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Chapter 12

BIOL 1000 Chapter 12 textbook notes

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BIOL 1000
Nicole Nivillac

BIOLOGY - Chapter 12 - TEXTBOOK NOTES Sex-Linked Genes - Genes located on male and female chromosomes (sex chromosomes) are called sex-linked genes - Chromosomes other than the sex chromosomes are called autosomes - In Humans, 1 to 22 are the autosomes Females & Male Chromosomes - Females = XX chromosomes - Males = XY chromosomes - ^ The Y chromosome has a short region of homology with the X chromosome that allows them to pair during meiosis - Half gametes produced by an XY male carry an X chromosome and half carry a Y - When a sperm cell carrying an X chromosome fertilizes an X-bearing egg cell, the new individual develops into an XX female - When a sperm cell carrying a Y chromosome fertilizes an X-bearing egg cell, the combination produces an XY male - Some insects = XX for females and XO for males (O means no Y chromosome) Sex Determination Depends on SRY Gene - SRY (gene on Y chromosome that determines sex) directs development toward maleness - After 6 to 8 weeks, the SRY gene becomes active in XY embryos and produces a protein that regulates the expression of other genes (stimulates development of testes) - XX embryos do not have SRY gene Discovering Sex-Linked Genes - Genes on chromosomes can be inherited in a process called sex linkage - Sex linkage arises from 2 differences between males and females: - 1) males have one X chromosome so only one allele for each gene and females have two copies of X chromosome so two alleles for all genes - 2) males have one copy of Y chromosome so one allele for each gene whereas females will not have any alleles since they don’t carry the Y - Morgan crossed the white-eyed male with a true-bleeding female with red eyes and observed that all the F1 flies had red eyes - F2 female flies had red eyes and males had both red and white eyes - The white eyed male parent had genotype: X Y and red-eyed female had X X w+ w+ - F1 female parent has genotype X X and male had X +Y w - A key indicator of this sex linkage is when all male offspring of a cross between a true breeding mutant female and a wild-type male have the mutant phenotype - ^ This occurs because a male receives his X chromosome from his female parent Sex-Linked Genes Are Inherited - Pedigree: shows all parents and offspring for many generations - In fruit flies, sex-linked recessive traits appear more frequently among males than females because males need to receive only one copy of the allele on the X chromosome inherited from their mothers to develop the trait - Females must receive two copies of the recessive allele, one from both parents, to express trait - Examples of human sex-linked traits are red-green colour blindness and a defect in blood clotting - Hemophiliacs – they bleed uncontrollably because a protein required for forming blood clots is not produced - Hemophilia affects only sons in the royal lines but could have affected daughters if a hemophiliac son had married a carrier female Inactivation of One X Chromosome - The inactivation occurs by a condensation process that folds and packs the chromatin of one of the two X chromosomes into a tightly coiled state - The inactivation occurs during embryonic development - If the two X chromosomes carry different alleles of a gene, one allele will be active in cell lines in which one X chromosome is active and the other allele will be active in cell lines in which the other X chromosome is active Chromosomal Alterations That Affect Inheritance - Chromosomes are sometimes altered by breaks in the DNA, which can be generated by agents such as radiation or certain chemicals or by enzymes encoded in some infecting viruses Most Common Chromosomal Alterations - Deletion: occurs if a broken segment is lost from a chromosome - Duplication: occurs if a segment is broken from one chromosome and inserted into its homologue - Translocation: occurs if a broken segment is attached to a different, nonhomologous chromosome - Inversion: Occurs if a broken segment reattaches to the same chromosome from which it was lost, but in reversed orientation, so that the order of genes is reversed Deletion and Duplications - A deletion may cause severe problems and disorders - A duplication may have effects that vary from harmful to beneficial - Some have important evolutionary change - That is because there are duplicate genes, one copy can mutate into new forms without seriously affecting the basic functions of the organism - For ex. Mammals have genes that encode several types of hemoglobin that are not present in vertebrates Translocation and Inversions - In many cases, a translocation is reciprocal, meaning that two nonhomologous chromosomes exchange segments - For ex. A particular cancer of the human immune system, Burkitt Lymphoma, is caused by a translocation that moves a segment of human chromosome 8 to the end of chromosome 14 - ^ The translocated segment contains genes that control cell division and the without this, it will cause uncontrolled cell division and development of cancer - Inversions have essentially the same effects as translocations – genes may be broken internally by the inversion, with loss of function, or they may be transferred intact to a new location within the same chromosome, producing effects that range from beneficial to harmful The Number of Entire Chromosomes May Also Change - Single choromosomes are lost or gained from cells entering or undergoing meiosis, resulting in a change of chromosome number - These changes occur through nondisjunction (the failure of homologous pairs to separ
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