Transcript 05. Chromosomal theory of heredity Genetics of sex

Theme: Chromosomal theory of
heredity. Genetics of sex
Lecturer: ass. prof. Tatyana Bihunyak
Questions:
1. Chromosome theory of inheritance by
Morgan.
2. Chromosome mapping.
3. Sex determination and sex
differentiation in humans.
4. Human sex-limited traits.
5. Human sex-influenced traits.
6. Sex-linked inheritance.
•Gene is a small segment of DNA that
codes the synthesis of a specific protein.
•Genes are located on the chromosomes.
•In human karyotype there are 46
chromosomes. In human diploid number
there are thousands of different genes.
•Many genes may be present on the same
chromosome. Such genes are said to be
linked, or to constitute a linkage group.
•Linked genes were discovered by great
American geneticist Thomas Hunt
Morgan of Columbia University in 1910.
Unlinked Genes
Linked Genes
Thomas Hunt
Morgan (1866-1945)
Nobel prize in 1933 for
his research on the fly
Drosophila in linkage and
crossing-over, which he
used to map the linear
arrangement of genes
along the chromosome.
T. H. Morgan studied chromosomes
of Drosophila melanogaster (fruit-fly)
The fruit-fly was selected because 1) it breeds rapidly, attaining
maturity in twelve days; 2) 30 generations can be bred in one
year; 3) it has only eight chromosomes.
Drosophila melanogaster has giant polytene chromosomes in
salivatory glands. Polytene chromosomes have numerous (5121024) chromonemes as result from repeated replication of DNA
without separation into daughter chromosomes.
Chromosomes of Drosophila carry as many as 2.500 genes.
L = long wings
l = short wings
G = gray body
g = black body
Autosomal Linkage. Dihybrid Testcross
4 types of gametes female are produced: GL and gl are
the noncrossovers (or “parentals”), most common
gametes; Gl and gL are the crossovers, formed as result
of crossing over.
In F2 there are two kinds of offspring: GgLl and ggll are
nonrecombinants (noncrossovers), they were formed
with noncrossover gametes; Ggll and ggLl are
recombinants (crossovers), they were formed with
crossover gametes.
F2:
offsprings
% of offsprings
Gl
Noncrossover gamete GL GgLl nonrecombinants
Noncrossover gamete gl
ggll
41.5
41.5
Crossover gamete gL
Crossover gamete Gl
8.5
8.5
ggLl recombinants
Ggll
F2 :
41.5%
8.5%
8.5%
41.5%
Distance between the linked genes
is measured in centimorgans (cM)
or map units.
1 cM = 1 map unit = 1% of
crossing over = 1% crossover
gametes = 1% recombinants
Crossing-over
Linkage between genes on the same
chromosome
1) complete linkage - when genes stay
together at a very short distance on the
chromosome; person with complete linked
genes can form only noncrossover gametes.
2) incomplete linkage - when genes stay
together at a far apart (under 50 cM); person
with incomplete linked genes can produce
crossover and noncrossover gametes,
because during meiosis crossing-over takes
place.
Incomplete Linkage
• Crossing over occurs primarily between non-sister chromatids
• One crossover will yield 2 recombinant and 2 nonrecombinant chromatids
• One crossover will yield 2 crossover and 2 noncrossover gametes
Q: if crossing over occurred once in each
of 10 meioses how many crossover
gametes would result?
10 meioses  40 gametes
 (1/2)(40)
= 20 crossover gametes
Chromosome theory of linkage:
1) Genes lie in a linear order on the chromosomes. The
position of a gene on a chromosome is locus.
2) Genes located on the same chromosome are linked
or constitute a linkage group. The number of linkage
groups is exactly the number of chromosome pairs in the
organism.
3) Linkage between two genes can be interrupted by
crossing-over (alleles exchanges between homologous
chromosomes during meiosis).
