Genetics Exam 3 Terms

Euploid

A chromosome number that is multiple of the basic monoploid number (=x)

Monoploid Number

the number of chromosomes in a basic chromosome set containing one copy of each chromosome (x)

Haploid

the number of chromosomes present in gametes (n)

Hexaploid wheat

contains 6 sets of 7 chromosomes per set

Homeologous chromosomes

pairs of chromosomes that were once completely homologous but became partially different after being separated into distinct species over evolutionary time

Haploid chromosome number in hexaploid wheat

n=21

Monoploid chromosome number in hexaploid wheat

x=7

Diploid

2 complete sets of chromosomes

Polyploid

more than 2 complete sets

triploid

3 complete sets

tetraploid

4 complete sets of chromosomes

pentaploid

5 complete sets of chromosomes

Hexaploid

6 complete sets of chromosomes

Monoploidy

only one complete set of chromosomes is present (usually lethal in higher organisms) due to ½ the normal amount of all gene products is produced

Monoploid

Male ants, wasps, and bees/ develop unfertilized eggs (parthenogenesis)/ produce gametes via mitosis

Diploid

Female ants, wasps, and bees/ form fertilized eggs/ produce gametes by meiosis

Plant monoploids

useful in the development on new plant varieties

Challenges of plant monoploids

• mutagenesis to produce new plant mutants was traditionally performed in diploid plants

• new recessive, mutant alleles must be homozygous in order to be expressed in phenotype

• alternative: monoploid plants

Producing monoploid plants

1. isolate products of meiosis (monoploid)

2. Cold treatment

3. transplant onto media that stimulates root and shoot growth

Fertile Monoploids

monoploids are sterile (can’t go through meiosis)

Solution:

• treat with colchicine

Colchicine

blocks formation of the mitotic spindle apparatus (complete mitotic failure = doubling of chromosome number)

Alternative method of creating monoploid plants in barley

Domestic barley (2n1) → ovum (n1) + Pollen (n2) ← Wild barley (2n2)

sterile hybrid (n1 n2)

selective loss of chromosomes from wild barley; retains chromosomes from domestic barley

monoploid domestic barley (n1)

select for desired trait

treat with colchicine = diploid domestic barley with desired trait

Polyploid Types

• Autoployploids

• Allopolyploids

Autopolyploids

contain multiple sets of chromosomes derived from one species

Allopolyploids

contain multiple sets of chromosomes derived from two or more different species

Triploids

often autopolyploids and may arise naturally (two sperm fertilize one egg)

Crossing a tetraploid (4x) and a diploid (2x) results in

a triploid

Triploid characteristics

• one chromosome lacks a pairing partner during meiosis

• result: unbalanced gametes (2 copies of some chromosome, one copy of others)

Examples of triploids

seedless watermelon, seedless grapes, and seedless bananas

Triploidy in humans

• lethal (most are miscarried)

• 2 sperm fertilize 1 egg

• major abnormalities of all organ systems

Autotetraploids

contains 4 sets of chromosomes from a single species

Origins of autotetraploids

• chromosomes separate but cytokinesis fails to occur

  • can occur during meiosis or in mitosis prior to entering meiosis

• complete failure of chromosome to separate

• can be induced by colchicine treatment

Characteristics of autotetraploids

• are fertile

  • all chromosomes have paring partners

• larger cells

• larger fruit

• larger flowers

Allotetraploids

contain four sets of chromosomes, derived from different but related species

Origins of allotetraploids

diploid species #1 (2n1) → n1 gametes + n2 gametes ← diploid species #2 (2n2)

sterile hybrid (n1n2)

spontaneous doubling of chromosome number

fertile allotetraploid (2n1 2n2) fertile !

Karpechenko’s Experiments

Hybrid between radish and cabbage

radish 2n =18 → gametes n=9 + gametes n=9 ← cabbage 2n=18

sterile hybrid

spontaneous doubling of chromosome number

fertile allotetraploid (had the leaves of a radish and the roots of a cabbage plant)

Commercially grown wheat is what

allohexaploid 2n = 6x = 42 chromosomes

Polyploidy in Animals

seen in flatworms, brine shrimp, and leeches

lizards

• reproduce via parthenogenesis (unfertilized eggs)

Frogs and toads

• reproduction involves both male and female

Certain fish

• tetraploid salmon contain twice as much DNA as diploids

Sterility of triploids (has been exploited for commercial oyster fishing)

• diploid oysters are unpalatable during spawning season

• triploid oysters can be harvested all year round

Changes in chromosome structure

• chromosome is broken

  • the break is not repaired

• unequal crossing-over

What factors cause changes in chromosome structure and breakage

spontaneous and agents

Spontaneous factors

• errors in DNA replication

• errors in recombination between chromosomes during meiosis

Agents that cause chromosome breakage

• ionizing radiation (x-rays, gamma rays)

• certain chemicals

• certain viruses

• exposure to heat

• mutations in genes involved in DNA repair or recombination

Deletions

loss of a chromosome segment

Duplications

extra copies of a chromosome segment

Inversions

a chromosomal segment is rotated 180 degrees from its normal orientation

Translocations

a chromosome segment is transferred from one chromosome to another, non-homologous chromosome

Terminal deletion

deletion at the end of the chromosome

Interstitial deletion

internal fragment of chromosome is lost

Cytological consequences 1

if the individual is heterozygous for the deletion, abnormal chromosome pairing occurs meiosis and a deletion loop forms in the area of the deletion

Cytological consequences 2

if breakage and deletion occurs at both ends of a chromosome, the two ends may fuse together and form a ring chromosome

Phenotypic consequences

in a heterozygous individual, if the region containing a dominant allele is deleted, the recessive allele will now be expressed

Pseudodominance

expression of a recessive allele when present in a single “dose”

Genetic consequences of deletions

• loss of variability

• mutant phenotype

• suppression of crossing over in the region containing the deletion

Cir-du-chat syndrome

deletion of 5p- infants have a cat-like cry; malformed larynx, facial abnormalities, intellectual impairment

Wilms tumor

deletion of 11q- kidney tumors, genital and urinary anomalies

Retinoblastoma

deletion of 13q- tumors of the eye, increased risk of other tumors

Prader-Willi syndrome

deletion of 15q- infants: weak, slow growth/ children and adults: obesity, severe compulsive overeating

Prader-Willi syndrome Treatment

• healthy diet

• control access to food

• behavioral modification

• sex hormone therapy

• growth hormone therapy if growth is impaired in childhood

Prader-Willi vs. Angelman Syndrome

• normal genets from BOTH mother and father are necessary for normal development

• normal chromosome 15 from mom - deletion of 15q from dad = prayer-willi syndrome

• normal chromosome 15 from dad - deletion of 15q from mom = Angelman syndrome

Angelman syndrome phenotype

• delayed development

• intellectual impairment

• problems with movement and balance

• severe speech impairment

• seizures

• Microephaly

• Hyperactivity

• difficulty sleeping

• happy, excited demeanor

Deletions are found in many tumors in humans

• neuroblastoma (brain tumor)

• retinoblastoma (tumor of retina of eye)

• wilms tumor (kidney tumor)

• melanoma (malignant skin cancer)

• small cell carcinoma of the lung

Williams Syndrome

deletion of part of the long arm of chromosome 7 (15q)

Phenotype of Williams Syndrome

• intellectual impairment

• heart defects

• characteristic facial features

• kidney abnormalities

• low muscle tone

• low birth weight

• failure to gain weight

• hypersensitivity to loud noises

• outgoing personality

How can we detect deletions

• karyotyping

• FISH: fluorescent in situ hybridization