Chapter 171 the x and y chromosomes an exception

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Chapter 17.1 The X and Y Chromosomes An exception occurs in the X and Y chromosomes The sex of an individual is determined by a distinctive pair of unmatched chromosomes known as the sex chromosomes, which are usually designated as the X chromosome and the Y chromosome. Other chromosomes are known as autosomes. Females have two X chromosomes and males only have one. Segregation of sex chromosomes predicts a 1:1 ratio of females to males Females are less likely to survive from conception to birth Males are less likely to survive from birth to reproductive maturity 17.2 Inheritance of Genes in the X chromosome Genes in the X chromosome are called X-linked genes. X-linked inheritance was discovered through studies of male fruit flies with white eyes The most common phenotype is called the wild type Genes in the X chromosome exhibit a "crisscross" inheritance pattern The Y chromosome does not carry an allele of the gene Drosophilia so the recessive mutation is expressed in the males phenotype. The Punnett square illustrates two important principles governing the inheritance of X-linked genes: Bio 281 Lecture Page 31
The Punnett square illustrates two important principles governing the inheritance of X-linked genes: The phenotypes of XX indicate a male transmits his X only to his daughters. 1. The phenotypes of XY indicate a male inherits his X chromosome from his mother. In this case, the mother carries the nonmutant gene. 2. These principles underlie the crisscross inheritance: An X-chromosome present in a male in one generation is transmitted to a female in the next and in the generation after that transmitted back to a male. X-Linkage provided the first experimental evidence that genes are in chromosomes Bridges crossed white eyed females with red eye males. The progeny consisted of red eyed females and white eyed males. Chromosomes sometimes fail to separate normally in meiosis, a process known as nondisjunction Genes in the X chromosome show characteristic patterns in human pedigrees The features of X-linked inheritance can be seen in human pedigrees for traits due to an X-linked recessive mutation. The key features of the inheritance of traits due to X-linked alleles are: Affected individuals are usually males Affect males have unaffected sons A female whose father is affected can have affected sons because such a female is a carrier 17.3 Genetic Linkage and Recombination Nearby genes in the same chromosome show linkage Genes that are sufficiently close together in the same chromosome are said to be linked. Bio 281 Lecture Page 32
The male progeny fall into two groups, one group represented by large numbers of progeny derives from maternal gametes, these are called nonrecombinants. The other group of male progeny is called recominants as they resulted from a crossover, which takes place in prophase I. The frequency of recombination is a measure of the genetic distance between linked genes. When two genes are on separate chromosomes, a ratio of 1:1:1:1 is expected for nonrecombinant and

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