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