Chromosomal Basis of Inheritance Flashcards
The behavior of chromosomes during meiosis accounts for what Mendelian laws?
Law of segregation
Law of independent assortment
What is the chromsome theory of inheritence, who proposed this and when did they?
This is the theory that states 1. mendelian genes haves specific loci (positions) along a chromosome and 2. it is the chromosomes that undergo segregation and independent assortment.
Proposed by Walter Sutton and Theodor Boveri in 1902
Look at this picture and understand how this dihybrid cross correlates with chromosomal behavior during meiosis.
Do it.
What was Thomas Hunt Morgans organism of choice in proving evidence of genes located on specific chromosomes? What are the 3 reasons this organism choice was important we discussed in class?
Fruit fly
- Bred at high rates
- New generations every 2 weeks
- They only have 4 pairs of chromosomes
What is a wild type? What did Morgan describe as the wild type for his experiment with fruit flies in sex-linked experiments? What is a mutant type? What did Morgan describe as the mutant type in his fruit fly experiments?
Wild type - phenotypic character most commonly observed in natural population. This is the red eyed fruit fly in Morgans experiment.
Mutant type - phenotypes that are alternative to the wild type that are assumed to be due to mutations to the wild type allele. This is the white eyed male fruit fly in Morgans experiment.
What was Morgans hypothesis in his experiment with his fruit flies was that the white-eye mutant allele must be located on the X chromosome only. Why was this?
The only white-eyed fly in the F2 generation were male, this made him believe that the allele must be related to the sex chromosomes. He knew that the male fruit fly was XY and the female was XX, which made him believe that the if the X had this white-eye allele and the Y didn’t have a coresponding allele it would make sense as to why only males show this phenotype.
When Morgan performed his experiment with red-eyed and white-eyed flies, what was his P generation? What did the crossing of the P generation produce? What did this mean?
P generation was a white-eyed male and a red-eyed female
F1 generation produced were all red-eyed, this meant that the white-eyed allele must be recessive to the wild-type red eye allele.
What were were the products when the F1 generation of fruit flies mated (red-eyed female to red eyed male)?
There was a 3:1 ratio of red-eyed to white eye offspring, hoever there were never any white-eyed females. This supported Morgans hypothesis. see the picture.
Did Morgans findings in red-eyed and white-eyed flies support the chromosome theory of inheritance? How so? What did it also show?
YES - A specific gene is carried on a specific chromosome
His work also showed the genes on sex chromosomes exhibit unique inheritance patterns
How many genes are located on each chromosome?
Hundreds or thousands
Do some genes get inherited together? Why or why not? What are these genes termed? What Law would this deviate from if this was the case?
YES
If they are located near each other on the same chromosome
Called linked genes
This deviates from the law of independent assorment - look at it as if during gamete formation, where one allele goes will dictate where the other linked allele will go. The law of independent assortment states allele assort independently to other alleles.
How did Morgan show that linked-genes existed? Wild-type? Mutant-type? what were the parental generation? F1?
He crossed two true-breeding flies that had two body traits that were thought to be linked, body color and wing-size.
Wild type: Gray body & normal wings
Mutant type: Black boy, vestigial wings
F1: all gray body & normal wings
In Morgans linked-gene experiments, what did he do with his F1 generation, why was this performed? What were the results vs the expected results? What can explain the results?
The Dihybrid F1 generation (gray body & normal wings) were testcrossed with the homozygous recessive (double mutant) flies.
This allowed Morgan to find out the genotype of the F1 generation females eggs because the male generation only donates recessive alleles.
The expected results if these genes were linked were for the F2 generation to all be either one of the two parental type phenotypes, however, a small amount of them showed nonparental, or recombinant type phenotypes.
Morgan explained these results as follows: These genes are linked based on the results, however, the production of a small portion of nonparental offspring must mean that there is a mechanism that occaisonally breaks the linkage between these two genes that are on the same chromosome.
What is a parental type offspring? What is a recombinant type (recombinants)?
Parental type: These offspring inherit a phenotype that matches one of the parents (P generation)
Recombinant type: These have a phenotype that match neither of the parents. In other words, they have a new combination of traits.
What would the frequency of recombination be in a test-cross of a heterozygous parent that has two genes located on different chromosomes?
50% recombination rate. SEE PICTURE.
