Chromosomal Variation + Rearrangements

Chromosome number can vary a lot across organisms. Does it relate to genome size or complexity?

No

Ploidy

The number of chromosome sets

Haploid (1n)

One set of chromosomes

Diploid (2n)

Two sets of chromosomes organized as homologous pairs

Polyploid

More than two sets of chromosomes

Three Basic Categories of Chromosome Mutations

• Structure is altered

• Number is altered

• One or more complete sets of chromosomes is added

n = haploid number =

number of nonhomologous (unique) chromosomes

Euploid

A cell/individual that contains the expected number of chromosomes

Aneuploid

Differ from expected by one or a few chromosomes

• Monosomy, trisomy, nullisomy

Polyploid (change)

Number is different by an entire set or sets of chromosomes

• triploid, tetraploid, etc.

Aneuploidy in humans

Humans are extremely sensitive to gene dosage changes, aneuploids do not usually survive

Forms of sex chromosome trisomies (humans)

XXY, XYY, XXX

Form of sex chromosome monosomy (humans)

XO

What can lead to a mosaic organism?

Nondisjunction in mitosis in soma - aneuploid cells are found in almost all normal tissues - common in cancer

Gynandromorph

Half male, half female body

Autopolyploidy

Additional sets of chromosomes from the same species

3n, 4n, 5n, etc.

Allopolyploidy

Additional sets of chromosome from another species (different species)

n1 + n2, 2n1 + 2n2, 3n1 + n2

Very important in ag.

Mechanisms that cause polyploidy

• Double/triple fertilization

• Cytokinesis failure in gamete precursor cells

• Mating between parents of different ploidy

Monosomy

2n-1

Trisomy

2n+1 or think of it as just +1

Nullisomy

2n-2

Autotriploidy

n x 3

Autotetraploidy

n x 4

Which chromosome sets (autopolyploids) can be fertile?

Even numbered → 4n, 6n, etc. → often give rise to new species

4n gets all diploid (2n) gametes

Odd-numbered chromosome sets (3n, 5n, etc) are usually

sterile → unbalanced gametes

gametes must be balanced (same # of every chromosome) to be viable

Interspecific Hybrid

Viable but usually infertile

Allodiploid

two full sets of chromosomes.. but each set comes from a different species

Unbalanced chromosomal alterations (gene dosage is changed)

Duplication

Deletion

Balanced chromosomal alterations (gene dosage is not changed + no loss or gain of DNA)

Inversion

Translocations

Duplication

Part of a chromosome is doubled → some pieces can’t pair

Deletion

part of a chromosome is lost → very unsafe

Inversion

Gene order on part of a chromosome is reversed

Translocation

Part of a chromosome moved to another chromosome (different from crossing over)

Haploinsufficiency

Mutant phenotype occurs when only one copy of a gene is present → half of the gene is completely insufficient

Unbalanced gene dosage leads to

developmental abnormalities

Unequal crossing over occurs between

repeated regions of chromosomes → can result in a homolog with a deletion and one with a duplication

What happens if a chromosome loses its centromere?

It degrades

Why are males more likely to have X-linked traits?

They only have one X chromosome, meaning a single mutated gene will cause the disease without a backup copy to compensate

Paracentric Inversions

Breaks and rotates then reinserts, but DOES NOT INCLUDE CENTROMERE

Pericentric Inversions

Inverted portion involves both chromosome arms AKA INCLUDES THE CENTROMERE

How do chromosome inversions affect phenotype? Mitosis? Meiosis?

Chromosome inversion effect homology, which most matters during meiosis → they trigger abnormal crossing over when homologous chromosomes pair up in meiosis 1

What do the chromosome strands have to do to line up matching genes?

They create an inversion loop, which creates broken and unbalanced chromosomes. Some will be dicentric and others will be acentric.

Paracentric inversions reduce fertility. True or False?

True → they reduce fertility up to 50% because half will be viable and half will be inviable in a sperm or egg inherits them.

What happens in meiosis with a pericentric inversion?

You get a normal, duplicated, and deletion chromosome. So about 50% are viable once again.

Where is recombination suppressed in chromosome inversions?

It is suppressed only within the inverted region of the heterozygote, depending on the size of the inversion. → It doesn’t stop recombination from happening, but when it does happen the cells are less viable

Reciprocal Translocation

Pieces of two nonhomologous chromosomes switch places → they both switch places with each other

Nonreciprocal Translocation

A piece of one chromosome is translocated to a nonhomologous chromosome → there’s a deletion on one chromosome because it gives a piece of itself to a nonhomologous chromosome → instead of blue and red switching, blue will add onto red

Reciprocal translocation effect on meiosis

Adjacent segregation - happens 50% of the time (splits along the midlines) and every gamete is unbalanced

Alternate segregation - happens 50% of the time (split across the gap) and half the gametes are viable

Most of the sequences of X and Y chromosomes (are/are not) homologous.

Most are NOT homologous. They have 2 small regions of homology at the end of the chromosome (PAR1 and PAR2).

Purpose of PARs

Allow homologous pairing and recombination of the X and Y chromosomes at meiosis

What is the sex-determining gene of mammals?

SRY gene on the Y chromosome

SRY Gene

A transcription factor needed for male-specific gene expression → no SRY gene means not phenotypically male

Aneuploidy Disorders

Turner’s Syndrome: XO

Klinefelter Syndrome: XXY, XXXY, etc

Poly-X-females: 1/1000 female births

Sex Reversal

Mechanisms that could cause XY females and XX males:

• If SRY gene gets deleted on Y → female

• If SRY gene gets moved over to an X → male