2 - Linkage, Recombination & Deviations from Mendelian Ratios Flashcards
which 4 processes contribute to genetic variation?
- mutation
- independent assortment of chromosomes during meiosis I
- crossing over between homologous chromosomes during meiosis I
- random fertilisation of ova by sperm
how does independent assortment + random fertilization promote genetic variation?
- The choice of which sperm fuses with which egg is random
- With 8,388,608 kinds of sperm and 8,388,608 kinds of eggs, the no. of possible chromosome combinations in the offspring from one couple is >70,000,000,000,000
- crossing over adds even more variation
recombinant phenotype
where the combination of phenotypes differs from that found in either of the parents
how do recombinant phenotypes occur?
through either independent assortment or crossing over
what is crossing over?
the process of genetic recombination that gives rise to new combinations of linked genes
crossing over explained
- Occurs during the pachytene phase of prophase I
- Begins with synapsis - pairing of homologous chromosomes
- The synaptonemal complex is a protein ‘zipper’ that holds homologous chromosomes together in the tetrad
what is the result of crossing over?
recombinant chromosomes with new combinations of linked genes
recombinant frequency
percentage of the progeny that inherit a combination of alleles that differs from either parent
what are the results of a testcross when genes are on different chromosomes?
50% parental, 50% recombinant
what are the results of a testcross when genes are on the same chromosomes?
> 50% parental
<50% recombinant
who was the first to discover gene linkage?
T.H.Morgan using fruit flies
testcross involving the linked genes black (b) and vestigial (vg)
Mutants are given a name that reflects their phenotype - usually abbreviated e.g. black (b) and vestigial (vg)
recombinant frequency = 17%
what are the 2 possible arrangements of alleles in a double heterozygote?
coupling and repulsion
coupling heterozygote testcross outcome
parentals = b+ vg+ and b vg recombinants = b vg+ and b+ vg
repulsion heterozygote testcross outcome
parentals = b vg+ and b+ vg recombinants = b+ vg+ and b vg
what do multiple crossovers lead to?
- an underestimate of the distance between 2 loci
- recombinant frequencies >50% are not possible
3 point cross
allows the detection of double crossovers such that genes can be put into order
- largest numbers = parental types
- smallest numbers = double crossover
who created chromosome maps?
A.H> Sturtevant (1913)
He saw that the recombination freq. between 2 genes should be related to their distance apart on the chromosome
what are the 6 deviations from Mendelian ratios?
sex-linkage, incomplete dominance, co-dominance, pleiotropy, polygenic inheritance, epistasis
sex linkage
occurs when certain traits are determined by genes on sex chromosomes
occurs when certain traits are determined by genes on sex chromosomes
the sex chromosome found only in males.
cannot carry a gene
incomplete dominance
Where a dominant allele does not completely mask the effect of a recessive allele at the same locus
co-dominance
Where each allele affects the phenotype in separate, distinguishable ways
incomplete dominance compared to codominance
- incomplete dominance = blending of traits and a ‘diluted’ phenotype
- co-dominance = no blending: traits appear together and remain distinct in the heterozygote
Pleiotropy
Where a single gene has multiple effects on the phenotype
polygenic inheritance
Where a single trait is determined by multiple genes
Epistasis
When one gene masks or modifies the expression of another gene
(B is epistatic to A)
ratio of F2 in dihybrid cross = 9:3:4
