Mendelian Genetics 2 Flashcards
What does a chi squared test do?
tells us whether the data are consistent with expectation
Chi Squared Formula
Χ² = Σ (O-E)²/E
Linkage
Definition
genes that are carried on the same chromosome are said to be linked they tend to segregate together, ie. they do not assort independently of each other
Linkage
Example
- 3 loci: A, B and C each with to alleles Aa, Bb and Cc respectively
- if not linked meiosis in the will generate gametes; ABC, abc, Abc, aBC, aBc, AbC, abC, ABcc
- but if they are linked. e.g if A and C are on the same chromosome, then the heterozygote genotype could be either AC/ac B/b, OR Ac/aC B/b
- for Mendelian segregation, we treat the linked alleles as if they are a single locus
i. e. if the genotype is Ac/aC B/b, then A and c will always segregate together as will a and C - BUT, we also have to take into account the effects of recombination
Coupling
Definition
two alleles are said to be in coupling if they are on the same chromosome
Repulsion
Definition
two alleles are said to be in repulsion if they are on different chromosomes
Haplotype
Definition
the genotype of a chromosome
Recombination Frequency
Definition
- recombination is more likely to occur between two linked loci if they are further apart on the chromosome than if they are closer together
- recombination frequencies of more than 50% cannot occur as this is independent assortment
Linkage Mapping
using the frequency with which recombinant offspring occur to determine the relative positions of genes on a chromosome
Recombination Frequency
Equation
number of recombinants / total number of offspring
-multiply this by 100 to find a percentage the ercentage is also equal to the distance between the genes on the chromosome in centiMorgans
Recombination With 3 Point Crosses
Rules
- Identify the parental combinations of alleles (most frequent classes)
- Identify the middle locus (which allele has ‘changed partners’ in the least frequent classes)
- Calculate the recombination frequnce between each pair of loci
Three Point Test Cross
Random Assortment
if unlinked, the genes will independently segregate producing the 8 different phenotypes in a 1:1:1:1:1:1:1:1 ratio
Three Point Test Cross
Linkage and Recombination
if linked, 8 different phenotypes will be produced, but the ratio will depend on the distance between the genes on the chromosome
Three Point Test Cross
Linkage and No Recombination
if the genes are linked but no recombination occurs, then 2 different phenotypes will be produced from two different phenotypes giving a 1:1 ratio
Three Point Test Cross
A+B+C+/abc x abc/abc
homozygous recessive individual crossed with an individual heterozygous for all three genes
Three Point Test Cross
How to determine the distance and arrangement of the genes?
1) the genotype for the phenotype that is the least frequent will be the one where two recombination events have taken place, this allows you to identify the middle gene e.g. if least common is A+bC+/aBc, then B is the middle gene
2) from here calculate the distance between the middle gene and the two outside genes separately
3) add together the frequency off all the times that A+ and B+ have segregated together, divide by total number of progeny and x100 to find the distance in cM
4) do the same for B+and C+, add together the frequency of the phenotypes where B+and C+ have segregated together and divide by the total number of progeny then multiply by 100 to find the distance in cM
Recombination Interference
Cross over at one point on a chromosome reduces the chance of crossing over occurring at nearby points on the same chromosome, to lower than would be mathematically predicted
Three Point Test Cross
Most Frequent Progeny
parental combinations
Three Point Test Cross
Least Frequent Progeny
double recombinations
1cM
1cM = one centimorgan
relative measure of distance so depends on the genome and rate of recombination in that particular organism
1cM in one genome does not equate to 1cM in another
Genotypic Sex Determination
based on organism having specific sex chromosomes that segregate from one another during meiosis
Sex Determination
Mammals
XX = female XY = male
Sex Determination
Butterflies
ZZ = male ZW = female
Sex Determination
Amphibians
ZZ = male ZW = female
Sex Determination
Insects
XX = female XY = male
Sex Determination
Some Plants
XX = female XY = male
Sex Determination
Birds
ZZ = male ZW = female
Sex Determination
Nematodes
XX = hermaphrodite X0 = male
Hermaphrodite
Definition
has the reproductive organs of both the male and the female sex
XY Sex Determination
50% of offspring will be male and 50% will be female
-each cell in the body has a 50:50 chance of displaying the maternal or paternal sex chromosome
Examples of Sex Linked Traits
- tortoiseshell cats
- barred plumage in chickens
Epistasis
Definition
interaction between two different genes so that an allele of one of them (the epistatic gene) interacts with or inhibits the phenotypic expression of the other (the hypostatic gene)
Sex Determination
Dominant Y Systems
- the presence of the Y chromosome is what determines sex
e. g. even XXY individuals are male (Klinefelter’s syndrome)
X Inactivation
-because the heterogametic sex has only one copy of the sex chromosomes whereas the homogametic sex has two copies, there has to be a mechanism to prevent overexpression of genes on the diploid chromosome
X Inactivation in Mammals
- known as dosage compensation
- achieved by inactivation of one or other of the X-chromosomes in females
- occurs randomly in different cells during development of the embryo
- its effects can be seen when the X chromosomes are heterozygous for traits like coat colour
Tortoiseshell Cats
- colour gene O located on the X chromosome
- two alleles, a recessive allele, ob which results in normal production of melanin (black coat pigment) and a dominant allele, O which results in the formation of an altered form of melanin which is orange
- if a female cat is X0/Xob, in some cells the XO will be inactivated, cells will be black and in others, Xob ill be inactivated so the cells will be ornage
- the sizes of the patches depend on how early in development that the X-chromosome was inactivated
- male cats only have one copy of the X chromosome so can only be either all black or all orange
How to determine how many genes control a phenotype?
-collect a population of mutants showing the SAME mutant trait
-interbreed all combinations of the mutant
-
Heterogametic Sex
produces two types of gametes, each carrying a different sex chromosome
Homogametic Sex
produces one type of gamete all carrying the same sex chromosome
Lethal Alleles
Recessive Lethal
absence of gene product results in death of homozygotes
e.g. yellow mice
Lethal Alleles
Dominant Lethal
both homozygotes and heterozygotes carrying the allele will die
e.g. Huntingdons
Recessive Epistasis
Aabb and AAbb mice have the same phenotype as aabb, epistasis of bb convers A-
Duplicate Recessive Epistasis
the two genes code for two proteins e.g. enzymes that take part in successive stages of a pathway i.e. both are required for the final product
-characterised by a 9:7 ratio in the progeny
Different Mutations in the Same Gene vs Mutations in Different Genes
- interbreed the mutants
- if any offspring are wildtype, then the mutations are in different genes
- this is the complementation test
- the wildtype allele in one mutant compliments the muntant allele in the other and vice versa at the second locus
Different Mutations in the Same Gene vs Mutations in Different Genes
- interbreed the mutants
- if any offspring are wildtype, then the mutations are in different genes
- this is the complementation test
- the wildtype allele in one mutant compliments the muntant allele in the other and vice versa at the second locus
Complementation Group
all carry mutations in the same gene
Codominance / Incomplete Dominance
a heterozygous individual shows a third phenotype, not the ‘dominant’ or ‘recessive’
What is the difference between codominance and incomplete dominance?
- incomplete dominance gives a blending of the ‘dominant’ and ‘recessive’ traits
- codominance is when the ‘recessive’ and ‘dominant’ traits appear together in the phenotype of the hybrid organism
