Lecture 8.7
Transcription
Lecture 8.7
8.7 IN Why are males more likely to be red-green color blind or hemophiliacs? Heredity and Genetic Variation Patterns of Inheritance They are X-Linked Traits; there is no corresponding allele on the Y chromosome. Whatever is on the X chromosome is expressed in the phenotype of males. Abnormal Chromosomes Can Affect Patterns of Inheritance Section 8.7 Review • Humans - 23 pairs of chromosomes • Autosomes - pairs 1-22 • Sex chromosomes - pair 23; determine gender (sex) Male = XY Female = XX Developmental Errors Chromosomal abnormalities • Some are caused by abnormal numbers or types of chromosomes. 1. Monosomy - 45 chromosomes; missing an autosome; death or still born • Chromosomal abnormalities are present from birth (often at fertilization). 2. Trisomy - 47 chromosomes; an extra autosome; usually death or miscarriage; some exceptions (Trisomy 21 - Down syndrome) Syndrome = group of symptoms associated with a disease or condition 3. Polyploidy - more than two sets of homologous chromosomes; miscarriage (not fatal in plants - used in agriculture) Caused by nondisjunction 1 Nondisjunction • Failure of one synaptic pair of chromosomes to separate. • 1 cell with 1 extra chromosome, 1 cell with 1 missing chromosome. Chromosomal Abnormalities Down syndrome (trisomy 21): The result of an extra copy of chromosome 21. People with Down syndrome are 47, 21+. • Down syndrome affects 1:700 children • Distinct facial features; affects hands and sometimes heart • Varying degrees of mental retardation Polyploidy • Triploidy 69 As a woman’s age increases, the risk of a baby with Down syndrome increases. 2 Sex chromosome abnormalities • 23rd pair (XX and XY) • Caused by nondisjunction of sex chromosomes • Few severe defects (unlike autosomal abnormalities) Sex chromosome abnormalities A. Super female - XXX (47; XX + X); an XX egg is fertilized by an X sperm • Usually fertile; mildly retarded; normal appearance B. Klinefelter syndrome - XXY (47; XY + X); an XX egg is fertilized by a Y sperm • 1/1,000 males • Sterility; low IQ; small testes; breast enlargement 3 Sex chromosome abnormalities C. Turner syndrome - XO (45; X). A 0 gamete is fused with an X gamete • 1/2,000 females • Short stature, web of skin, sterile, poor development of breasts and 2° sex characteristics. Normal intelligence. Sex chromosome abnormalities D. Super male - XYY (47; XYY). A YY sperm fertilizes an X egg. Sex Chromosomes • 23rd pair (XX and XY) • Y chromosome nondisjunction • 20/1,000 males in prison Which are abnormal types? • Charles Manson • Genetic predisposition to violence? Chromosome Numbers in Cells • There must be at least two of each type of chromosome in every cell for the embryo to survive. • Exception - X chromosome (45; X0) • Extra sex chromosomes permit the fetus to develop. • Extra autosomes rarely permit fetal development. Barr Bodies - an inactivated X chromosome in all female cells • Dark stain in the nuclei • Only one X chromosome remains active in females. • Normal males do not have Barr bodies. Why? • Used in Olympic screening to determine athlete gender. 4 If a woman has a sex change operation will she still have Barr bodies in her cells after the operation? 1. How many Barr bodies does this female have? XXX (47; XXX) Abnormalities may result from altered chromosome structures. 1. Deletion - a piece of chromosome is missing. 2. How many Barr bodies does this female have? X0 (45; X) 2. Duplication - a missing piece of chromosome may attach to its partner. 3. Inversion - a section is reversed. 3. How many Barr bodies does a normal male have? (46; XY) 4. Translocation - a missing piece may attach to an unrelated chromosome. 8.7 OUT Which process occurs when homologous chromosomes do not separate during meiosis? a. X-linked inheritance b. incomplete dominance c. translocation d. nondisjunction 5 Read Chapter 8.7 Homework: 8.4-8.7 Parts B and C Due 3/23/15 Quiz: 8.4-8.7 3/24/15 6