Human genetics

2. GENETICS
TERMINOLOGIES AND BASIC CONCEPTS
Sexes
Are defined by mutual incompatibility between the same mating type Mutual
incompatibilities between the same mating
Type.
Anisogamy (i.e. the production of large and small gametes).
Female gametes (eggs): Few, large, immobile, include resources.
Male gametes (sperm): Numerous, small, motile.
For most diploid eukaryotes, sexual reproduction is the only mechanism resulting
in new members of a species.
Meiosis in the sexual organs of parents produces haploid gametes, which unite
during fertilization to restore the diploid phenotype in the offspring.
Dioecious
The majority of animals exist as one of two sexes, with males producing sperm and
females producingeggs.
OR
Unisexual=dioecious=gonochoric: Refer to an individual who possesses only male
or female sexual organs, notboth.
Monoecious
All individuals of a species look alike and produce eggs and sperm. Common in
invertebrates. Individuals with both gonads are hermaphrodites.
OR
Bisexual=monoecious=hermaphroditic: Refer to individuals who possess both
male and female reproductive organs.
Intersex:

3. GENETICS
Usually reserved for individuals of intermediate or indeterminate sexual
differentiation. This state is not normal and the affected individuals are often sterile
Sexual dimorphism:
In many species, the differences between sexes are not limited to the reproductive
organs, but extend to other characteristics such as size, ornaments, and bodyshape.
For most organisms, sexual reproduction requires some form ofsexual
differentiation.
In higher forms of life, this is manifested as phenotypic dimorphism between males
and females of a species.
Traditionally, the symbol ♂ designates male
And the symbol ♀ designates female
Primary Sex Characteristics
Refers to the gonads (ovaries and testes) and associated structures i.e. sex organs.
SecondarySex Characteristics:
Refer to the overall appearance of the organism, external genitalia and mammary
glands
Homogametic:
Gender of an organism due to presence of two of the same sex chromosome. (E.g.
XX)
Heterogametic:
Gender of an organism due to presence of two different sex chromosomes (E.g.
XY) in mammals, females are the homogametic sex, and males are the
heterogametic sex

4. GENETICS
Lyon hypothesis
(Proposed byMary Lyon)
Genes in Dosage compensation in mammalian females.
Random inactivation of one X chromosome in females equalizes the activity of X-
linked males andfemales.
Barr body .A densely staining mass in the somatic nuclei of mammalian females
an inactivated X chromosome tightlycoiled.
Sex-linkedtraits
Are traits controlled by genes located on the sex chromosome?
Sex-influenced traits
Traits controlled by autosomal genes that are usually dominant in one sex but
recessive in the other sex.
Sex limitedtraits.
Genes that produce a phenotype in only one sex.
Example: Precocious puberty in heterozygous males but not in heterozygous
females.

5. GENETICS
Dosagecompensation
Dosage compensation is the mechanism that keeps females (XX) from expressing
twice as much of X-chromosome a mechanism that regulates the expression of sex-
linked gene products. OR chromosome genes as males (XY), who have only one X
chromosome.
Both sexes are rendered roughly equal by inactivation of one X chromosome in
females.
Sex differentiation:
Subsequent events that ultimately produce either the male or female sexual
phenotype.
Sex determination
Definition
Sex determination is the natural event by which an individual of a dioecious
species becomes male or female. There are two main mechanisms for sex
determination. Which are environmental and genetic determination
respectively. Or.
Mechanisms or Developmental decisions that occur during embryogenesis
HISTORY.
Aristotle (ca. 335B.C.):
Sex is determined by “the heat of the male partner duringintercourse”
Vesalius (~ 1543).
Held the same view
During the 1600s and1700s:
Females were seen as producing eggs that could transmit parental traits, and the
physiology of sex traits, began to be studied.

