Genetics - part two

Chapter 15

Review

• Genes are located on chromosomes

• Diploid organisms have two copies of every chromosome

• Therefore, two alleles for every gene (except X,Y)

Chromosomal basis of Mendel’s Laws

• Meiosis

• Segregation

• Independent assortment

Fruit Flies

Fruit Flies in Genetics

• Small genome – 4 chromosomes

• Short generation time

• Wild type – most common phenotype– Red eyes

• Studies revealed– Sex-linked genes

Sex-linked Inheritance

Gene Recombination & Crossing Over

Constructing a Gene MapThe farther two genes are from each other, the higher the probability that crossing over will occur between them. This can be used to figure out where genes are on the chromosome.

Some chromosomal systems of sex determination

Sex-linked Inheritance

Inactivation of the X & Barr Bodies• One X is inactivated during development – becomes Barr body• Same X is not inactivated in all cells• Mosaic in heterozygotes

Nondisjuction (during Meiosis)

Down Syndrome

Genetic disorders caused by abnormal chromosome number (due to nondisjunction)

Name Chromosome Description

Down syndrome 21 Trisomy

Edward’s syndrome 18 Trisomy

Patau syndrome 13 Trisomy

Turner’s syndrome Single X XO

Klinefelter’s syndrome Extra X (in males) XXY

XYY syndrome Extra Y (in males) XYY

Triple X syndrome Extra X (in females) XXX

Alterations of Chromosomes

Genomic Imprinting

• Some genes behave differently if they come from ova/ sperm

• One gene can be “silenced” and the other is expressed

• Imprinting is not inherited

• Genes are re-imprinted for in the next generation

Example: Imprinting• Prader-Willi/ Angelman syndromes• Deletion on chromosome 15

• If inherited from father– Prader-Willi syndrome

• Mental retardation, obseity, short stature, small hands and feet

• If inherited from mother– Angelman syndrome

• Spontaneous uncontrollable laughter, jerky movements, motor and mental problems