4) The distance between the linked genes on the
chromosome determines the strength of linkage. Linkage
strength between two genes turn out to the distance
between them.
Order of genes on the 21 human chromosome
•Chromosome maps are diagrams of order of
genes and distance between them on chromosomes.
•Crossover frequency can be used to determine the
order of genes on the chromosome.
Chromosome map of Drosophila
True or False?
1.____Genes of wing length and body
color are linked in Drosophila
2. ____ Crossover frequency can be used
to determine the order of genes on the
chromosome
3. ____ Person with incomplete linked
genes can forms only noncrossover
gametes
4._____ Human has 23 linkage groups
5. _____ Gene map units are expressed as
centimendels
The Sex Chromosomes
Discovered in late 1800s
Mammals, fruit flies
XX is female, XY is male
Human X and Y
chromosomes function as
homologues during meiosis
Sex determination in humans
P: ♀ 44 A + XX  ♂ 44A + XY
Gametes: 22A + X
22A + X; 22A + Y
F1: 44 A + XX; 44A + XY.
In human being sex inherits as Mendelian Trait.
•If fertilization is by an X-bearing sperm, the
resulting zygote will be XX and will develop into a
female.
•If fertilization is by a Y-bearing sperm, the
resulting zygote will be XY and will develop into
male.
Sex determination in humans
The sex of the offspring is determined
by the kind of sperm that will fertilize
an egg.
Sex Determination
eggs
sperm
X
Y
X
X
Female germ cell
Male germ cell
X
X
X
XX
XX
Y
XY
XY
sex chromosome combinations possible
in new individual
Sex determination in humans
Ratio of Sexes in Human population
The stages
of Human
Development
Male
Female
Zygotes
114-135
100
At birth (newborns)
106
100
10 years
100
100
50 years
85
100
80 years
50
100
Sex-determination in organisms
Type of
Sexdetermi
nation
XY
Organisms
Man (Homo
sapiens),
Mammals, some
fishes, flies,
Nemathelminthes
Heterochromosomes of
Somatic cells
Gametes
females
males
eggs
sperms
XX
XY
X
X and
Y
Heterogametic
male
XY
Birds, butterflies,
reptiles
XY
XX
X and
Y
X
XO
Grasshoppers
(Tettigoniidae)
XX
XO
X
X and
O
male
Clothes moth,
common lizard
(Lacerta vivipara)
XO
XX
X and
O
X
female
XO
Homogametic sex produces one type of
heterogametic sex produces two types of gametes.
female
gametes;
Other Sex Determination
Other Sex Determination
• The Y chromosome sometimes
does not dictate its maleness
• XY fruit fly and absence of a
second X is male
• XXY fruit fly is female
Environmental
Sex Determination
• Sex may be determined after fertilization
• Determined by temperature during early
embryonic development
– Turtles produce more females at a higher
temperature
– Alligators and many lizards produce more
males at a higher temperature
•Sex differentiation in humans takes place
during embryonic development.
•Gene Sry (for sex determining region of the
Y chromosome) that directs development of
gonad to testis and secreting of androgens is
presence on the short arm of the Ychromosome.
•If it is absence, the gonad becomes an ovary
and female structures (uterus, Fallopian
tubes) develop.
Sry on
Duct system in the early embryo that develops into a
male or female reproductive system.
The Sex Chromosomes
The X Chromosome
• Carries more than 2,300 genes
• Most genes deal with nonsexual
traits
• Genes on X chromosome can be
expressed in both males and
females
The Sex Chromosomes
The Y Chromosome
• Fewer than two dozen genes
identified
• One is the master gene for male
sex determination
• SRY gene (Sex-determining
region of Y)
• SRY present, testes form
• SRY absent, ovaries form
Hermaphroditism – a state characterized
by the presence of both male and female
gonadal tissue.
1) True hermaphroditism is caused by abnormal
differentiation of the gonads, with the presence
of both ovarian and testicular tissue and
genitalia.
