chapter 15 the chromosomal basis of inheritance section a: relating mendelism to chromosomes...

CHAPTER 15 THE CHROMOSOMAL BASIS OF

INHERITANCE

Section A: Relating Mendelism to Chromosomes

1. Mendelian inheritance has its physical basis in the behavior of

chromosomes during sexual life cycles

2. Morgan traced a gene to a specific chromosome

3. Linked genes tend to be inherited together because they are located on the

same chromosome

4. Independent assortment of chromosomes and crossing over produce genetic

recombinants

5. Geneticists use recombination data to map a chromosome’s genetic loci

生物醫學暨環境生物學系張學偉助理教授

• Mendel’s hereditary factors are the genes located on chromosomes.

chromosomes theory of inheritance.

Introduction

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

1. Mendelian inheritance has its physical basis in the behavior of chromosomes during sexual life cycles


Chromosomal basis ofMendel’s laws.

Fig. 15.1

• Thomas Hunt Morgan was the first to associate a specific gene with a specific chromosome.

2. Morgan traced a gene to a specific chromosome


•Morgan choice an experimental animal, Drosophila melanogaster

•fruit fly species that eats fungi on fruit

•prolific breeders

•generation time of two weeks.

•Fruit flies have three pairs of autosomes and a pair of sex chromosomes (XX in females, XY in males). 3+1

• The normal character phenotype is the wild type (red).

• Alternative traits are mutant phenotypes.


Fig. 15.2

Genes located on a sex chromosome are called sex-linked genes.

female male


Fig. 15.3

Morgan concluded that a fly’s eye color was linked to its sex (x).

• Results of crosses with linked genes deviate from those expected according to independent assortment.

3. Linked genes tend to be inherited together because they are located on the same chromosome

• Morgan reasoned that body color and wing shape are usually inherited together because their genes are on the same chromosome.


Fig. 15.4

(b+vg+

Or b vg+)(b vg)

result from:

• (1) independent assortment of genes located on

nonhomologous chromosomes

(2) or crossing over of genes located on homologous chromosomes.

4. Independent assortment of chromosomes and crossing over produce genetic recombinants


The production of offspring with new combinations of traits inherited from two parents is genetic recombination.

•The physical basis of recombination between unlinked genes is the random orientation of homologous chromosomes at metaphase I of meiosis. leads to the independent assortment of alleles.

• linked genes, genes located on the same chromosome, tend to move together through meiosis and fertilization. [Don’t assort independently]

• Under normal Mendelian genetic rules, we would not expect linked genes to recombine into assortments of alleles not found in the parents.

• But in fact, recombination between linked genes does occur. (see Morgan)

• This switched alleles between homologous chromosomes.


Fig. 15.5a

•A 50% frequency of recombination is observed for any two genes located on different chromosomes.


Fig. 15.5b

• The results of Morgan’s testcross for body color and wing shape did not conform to either independent assortment or complete linkage.

• Most of the offspring had parental phenotypes, suggesting linkage between the genes.

• However, 17% of the flies were recombinants, suggesting incomplete linkage.


• One of Morgan’s students, Alfred Sturtevant, used crossing over of linked genes to develop a method for constructing a chromosome map.

• This map is an ordered list of the genetic loci along a particular chromosome.

5. Geneticists can use recombination data to map a chromosome’s genetic loci


• Sturtevant hypothesized :

that the frequency of recombinant offspring reflected the distances between genes on a chromosome.


The farther apart two genes are, the higher the probability that a crossover will occur between them and therefore a higher recombination frequency.

Sturtevant used recombination frequencies from fruit fly crosses to map the relative position of genes along chromosomes, a linkage map.


Fig. 15.6

Using recombination frequencies to construct a genetic map

One map unit (sometimes called a centimorgan) is equivalent to a 1% recombination frequency.

• Some genes on a chromosome are so far apart, the frequency of recombination reaches is its maximum value of 50% and the genes act as if found on separate chromosomes and are inherited independently.



Fig. 15.7

More than one gene can affecta given phenotypic characteristic(eye color)

Partial genetic map

• Map units (linkage map) indicate relative distance and order, not precise locations of genes. The frequency of crossing over is not actually uniform over the length of a chromosome.


•Combined with other methods like chromosomal banding, geneticists can develop cytological maps. the positions of genes with respect to chromosomal features.

•More recent techniques show the absolute distances between gene loci in DNA nucleotides.


INHERITANCE

Section B: Sex Chromosomes

1. The chromosomal basis of sex varies with the organism

2. Sex-linked genes have unique patterns of inheritance

• In human and other mammals, there are two varieties of sex chromosomes, X and Y.

1. The chromosomal basis of sex varies with the organism


Fig. 15.8

Avoid confusion with XY system

Male develop from unfertilized eggs(they have no father)

•In humans, the anatomical signs of sex first appear when the embryo is about two months old.

