Genetics Chapter 8 Flashcards- Variation in Chromosome Structure and Number

genetic variation

genetic variation

Genetic differences among members of the same species or among different species.

Three features of chromosomes that aid in their identification are:
location of the centromere, the size of the chromosome, and banding pattern

allelic variation

Genetic variation in a population that involves the occurrence of two or more different alleles for a particular gene.

cytogeneticist

A scientist who studies chromosomes using microscopy.

Chromosome classification with regard to centromeric location

metacentric- centromere is near the middle

submetacentric- the centromere is slightly off center

acrocentric- the centromere significantly off center but not at the end

telocentric- the centromere is at one end

karyotype

An organized representation of a micrograph of all the chromosomes within a cell. It reveals how many chromosomes are found within an actively dividing somatic cell.

G bands

The chromosomal banding pattern that is observed when the chromosomes have been treated with the stain called Giemsa.

deletion (deficiency)

The condition in which a segment of chromosomal material is missing.

total amount of genetic material is changed. Usually phenotypically detrimental but depends on the size of the deletion

terminal deletion

Loss of a segment from the end of a linear chromosome.

Usually caused by a chromosome break

interstitial deletion

Loss of an internal segment from a linear chromosome.

Usually caused by a chromosome break

repetitive sequences

Short DNA sequences that are present in many copies in the genome.

duplication

Repetition of a segment of DNA more than once within a genome.

total amount of genetic material is changed. duplication is usually less harmful than deletions and is dependent on the size of the duplication

nonallelic homologous recombination

Recombination that occurs at nonhomologous sites within chromosomes due to the occurrence of repetitive sequences.

The result is that one chromatid has an internal duplication (gene duplication), and another chromatid has a deletion

gene duplication

An increase in the copy number of a gene. Can lead to the evolution of gene families.

over time, gene duplications can lead to the formation of gene families, such as the globin gene family

gene family

Two or more different genes within a single species that are homologous because they were derived from the same ancestral gene.

homologous

(1) describes two genes that are derived from the same ancestral gene and have similar DNA sequences
(2) the two homologs of a chromosome pair are said to be homologous to each other.

copy number variation (CNV)

Variation in the copy number of a gene within a species. Fairly common among members of a species

segmental duplication

A tandem duplication of a small segment of a chromosome.

paralogs

Homologous genes within a single species

inversion

A change in the direction of genetic material along a chromosome in which a segment is flipped so it runs in the reverse order.

chromosomal rearrangement- does not change the total amount of genetic material

dicentric

Describes a chromosome with two centromeres.

pericentric inversion

An inversion in which the centromere is located within the inverted region of the chromosome.

paracentric inversion

An inversion in which the centromere is found outside of the inverted region.

position effect

A change in phenotype that occurs when the position of a gene is changed from one chromosomal site to a different one.

may occur when an inversion or translocation may reposition a gene on a chromosome in a way that alters its normal level of expression.

inversion heterozygote

A diploid individual that carries one normal chromosome and a homologous chromosome with an inversion.

inversion loop

The loop structure that is formed when the homologous chromosomes of an inversion heterozygote attempt to align themselves (i.e., synapse) during meiosis.

The chromosomes form an inversion loop so that the homologous regions can pair with each other. For the inverted and noninverted regions to pair, a loop must form.

for situations with a pericentric inversion- 2 chromosomes will have a deletion and duplication along with 2 regular chromatids.

for situations with a paracentric inversion- 1 chromosomes will be acentric and degrade and the other will be dicentric (have 2 centromeres) with a dicentric bridge

dicentric bridge

The region between the two centromeres in a dicentric chromosome.

acentric fragment

A fragment of a chromosome that lacks a centromere.

translocation (simple translocation)

(1) the phenomenon in which one segment of a chromosome breaks off and becomes attached to a different chromosome or a different part of the same chromosome

chromosomal rearrangement- does not change the total amount of genetic material

reciprocal translocation

A translocation in which two different chromosomes exchange pieces.

2 mechanisms that can cause a reciprocal translocation:

1) chromosomal breakage and DNA repair- DNA repair enzymes recognize the reactive ends of a break and incorrectly connect them

2) crossover between nonhomologous chromosomes (ex. pairing of transposable elements (repetitive sequences) found in different chromosomes)

chromosomal rearrangement- does not change the total amount of genetic material

telomeres

Specialized DNA sequences found at the ends of linear eukaryotic chromosomes.

balanced translocation

A translocation, such as a reciprocal translocation, that does not alter the total amount of genetic material.

individuals that carry a balanced translocation have a greater risk of producing unbalanced gametes due to the formation of a translocation cross

unbalanced translocation

A translocation that results in a cell having too much genetic material compared with a normal cell.

