Genetics - 10.2 Inheritance (HL) Flashcards
Understandings:
- Gene loci are said to be linked if on the same chromosome
- Unlinked genes segregate independently as a result of meiosis
- Variation can be discrete or continuous
- The phenotypes of polygenic characteristics tend to show continuous variation
- Chi-squared tests are used to determine whether the difference between an observed and expected frequency distribution is statistically significant
The Dihybrid cross
- determine the genotypic and phenotypic combination of offspring for 2 particular genes that are UNlinked (ie 2 genes with 2 alleles = up to 4 different gamete combinations
= A cross between organisms with 2 different characteristics
How to work out dihybrid cross
dihybrid cross determines the genotypic and phenotypic combinations of offspring for two particular genes that are unlinked
Because there are two genes, each with two alleles, there can be up to four different gamete combinations
The easiest way to work out potential gamete combinations in a dihybrid cross is to use the FOIL method:
FOIL = First / Outside / Inside / Last
FOIL method
https://ib.bioninja.com.au/higher-level/topic-10-genetics-and-evolu/102-inheritance/dihybrid-crosses.html
The dihybrid cross and law of independant assortment (+ why)
according to law of independent assortment, pairs of alleles are inherited independently of one another if their gene loci are on desperate chromosomes (= genes = UNLINKED)
- DUE TO: RANDOM ORIENTATION OF HOMOLOGOUS PAIRS DURING METAPHASE 1 OF MEIOSIS
independent segregation of unlinked genes increases what?
(the independent segregation of unlinked genes = increases number of potential gamete combinations and an increased variety of possible phenotypes = more complex inheritance patterns)
Thomas Hunt Morgan’s genetic studies in the 1930s on DROSOPHILA FRUIT FLIES
= shows: genes are arranged in a linear order on the chromosome (some one chromosome has many genes)
- his fruit fly experiments included an understanding of mutations, gene linkage, gene interactions and sex linkage
Recombinants
- can be identified with TEST CROSS
Recombinants of linked genes = combinations of genes NOT found in the parents
- occur as a result of crossing over of genetic material during phrophase 1 of meiosis
- if linked genes = separed by a chiasma, there will be an exchange of alleles between the non-sister chromatids
- = creates new allele combinations that are different to those of the parent
AB = normal
Ab = recombinant
aB= recombinant
ab = normal
frequency of recombinant phenotypes (in a population) = < frequency of non-recombinant phenotypes
(DUE TO and DEPENDANT ON notes!)
= DUE TO: crossing over being a random process and chiasmata do not form at the same location with every meiotic division
= DEPENDANT ON: distance between linked genes
(recombination frequency between 2 genes = increases when genes = further apart on the chromosomes
—> as: more possible locations where a chiama could form between the gene
Chi squared test
(it is possible to infer whether two genes are linked or unlinked by looking at the frequency distribution of potential phenotypes)
= statical measure that are used to determine whether the difference between an observed and expected frequency distribution is statistically significant
= can be applied to data generated from a dihybrid cross to determine if there is a statistical correlation between observed and expected frequencies
Chi squared test - unlinked
equal possibility of inheriting any potential phenotypic combination = due to random segregation of alleles via independent assortment
Chi squared test - linked
only express the phenotupic combinations present in either parent UNLESS: crossing over occours, therefore = unlinked recombinant phenotypes occour less frequently than ‘linked’ parental phenotypes
Chi squared test - if observed frequence do NOT conform to those expected for an UNLINKED dihybrid cross
= suggest =
1) genes are linked and hence not independently assorted
2) the inheritance of the traits are not random, but are potentially being affected by NATURAL SELECTION
The Dihybrid cross ration
9 : 3 : 3 : 1
The Dihybrid cross (HOW TO DO)
refer to pgs 23/24 of booklet
The Dihybrid TEST cross
1) Heterozygous + homozygous (recessive or dominant) = 4/5 columns, 1/2 rows
2) homozygous (recessive or dominant) + homozygous (recessive or dominant)
= 1/2 columns, 1/2 rows
- REFER TO PG 25 OF BOOKLET
Linkage / linked genes - man in charge
Thomas Hunt Morgan’s genetic study in 1930s (on drospophila fruit flies showed that genes are arranged in a linear order on the chromosome, so one chromosome has many genes. Genes on the same chromosome are called LINKED GENES and move together during meiosis)
Linkage group =
= group of genes whose loci are on the same chromosome = doesn’t independently assort
Facts about linked genes
1) tend to be inherited together and hence don’t follow normal mendelin inheritance for a dihybrid cross
2) linked genes may become separated via recombination (ie due to crossing over in meisosis 1)
3) instead phenotypic ratio will be more closely aligned to a monohybrid cross as the 2 genes are inherited as a single unit
Non linked geneotypes = written as
Aa Bb
linked written as
AB / ab
Radio for dihybrid test-cross of two linked genes =
cross AaBb x aabb = ration of 1:1:1:1
Effect of crossing over on linkage (RECOMBIANTS)
when the chromosomes line up in meiosis, two chromatids (one from each member of the par) may cross each other and break off, with the broken parts rejoining to the opposite chromatid - when fully separated out of the independent chromosomes the ones with the new combination of alleles = RECOMBIANTS
Gene linkage (2 key proposeals)
- the alleles for traits = located on a shared chromosome (= gene linkage) and hence did not independently assort
- linked alleles could be uncouple via recombination (= crossing over!) to create alternative phenotypic combinations but these new pheotupes would cccour at a much lower frequency
also observed by morgan
= the # of crossing over between linked genes differed depending on the combination of traits
- the idea = crossover frequency may be a product of the distance between genes on a chromosomes and genes with increased crossover frequency = further apart = genes decrease (CF) = closer together
- MORGAN: used these concepts to develop first gene linkage maps that showed the relative positions of genes on chromosomes
Explain why linked gene ratios are not the same non-linked gene ratios
you only have 2 possible gametes as they are on the same chromosome = do not separate in miosis
linked vs crossing over (gametes)
linked = 2 possible gametes (=> phenotypes/genotypes)
crossing over = more possible gametes (=> phenotypes/genotypes), therefore, increase of genetic diversity
Chi-squared test
REFER TO BOOKLET PG 32 FOR HOW TO DO
Cross over frequency (equation)
REFER TO PG 33 OF BOOKLET:
Crossover value = (number of recombinants / total number of offspring) x (100/1)
Polygenic inheritance =
increase of number of loci responsible for a particular trait increases number of possible phenotypes = results in a phenotypic distribution that follows a normal distribution curve
Polygenes (+ polygenic traits)
= variation in phenotypes for a particular characteristic can be either discrete (discontinuous) or continuous
POLYGENIC INHERITANCE
A single phenotypic character may be determined by 2 or + genes, it is usually indicated by characters that show quantitative variation in a population
= characteristics controlled by more than 2 gene loci, tend to exhibit CONTINOUS VARIATION w/ an individual phenotype existing somewhere along a continuous spectrum of potential phenotypes
Polygenes - monogentic tratis
= characteristics controlled by a single gene loci tent to exhibit discrete variation, with individual expressing one of a # of distinct phenotyps
Polygenes - phenotypic characteristics
= are not solely determined by genotype but are also influenced by environmental factors - the added effect of environmental processes functions to increase the variation seen for a particular trait
Chromosome mapping
PAGE 34 OF BOOKLET!!!
https://youtu.be/iMTzdjxxWiY
