Chromosomal Basis of Inheritance Chapter 15

Most genetics work done on fruit flies (little time to observe many generations) Thomas Morgan - fruit fly eye color. Wild type (normal) eye color - red. Mutant - white.

Discovered mutant eye color appeared more often in males - some traits sex-linked (carried on sex chromosomes).

Chromosomes have hundreds or thousands of genes. Genes located on same chromosome, linked genes, inherited together - chromosome passed as unit.

"A" and "B" are linked due to their occurrence in the same chromosome. Similarly, "a" and "b" are linked in the other chromosome.

Production of offspring with new combinations of traits inherited from 2 parents - genetic recombination. Can occur during crossing over - sections of homologous chromosomes exchanged during meiosis I.

Genetic map - list of the loci along the chromosomes. Further apart genes are, higher probability they will switch places.

Sex chromosomes 2 sex chromosomes - X and Y. Males – XY, females - XX. Other species - X-0 system, Z-W system, haplo-diploid system. Humans - X-Y system like normal chromosomes - 50/50 chance of having male or female.

Until embryo is 2 months old - fetus female. If fetus XY - SRY gene turned on making fetus male.

Sex chromosomes, have genes for traits other than sex. Trait recessive - female will only inherit it if both parents pass it on. Males - 50% chance of inheriting it (only have 1 X chromosome) Males have higher rate of sex- linked diseases than females.

Muscular dystrophy - sex-linked disease. Affects far more males than females. Hemophilia (excessive bleeding) - sex-linked.

Only 1 X in females turned on. Other - Barr body - reactivated in ovaries during egg production (to pass genes on). Females exhibit characteristics from mother, some from father (sex chromosomes only).

Barr body replicated, not transcribed

Pattern responsible for mosaic of effects (tortoiseshell cats) Due to patches of cells expressing orange allele, others have nonorange allele.

Errors Errors can occur both in DNA and in chromosomes. Nondisjunction - homologous chromosomes fail to separate during meiosis I, or chromatids - during meiosis II. Some gametes receive 2 of same type of chromosome; another gamete receives no copy.

Abnormal # of chromosomes - aneuploidy. Trisomy - gamete receives 3 of same chromosomes (2n + 1). Monosomy - gamete receives 1 of same chromosome (2n – 1). Earlier in development - more profound effect - those cells go through mitosis.

Organisms with more than 2 complete sets of chromosomes - polyploidy. Happens more often in plants than animals. Species with polyploidy usually more normal than aneuploidy (no missing chromosomes)

A rodent species that is the result of polyploidy

Deletion - piece of chromosome broken off during cell division. Duplication - fragment becomes attached as extra segment to sister chromatid. Inversion - piece breaks off, turns around, reattaches (backwards). Translocation - chromosomal fragment joins nonhomologous chromosome.

Down syndrome - trisomy (Trisomy 21). Chromosome 21 - smallest chromosome - individual can survive.

Aneuploidy – can occur in sex chromosomes. Klinefelter’s syndrome - male XXY (nondisjunction) Trisomy X (XXX) can occur in females. Monosomy X - Turner syndrome - nondisjunction.

Deletion disease - cri-du-chat - chromosome 5. Chronic myologenous leukemia - example of translocation between chromosome 9 and chromosome 22.

Some traits dependent on who passes alleles (mother or father) Prader-Willi syndrome caused by deletion on chromosome 15 (father) Deletion from mother - Angelman syndrome. Due to genomic imprinting - gene on 1 homologous chromosome silenced, allele on homologous chromosome expressed.

Some eukaryotic genes located in mitochondria. These genes all passed from mother to offspring; none of father’s mitochondrial genes passed on.

Coloration due to mitochondrial genes