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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)

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

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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)


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!



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!



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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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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