6. GENETICS
Until 20th century.
The environment –temperature and nutrients, in particular –was believed to be
important.
The Factors favoring the storage of energy and nutrients predisposed one to have
female offspring, whereas factors favoring utilization of energy and nutrients
influenced one to have male offspring. (Geddes and Thomson,1890).
In 1905:
Establishment correlation (in insects) of the female sex with XX sex chromosomes
and the male sex with XY or XO chromosomes. (Stevens;Wilson).
•A specific nuclear component is responsible for directing the development of the
sexual phenotype
•Evidence accumulated that sex determination occurs by nuclear inheritance rather
than by environmental influence.
Nowadays:
Both environmental and internal of sex determination mechanisms can operate in
different species.
Mechanism of sex determination:-
Sexual reproduction is the formation of offspring that are genetically distinct from
their parents; most often, two parents contribute genes to their offspring and the
genes are assorted into new combinations through meiosis.
Among most eukaryotes, sexual reproduction consists of two processes that lead to
an alternation of haploid and diploid cells: meiosis produces haploid gametes
(spores in plants), and fertilization produces diploid zygotes.
The term sex refers to sexual phenotype. Most organ-isms have only two sexual
phenotypes: male andfemale.
The fundamental difference between males and females is gamete size: males
produce small gametes; females produce relatively larger gametes

7. GENETICS
The mechanism by which sex is established is termed sex determination. We
define the sex of an individual organism in reference to its phenotype. Sometimes
an individual organism has chromosomes or genes that are normally associated
with one sex but a morphology corresponding to the oppositesex.
Sex determination mechanisms include.
A. Chromosomal determination mechanism
B. Genetically controlled sex determination.
C. Hormonally controlled sex determination.
D. Environmentally controlled sex determination.
 ƒ Sex determination is associated with sex chromosomes that are
different between male and female individuals. Many specieshave
Chromosomal sex-determining systems:
XX –XY system
XX –XO system
And ZZ-ZW system.
 Genetic sex determination: sex is determined at fertilization by the
combination of genes that the zygote receives.
 Environmental sex determination: in some species, sex is
determined after fertilization by environmental factors (temperature,
population size, or sex of others).
 Hormonal determination refers to the extent of the body coordination
system and its effects.
Types
A) Chromosomal Sex-DeterminingSystems
 The chromosome theory of inheritance (states that genes are located on
chromosomes, which serve as vehicles for the segregation of genes in
meiosis.
 Definitive proof of this theory was provided by the discovery that the sex
of certain insects is determined by the presence or absence of particular
chromosomes.

8. GENETICS
 In 1891, Hermann Henking noticed a peculiar structure in the nuclei of
cells from male insects. Understanding neither its function nor its relation
to sex, he called this structure the X body.
 Later, Clarence E. McClung studied the X body in grasshoppers and
recognized that it was a chromosome.
 McClung called it the accessory chromosome, but it eventually became
known as the X chromosome, from Henking’s original designation.
 McClung observed that the cells of female grasshoppers had one more
chromosome than the number of chromosomes in the cells of male
grasshoppers, and he concluded that accessory chromosomes played a
role in sex determination.
 In 1905, Nettie Stevens and Edmund Wilson demonstrated that, in
grasshoppers and other insects, the cells of females have two X
chromosomes, whereas the cells of males have a singleX.
 In some insects, they counted the same number of chromosomes in the
cells of males and females but saw that one chromosome pair was
different.
 Two X chromosomes were found in female cells, whereas a single X
chromosome plus a smaller chromosome, which they called Y, was found
in male cells.
 Stevens and Wilson also showed that the X and Y chromosomes separate
into different cells in sperm formation; half of the sperm receive an X
chromosome and the other half receive aY.
 All egg cells produced by the female in meiosis receive one X
chromosome. A sperm containing a Y chromosome unites with an X-
bearing egg to produce an XY male, whereas a sperm containing an X
chromosome unites with an X-bearing egg to produce an XX female.
This distribution of X and Y chromosomes in sperm accounts for the
 1 : 1 sex ratio observed in most Dioecious organisms Because sex is
inherited like other genetically deter-mined characteristics, Stevens and
Wilson’s discovery that
Sex is associated with the inheritance of a particular chromo-some also
demonstrated that genes are on chromosomes.

