Chromosomal Inheritance Flashcards
Chromosome Theory of Inheritance
Mendelian genes have specific loci along chromosomes and it is the chromosomes that undergo segregation and independent assortment
What does chromosomal theory of inheritance attempt to explain?
Mendel’s Principles of Inheritance
What did TH Morgan contribute?
Provided the first solid evidence associating a specific gene with a specific chromosome
–> Also completed work on the founding of recombination and sex-linked inheritance
Morgan’s Experimental Model
Drosophila Melanogaster (fruit flies)
Benefits of the Fruit Fly Model
1) Prolific breeders (one breeding could produce hundreds of progeny)
2) Cheap to grow
3) Fast generation time
4) Offered easily distinguishable phenotypes
5) Have only 4 pairs of chromosomes (homologs)
–> After Morgan’s research we know 3 are autosomal pairs and then they have a sex chromosome pair
Wild Type vs Mutant Notation
MUT = letter of mutated variant
WT= (letter of mutated variant)+
Ex: w+ = wild type
w = mutant
What was Morgan first looking for?
A variant/mutation in the drosophila
What was Morgan’s first variant?
A white-eyed male –> (wild type is red eyes)\
–> Found after 2 years of breeding
Wild Type
Phenotype for a character most commonly observed in natural populations
Mutant Phenotype
Alternatives to the wild type
Morgan’s First Cross (to make F1)
P Gen = True-breed Red-Eye (WT) Female + Mutant White-Eye Male
–> Produced all red-eye F1 generation (heterozygotes)
Morgan’s Second Cross (to make F2)
Bred F1 offspring (heterozygotes) with each other
= 3:1 ratio of red to white eyed flies (just like Mendel’s)
BUT, they observed the white-eyed flies were only ever MALE
Morgan’s Second Cross (to make F2) Conclusion (of the white fly males)
Eye color in fruit flies is linked to sex
Female Sex Chromosomes + Gametes
XX
= All gametes have X
Male Sex Chromosomes
XY
= 1/2 gametes have X and 1/2 have Y
Males are _________ in sex chromosomes
hemizygous
–> Whatever allele is present is the one that is expressed (no second allele to mask/affect phenotype)
Morgan’s additional crosses to exam six linkage
+
What was his conclusion?
1) Homo. Dom. Female + Recessive Male
–> Produced all heterozygotes (dom phenotype)
2) Homo. Rec. Female + Dominant Male
–> Produced white (rec) males
Conclusion: Allele producing eye color must reside on X chromosome which is derived from mother
Main finding of Morgan:
Found that genes reside on chromosomes and that each gene resides on a SPECIFIC chromosome
Linkage
Genes on the same chromosome tend to be inherited together
–> Because the chromosome is passed along as a unit
What does linkage deviate from?
Mendel’s Law of Independent Assortment
In linked genes all offspring are predicted to…
have a parental phenotype!!
1:1 Ratio (or 1:1:0:0)
In unlinked genes, offspring are predicted to…
have either a parental or non-parental phenotype
(1:1:1:1)
Recombinant Type
A new combination compared to parents
Non-Parental phenotype
What type of cross did Morgan use to test linkage vs non-linkage phenomena?
Test crosses, examining two characters at a time
Morgan’s Test Crosses
P Gen = Double Mutant + Double WT
F1 Gen = Double heterozygote
Crossed double heterozygote (F1) female with a double mutant male (test cross)
Offspring =
–> 2 of parental type
–> 2 of non-parental type
Found the 1:1:1:1 ratio
Linked Genes vs Unlinked Genes Offspring Ratios
Linked = 1:1:0:0 (only parental)
Unlinked = 1:1:1:1 (parental + recombinant)
Recombination
The production of offspring with combinations of traits that differ from those found in the parents
Recombination is a result of
crossing over of non-sister chromatids in which homologs exchange DNA segments
Crossing over in meiosis =
production of recombinant gametes = recombinant offspring
Recombinant Frequency
(# recombinant offspring) / (total offspring) = %
Recombinant Frequency < 50%
Genes assort independently = Genes linked
Recombinant frequency = 50%
Genes are unlinked
(50% is the max)
Complete linkage
Recombinant frequency = 0%
All offspring are identical to one parent in phenotype for the specific genes (1:1:0:0)
Unlinked Genes (recomb. freq)
Recombinant Frequency = 50%
Equal # of parental vs non parental phenotypes among offspring (1:1:1:1)
When genes on the same chromosome are far apart…
crossover between them is almost certain –> Recombinant frequency reaches 50% and genes appear to be unlinked
The more distance between genes…
The more points there are between them where crossing over can occur (= more recombination)
Recombinant frequency can reflect…
gene distances
The greater the recombinant frequency,
the greater the gene distance
How was recombinant frequency used?
Used to map the genome in terms of relative distances between genes
