Human Sex Chromosomes, Biology, McDougal Littell, 2008.
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Dihybrid Punnett Square, http://cccmkc.edu.hk/index.en.php |
10.1 Meiosis
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Cartoon Guide to Genetics 1, 2, 3 Gonick, Larry, and Mark Wheelis. The Cartoon Guide to Genetics. New York, NY: Collins Reference, 2007. Print.
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10.1.1 Describe the behaviour of the chromosomes in the phases of meiosis. 10.1.2 Outline the formation of chiasmata in the process of crossing over. 10.1.3 Explain how meiosis results in an effectively infinite genetic variety in gametes through crossing over in prophase I and random orientation in metaphase I.
Meiosis II http://www.biologycorner.com/APbiology/inheritance/10-1_meiosis.html
Chiasmata
Sadava, D. et al. Life: The Science of Biology, 8th ed
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DNA is replicated before meiosis starts
Meiosis I - key points: crossing over and independent assortment
Prophase I
Metaphase I
Anaphase I
Telophase I and Cytokinesis
Result: two haploid cells
Meiosis II - key point: sister chomatids are separated
Prohpase II
Metaphase II
Anaphase II
Telophase II and Cytokinesis
Result: four unique haploid cells
Important Ideas
Crossing over and independent assortment create unique gametes.
Random fertilization happens during sexual reproduction - one random sperm and one random egg meet and create one unique offspring.
Sexual reproduction's most important characteristic is its ability to create genetic variation in offspring and increase the evolutionary success of the species.
Thanks meiosis and random fertilization!
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10.1.4 State Mendel’s law of independent assortment.
10.1.5 Explain the relationship between Mendel’s law of independent assortment and meiosis.
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Mendel observed the inheritance of many traits in his pea plants. The inheritance of one trait was not connected to the inheritance of another trait.
For example, seed color could be yellow or green, and seed shape could be round or wrinkled.
The inheritance of seed color did not affect the inheritance of seed shape.
The offspring could have yellow and round seeds, yellow and wrinkled seeds, green and round seeds, or green and wrinkled seeds.
From these observations, Mendel wrote his Law of Independent Assortment which predicted the random assortment of homologous chromosomes during metaphase I of meiosis !!!!
Why can't I be more like Mendel? |
10.2 Dihybrid crosses and gene linkage
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Dihybrid crosses follow the inheritance pattern of two traits. (Di- = two) In the following cross, two traits of each parent are observed: 1) seed color: Y=yellow / y=green 2) seed shape: R=round / r=wrinkled
P1: Phenotypes = Yellow/Round X Green/Wrinkled Genotypes = YYRR X yyrr
F1: Phenotypes = 100% Yellow/Round Genotypes = 100% YyRr
Yellow/Round X Yellow/Round YyRr X YyRr
F2: Phenotypes = 9 Yellow/Round 3 Yellow/Wrinkled 3 Green/Round 1 Green/Wrinkled Genotypes = ?
9:3:3:1 ratio
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10.2.1 Calculate and predict the genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes.
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Practice Problems Write the genotypic and phenotypic ratios for the following crosses:
Alleles A = long wings a = short wings B = wide beak b = thin beak
Crosses AABB X aabb aaBB X AAbb AaBb X AaBb AaBB X Aabb
Determine the genotype of a long winged, wide-beaked bird. It could be AABB or AaBb. Use a testcross.
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10.2.2 Distinguish between autosomes and sex chromosomes.
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Autosomes are chromosomes that do not determine male or female characteristics. Sex chromosomes have genes for determining male or female characteristics.
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10.2.3 Explain how crossing over between non-sister chromatids of a homologous pair in prophase I can result in an exchange of alleles.
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10.2.4 Define linkage group.
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Linkage groups are genes that are on the same chromosome. They do not separate by independent assortment - genes on different chromosomes do separate randomly during independent assortment. They can separate by crossing over.
Studying how often linkage groups separate can show where the genes are located on the chromosome.
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10.2.5 Explain an example of a cross between two linked genes.
Alleles are usually shown side by side in dihybrid crosses, for example, TtBb. In representing crosses involving linkage, it is more common to show them as vertical pairs, for example
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Recombinant: An organism, cell, or chromosome that is the result of crossing over ("re-combined")
Linkage groups are written horizontally to show which alleles are on the same chromosome: T and B are on one chromosome; t and b are on the other homologous chromosome. |
10.2.6 Identify which of the offspring are recombinants in a dihybrid cross involving linked genes.
In a test cross of the recombinants will be and
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Usually linked genes are inherited together. If crossing over occurs between linked genes, the result is a recombinant.
In a testcross, TtBb X ttbb usually produces offspring like TtBb and ttbb. Same cross written differently:
TB tb TB tb __ X __ usually produces offspring like __ and __ tb tb tb tb
But if crossing over occurs, then the same cross can produce recombinants:
TB tb Tb tB __ X __ produces recombinant offspring like __ and __ tb tb tb tb
Without crossing over, a TtBb parent produces TB and tb gametes. With crossing over, a TtBb parent produces Tb and tB gametes.
Recombinants can be identified because they
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10.3 Polygenic inheritance
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10.3.1 Define polygenic inheritance.
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Polygenic inheritance: more than one gene controls the trait. ("poly" = many, "genic" = genes)
Examples:
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10.3.2 Explain that polygenic inheritance can contribute to continuous variation using two examples, one of which must be human skin colour.
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Human skin color is controlled by at least 4 genes.
This Punnett square shows the possible variation for two parents that are heterozygous for 3 skin color genes.
This variation creates a continuous range of phenotypes.
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