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Ch 15.1-15.4. CHROMOSOMES & INHERITANCE Locating genes on chromosomes T. H. Morgan’s studies with fruit flies Incomplete linkage - crossing over Alteration in chromosome number Changes in large portions of a chromosome
Review Monday in UNLH 1000 (Exam W in class) 6:10 (Paine) – 9 p.m. (Morse starts midway)
Paine and Morse office hours: T, W 2:10-4:00 Practice exam questions on Ilearn today (Paine
answers up just before the review session)
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Overview: Locating Genes on Chromosomes
• A century ago the relationship between genes and chromosomes was not obvious
• Today we can show that genes are located on chromosomes
• The location of a particular gene can be seen by tagging isolated chromosomes with a fluorescent dye that highlights the gene
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Fig. 15.1. Fluorescent yellow dye used to tag a specific gene on homologous chromosomes (key – gene’s physical location)!
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See Fig. 15.2
What we now know about meiosis and linkage explains Mendel’s Laws of Segregation and Independent Assortment
Gametes
Yellow-round seeds (YYRR)
Green-wrinkled seeds (yyrr)
All F1 plants produce yellow-round seeds (YyRr)
Meiosis
Two equally probable
arrangements of chromosomes
at metaphase I
LAW OF INDEPENDENT ASSORTMENT
Fertilization among the F1 plants
P Generation
Meiosis
Fertilization
Anaphase I
Metaphase II
F2 Generation
Gametes
F1 Generation
LAW OF SEGREGATION
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Meiosis
An F1 x F1 cross-fertilization
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Morgan’s Experimental Evidence: Scientific Inquiry
• The first solid evidence associating a specific gene with a specific chromosome came from T. H. Morgan (Columbia University)
• Morgan’s experiments with fruit flies provided convincing evidence that chromosomes are the location of Mendel’s heritable factors
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Morgan’s Choice of Experimental Organism
• Characteristics that make fruit flies a very convenient organism for genetic studies:
– Several hundred offspring from a single pair
– A generation can be bred every two weeks
– Small amount of space, cost for rearing
– They have only four pairs of chromosomes; chromosomes easily visible with a microscope
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• Morgan noted wild type, or normal, phenotypes that were common in the fly populations
• Traits alternative to the wild type are called mutant phenotypes
– w = white-eye mutant
– w+ = wild type (red eye)
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Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair
• In one experiment, Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type)
– The F1 generation all had red eyes
– The F2 generation showed the 3:1 red:white eye ratio, but only males had white eyes
• Morgan determined that the white-eye mutant allele must be located on the X chromosome
• Morgan’s finding supported the chromosome theory of inheritance 8
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Figure 15.3 T. H. Morgan’s first mutant - whited-eyed Drosophila melanogaster.!Sex-linked Inheritance 9!
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Figure 15.4 Sex-linked inheritance!
Males are the heterogametic sex.
Females are the homogametic sex.
w+ - wild type (red eye) w - mutant (white eye)
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Figure 15.4 Sex-linked inheritance!
Males are the heterogametic sex.
Females are the homogametic sex.
w+ - wild type (red eye) w - mutant (white eye) All F1 progeny
have red eyes.
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See Figure 15.4 Sex-linked inheritance!
Males are the heterogametic sex.
Females are the homogametic sex.
w+ - wild type (red eye) w - mutant (white eye) All F1 progeny
have red eyes.
White-eyed males reappear in the F2 generation.
Why do white eyes disappear in the F1?
Because it is a recessive AND sex-linked trait (on the X chromosome)
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Concept 15.3: Linked genes tend to be inherited together because they are located near each other on the same chromosome
• Each chromosome has hundreds or thousands of genes (only 78 identified so far on human Y)
• Humans: 2n = 46 chromosomes (23 homologous pairs); Fruit flies: 2n = 8
• Genes located on the same chromosome that tend to be inherited together are called linked genes
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Recombination of Linked Genes: Crossing Over
• Morgan discovered that genes can be linked, but the linkage was incomplete, as evident from recombinant phenotypes
• Morgan proposed that some process must sometimes break the physical connection between genes on the same chromosome
• That mechanism was the crossing over of homologous chromosomes
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Concept 15.2: Sex-linked genes exhibit unique patterns of inheritance
• In humans and other animals, there is a chromosomal basis of sex determination
• An organism’s sex is an inherited phenotypic character determined by the presence or absence of certain chromosomes
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The Chromosomal Basis of Sex
• In humans and other mammals, there are two varieties of sex chromosomes, X and Y; females are XX, males XY
• SRY (Sex-determining Region of Y) is required for development of the testes in males
• In the absence of SRY, the gonads develop into ovaries in females
• Some animals have alternative methods of sex determination
Fig. 15.6
The X-Y system
The X-0 system
The Z-W system
The haplo-diploid system
Parents
Sperm Ova
Zygotes (offspring)
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California red scale on oranges (#1 pest of citrus)
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Aphytis melinus parasitoid (from Pakistan in the 1950’s) “stinging” a scale
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X Inactivation in Female Mammals
• One X chromosome in each cell in females becomes almost completely inactivated during embryonic development
• Inactive X condenses into a compact body called a Barr body (most genes are not expressed in this X)
• This occurs randomly in each embryonic cell – thus, females are a mosaic of two cell types (X from father vs. mother)
• In the ovaries, Barr-body chromosomes are reactivated in the cells that will give rise to eggs – so every female gamete has an active X chromosome
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X Inactivation – Tortoiseshell Cats
• Tortoiseshell (calico) female cats – mottled with patches of orange, cream, black, brown, red, or chocolate
• Markings are usually asymmetrical
• Primary gene for coat color can be masked by a co-dominant gene for orange color located on the X chromosome
• Third gene softens colors (orange to cream, black to gray, etc.)
