Chromosomal Basis of Inheritance Chapter 15

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

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

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

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

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

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

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

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

Muscular dystrophy - sex-linked disease. Affects more males than females. Hemophilia, excessive bleeding, - sex-linked.

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

Pattern responsible for mosaic of effects like patterns seen in tortoiseshell cats. Occurs due to patches of cells expressing orange allele while others have nonorange allele.

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

Abnormal number of chromosomes - aneuploidy. Trisomy - gamete receives 3 of same chromosomes (2n + 1). Monosomy - gamete receives only 1 of same chromosome (2n – 1). Earlier in development this occurs more profound effect because cells will go through mitosis.

Organisms with more than 2 complete sets of chromosomes - polyploidy. Happens more often in plants than animals. Species with polyploidy - more normal than aneuploidy because not 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 with this abnormality can survive.

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

Deletion - cri-du-chat found on chromosome 5. Chronic myologenous leukemia - translocation between chromosome 9 and chromosome 22.

Some traits dependent on who passes on alleles, mother or father. Prader-Willi syndrome - deletion on chromosome 15 inherited by father. Same deletion from mother - Angelman syndrome. Genomic imprinting - gene on 1 homologous chromosome silenced - allele on homologous chromosome expressed.

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

Coloration due to mitochondrial genes