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Chapter 12 Genes, Chromosomes, Human Genetics.docx

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BIOL 1500
Tamara Kelly

Heena Loomba 1 Chapter 12: Genes, Chromosomes, and Human Genetics 12. 2 Sex-Linked Genes  Sex-linked genes: genes located on sex chromosomes, they are inherited differently in males and females  Autosome: chromosomes other than sex chromosomes, the genes have the same patterns of inheritance in both sexes  Females have two copies of the X chromosome, a fully homologous XX pair  Males have one X chromosome and one Y chromosome  The Y chromosome has a short region of homology with the X chromosome that allows them to pair during meiosis  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 cell, the combination produces an XY male  SRY gene carried on the Y chromosome is the “master switch” that directs developments toward maleness during embryonic development  After 6-8 weeks of embryonic development, the SRY gene becomes active in XY embryos, producing a protein that regulates the expression of other genes which stimulates the structures to develop as testes, penis, and scrotum  For the first month of embryonic development, the structures that give rise to reproductive organs and tissues are the same in XX or XY embryos  In XX embryos, development proceed toward female reproductive structures because rudimentary male structures degenerate because the hormones released by the developing XY embryos are not present  Since males and females have different sets of sex chromosomes, the genes carried on these chromosomes can be inherited by sex linkage  Sex linkage arises from two differences between males and females:  Males have one X chromosome, so one allele for each gene on this chromosome. While females have two X chromosomes, so two alleles for all genes on the X chromosome  Males have one copy of Y chromosome and one allele for each gene on this chromosome. While females do not have a Y chromosome, so no Y alleles.  Sex linkage was discovered when Morgan crossed a white-eyed male fly with a true-breeding female with red eyes. He observed that all the F1 flies had red eyes. Next, he allowed the F1 flies to interbreed. He found that all the F2 females had red eyes and half of the F2 males had red eyes and half had white eyes Heena Loomba 2 Chapter 12: Genes, Chromosomes, and Human Genetics  Reciprocal cross was always made between a white-eyed female fly with a red-eyed male fly. The F1 males all had white eyes and the females all had red eyes. In the F2 generation, the males and females flies showed a 1:1 ratio of red eyes to white eyes  In humans (and fruit flies) sex-linked recessive traits are more frequent in 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 each parent to express the trait  Although females have twice as many copies of genes carried on the X chromosomes, they do not require twice as much of the products of those genes. In most body cells, females inactivate most of the genes on one X chromosome  Inactivation occurs by a condensation process that folds and packs the chromatin of one of the two X chromosomes into a tightly coiled state  Inactivation occurs during embryonic development. Once of the X chromosomes is inactivated in a cell, that same X is inactivated in all descendants of the cell 12.3 Chromosomal Alterations That Affect Inheritance  Chromosomes can be altered by breaks in the DNA  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. In a receiving homologue, the alleles in the inserted fragment are added to the ones already there  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  To be inherited, chromosomal alterations must occur or be included in cells of the germ line leading to development of eggs or sperm  At times, whole, single chromosomes are lost or gained from cells under
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