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Inheritance of Sex-Linked Genes
• The sex chromosomes have genes for many characters unrelated to sex
• A gene located on either sex chromosome is called a sex-linked gene (by convention, typically on X in humans)
• Sex-linked genes follow specific patterns of inheritance
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• Disorders caused by recessive alleles on the X chromosome in humans include:
– Color blindness (ca. 1 in 10 males; understand why it is rare in females – calculate what percent)
– Duchenne muscular dystrophy (1 in 3,500 males in the U.S.)
– Hemophilia (1 in 10,000 males)
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MUTATION
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Initial population
A change in the genetic material of a cell or virus
Mutation =
Mutation is the ultimate source of all genetic variation!
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Normal Mouse
Mutant
“Kinky Tail” Mouse
Tails as long as 23 cm have been reported in humans
Mutant
Normal Mouse
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“normal” (wild type) Drosophila melanogaster
White eyes!
Dark body!
Stubby wings!
Eyes absent!
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Concept 15.4: Alterations of chromosome number or structure cause some genetic disorders
• Large-scale chromosomal alterations often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disorders
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Abnormal Chromosome Number
• In nondisjunction, pairs of homologous chromosomes do not separate normally during meiosis
• As a result, one gamete receives two of the same type of chromosome, and another gamete receives no copy
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CHROMOSOMAL MUTATIONS (Number)!
Mutation supplies the raw material for evolution
See Figure 15.13
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• Aneuploidy results from the fertilization of gametes in which nondisjunction occurred
• Offspring with this condition have an abnormal number of a particular chromosome
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Human Disorders Due to Chromosomal Alterations
• Alterations of chromosome number and structure are associated with some serious disorders
• Some types of aneuploidy appear to upset the genetic balance less than others, resulting in individuals surviving to birth and beyond
• These surviving individuals have a set of symptoms, or syndrome, characteristic of the type of aneuploidy
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Down Syndrome
• Down syndrome is an aneuploid condition that results from three copies of chromosome 21
• It affects about one out of every 700 children born in the United States
• The frequency of Down syndrome increases with the age of the mother, a correlation that has not been well explained
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CHROMOSOMAL MUTATIONS (Number)!
3 copies of chromosome 21 (trisomy 21)
Down Syndrome
Dosage = The number of copies of a given gene present in a cell or nucleus
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• A trisomic zygote has three copies of a particular chromosome
• A monosomic zygote has only one copy of a particular chromosome
• Polyploidy is a condition in which an organism has more than two complete sets of chromosomes
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CHROMOSOMAL MUTATIONS (Polyploidy)!
Mutation supplies the raw material for evolution
Polyploid = cell or organism containing more than 2 sets of chromosomes.
Haploid = one set of chromosomes in the cell (1n) Diploid = 2 sets of chromosomes in the cell, one from each parent (2n)
Oenothera gigas (4n = 28) !- Thousands of plant species are polyploid
- Rare in vertebrates
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First polyploid vertebrate discovered in Chile - a tetraploid burrowing rodent (1999)!
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37!Figure 15.14
CHROMOSOMAL MUTATIONS (Structure)!(REVIEW of MEIOSIS + MITOSIS -- SEE pp. 250-259 and FIGURE 13.8)
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Key to Impacts -- Where Mutations Occur! Where in the genome might mutations occur?
* Intron -- non-coding region of a eukaryote gene
* Most mutations occur here
* Potentially little impact (modulate gene expression?)
* Exon -- coding region of a eukaryote gene
* Crossing over that includes part of an intron and an exon (See Fig. 17.13)
* This could lead to new proteins
* Most would be non-beneficial
* An occasional beneficial change (in a particular environment)
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There are several globin genes within a single mammalian genome.
Hemoglobin (4 subunits)
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Practice Question 1
T. H. Morgan found a white-eye mutation in male fruit flies that was recessive, with the allele located on the X chromosome. Assume males are the heterogametic sex (XY). What phenotypes result from crossing a white eye female and a wild type male?
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Practice Question 2
In Mendel’s studies, the allele for round seed was dominant over that for wrinkled seed and the unlinked allele for yellow seed was dominant over green seed. What offspring are produced by a dihybrid cross of a wrinkled green seed by the double heterozygous genotype?
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Practice Question 3
You are running a cross involving 4 different genes. Three different chromosomes are involved and only the A/a and C/c genes are linked. With diploid parents AaBbccDd x AaBbcCdd (Ac and aC are linked in the second parent) and no crossing over, what are the odds of progeny with genotypes (a) aaBbccdd, (b) aabbCcDd, (c) AABbccdd?
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