the chromosomal basis of inheritance
DESCRIPTION
The Chromosomal Basis of Inheritance. Chapter 15. The Chromosomal Theory of Inheritance. Mendelian genes have specific loci (positions) along chromosomes. It is the chromsomes that undergo segregation and independent assortment. Fig. 15-2. P Generation. Yellow-round - PowerPoint PPT PresentationTRANSCRIPT
The Chromosomal Basis of Inheritance
Chapter 15
The Chromosomal Theory of Inheritance
• Mendelian genes have specific loci (positions) along chromosomes.
• It is the chromsomes that undergo segregation and independent assortment.
Fig. 15-2P Generation Yellow-round
seeds (YYRR)
Y
F1 Generation
Y
R R
R Y
r
r
r
y
y
y
Meiosis
Fertilization
Gametes
Green-wrinkledseeds ( yyrr)
All F1 plants produceyellow-round seeds (YyRr)
R R
YY
r ry y
Meiosis
R R
Y Y
r r
y y
Metaphase I
Y Y
R Rrr
y y
Anaphase I
r r
y Y
Metaphase IIR
Y
R
y
yyy
RR
YY
rrrr
yYY
R R
yRYryrYR1/41/4
1/41/4
F2 Generation
Gametes
An F1 F1 cross-fertilization
9 : 3 : 3 : 1
LAW OF INDEPENDENTASSORTMENT Alleles of geneson nonhomologouschromosomes assortindependently during gameteformation.
LAW OF SEGREGATIONThe two alleles for each geneseparate during gameteformation.
1
2
33
2
1
Thomas Hunt Morgan
• Drosophila melanogaster • Wild Type- the phenotype for a character most
commonly observed in natural populations.• Mutant Phenotypes- traits that alternatives to the
wild type.• A student at UK
EXPERIMENT
PGeneration
GenerationF1 All offspring
had red eyes
Fig. 15-4b
RESULTS
GenerationF2
The Chromosomal Basis of Sex Determination
SRY- Sex determining Region of Y-The presence of this gene on the Y chromosome codes for the development of testes. In the absence of this gene, the gonads develop into ovaries
Sex Linked Genes
• A gene located on either sex chromosome is called a sex-linked gene• In humans, sex-linked usually refers to a gene on the larger X chromosome• For a recessive sex-linked trait to be expressed
-A female needs two copies of the allele -A male needs only one copy of the allele
• Sex-linked recessive disorders are much more common in males than in females
Fig. 15-7
XNXN XnY XNXn XNY XNXn XnY
N= normal color vision
Fig. 15-7
(a) (b) (c)
XNXN XnY XNXn XNY XNXn XnY
YXnSpermYXNSpermYXnSperm
XNXnEggs XN
XN XNXn
XNY
XNY
Eggs XN
Xn
XNXN
XnXN
XNY
XnY
Eggs XN
Xn
XNXn
XnXn
XNY
XnY
Morgan’s Experiments
• Linked Genes- genes located on the same chromosome that tend to be inherited together in genetic crosses.
What type of ratios would you expect to see in the testcross offspring if the genes were located on different chromosomes?
What if they were located on the same chromosome and parentalalleles are always inherited together?
Morgan’s Experiments
• Linked Genes- genes located on the same chromosome that tend to be inherited together in genetic crosses.
• Genetic Recombination- the production of offspring with combinations of traits that differ from those found in either parent.
YyRr
Gametes from green-wrinkled homozygousrecessive parent ( yyrr)
Gametes from yellow-roundheterozygous parent (YyRr)
Parental-type
offspring
Recombinantoffspring
yr
yyrr Yyrr yyRr
YR yr Yr yR
Parental Types- offspring that inherit a phenotype that matchesone of the parental phenotypes.
Recombinant Types or Recombinants- offspring that have new combinations of phenotypes.
Recombination
• Crossing Over- accounts for the recombination of linked genes.
• Genetic Map- an ordered list of the genetic loci along a particular chromosome.
• Linkage Map- a genetic map based on recombination frequencies. Displays order but not precise location.– Distances are expressed in map units- equivalent to 1%
recombination frequency. (Centimorgans)
Exit Slip
• In humans, pseudohypertrophic muscular dystrophy is a condition in which the muscles gradually waste away, ending in death in the early teens. This type of muscular dystrophy is dependent upon a sex-linked recessive gene that occurs only in boys and has never been reported in girls. Hypothesize why it does not appear in girls?
• Why does the sex-linked gene for pseudohypertrophic muscular dystrophy not become eliminated from the human race since all boys showing the trait die before reaching maturity?
Alteration of Chromosome Number
• 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 the other gamete doesn’t receive a copy.
• Aneuploidy- a chromosomal aberration in which one or more chromosomes are present in extra copies or deficient in number
• Polyploidy- a chromosomal alteration in which the organism possesses more than two complete chromosome sets.
Fig. 15-13-3
Meiosis I
Nondisjunction
(a) Nondisjunction of homologous chromosomes in meiosis I
(b) Nondisjunction of sister chromatids in meiosis II
Meiosis II
Nondisjunction
Gametes
Number of chromosomes
n + 1 n + 1 n + 1n – 1 n – 1 n – 1 n n
Alteration of Chromosome Structure
• Deletion- removes a chromosomal segment• Duplication- repeats a segment• Inversion- reverses a segment within a
chromosome• Translocation- moves a segment from one
chromosome to another
Exceptions to the Standard Chromosome Theory
• Genomic Imprinting- a variation in phenotype depending on whether an allele is inherited from the male or female parent.
• Extranuclear Genes- genes outside of the nucleus. Some are located in organelles in the cytoplasm.