Chromosomes and Inheritance

Using Fruit Flies to Study Genetics Thomas Hunt Morgan, 1908 Among normal red-eyed Drosophila he found a

mutated white-eyed fly (male). Since it was a recessive gene that was expressed

only in male flies, Morgan suggested that the chromosome was located on the X chromosome.

Miniature winged mutation was transmitted on the same gene. These two characteristics did not always travel together.

Morgan suggested that the chromosomes could swap parts and received the Nobel prize in 1934.

Drosophila melanogaster

X-linked Inheritance

A situation where the genes that cause a disorder are located on the X chromosome.

More male victims than female Hemophilia Color-blindness Duchenne muscular dystrophy

Color Blindness The inability to distinguish among certain

shades of green and red. These genes are found on the X

chromosome. A recessive disorder where the genes do

not code for the proper pigment protein. Passed from a mother who is heterozygous

for the trait, she will see correctly Since the son only gets one X chromosome,

he has no other allele to prevent the condition.

The Ishihara color blind test.

Autosomal Genetic Disorders Autosomal recessive disorders – a genetic

disorder related to a recessive trait found on an autosomal chromosome. Sickle-cell anemia Albinism Cystic fibrosis – impaired lung function Phenylketonuria – mental retardation Tay-Sachs disease – nervous system

degeneration Werner syndrome – premature aging

Sickle-cell Anemia Called sickle cell because of the shape that

the red blood cell takes in the victim. These cells clog capillaries, resulting in

decreased oxygen supply to brain, tissues. Lower life expectancy (42, men 48, women) Normal hemoglobin is type A Sickle-cell anemia hemoglobin is type S

Causes cell shape distortion

Result of one single amino acid substitution:Sickle cell anemia

Because it is a recessive condition, the child must inherit two recessive genes, one from each parent.

The parent who carries the recessive allele is called a carrier. The individuals with hemoglobin S are

resistant to Malaria.

Dominant Autosomal Disorders A single faulty allele can cause damage,

even when a second functional allele exists. Huntington’s Chorea – brain tissue

degeneration Mental impairment, uncontrollable spastic

movements Polydactyly – extra fingers or toes Campodactyly – inability to straighten little

finger

Huntington’s Chorea

Polydactyly, an autosomal dominant trait

Pedigrees

Can be used to determine if a condition is dominant or recessive, X-linked or autosomal. Circle = female Square = male Filled = has the condition Half filled = carrier

Pedigree of a family with polydactyly

Polyploidy

A condition in which one or more entire sets of chromosomes has been added to the genome of a diploid organism. When two sperm fertilize one egg. During a meiosis malfunction. Happens frequently in plants, not in

humans.

Incorrect Chromosome Number

Aneuploidy is when an organism has more or fewer than the normal number of chromosomes.

Occurs commonly, often unnoticed. Responsible for 30% of miscarriages. Down Syndrome Turner syndrome Klinefelter syndrome

Nondisjunction A failure of homologous chromosomes or sister

chromatids to separate during meiosis. Meiosis I or meiosis II This produces an egg or sperm with a

chromosome count of 22 or 24. The embryo formed has a chromosome count of

45 or 47. Presence of the extra chromosome causes an

unbalance in the proportion of biological ingredients.

All cause miscarriage except in the cases of chromosomes 13, 18, 21

Normal homologous chromosome separation at anaphase I

Nondisjunction : Incomplete separationof homologous chromosomes at anaphase I

Normal human karyotype, male

Normal Human karyotype, female

Human karyotype indicating Trisomy 21

Down Syndrome 90% from egg nondisjunction 10% from sperm nondisjunction

Smallish, oval heads IQ well below normal Reduced life span

Risk increases with mothers over 35.

Turner’s Syndrome

Only one X chromosome Called XO

Ovaries don’t develop properly Short in stature Brown spots on their bodies

Klinefelter Syndrome Phenotypically male XXY

Some breast development Lack of facial hair More feminine figure Tall stature Non-functional testis

Deletions

When a chromosome fragment breaks off and does not rejoin any chromosome. Cri-du-chat (cry of the cat) Syndrome

Mental retardation Improperly constructed larynx

A deletion of the short arm of chromosome #5.

Inversions and Translocations When a fragment of a chromosome

rejoins the chromosome it came from it may do so in a flipped manner, this is an inversion

A translocation is an abnormality where two chromosomes that are not homologous exchange pieces, leaving both with improper gene sequences.

Inversion

Translocation, called the PhiladelphiaChromosome is found in tumor cells.

Duplication

Duplications When two homologous chromosomes

exchange unequal pieces, one loses genetic material and the other gains.

When duplicated, the chromosome with the extra material will function normally.

In evolution, the extra gene could be altered through mutation and eventually produce a new protein.

Summary of ChromosomalStructural Changes