• In individuals

with the SRY gene (sex determining region of the Y chromosome), the generic embryonic gonads are modified into testes.

lacking the SRY gene, the generic embryonic gonads develop into ovaries.

• Activity of the SRY gene triggers a cascade of biochemical, physiological, and anatomical features because it regulates many other genes.

• In addition, other genes on the Y chromosome are necessary for the production of functional sperm.


• the sex chromosomes, especially the X chromosome, have genes for many characters unrelated to sex.

• E.g., white-eye locus in Drosophila.



Fig. 15.9

Because males have only one X chromosome (hemizygous), any male receiving the recessive allele from his mother will express the trait.

males are far more likely to inherit sex-linked recessive disorders than are females.


human sex-linked disorders

• Duchenne muscular dystrophy.

• due to the absence of an X-linked gene for a key muscle protein, called dystrophin.

• characterized by a progressive weakening of the muscles and loss of coordination.


• Hemophilia is a sex-linked recessive trait defined by the absence of one or more clotting factors.

• Individuals can be treated with intravenous injections of the missing protein.


Although female mammals inherit two X chromosomes, only one X chromosome is active and one X chromosome condenses into a compact object, a Barr body

After Barr body formation, all descendent cells have the same inactive X.

• The condensed Barr body chromosome is reactivated in ovarian cells that produce ova.

involves the attachment of methyl (CH3) groups to

cytosine nucleotides on the X chromosome (inactivates most of its genes).

Fig. 15.10

• Barr body occurs randomly and independently in embryonic cells at the time of X inactivation.

• As a consequence, females consist of a mosaic of cells, some with an active paternal X, others with an active maternal X.

•If a female human is heterozygous for a sex-linked trait, will have patches of normal skin and skin patches lacking sweat glands.


INHERITANCE

Section C: Errors and Exceptions in Chromosomal Inheritance

1. Alterations of chromosome number or structure cause some genetic

disorders

2. The phenotypic effects of some mammalian genes depend on whether they

are inherited from the mother or the father (imprinting)

3. Extranuclear genes exhibit a non-Mendelian pattern of inheritance

1. Alterations of chromosome number or structure cause some genetic disorders

Fig. 15.11 Meiotic nondisjunction

Nondisjunction problems with the meiotic spindle

• after nondisjunction cells will have an abnormal chromosome number or aneuploidy.

Trisomic = 2n + 1; Monosomic = 2n - 1.

• Although the frequency of aneuploid zygotes may be quite high in humans, most of these alterations are so disastrous that the embryos are spontaneously aborted long before birth.

• Organisms with more than two complete sets of chromosomes polypoidy. (nondisjunction) e.g, triploid (3n). tetraploid (4n)

• e.g., both fishes and amphibians have polyploid species.

• Polyploidy is relatively common among plants and much less common among animals.

• Polyploids are more nearly normal in phenotype than aneuploids.

• Down syndrome: three copies of chromosome 21 aneuploid (nondisjunction)

Fig. 15.14

•The frequency of Down syndrome correlates with the age of the mother. age-dependent abnormality

Fig. 15.13

Breakage of a chromosome can lead to four types of changes in chromosome structure.

common in meiosis

Typically harmful

chronic myelogenous leukemia

(CML).[ch9/ch22]

• For most genes it is a reasonable assumption that a specific allele will have the same effect regardless of whether it was inherited from the mother or father.

2. The phenotypic effects of some mammalian genes depend on whether they were inherited from the mother or the father (imprinting)

• However, for some traits in mammals, it does depend on which parent passed along the alleles for those traits.

The imprinting genes involved are not sex linked and may or may not lie on the X chromosome.

• . a deletion of a specific segment of chromosome 15

• Prader-Willi syndrome abnormal chromosome from father.

• Angelman syndrome from the mother.

• In genomic imprinting process, a gene on one homologous chromosome is silenced, while the other is expressed.

• cp: barr body is for x chromosome inactivation.

• same alleles may have different effects on offspring, depending on whether they arrive in the zygote via ovum or sperm.

• The imprinting status of a given gene


Fig. 15.15

• In many cases, genomic imprinting occurs when methyl groups are added to cytosine nucleotides on one of the alleles.

• Fragile X syndrome, which leads to various degrees of mental retardation, also appears to be subject to genomic imprinting.

(inherited from the mother)

• This disorder is named for an abnormal X chromosome with extra CGG repeat.

Not all of a eukaryote cell’s genes are located in the nucleus.

3. Extranuclear genes exhibit a non-Mendelian pattern of inheritance

phenotype of the offspring was determined only by the maternal parent (maternal inheritance).

Extranuclear genes are found on small circles of DNA in mitochondria and chloroplasts.

These organelles reproduce themselves.

Their cytoplasmic genes do not display Mendelian inheritance.

• They are not distributed to offspring during meiosis.

Mendel didn’t include:

• Sex-linked traits

• Major chromosomal aberrations

• imprinting & extranuclear genes

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chapter 15 the chromosomal basis of inheritance section a: relating mendelism to chromosomes...

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