Genetics: Genetics Exam 3 Ch 8 Flashcards

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Chapter 8
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Genetic variation

refers to differences between members of
the same species or those of different species

- allelic variation: due to differences in particular genes

- chromosomes: vary in structure (chromosome segments can be deleted, duplicated, and/or rearranged) & number (# individual chromosomes may vary (3 not 2) or numbers of chromosome sets (4 sets not the usual 2)



field of genetics that involves the microscopic examination of chromosomes



examines the chromosomal composition of a particular cell or organism

- allows detection of individuals with abnormal chromosome number or structure

- provides a way to distinguish b/w species (human vs fruit fly vs corn)


Cytogeneticists use three main features to identify and classify chromosomes:

1. Location of centromere

2. Size

3. Banding patterns

N: these features are all seen in a karyotype



an individual's complete set of chromosomes

N: seen by micrograph


G banding

Chromosomes are exposed to the dye Giemsa

- shows dark and light bands

- 300 G bands seen in metaphase, 800 G bands in prometaphase


The G banding pattern is useful because:

- distinguishes individual chromosomes from each other

- detects changes in chromosome structure

reveals evolutionary relationships among the chromosomes of closely related species


Comparison of centromeric locations:

metacentric, submetacentric, acrocentric, telocentric

card image

See image


2 Two primary ways chromosome structures can be altered

1. Total amount of genetic material in the chromosome can change

- deletion (deficiency): loss of a chromosomal segment

- duplication: repetition of a chromosomal segment compared to a normal chromosome

2. Total amount of genetic material remains the same but is rearranged

- inversion: change in the direction of part of the genetic material along a single chromosome

- translocation: a segment of one chromosome becomes attached to a different chromosome


Simple translocation

A piece of a chromosome is attached to another chromosome


Reciprocal translocations

Two different types of chromosomes exchange pieces, producing two abnormal chromosomes with translocations.

Two non-homologous chromosomes exchange genetic material
- Reciprocal translocations arise from two different mechanisms
- Chromosomal breakage and DNA repair
- Abnormal crossovers

Leads to a rearrangement of the genetic material, not a change in the total amount
- Thus aka balanced translocations
- like inversions, are usually without phenotypic consequences
- In a few cases, they can result in a breakpoint effect or a position effect

Unlike reciprocal translocations, some translocations alter the total amount of genetic material
- Called unbalanced translocations, when genetic material is duplicated and/or deleted
- Unbalanced translocations are associated with phenotypic abnormalities or even lethality
Example: Familial Down Syndrome
• In this condition, the majority of chromosome 21 is attached to chromosome 14
- Offspring may have three copies of genes found on a large segment of chromosome 21


Terminal deletion

a deletion that occurs towards the end of a chromosome; during chromosomal deletion, the chromosome breaks in two pieces and the fragment w/o the centromere is lost


Interstitial deletion

a deletion that occurs from the interior of a chromosome; chromosome breaks in two places, central fragment is lost, two outer pieces reattach


Phenotypic consequences of deletion depends on

- size of deletion

- chromosomal material deleted (are they vital)


Cri-du-chat syndrome

detrimental condition caused by chromosomal deletion in the short arm of chromosome 5



chromosomal duplication is usually caused by abnormal events during recombination

- repetitive sequences (shown in red): can cause misalignment between homologous chromosomes

- If a crossover occurs, nonallelic homologous recombination results


Phenotypic consequences of duplications

- tend to be correlated to size (bigger piece, more likely to have effect)

- end to have less harmful effects the deletions of comparable size


Gene family

consists of two or more genes in a single species that are derived from the same ancestral gene.

N: The majority of small chromosomal duplications have no
phenotypic effect but are vital because they provide the raw material for the addition of genes to a species. This can ultimately lead to the formation of gene families.



homologous genes within a single species

N: Homolog is the umbrella term for a genes that share origin. Orthologs are two genes in two different species that share a common ancestor, while paralogs are two genes in the same genome that are a product of a gene duplication event of the original gene.


Copy Number Variation (CNV)

segment of DNA that varies in copy number among members of the same species

- may be missing a particular gene

- may be a duplication


Possible mechanisms of CNV (Copy Number Variation)

- nonallelic homologous recombination

- proliferation of transposable elements (mobile DNA sequences capable of replicating themselves within genomes independently of the host cell DNA)

- errors in DNA replication


CNV associated w some human diseases


- Autism and some certain learning disabilities

- Susceptibility to infectious diseases

- Cancer


Pericentric inversion

the inverted segment includes the centromere (with both arms involved)


Paracentric inversion

the two breaks appear on the same side of the centromere (in the same arm).


