Chapters 4 & 6_post

Genetics: A Conceptual Approach

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Chapter 4 Sex Determination and Sex-Linked Characteristics Dr. Ed Otto George Mason University
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Key objectives Review different ways that sex is determined in different organisms Examine the pattern of inheritance for sex- Discuss mechanisms for dosage compensation
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Sex Determination
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Sexual Reproduction Sexual reproduction is the formation of offspring that are genetically distinct from their parents Sexual reproduction includes 2 processes that lead to an alternation of haploid and diploid cells 1)Meiosis produces haploid gametes 1)Fertilization produces diploid zygotes
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Sex determination A fundamental difference between males and females is gamete size: males produce smaller gametes than females The term sex refers to sexual phenotype: male and female The mechanism by which sex is established is called sex determination Sex is determined by a number of different mechanisms, including chromosomal, genic, and environmental systems.
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I. Chromosomal Sex-determining Systems Sex in many organisms is determined by a pair of Sex Chromosomes , that differ in males and females. The non-sex chromosomes, called autosomes, are the same for males and females
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Human Karyotype Humans have 23 pairs of chromosomes: 22 pairs of autosomes & 1 pair of sex chromosomes
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XX-XY Sex Determination Male: - Has one X chromosome & one Y chromosome (XY) - X and Y separate during meiosis: ½ sperm receive X chromosome, ½ sperm receive Y chromosome - Because male makes 2 different kinds of gametes, male is referred to as the heterogametic sex Female: - Has two X chromosomes (XX) - All egg cells produced receive a single X chromosome (female is homogametic sex) Random fertilization results in production of equal numbers of males (XY) and females (XX)
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XX-XY Sex Determination X and Y chromosomes do not share extensive homology (i.e. most genes are different) X and Y pair during meiosis because there are small regions of homology called the pseudoautosomal regions XX-XY system for sexual determination is found in all mammals; in some plants, insects, reptiles
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XX-XO Sex Determination Male: - Has one X chromosome (XO) - “O” signifies the absence of a sex chromosome - Heterogametic sex: ½ sperm receive X chromosome, ½ sperm receive no sex chromosome Female: - Has two X chromosomes (XX) - Homogametic sex (all egg cells produced receive a single X chromosome) System is found in grasshoppers and some beetles
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II. Genic Sex Determining Systems In some organisms sex is genetically determined, but there is no visible difference in the chromosomes of males and females These organisms have genic sex determination: genotypes at one or more loci determine sex Important to understand that even in chromosomal sex-determining systems, sex is determined by individual genes So both genic and chromosomal sex determination are controlled by genes; the difference is that with chromosomal sex determination, the chromosomes that carry those genes look different in males and females
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III. Environmental Sex Determining Systems
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This note was uploaded on 05/06/2010 for the course BIO 311 taught by Professor Otto during the Spring '10 term at George Mason.

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Chapters 4 - Chapter 4 Sex Determination and Sex-Linked Characteristics Dr Ed Otto George Mason University Key objectives Review different ways

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