9. GENETICS
 As Stevens and Wilson found for insects, sex in many organisms is
determined by a pair of chromosomes, the sex chromosomes,which
differ between males and female
Chromosomal determination is found in,
XX/X0 mechanism,
XX/XY mechanism,
ZW/ZZ mechanism,
B) Genetic SexDetermination
 In some plants, fungi, and protozoans, sex is genetically determined, but
there are no obvious differences in the chromosomes of males and
females there are no sex chromosomes. These organisms have genic sex
determination;
 Genotypes at one or more loci determine the sex of an individual plant,
fungus, or protozoan. It is important to understand that, even in
chromosomal sex-determining systems, sex is actually determined by
individual genes.
 For example, in mammals, a gene (SRY discussed later in this chapter)
located on the Y chromo-some determines the male phenotype. In both
genic sex determination and chromosomal sex determination, sex is
controlled by individual genes; the difference is that, with chromosomal
sex determination, the chromosomes also look different in males and
females.
C) Hormonal determination
Sex determination is not determined by hormones but they influences the
secondary characters which can be termed as differentiation.
All hormonally controlled sex development beginning at embryogenesis and
progressing into adulthood. This includes sex characteristics such as external
genitalia, breasts, body build, and behavior.

10. GENETICS
Sex controldevelopment of the brain hormones
Males and females of any species usually differ in complex behaviors such as
mating, parenting, and aggression. How does thisoccur?
For each sex hormone, there is a unique distribution of receptors throughout the
brain.
Androgen receptor is concentrated in areas that control aggression and mating.
Estrogen receptors are concentrated in areas that controlovulation.
Songbirds are a good example. Male birds attract females by singing, and the songs
are learned from older males. If male birds are castrated, the amount and quality of
their singing decreases.
If they are subsequently treated with testosterone, singing resumes. Specific areas
in the brain of male birds that are associated with singing are larger than in female
birds.
Interesting effects of sex hormones are seen in mammals that produce litters of
multiple offspring. The growing fetuses exchange sex hormones via the placental
circulation.
Females that develop between 2 males (2M females) are exposed to higher levels
of testosterone than siblings that develop next to 1 or no males.
These 2M females have masculinized genitals, have shorter reproductive cycles,
and are less attractive to males.
The converse effects are observed in males that develop next to 2 females. They
have smaller seminal vesicles and are less aggressive than 0F males
D) Environmental Sex Determination
 Genes have had a role in all of the examples of sex determination discussed
thus far. However, in a number of organ-isms, sex is determined fully or in
part by environmental
Factors.

11. GENETICS
 A fascinating example of environmental sex determination is seen in the
marine mollusk Crepidula fornicate,
 Slipper limpets live in stacks, one on top of another. Each limpet begins life as
a swimming larva. The first larva to settle on a solid, unoccupied substrate
develops into a female limpet. It then produces chemicals that attract other
larvae, which settle on top of it.
 These larvae develop into males, which then serve as mates for the limpet
Below.
 After a period of time, the males on top develop into females and, in turn,
attract additional larvae that settle on top of the stack, develop into males,
and serves.
 Environmental factors are also important in determining sex in many
reptiles. Although most snakes and lizards have sex chromosomes, the
sexual phenotype of many turtles, crocodiles, and alligators is affected
by temperature during embryonic development. In turtles, for example.
 Warm temperatures produce females during certain times of the year,
whereas cool temperatures produce males. In alligators, the reverseis
true.
Sex differentiation in mammals.
Sex differentiation is the Subsequent events that ultimately produce either the
single phenotype.
Primary sex differentiation: mammalian sex determination is controlled by the
SRY gene. SRY expression induces testes, lack of SRY results inovaries.
Secondary sex differentiation: refers to all hormonally controlled sex
development beginning at embryogenesis and progressing into adulthood. This
includes sex characteristics such as external genitalia, breasts, body build, and
behavior.
Most sex hormones are steroids derived from cholesterol. These include androgens
(testosterone and DHT) and female sex hormones (estrogen and progesterone).
The formation of male and female reproductive structures depends on:

