Chapter-15-The Chromosomal Basis of Inheritance

                                        C-15-The Chromosomal Basis of Inheritance

Sex Linked Genes-Thomas Hunt Morgan. Studied the fruit fly.

Sex Linked Genes-Genes that are located on the sex chromosomes; usually the X.

Example eye color in fruit flies

Heterozygous Red eyed female (x) Red male

Males only need 1 copy of a sex linked gene to express it.

                                                     Linked Genes

Linked Genes-genes that are located on the same chromosome. Crossing over may occur among linked genes.

Example-Body color and wing size in the fruit fly

Genetic maps-show the sequence of genes along a chromosome.

Fig 15-7

                                All organisms are not based on the XX- XY System

X-Y system- Humans and other mammals

X-O system- Some insects. Here there is no Y chromosome. Sperm are either X or O

Z-W system- Birds, some fish and insects

Haplo-Diploid system-Bees and ants.The male develops from an unfertilized egg and are haploid. Females develop from fertilized eggs and are diploid.

                                         Sex Linked Diseases in Humans

Color Blindness

Hemophilia

Duchenne Muscular Dystropy- progressive weaking of the muscles and loss of coordination

         Nondisjunction causes an abnormal # of chromosomes

Nondisjunction-in which members of a pair of homologous chromosomes fail to separate during meiosis

XXX Female

X0 Female (Turners Syndrome

XXY Male (Kleinfelters syndrome)

Y0 a nonviable situation

Trisomy 21

Some facts on abnormal # of Sex Chromosomes

XXX Female-about 1 in 1000 woman. Higher incidence of below normal intelligence

X0-Turners syndrome-Female. About 1 in 5000. Fail to menstruate or develop normal secondary characteristics such as breasts.Sterile,short. Mentally OK.

XXY-male. 1in 1000. Partial breasts, small testes, sterile,increased incidence of mental deficiency.

Trisomy 21

Trisomy 21-Down Syndrome. Here you have an extra chromosome at pair # 21. Page 281 in text.

1 in 900 births. Weak muscles,small mouth,low resistance to disease,heart defects and mental retardation.

Increases with the age of the mother (40)

  Aneuploidy-an abnormal number of chromosomes 

Trisomic- more than 46. XXX or XXY or Trisomy-21

Monosomic-less than 46. X0 or Y0

         Polyploidy-abnormal # of sets of chromosomes

3N- fertilization of an abnormal egg (2N) by a normal sperm(N). (69 chromosomes)

4N- failure of the 2n zygote to divide after replicating its chromosomes.Subsequent mitosis would then produce a 4n embryo which will have 92 chromosomes

                               Alterations of Chromosome Structure (#of Genes)

Deletion

Duplication

Inversion

Translocation

                                                       Imprinting:

Imprinting- this is when the same abnormal gene can have a different effect depending on whether you inherited it from your mother or your father.

Example- there is a defective(deletion) gene on chromosome # 15 which will cause 2 different diseases depending on who you got the same bad gene from.

                                                    Extranuclear Genes

These are genes (DNA) that are found outside the nucleus.

Mitochondria- contains its own DNA (genes)

Plastids- in plants (chloroplasts) contain their own DNA

                                           Example of inheriting extranuclear DNA

Mitochondrial Myopathy- here you have inherited defective DNA(genes) located in your Mother’s mitochondria.

Weakness and muscle deterioration due to reduced ATP production in the mitochondria

Note- this defect is always inherited from your mother.