2) Male pseudohermaphroditism – if only
testicular tissue is present, but there are some
female morphological characteristics.
3) Female pseudohermaphroditism – if only
ovarian tissue is present, but there are some
male morphological characteristics.
Testicular feminization syndrome
•this condition was described by
Morris (1953)
•karyotype: 46, XY;
•female external development
(phenotype);
•presence of testes;
•absence of uterus and tubes;
•resistance to testosterone;
•frequency: 1/40 000 births
•Sex-limited traits - affect a body
structure or function and they are present in
only one gender.
•The gene that controls such trait is
autosomal.
•The sex hormones influence the activity of
this gene.
•The traits are generally associated with
primary
or
secondary
sexual
characteristics, and thus are expressed only
in the gender which utilizes those
characteristics.
Sex-limited traits
• For example, there are genes which
influence how much milk a lactating
mother produces when she’s nursing a
baby. These genes are carried by both
males and females, but only females
ever express them.
• Beard growth and breast size are sexlimited traits too.
•In a sex-influenced trait the difference is in the
ways the two genders express the genes.
•Allele is dominant in one sex but recessive in the
other.
•Hormonal differences can cause this difference in
expression.
For example, a gene for hair growth pattern has two
alleles, one that produces hair all over the head and
another that causes pattern baldness. The baldness
allele is dominant (A) in males but recessive (a) in
females, which is why more men than women are
bald. A heterozygous male (Aa) is bald, but a
heterozygous female is not. The genotype of a bald
women is aa (many develop bald spots or has
receding hairlines).
Male pattern baldness
• The products of these genes are
highly influenced by the
hormone testosterone.
• In the presence of high levels of
testosterone, the baldness allele
has a very powerful influence.
• In the presence of low levels of
testosterone, this allele is quite
ineffectual.
Figure 1 : aa
Figure 2 : AA or Aa
Sex-Linked Genes
Is there a special pattern of inheritance
for genes located on the X chromosome
or the Y chromosome?
Because these chromosomes determine sex,
genes located on them are said to be sexlinked genes
Many sex-linked genes are found on the X
chromosome
More than 100 sex-linked genetic
disorders have now been mapped to the X
chromosome
The human Y chromosome is much smaller
than the X chromosome and appears to
contain only a few genes
Sex-linked Traits
on Sex Chromosomes
Hemophilia
XX chromosome - female
Xy chromosome - male
Hemophilia (absence of clotting factors VIII or IX, blood
fails to coagulate; coagulates very slowly after an external
or internal injury) is a sex-linked recessive disorder.
XN
y
XN XN XN XN y
Xn
XN Xn
Xn y
N = normal
n = Hemophilia
Royal Hemophilia Pedigree
Colorblindness
XN
y
XN XN XN XN y
Xn
XN Xn
Xn y
N = normal
n = Colorblind
XN XN
XN Xn
XN y
Enamel hypoplasia (hereditary defect that cause
holes and cracks to appear around the crowns of
the teeth) is sex-linked dominant trait
Holandric genes are on the Y chromosome
Hairy pinnae (hairy ears) – Y-linked trait
Y-linked Ear-Hair
X
X
X
Yh
XX
XX
X Yh
Yh = Ear Hair
Q: A normal sighted woman get married with a
man with color blindness. What is the
probability that their children will have normal
sight and color blindness ?
Solution: XH – normal, Xh – color blindness
P:
♀ XHXH

♂ XhY
Gametes: XH
Xh, Y
F1:
XHXh; XHY
Answers: 100 % of children will be normal
True or False?
1. ____ In human being sex inherits as
Mendelian trait
2. ____ Testicular feminization syndrome is
female pseudohermaphroditism
3. ____ Beard growth is sex-limited trait
4. ____Hemophilia is sex-linked recessive
disorder
5. _____Homogametic sex produces two types
of gametes
Thank you for attention !