Chapter 8Chapter 8

Human Human Karyotypes Karyotypes

and and Chromosome Chromosome

BehaviorBehavior

The banding patterns of human chromosomesThe banding patterns of human chromosomes

The short arm of a chromosome is

called “p” and the long arm

“q”. A, B, C, etc. refer to grouping of chromosomes

by size.

The 3 shapes of chromosomes The 3 shapes of chromosomes based on the location of the centromerebased on the location of the centromere

The proposed mechanism for the creation of the The proposed mechanism for the creation of the long human chromosome 2 by fusion of two long human chromosome 2 by fusion of two

shorter chromosomesshorter chromosomes

In human females, one of the 2 X-chromosomes In human females, one of the 2 X-chromosomes is inactivated early in developmentis inactivated early in development

Proposed steps in the creation of the modern Proposed steps in the creation of the modern Y-chromosome by rearrangement of segments Y-chromosome by rearrangement of segments

in an ancient Y-chromosomein an ancient Y-chromosome

Distribution of Y-chromosome haplotypes, Distribution of Y-chromosome haplotypes, presumed to have descended from Genghis Khanpresumed to have descended from Genghis Khan

The probability of a child with Down’s The probability of a child with Down’s syndrome increases with Mother’s agesyndrome increases with Mother’s age

Pairing and segregation of 3 homologous Pairing and segregation of 3 homologous chromosomes in meiosis Ichromosomes in meiosis I

This kind of chromosome

pairing occurs in trisomics (2n+1) or

triploids (3n)

Mapping the deletion of part of a chromosome segment by Mapping the deletion of part of a chromosome segment by testcrosses and uncovering of recessive genestestcrosses and uncovering of recessive genes

Mapping of genes in Drosophila using overlapping Mapping of genes in Drosophila using overlapping deletions and polytene chromosomesdeletions and polytene chromosomes

Unequal crossing over of misaligned repeat Unequal crossing over of misaligned repeat sequences leads to gain or loss of repeatssequences leads to gain or loss of repeats

Unequal crossing over involving eye Unequal crossing over involving eye

pigment genespigment genes

Mechanism of creation of a chromosomes Mechanism of creation of a chromosomes with an inverted segmentwith an inverted segment

Pairing of homologous chromosomes Pairing of homologous chromosomes

in an inversion heterozygotein an inversion heterozygote

An inversion which does not

involve the centromere is called a

paracentric inversion

Absence of recombination within an inversion Absence of recombination within an inversion

loop does not create deletions or duplicationsloop does not create deletions or duplications

A crossover within an inversion loop of a A crossover within an inversion loop of a paracentric inversion creates dicentric and paracentric inversion creates dicentric and

acentric chromosomesacentric chromosomes

When an inversion involves the

centromere, it is called a pericentric inversion.

Crossing over in a pericentric inversion

does not create dicentric and acentric

chromosomes

A crossover within an inversion loop of a A crossover within an inversion loop of a pericentric inversion creates chromosomes pericentric inversion creates chromosomes

with deletion and duplicationwith deletion and duplication

Structure of chromosomes with a Structure of chromosomes with a reciprocal translocationreciprocal translocation

Pairing and segregation of chromosomes with a reciprocal Pairing and segregation of chromosomes with a reciprocal translocation during meiosis Itranslocation during meiosis I

Mechanism of creation of a Mechanism of creation of a Robertsonian translocationRobertsonian translocation

Pairing and segregation with a Robertsonian translocation Pairing and segregation with a Robertsonian translocation involving human chromosomes 14 and 21involving human chromosomes 14 and 21

Such a translocation

results in a high probability of having a child with Down’s syndrome.

Variegation (mottling) of eye color due to positioning of the eye Variegation (mottling) of eye color due to positioning of the eye color gene near centromeric heterochromatincolor gene near centromeric heterochromatin

When the expression of a

gene is affected by its location on a

chromosome (even though the gene

itself is not changed), such a variation is called “position effect”

Two kinds of polyploidyTwo kinds of polyploidy

Multiplication of the entire chromosome complement is called polyploidy. When all the genomes are the same, it is called autopolyploidy. When two (or more) different genomes

are duplicated, it is called allopolyploidy.

Formation of a tetraploid organismFormation of a tetraploid organism

Creation of a totally homozygous diploid cell by Creation of a totally homozygous diploid cell by doubling of chromosome number in a doubling of chromosome number in a

monoploid cell by colchicinemonoploid cell by colchicine

Monoploid cells can only be

grown in plants. In humans, the

only viable monoploid cells are the egg and

the sperm. Monoploidy in somatic cells is

lethal.

Evolution of wheat genome through Evolution of wheat genome through allopolyploidyallopolyploidy

Evolution of wheat genomeEvolution of wheat genome