How can it occur in higher organisms?
- The process by which offspring derive a combination of alleles different from that of either parent; the generation will be of new allelic combinations.
- In higher organisms, this can occur by crossing-over
In 1928, Doctors completed a four-generation pedigree tracing two known X-linked traits: red-green colorblindness and hemophilia (X-linked "bleeder's" disease) The maternal grandfather of the family exhibited both traits. What does this infer about his X chromosome? What would be expected of his immediate offspring?
- His single X chromosome carried mutant alleles of the two corresponding genes
- Neither colorblindness nor hemophilia showed up in his sons and daughters.
His grandsons and one great grandson inherited both of the X-linked conditions. The fact that none of the descendants manifested one of the traits without the other suggests?
The mutant alleles did not assort independently during meiosis. Instead they traveled together in the gametes forming one generation and then into the gametes forming the next generation, producing grandsons and great-grandsons with an X chromosome specifying both conditions.
Genes that travel together more often than not exhibit _______ _______
In another pedigree following colorblindness and the slightly different B form of hemophilia, which also arises from a mutation on the X chromosome, revealed a different inheritance pattern. A grandfather with hemophilia B and colorblindness had four grandsons, but only one of them exhibited both conditions. In this family, what can be said of the genes for colorblindness and hemophilia
- This time the genes appeared to assort independently, producing in the male progeny all four possible combinations of the two traits: normal vision and normal blood clotting, colorblindness and hemophilia, colorblindness and normal clotting, and normal vision and hemophilia
- Additionally, the combinations were produced in approx. equal frequencies
In the pedigree (b), even though the mutant alleles of the two genes were on the same X chromosome in the grandfather, they had to separate to give rise to grandsons III-2 and III-3. What was the result of this separation of genes on the same chromosome?
The result was recombination, the occurrence of progeny displaying new gene combinations not seen in previous generations
Name two ways recombinant progeny can result
- From the recombination of genes on the same chromosome during gamete formation
- From independent assortment of genes on nonhomologous chromosomes
The two X-linked genes that determine a fruit fly's eye color and body color are said to be syntenic. Why?
They are located on the same chromosome
The slash symbol (/) represents?
Separation of genes found on chromosomes of a pair (either X and Y chromosomes or a pair of X chromosomes or homologous autosomes)
w y / Y represents the genotype of a male fly with an X chromosome bearing _______, as well as a Y chromosome; phenotypically, this male has ______ eyes and a ______ body
- w and y
- white eyes (not wild type red)
- yellow body (not wild type brown)
In a cross between a female with mutant white eyes and a wild-type brown body (wy+
) and a male with wild-type red eyes and a mutant yellow body (w+y
/Y), the F1 offspring are _______ divided between brown-bodied females with normal red eyes (___/___) and brown-bodied males with mutant white eyes (___/___)
The male progeny look more like their mother why? Is the same true of the female F1s?
- Their phenotype directly reflects the genotype of the single X chromosome they received from her.
- Nope, they received w and y+ on the X from their mother and w+y on the X from their father making them dihybrids
With two alleles for each X-linked gene, one derived from each parent, the _______ relations of each pair of alleles determine the female phenotype.
What kind of gametes will the dihybrid F1 female generation produce if the two fly genes for eye and body color assort independently?
The dihybrid F1 females should make four kinds of gametes, with four different combinations of genes on the X chromosome: wy+, w+y, w+y+ and wy
The four types of gametes should occur in ______ (ratio?). If it happens that way, approx. half of the gametes will be _______ types (carrying?) and the other half will be ______ types (carrying?)
- equal (1:1:1:1)
- parental types (wy+ combo from the P gen. female and w+y from the P gen. male)
- recombinant types (w+y+ or wy allele combo not seen in P gen. due to shuffling)
What do the results of the breeding study that produced 9026 F2 males tell us about the expected 1:1:1:1 ratio?