Robertsonian translocation

A rearrangement of genetic material that arises from breaks near the centromeres of two nonhomologous acrocentric chromosomes.

translocation cross

The structure that is formed when the chromosomes of a reciprocal translocation attempt to synapse during meiosis. This structure contains two normal (nontranslocated chromosomes) and two translocated chromosomes.

These chromosomes form a translocation cross because their homologous regions are pairing with each other.

A total of eight chromatids are found within the cross.

3 results are possible

1) alternate segregation- one daughter cell receives 2 normal chromosomes, and the other cell gets 2 translocated chromosomes. then after meiosis II- 4 haploid cells are produced- 50% normal and 2 have reciprocal balanced translocations

2) adjacent-1 segregation happens when adjacent chromosomes (one with each type of centromere) segregate into the same cell. After meiosis II all 4 haploid daughter cells are genetically unbalanced. Usually, these haploid cells are not viable (leading to semisterility)

3) adjacent-2 segregation (very rare) the centromeres do not segregate as they should and so one daughter cell receives both copies of one chromosome and the other d.c. gets the other chromosome. after meiosis II- 4 haploid abnormal cells have unbalanced combination of chromosomes.

semisterility

The condition in which an individual has a lowered fertility.

euploid

Describes an organism in which the chromosome number is an exact multiple of a chromosome set.

among animals, variations euploidy are relatively rare. Some tissues within a diploid animal may exhibit polyploidy.

triploid

Describes an organism or cell that contains three sets of chromosomes. 3n

polyploid

Describes an organism or cell with three or more sets of chromosomes.

polyploidy in plants is relatively common and has many advantages for agriculture. Triploid plants are usually seedless because their chromosomes cannot segregate properly during meiosis

aneuploid

Not euploid; a variation in chromosome number such that the total number of chromosomes is not an exact multiple of a set orn number. Aneuploidy is the condition of being aneuploid.

trisomic

Refers to a diploid cell or organism with one extra chromosome (i.e., 2n+ 1).

monosomic

Refers to a diploid cell or organism that is missing a chromosome (i.e., 2n− 1).

nondisjunction

Event in which chromosomes do not segregate properly during mitosis or meiosis.

aneuploidy

often detrimental because it results in an imbalance in gene expression. Down syndrome (trisomy 21- aneuploidy in humans- 150% gene expression) increases in frequency with maternal age.

Those individuals who survive an incorrect number of chromosomes 13, 18 or 21 because the chromosomes are small and carry relatively few genes.

Those that survive with an incorrect number of one of the sex chromosomes do so likely because X-chromosome inactivation allows only one X chromosome to remain active

haplodiploid system

A mechanism of sex determination found in some species, such as bees, in which one sex is haploid (i.e., male) and the other sex is diploid (i.e., female).

endopolyploidy

In a diploid individual, the phenomenon in which certain cells of the body are polyploid

polytene chromosome

A type of chromosome found in certain cells, such as Drosophila salivary cells, that consists of many copies of a chromosome that lie side by side.

chromocenter

The central point where chromosomes making up a polytene chromosome aggregate.

why is a triploid individual usually infertile?

During meiosis in a triploid individual, the homologs cannot pair properly. This results in highly aneuploid gametes, which are usually nonviable. Also, if aneuploid gametes participate in fertilization, the offspring are usually nonviable.

in agriculture, an advantage of triploidy in plants is that the plants are often seedless

meiotic nondisjunction

The event in which chromosomes do not segregate equally during meiosis.

can lead to variation in chromosome number

mitotic nondisjunction

An event in which chromosomes do not segregate equally during mitosis.

can lead to variation in chromosome number

alloploid

Describes an organism that contains sets of chromosomes from two or more different species.

complete nondisjunction

Event that may occur during meiosis or mitosis when all of the chromosomes fail to segregate properly and remain in one of the two daughter cells.

autopolyploid

A polyploid produced within a species as a result of nondisjunction. It is an increased number of SETS of chromosomes within a single species.

alloploidy

The phenomenon in which a cell or organism contains sets of chromosomes from two or more different species. interspecies mating results in alloploids

allodiploid

Describes an organism that contains one set of chromosomes from each of two different species.

allopolyploid

Describes an organism that contains two (or more) sets of chromosomes from two (or more) species.

allotetraploid

Describes an organism that contains two complete sets of chromosomes from two different species, for a total of four sets.

usually arise from allodiploids when a somatic cell in an allodiploid undergoes complete nondisjunction to create an allotetraploid cell.