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.