12. GENETICS
• Gene action.
• Interactions within theembryo.
• Interactions with other embryos in theuterus.
• Interactions with the maternalenvironment.
The role of sex chromosomes.
The phenotypes associated with sex-chromosome anomalies allow us to make
several inferences about the role of sex chromosomes in human sex determination.
 Genetic sex (XX or XY) is determined by the type of sperm (X-bearing or
Y-bearing) that fertilizes the egg.
 Early gonads have potential to be either ovaries or testes for ~6 weeks
 Sex-determining region of the Y chromosome (SRY) is a gene producing a
protein which causes the middle of the neuter gonads to become testes.
 If testes develop, they begin to produce androgens like testosterone.
 If this gene is not present, the outside of the neuter gonads turn into ovaries.
Levels of differentiation.
The chromosomal sex of an individual (XX or XY) can differ from the phenotypic
sex
Sex of an individual is defined at three levels
Chromosomal sex or genetic sex. I.e. xx or xy
Gonadal sex i.e. Testis and ovaries.
Phenotypic sex i.e. male and female.

13. GENETICS
Sex Determination in Humans
Humans, like Drosophila, have XX-XY sex determination, but, in humans, the
presence of a gene (SRY) on the Y chromosomes determinesmaleness.
The phenotypes that result from abnormal numbers of sex chromosomes, which
arise when the sex chromosomes do not segregate properly in meiosis or mitosis,
illustrate the importance of the Y chromosomes in human sex determination.
Example.
Turner syndrome Persons who have Turner syndrome are female and often have
underdeveloped secondary sex characteristics. This syndrome is seen in 1 of 3000
female births.
Affected women are frequently short and have a low hairline, a relatively broad
chest, and folds of skin on the neck. Their intelligence is usually normal. Most
women who have Turner syndrome are sterile.
In 1959, Charles Ford used new techniques to study human chromosomes and
discovered that cells from a 14-year-old girl with Turner syndrome had only a
single X chromosome. This chromo-some complement is usually referred to as
XO.
There are no known cases in which a person is missing both X chromosomes, an
indication that at least one X chromosomes is necessary for human development.
Presumably embryos missing both Xs spontaneously abort in the early stages of
development.
The male-determining gene in humans
The Y chromosomes in humans and all other mammals is of paramount importance
in producing a male phenotype. However, scientists discovered a few rare XX males
whose cells apparently lack a Ychromosome.
For many years, these males presented an enigma: How could a male phenotype
exist without a Y chromosome? Close examination eventually revealed a small part
of the Y chromosome attached to anotherchromosome.

14. GENETICS
This finding indicates that it is not the entire Y chromosome that determines
maleness in humans; rather, it is a gene on the Y chromosome.
Early in development, all humans possess undifferentiated gonads and both male
and female reproductive ducts.
Then, about 6 weeks after fertilization, a gene on the Y chromosomes becomes
active. By an unknown mechanism, this gene causes the neutral gonads to develop
into testes, which begin to secrete two hormones: testosterone and Mullerian-
inhibiting substance.
Testosterone induces the development of male characteristics, and Mullerian-
inhibiting substance causes the degeneration of the female reproductive ducts.
In the absence of this male-determining gene, the neutral gonads become ovaries,
and female features develop.
The male-determining gene in humans, called the sex-determining region Y (SRY)
gene, was discovered in1990
This gene is found in XX males and is missing from XY females; it is also found
on the Y chromosomeof other mammals.
Definitive proof that SRY is the male-determining gene came when scientists
placed a copy of this gene into XX mice by means of genetic engineering. The XX
mice that received this gene, although sterile, developed into anatomical males.
The SRY gene encodes a protein called a transcription factor that stimulates sex
differentiation.