It was drastically off, the parental types accounted for most of the F2 population at 99% and the new recombinant types barely made a dent at 1%
What are the two possible explanations as to why the two genes failed to assort independently?
- The wy+ and w+y combo could be preferred because some intrinsic chemical affinity exists between these particular alleles
- Alternatively, these combos of alleles might show up most frequently because they are parental types. The F1 female is more likely to pass on parental combo as opposed to recombinant combos to her own progeny
Two genes are linked if the _______ types outnumber the ________ types
A second set of crosses (Cross B) involving the same genes but with a different arrangement of alleles explains why the dihybrid F1 females do not produce a 1:1:1:1 ratio of the four possible types of gametes. What are the parental phenotypes. What are all of the F1 females?
- Red-eyed, brown bodied females and white eyed, yellow bodied males
- The F1 females are all w+y+/wy dihybrids
In both experiments, it is the ________ classes that show up most frequently in the F generation while the ________ class occurs less frequently.
- parental class
- recombinant class
It is important to appreciate that the designation of "parental" and "recombinant" gametes or progeny of a doubly heterozygous F1 female is operational (explain)
It is determined by the particular set of alleles she receives from each of her parents
When genes assort independently, the numbers of parental and recombinant F2 progeny are equal. How?
A doubly heterozygous F1 individual produces an equal number of all four types of gametes.
In contrast, two genes are considered _______ when the number of F2 progeny with parental genotypes exceeds the number of F2 progeny with recombinant genoytpes. Instead of assorting independently, the genes behave as if they are ________ to each other most of the time
Linked autosomal genes are not inherited according to the 9:3:3:1 Mendelian ratio. Why?
That ratio is expected for two independently assorting, non-interacting genes, each wit one completely dominant and one recessive allele.
The figure shows the consequences of _____ if the F1 dihybrid individuals were both of genotype AB/ab. Which classes would increase at the expense of which classes? What if the alleles of the parents are configured differently (Ab/Ab * aB/aB)
- The 9/16 and 1/16 classes would increase and the 3/16 classes would decrease
- The F1 are therefore Ab/aB, then the two 3/16 genotypic classes would increase at the expense of the 9/16 and 1/16 classes
Why does linkage undo the basis of the 9:3:3:1 ratio?
unequal numbers of the four gametes are produced, so each box of the punnett square no longer represents an equally likely fertilization
Early twentieth-century geneticists had trouble interpreting crosses like the figure below because it was hard to trace which alleles came from which parent. State examples
All the F2 with genotype A- B- would have the same phenotype, but they could have arisen from fertilizations involving two parental gametes (dar blue squares), two recombinant gametes (light blue squares) or one of each kind (medium blue squares)
Fruit flies carry an ________ gene for body color (in addition to the X-linked y gene) ; the wild type is once again brown, but a recessive mutation in this gene gives rise to black (b). A second gene on the same _______ determines wing shape, the wild type being straight (edges) and the recessive mutation being curved (c)
In a cross between two pure-breeding strains: black-bodied females with straight wings (___/___) and brown-bodied males with curved wings (___/___), all the F1 progeny are ______ ________ and are phenotypically _______ _____.
- double heterozygotes
- wild type
In the test-cross, of the F1 group, all of the offspring receive the recessive b
alleles from their _______. The phenotype of the offspring thus indicate the kinds of gametes received from the _______. For example, a black fly with normal wings would be bc+/bc
; because we know it received the bc
combination from its _______, it must have received bc+
from its _______
- Physical Markers: cytologically visible abnormalities that make it possible to keep track of specific chromosome parts from one generation to the next
- Genetic Markers: genes identifiable through phenotypic variants that can serve as points of reference in determining whether particular progeny are the results of recombination
The figure depicts the chromosomes of these heterozygous females. One X chromosome carried mutations producing carnation eyes (dark ruby color abbrv. car
) that were kidney shaped (Bar
); in addition, this chromosome was marked physically by a visible discontinuity. What caused it?
