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 separation of 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 Philadelphia Chromosome 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 Chromosomal Structural Changes