Sex-Linked Inheritance
The pair of chromosomes that determine the
sex in humans is called the sex chromosomes,
XX for female and XY for male. The nonsex
chromosomes are called autosomes
Drill
1. The pair of chromosomes that determine the
sex in humans is called the sex chromosomes,
XX for female and XY for male. What are the
nonsex chromosomes called?
Answer: AUTOSOMES
Drill#5 Use a Punnett Square to…
• Determine the chance of having a boy…
Use a Punnett Square to…
• What is the chance of having a boy?
X
X
Y
X
Use a Punnett Square to…
• What is the chance of having a boy? 50%
X
Y
X
X
XX
XX
XY
XY
Drill
2. In cats, curled ears (Cu) result from an allele that is
dominant over an allele for normal ears (cu). Black
color results from an allele (B) that is dominant over an
allele for gray (b). A gray cat homozygous for curled
ears is mated with a homozygous black cat with normal
ears. All the F1 cats are black and have curled ears.
a. If two of the F1 cats mate, what phenotypes and
proportions are expected in the F2?
b. An F1 cat mates with a stray cat that is gray and has
normal ears. What phenotypes and proportions of
offspring are expected from this cross?
Preparation of Karyotype
• Cells from blood or amniotic
fluid are grown in a culture
dish.
• Cell division is then stopped in
metaphase with colchicine, a
protein that inhibits mitotic
spindle from forming
• Cells are centrifuged to release
the chromosomes.
• Chromosomes are stained,
photographed, and grouped by
size and banding patterns
Karyotype
Karyotype
Sex Chromosomes
• The Y chromosome is acrocentric and much
smaller than the X chromosome.
Sex Determination in Humans
• Humans have XX-XY sex determination.
• The presence of a gene (SRY) on the y
chromosome codes for maleness.
• Certain phenotypes result from abnormal
numbers of sex chromosomes which arise
when sex chromosomes do not segregate
properly in meiosis or mitosis.
• Studying these abnormalities illustrates the
importance of the Y chromosomes.
Turner Syndrome
Turner Syndrome
Klinefelter’s syndrome
Klinefelter Syndrome
Poly – X Female
Poly – X Female
• Triplo X or poly X females occur in about 1 in
1000 female births
• No distinctive features other than being tall
and thin
• Many are fertile
• Slightly greater incidence of retardation as the
number of X chromosomes increase.
Nondisjunction
Role of Sex Chromosomes
• The x chromosome contains genetic information essential
for both sexes; at least one copy of an X-chromosome is
required for human development
• The male determining gene is located on the y
chromosome. A single copy of this chromosome even in
the presence of several X chromosomes produces a male
phenotype.
• The absence of the Y chromosome results in a female
phenotype.
• A female needs at least 2 copies of the X chromosome to
be fertile.
• Additional copies of the X chromosome may upset normal
development of both males and females producing physical
and mental problems.
Sex linked inheritance
• Characteristics determined by genes on the
sex chromosomes are called sex linked
characteristics.
• Diploid females have 2 alleles at each x-linked
locus
• Diploid males possess a single allele on 1 xlinked locus.
Sex linked inheritance
• Females inherit X-linked alleles from both
parents
• Males inherit a single X-linked allele from their
mother.
Sex linked inheritance
Sex-Linked Inheritance
• Color blindedness
• Hemophilia
Summarizing Questions
1. Colorblindness in humans is most commonly
due to an X-linked recessive allele. Betty has
normal vision, but her mother is colorblind.
Bill is color blind. If Bill and Betty marry and
have a child, what is the probability that the
child will be color blind?
Summarizing Question
2. Hemophilia results from a recessive X-linked
gene. Jill has hemophilia. She marries Bill,
who has normal blood clotting. What
proportion of their children are expected to
have hemophilia?
Summarizing Question
3. Joe has hemophilia, an X-linked recessive
disease. Could Joe have inherited the gene for
the disease from the following persons?
a. his mother’s mother
b. his mother’s father
c. his father’s mother
d. his father’s father
Home Assignment
• Due Monday, January 9th. Use Chapters 10 and 11
or the internet to answer the following questions.
1. Describe how one DNA strand directs the
synthesis of another.
2. How does a cell copy both strands of DNA
simultaneously?
3. Draw a strand of DNA, label the phosphate
groups, the ribose, the nitrogen bases (ATCG),
the 3’ end and the 5’ end.
4. What are Okazaki fragments?
5. What is the ultimate source of genetic
diversity?
6. There are two types of mutations, point and
chromosomal. List 3 types of point mutations
and 4 types of chromosomal mutations.
7. Explain Francis Crick’s “Central Dogma”.
Define transcription and translation.