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23 Cards in this Set
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X-Linked Recessive Disorders |
Abnormal disorder-causing allele is recessive and is located on the X-chromosome where the normal, wild type allele is dominant - More males than females show the disorder Offspring of an affected male: - Sons: None are affected nor will they transmit the condition to their offspring - Daughters: None are affected, but all are carriers and half their sons are affected with half their daughters as carriers |
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Sex Linkage |
In Drosophila (flies), the white gene (w) is on the X-chromosome - The normal allele "+" at this locus gives red eyes - Gender differences include abdomen sizes |
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Reciprocal cross: Flies 1 - Pure/true breeding = Homozygous for the gene |
True-breeding red-eyed female x white-eyed male in P generation:
- Red eyes w+w+ produces only Xw+ haploid gametes - White eyes w produces only Xw and Y gametes - All F1 flies have red eyes (white-eye is recessive) where females inherit one X from each parent (XwXw+) and males inherit X chromosome from mothers (Xw+Y) - F2 females have Xw+ from their father, and either Xw+ or Xw from their mothers (red eyes) - F2 males have 1/2 red (Xw+Y) and half white (XwY) |
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Reciprocal cross: Flies 2 |
True-breeding white-eyes female x red-eyed male in P generation: - Female flies produce only Xw haploid gametes - Male flies produce Xw+ and Y haploid gametes - Half F1 females have red eyes (Xw+Xw), half white (XwXw) - Half F1 males have red eyes (Xw+Y), half white (XwY) which they receive the Xw from their mother - F2 females 1 red (Xw+Xw) : 1 white (XwXw) : male 1 red (Xw+Y) to make a combined phenotypic ratio of 1red:1white |
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Autosomal Genes on Different Chromosomes |
- If there are two genes on two different chromosomes A and B - How many / what are the possible combinations in the gametes of a mother heterozygous for both A and B (AaBb)? - There are 4 - AB Ab aB ab - If the mother is heterozygous for both A and B (AaBb) and the father is homozygous recessive at both A and B (aabb) you can use a test cross - Remember Mendel's principle of independent assortment |
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Two Genes on Different Chromosomes |
Ex. In dogs, a dark coat colour (C) > albino (c) and short hair (S) > long hair (s) - Parents: C/C ; S/S x c/c ; s/s - F1: C/c ; S/s - A testcross to the homozygous recessive is: CcSs x ccss leading to four possible outcomes - CcSs / ccss / Ccss / ccSs |
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Flies: Purple eyes (pr) and vestigial winds (vg) - Thomas Hunt Morgan |
Normal red eyes & normal wings: pr+pr+ ; vg+vg+ Recessive: prpr ; vgvg - In the testcross, Morgan found an unusually high number of parental phenotypes and a low number of recombinant phenotypes - The behaviour of these linked genes is due to chromosome recombination (swapping pieces of DNA during prophase 1 of meiosis) |
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What progeny do you expect from the cross of a [female heterozygous fly for both pr and vg] and [male homozygous for both pr and vg] ? * Testcross because homozygous recessive |
Female: pr+ / pr ; vg+ / vg Male: pr / pr ; vg / vg - One type of sperm produced by male x four types of eggs produced by female - There will be more parental phenotypes (red eyes / normal wings * purple eyes / vestigial wings) than recombinant phenotypes (red eyes / vestigial wings * purple eyes / normal wings) - This means that since they didn't sort into a 1:1:1:1 ratio for testers progeny, the purple-eye and vestigial-wings DO NOT assort independently - Simplest alternative is that the two genes are linked on the same chromosome |
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Genetic Combination at the DNA Level |
A process where two homologous chromosomes exchange segments with each other by crossing-over during meiosis - The frequency of this recombination is a function of the distance between linked genes - The near the two genes are, the greater chance they will be inherited than if the two genes were further apart - To determine the distance between two genes on the chromosome, we calculate the recombination frequency (percentage of testers progeny that are recombinants) |
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Recombinant Percentage |
After you do the testcross and get the progeny produced by pairing the parental and recombinant gametes, you count the parental progenies, recombinant progenies, and then add them to get the total progeny
- Divide the number of recombinant progenies by the total progeny and multiply by 100 to get the recombination frequency |
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Verifying Linkage |
- If the recombination frequency is less than or equal to 50%, the genes are not linked (they're on different chromosomes or too far apart on the same chromosome) - If the recombination frequency is <50%, the genes are linked and are more likely to be on the same chromosome |
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Chromosome/Linkage Maps |
Made using recombinant offspring frequencies to show the relative locations of genes on a chromosome - Genes farther apart on a chromosome are more likely to have more than one crossover occur between them - A single crossover between two genes gives recombinant chromatids - 1 map unit = 1% recombination = 1 centimorgan - Widely separated linked genes often recombine (seem to assort independently) |
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Deletion |
Broken segment is lost from the chromosome - May cause severe problems if the missing segment contains genes that are essential for normal development or cellular functions - Ex. One deletion from human chromosome 5 typically leads to severe mental retardation and a malformed laryx (Cri-du-chat) |
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Duplication |
Broken segment is inserted into a homologous chromosome (amplified multiple times) - May have effects that vary from harmful to beneficial, depending on the genes and alleles contained in the duplicated region - Ex. The additional hemoglobin genes of mammals are believed to have appeared through duplications, followed by mutations in the duplicates that created new and beneficial forms of hemoglobin as further evolution took place - Can occur during recombination in meiosis if crossover is unequal |
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Translocation |
Broken segment attached to a non homologous chromosome - Usually reciprocal, meaning that two non homologous chromosomes exchange segments - Resemble genetic recombination except that the two chromosomes involved in the exchange don't contain the same genes - Ex. Burkitt Lymphoma |
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Inversion |
Broken segment reattaches to the same chromosome but in reverse order - Have the same effects as translocations and can be beneficial or harmful - Ex. Five human chromosome pairs show evidence of translocations and inversions that aren't present in one of our nearest primate relatives, gorillas |
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Changes in Chromosome Number |
Due to - Failure of homologous pair separation during meiosis 1 (one daughter gets too many vs. too few) - Failure of chromatid separation during meiosis 2 (sister chromatids may go to the same side) |
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Euploids |
Eu = true - The normal number of chromosomes |
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Aneuploids |
Extra or missing chromosomes - Few in humans (sex chromosomes) - Not normal |
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Polyploids |
Mutants - Extra whole sets of chromosomes (triploids, tetraploids) where spindle fails during mitosis - In animal cells it's not tolerated - It's fine in plant cells because it can help make it bigger; usually engineered with fish and oysters (GMO) - Humans never show this |
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Nondisjunction |
During the first meiotic division, it causes both chromosomes of one pair to be delivered to the same pole of the spindle - Produces two gametes with an extra chromosome and two with a missing chromosome |
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Prenatal Diagnosis |
Tests the fetus' cells for mutant alleles or chromosomal alterations - Cells are obtained from the embryo, amniotic fluid around embryo (amniocentesis) or the placenta (chorionic villus sampling) - Used when it's necessary to check females over 40 or when the male has a point mutation |
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Genetic Screening |
Done after birth using biochemical and molecular tests |