Morgans findings showed that genes can be linked, but the linkage is incomplete. What is evident to this? What causes the recombination of linked genes? What causes the recombination of unlinked genes?
UNDERSTAND PICTURE
The presence of recombinant phenotypes in offspring of breeding with organisms with linked genes.
Crossing over of homologous chromosomes causes recombination of linked genes
Independent assortment of homologous chromosomes causes recombination of unlinked genes
What is a linkage map? Who came up with this?
A genetic map of a chromosome based on recombination frequencies.
Alfred Sturtevant
What did Alfred Sturtevant predict the relationship was between distance between genes on the same chromosome and their probability of crossing over?
He predicted that the further apart two genes are the higher probability that a crossover will occur between them.
This would make the frequency of recombination higher.
What are the units on a linkage map, what do they represent?
one map unit is a centimorgan and represents a 1% recombination frequency.
SEE PICTURE
Be able to look at this picture and understand the recombination frequencies based on the distance between the two traits.
Look at the gray and normal wings and the black and vestigial wings. You know the results from Morgans experiments, this may help you understand the linkage maps.
DO IT.
Do all animals have the same sex chromosome system as humans? Some examples?
NO:
- Some like grasshoppers and cockroaches only have an X chromosome. Females have XX (2 chromosomes), and males Have XO (only one chromosome)
- Some birds and fishes have a system where their eggs (not the sperm) determine the sex. Designated Z and W. Females are ZW and males are ZZ.
- Most species of bees and ants dont have sex chromosomes. In this case the females arise from fertilized eggs and are diploid, while the males have no fathers and arise from unfertilized eggs and are haploid.
See picture:
Look at this picture and be able to understand how a recessive trait being linked to the x chromosome effects inheritance between males and females.
Do it.
How does a female who is heterozygous for an x-linked gene act special in regards to the formation of Barr bodies? Meaning what two types of cells will arise from this process?
What does this mean in regards to allele expression?
They will be a mosaic of two types of cells:
Cells that have an active X derived from the father
Cells that have an active X derived from the mother
- This means that, if heterozygous, approximately half will express one allele and the other half the other allele. Look at the cat picture to understand this further.
What is nondisjunction? What is the result of this?
This is when either:
Homologous chromosomes fail to separate normally during meiosis I
OR
Sister chromatids fail to separate during meisosi II
This results it one of the gametes receiving two of the same type of chromosome while the other receives no copy of the chromosome.
SEE PICTURE
What are some alterations of chromosome structure that can occur as a result of an error in meiosis? (4)
What was an example in book given that could cause these to occur?
- Deletion - removes chromosomal segment
- Duplication - repeats chromosomal segment
- Inversion - reverses segment within a chromosome
- Translocation - moves segment from one chromosome to a non-homologous chromosome
Radiation was an example given that could cause any one of these to occur.
What are some conditions caused by alterations in chromosome structre (the 4 alterations discussed earlier)? There is one due to deletion and one due to translocation.
Cri du chat (cry of the cat) - specific deletion in chromosome 5 causes this. Cognitively challenged with catlike cry, usually die in infancy or early childhood.
Chronic myelogenous leukemia (CML) - caused by translocations of chromosomes. Abnormally short chromosome 22 (philadelphia chromosome) caused by translocation of material to chromosome 9 which becomes abnormally long.
What is genomic imprinting? What is an example of this? Why is this significant? Does this occur in sex chromosomes or autosomes mostly?
Some mammalian traits have phenotypes that are dependent on which parent passed along the allele for the traits.
Another way of looking at this is that certain genes are silenced that are “stamped with an imprint” during gamete production.
If a gene is imprinted, no allele from that gene will be expressed coming from that parent. This not only means that mutant alleles will not be expressed, but it also means that the normal alleles wont be expressed. So if the other parent expresses a mutant allele then that phenotype will show. See mouse example.
This mostly occurs in autosomes.
IF HE POSTS LECTURE ABOUT THIS LISTEN TO IT TO UNDERSTAND FULLY
What is the common “imprint” noted on imprinted genes? What is their normal affect?
Ususally methylation (addition of a methyl groups) is what imprints a gene and turns it off.
In some cases, this methylation help a gene express its allele. Like in the rat picture, in males the methylated gene is turned on, in females it is turned off.
