkage
The gene for tail length has two alleles :
Dominant allele T produces a normal length tail
Recessive allele t produces a shorter length tail
The gene for scale colour has two alleles:
Dominant allele G produces green scales
Recessive allele g produces white scales
A newt heterozygous for a normal tail and green scales is crossed with a newt that has a shorter tail and white scales
Parental phenotypes: normal tail, green scales x short tail, white scales
Parental genotypes: (TG)(tg) (tg)(tg)
Parental gametes: (TG) or (tg) (tg)
Predicted ratio of phenotypes in offspring – 1 normal tail, green scales : 1 short tail, white scales
Predicted ratio of genotypes in offspring – 1 (TG)(tg) : 1 (tg)(tg)
how do the genes ally on the chromosomes in autosomal linkage
corresponding letter on the same side - due to semi-conservative DNA replication
but due to independent assortment they can separate and the two letters will be different
what is a linkage group
genes which are found on the same chromosome- assuming that there are no cross-over events and the genes should remain together
how many types of gametes are produced from autosomal linkage
2 as the gametes must stay together in the original format eg AB or ab
how do cross-overs effect inheritance
produce recombinant gamete types - so the locus switches
-however only small chance of crossover event happening in the right position
what are the autosomal linkage ratios
dominant x dominant = 3:1
dominant x recessive = 2:1:1
instead of normal dihybrid ratio of 9:3:3:1
what is epistasis
where the expression of one gene affects the expression of another
what is a common example of epistasis
metbolic pathways ocnsitst of several steps whihc is controlled by a specific enzyme - production of each enzyme is controlled by a specific gene
explain the example of a metabolic pathway of epistasis
expresssion of gene B is dependent on the expression of gene A
-if there is no A allele present then there is no functional enzyme A so the white substrate cannot be converted to the pale blue pigment
-therefore, if there is no pale blue pigment present it does not matter wether or not there is a B allele present as thee is nothing for enzyme B to work on
what is the gene pool
all of the alleles of all of the genes of all of the organisms in a breeding pop
what is the allele frequency
number of times in which an allele occurs in the gene pool
what is the hardy-weinberg principle
mathematical model or equation which can be used to determine the frequencies of alleles of a particular gene in a population - but has issues
what does the hardy-weinberg principle state
allele frequency will remain constant from one generation to the next, (providing no mutations)
what is the notation for allele frequency
dominant = p
recessive = q
what are the two equations of the hardy-weinberg principle
p + q = 1
p^2 + 2pq + q^2 = 1
why does p + q = 1
there are only two alleles in the frequency then the frequency of one plus the frequency of the other must account for 100%
what are the four types of allele arrangements possible
Aa / AA / aA / aa
what is the frequency of AA
p^2
what is the frequency of aa
q^2
what is the frequencies of aA and Aa
2pq
what does p^2 equate to
the homozygous dominant
what does q^2 equate to
the homozygous recessive
what does 2pq stand for
heterozygote
what are the 5 assumptions of the hardy-weinberg principle
-no mutation as would introduce new alleles
-breeding pop is isolated
-no selection - alleles equally likely to be expressed
-breeding pop is large
-mating is entirely random
example of p + q = 1
E.g. in a population of 100 individuals there would be 200 alleles because every individual has two versions of each gene
If 120 of those alleles were the dominant allele then the frequency of the dominant allele would be 120/200
It could be said that p = 120 ÷ 200 = 0.6
If p = 0.6 then q = 1 - 0.6 = 0.4
worked example of p^2 + 2pq + q^2
In a population of birds 10% of the individuals exhibit the recessive phenotype of white feathers. Calculate the frequencies of all genotypes.
Solution:
We will use F / f to represent dominant and recessive alleles for feather colour
Those with the recessive phenotype must have the homozygous recessive genotype, ff
Therefore q2 = 0.10 (as 10% of the individuals have the recessive phenotype and q2 represents this)
To calculate the frequencies of the homozygous dominant ( p2 ) and heterozygous ( 2pq ):
Step 1: Find q
square root of q^2 = 0.32
Step 2: Find p (the frequency of the dominant allele F). If q = 0.32, and p + q = 1
p + q = 1
p = 1 - 0.32
p = 0.68
Step 3: Find p2 (the frequency of homozygous dominant genotype)
0.682 = 0.46
p2 = 0.46
Step 4: Find 2pq = 2 x (p) x (q)
2 x (0.68) x (0.32) = 0.44
Step 5: Check calculations by substituting the values for the three frequencies into the equation; they should add up to 1
p2 + 2pq + q2 = 1
0.46 + 0.44 + 0.10 = 1
when calculating carriers what is important to include in the calc
p^2 + 2pq
What do the symbols q^2 and p^2 mean
‘homozygous’ number represents a P squared or Q squared value
What do the symbols p and q mean
any ‘allele’ frequency/number represents a P or Q value
How to word cross over event questions
Two dominant alleles and two recessive are linked
F1 gen has the genotype RrBb and produce mainly RB and rb gametes (parental gametes)
Cross over in meiosis produces some Rb and rB gamete’s (recombinant)
So fewer Rrbb and RrBB
OR SUB IN GENO IN THE QUESTION
In a male with disorder, where would the sex-linked mutation be located?
The non-homologous section of an X
chromosome
