Lec12F08Complete - Biology 1A Lecture 12 Complete 1 Chapter...

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Biology 1A Lecture 12 Complete 1 Chapter 15: The Chromosomal Basis of Inheritance The Chromosomal Basis of Sex In humans and other animals, there is a chromosomal basis of sex determination An organism’s sex is an inherited phenotypic character determined by the presence or absence of certain chromosomes In humans and other mammals, there are two varieties of sex chromosomes, X and Y Other animals have different methods of sex determination Fig. 15-9 Inheritance of Sex-Linked Genes The sex chromosomes have genes for many characters unrelated to sex A gene located on either sex chromosome is called a sex-linked gene Sex-linked genes follow specific patterns of inheritance Fig. 15-10 X Chromosome Some disorders caused by recessive alleles on the X chromosome in humans: Color blindness Duchenne muscular dystrophy Hemophilia X inactivation in Female Mammals In mammalian females, one of the two X chromosomes in each cell is randomly inactivated during embryonic development (Barr body) If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character Fig. 15-11 Genetic disorders due to chromosomal alterations Large-scale chromosomal alterations often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disorders Abnormal Chromosome Number In nondisjunction , pairs of homologous chromosomes do not separate normally during meiosis As a result, one gamete receives two of the same type of chromosome, and another gamete receives no copy Aneuploidy results from the fertilization of gametes in which nondisjunction occurred. Offspring with this condition have an abnormal number of a particular chromosome Fig. 15-12 Aneuploidy A trisomic zygote has three copies of a particular chromosome A monosomic zygote has only one copy of a particular chromosome Polyploidy is a condition in which an organism has more than two complete sets of chromosomes
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Biology 1A Lecture 12 Complete 2 Alterations of Chromosome Structure Breakage of a chromosome can lead to four types of changes in chromosome structure: Deletion removes a chromosomal segment Duplication repeats a segment Inversion reverses a segment within a chromosome Translocation moves a segment from one chromosome to another Fig. 15-14 Human Disorders Due to Chromosomal Alterations Alterations of chromosome number and structure are associated with some serious disorders Some types of aneuploidy appear to upset the genetic balance less than others, resulting in individuals surviving to birth and beyond These surviving individuals have a set of symptoms, or syndrome, characteristic of the type of aneuploidy Down Syndrome Down syndrome is an aneuploid condition that results from three copies of chromosome 21 It affects about one out of every 700 children born in the United States The frequency of Down syndrome increases with the age of the mother, a correlation that has not been
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This note was uploaded on 09/16/2010 for the course BIO 1A taught by Professor Mccray during the Fall '09 term at Merritt College.

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Lec12F08Complete - Biology 1A Lecture 12 Complete 1 Chapter...

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