Break point effect:

An inversion break point occurs within a vital gene, thereby separating it into 2 nonfunctional parts


Position effect:

A gene is repositioned in a way that alters its gene expression


Inversion heterozygote

Individuals with one copy of a normal chromosome and one copy of an inverted chromosome
- may have a high probability of producing gametes that are abnormal in their total genetic contentN: The abnormality is due to crossing over within the inverted segment


Inversion Heterozygotes:

During meiosis I, pairs of homologous sister chromatids synapse with each other
- For the normal and inversion chromosome to synapse properly, an inversion loop must form
- If a crossover occurs within the inversion loop, highly abnormal chromosomes are produced


Robertsonian Translocation

Familial Down Syndrome is an example of Robertsonian
This translocation occurs from the following:
- Breaks occur near the centromeres of two non-homologous acrocentric chromosomes
- The small acentric fragments are lost
- The larger fragments fuse at their centromeric regions to form a single chromosome which is metacentric or submetacentric
Most common rearrangement in humans
- Approximately one in 900 births



Variation in the number of complete sets of chromosome



Variation in the number of particular chromosomes within a set

- trisomic: organism has 3 copies of a chromosome instead of 2

- monosomic: 1 copy instead of 2

Commonly causes an abnormal phenotype
- It leads to an imbalance in the amount of gene products
- Three copies of a gene will lead to 150% production
- A single chromosome can have hundreds or even
thousands of genes... Large excess of gene products

Imbalance of gene products produce individuals that are less likely to survive than a euploid individual.

Alterations in chromosome number occur frequently during gamete formation.

The autosomal aneuploidies compatible with survival are trisomies 13, 18 and 21. Aneuploidies involving sex chromosomes generally have less severe effects than those of autosomes.



Organisms w/ 3 or more sets of chromosomes


Gene mutations

molecular changes in the DNA sequence of a gene

- can involve a base substitution

e.g. T -> G



a change of a pyrimidine (C. T) to another pyrimidine or a purine (A, G) to another purine



a change of a pyrimidine to a purine or vice versa


Silent mutations

base substitutions that don't alter the amino acid sequence of the polypeptide


Missense mutations

base substitutions in which an the amino acid change does occur


Nonsense mutations

base substitutions that change a normal codon to a stop codon


Frameshift mutations

involve the addition or deletion of a number of nucleotides that is not divisible by 3

shifts the reading frame so that translation of mRNA results in a completely different amino acid downstream of the mutation



Diploid animals that sometimes produce polyploid tissues


Polytene Chromosomes

bundle of chromosomes that lie together in a parallel fashion

- bundle is made from doublings of drosophila chromosome pairs wherein chromosomes undergo repeated rounds of replication w/o cellular division


3 natural mechanisms that cause the
chromosome number to vary

1. Meiotic nondisjunction
2. Mitotic nondisjunction
3. Interspecies crosses


Nondisjunction in meiosis I

failure of chromosomes to segregate properly during anaphase

During fertilization, some gametes produce an individual that is trisomic
Some gametes produce an individual that is monosomic for the missing chromosome
All four gametes are abnormal


Nondisjunction in Meiosis II

If nondisjunction occurs in meiosis II,

50% Abnormal gametes
50% Normal gametes


Meiotic Nondisjunction

In rare cases, all the chromosomes can undergo nondisjunction and migrate to one daughter cell
This is termed complete nondisjunction
- It results in one diploid cell and one without chromosomes
- The cell without chromosome is inviable
- The diploid cell can participate in fertilization with a normal
gamete (this yields a triploid individual)


Mitotic Abnormalities

Abnormalities in chromosome number often occur after fertilization.

- if so, abnormality occurs during mitosis not meiosis

1. Mitotic nondisjunction: sister chromatids separate improperly (leads to trisomic and monosomic daughter cells)

2. Chromosome loss: one of sister chromatids do not migrate to a pole (leads to normal and monosomic cells)



genetic abnormalities that occur after fertilization

- contains a subset of cells genetically different from the rest of the organism

- size and location of the mosaic region depends on the timing and location of the original abnormality

extreme case: abnormality takes place early embryonic development (precursor to be large segment of organism)


Comparison of Autopolyploidy, Alloploidy, and Allopolyploidy

The number of chromosome sets in a given individual or species can vary in three ways
- Autopolyploidy: increase in the number of sets within a single species
- Alloploidy: the combining of chromosome sets from different species
- Allopolyploidy: when the number of chromosome sets increases in an alloploid

Complete nondisjunction can produce an individual with one or more sets of chromosomes
- diploid species -> autopolyploidy (tetraploid)
A common mechanism for changes in the number of sets of chromosomes is alloploidy
- the result of interspecies crosses
- Most likely occurs between closely related species

- species 1, species 2 -> alloploidy (allodipoid)

An allodiploid has one set of chromosomes from two different species.
An allopolyploid contains a combination of both autopolyploidy and alloploidy.
An allotetraploid contains two complete sets of chromosomes from two different species.

- species 1, species 2 -> alloploidy (allotetraploid)