The end of the X chromosome was broken off and attached to an autosome.
The other X chromosome had wild-type alleles (+) for both the car and Bar genes, and its physical marker consisted of?
Part of the Y chromosome that had become connected to the X-chromosome centromere
The figure shows how the chromosomes in these car Bar/car+ Bar+
females were transmitted to male progeny. The results stated that all sons showing a phenotype determined by one or the other parental combination of genes (either car Bar
or car+ Bar+
) had an X chromosome that was structurally __________ from one of the original X chromosomes in the mother. Was the same true for the recombinant sons?
- Nope, recombinant sons like the ones with carnation eye color and normal eye shape (car Bar+/Y), had an identifiable exchange of the abnormal features marking the ends of the homologous X chromosomes accompanying the recombination of genes
The evidence thus tied an instance of phenotypic _______ to the ______ _____ of particular genes located in specifically marked parts of particular chromosomes
- phenotypic recombination
- crossing over
This experiment demonstrated elegantly that genetic recombination is associated with the actual ______ _______ of segments between homologous chromosomes during _______
- reciprocal exchange
Most critical role of crossing over (after crossover)
To ensure that chromosomes segregate properly when they are transmitted between parents and their progeny
Proper chromosome segregation requires homologous chromosomes to be pulled to opposite spindle poles, which in turn requires the homologous chromosome not only to pair with each other during _______, but also to be linked to each other physically through metaphase until they separate at _______
What provides the necessary link between homologous chromosomes until anaphase of meiosis I?
Why can't it be the synaptonemal complex or recombination nodules?
Once a crossover takes place, the cohesin complexes distal to the crossover point (further away from the centromere than chiasmata) that keep the homologous chromosomes other at the metaphase plate and thus ensure proper chromosome segragation
It cant be the synaptonemal complex or recombination nodules because both disappear by the end of prophase I
Why do different gene pairs exhibit different linkage frequencies? pg 134
Genes are arranged in a line along a chromosome, the closer together two genes are on the chromosome, the less their chance of being separated by an event that cuts and recombines the line of genes
Alfred H. Sturtevant proposed that the percentage of total progeny that were recombinant types, the _____________ __________ (__), could be used as a gauge of the physical distance separating any two genes on the same chromosome
recombination frequency (RF)
centimorgan (cM) (define)
One cM is equal to a __% chance that a marker at one genetic locus will be _______ from a marker at a second locus due to crossing over in a single generation
AKA the map unit (m.u.) a unit of measure of recombination frequency.
Going back to the two pairs of X-linked Drosophila genes we analyzed earlier (cross series A), we see that because the X-linked genes for eye color (w) and body color (y) recombine in 1.1% of F2 progeny, they are ____m.u. apart.
- 1.1 m.u. apart
The X-linked genes for eye color (w) and wing size (m) have a recombination frequency of 32.8 and are therefore _____ m.u. apart
- 32.8 m.u.
In the figure, genes A and B are close together on a chromosome, so, meiosis will usually take place with no _________ between the genes. In a dihybrid such as A B/a b, the outcome of such a meiosis will be _______ _______ type gametes
- four parental type gametes
Occasionally, during a meiosis a crossover will occur between two genes, resulting in _____ recombinant and ____ parental type gametes. Why will these crossovers be rare?
As the distance between the two genes increases, the frequency of meioses with a single crossover between the genes ________, and the fraction of recombinant gametes _________
- Most of the length of the chromosome lies outside of the region between genes A and B.
Why are the two types of parental gametes produced at roughly equivalent frequencies? Why are the two types of recombinant gametes appearing in approximately equal numbers with respect to each other?
- Whenever nonsister chromatids do not crossover between genes A and B, one of each type of parental chromosome is generated.
- Likewise, every time two nonsister chromatids do crossover between the two genes, both reciprocal recombinants are produced