15. GENETICS
Sex linked inheritance.
It is revealed by the Mendel discovered from his crosses Sex linked inheritance is
defined as,
The characteristics that inherited from the parental generation to the progeny in
true breeding of pure lines regardless of the phenotype of parents.
Sex linked characteristics are always depend upon the sex chromosomes of parents
and the coordination frequencies of the genes respectively.
It is evidently shown among pea plants.
Explanation.
A major extension of these Mendelian principles is the pat-tern of inheritance
exhibited by sex-linked characteristics, characteristics determined by genes located
on the sex chromosomes.
Genes located on the autosomal Genes on the X chromosome determine X-linked
characteristics.
Genes on the Y chromosome determine Y-linked characteristics. Because the Y
chromosome of many organisms contains little geneticinformation,
Most sex-linked characteristics are X linked.
Males and females differ in their sex chromosomes; so the pattern of inheritance
for sex-linked characteristics differs from that exhibited by the
Chromosomes.
Sex-linked traits are traits controlled by genes located on the sex chromosome. For
example
Color blindness and hemophilia.

16. GENETICS
X linked inheritance.
The x-linked type sex inheritance is performed by those genes which are localized
in the non-homologous section of x chromosome and that have no corresponding
allele on Y-chromosome.
The x linked genes are commonly known as sex linked genes.
Y linked inheritance,
The y-linked type sex inheritance is performed by those genes which are localized
in the non-homologous section of Y-chromosome and that have no corresponding
allele on X-chromosome.
The x linked genes are commonly known as holandric genes (Greek. Complete
man)
XY_linked inheritance
The xy-linked type sex inheritance is performed by those genes which are localized
on the homologous section of both xand Y-chromosome.

17. GENETICS
Characteristics of sex linked inheritance.
The x linked genes exhibits following characteristic pattern ofinheritance.
The differential region of each chromosome contain genes that have no counterpart
on the other kind of chromosome which may be dominant or recessive.
Genes in male are homozygous.
The x-linked recessive genes show the following two more peculiar features:
crisscross pattern of inheritance (i.e., in crisscross inheritance, a X-linked recessive
gene is transmitted from P1 male parent (father) to F2 male progeny (grandson)
through its F1 heterozygous females (daughters), which are called carriers) and
different F1 and F2 results (ratios) in the reciprocal crosses.
The X-linked recessives can genes located on the autosomal Genes on the X
chromosome determine X-linked characteristics.
1) The X-linked recessive be detected in human pedigrees (also in Drosophila)
through the following clues: phenotype is usually found more frequently in
the male than in the female. This is because an affected female can result
only when both mother and father bear the X-linked recessive allele (e.g.
, 𝑋 𝐴 𝑋 𝑎 ∗ 𝑋 𝑎 𝑌), whereas an affected male can result when only the mother
carries the gene. Further if the recessive X-liked gene is very rare, almost all
observed cases will occur in male.
2) Usually the affected male characters is transferred to daughters. None any
son is affected.
3) Dominant x linked genes are frequently found infemales.
X-Linked White Eyes in Drosophila
The first person to explain sex-linked inheritance was American biologist Thomas
Hunt Morgan.
He began his career as an embryologist, but the discovery of Mendel’s principles
inspired him to begin conducting genetic experiments, initially on mice and rats.

18. GENETICS
In 1909, Morgan switched to Drosophila melanogaster; a year later, he discovered
among the flies of his laboratory colony a single male that possessed white eyes, in
stark contrast with the red eyes of normal fruit flies.
This fly had a tremendous effect on the future of genetics and on Morgan’s career
as a biologist.
To investigate the inheritance of the white-eyed characteristic in fruit flies, Morgan
systematically carried out a series of genetic crosses.
First, he crossed pure-breeding, red-eyed females with his white-eyed male,
producing F1 progeny of which all had red eyes (In fact, Morgan found 3 white-
eyed males among the 1237 progeny, but he assumed that the white eyes were due
to new mutations.)
Morgan’s results from this initial cross were consistent with Mendel’s principles:
a cross between a homozygous dominant individual and a homozygous recessive
individual produces heterozygous offspring exhibiting the dominant trait.
His results suggested that white eyes are a simple recessive trait. However, when
Morgan crossed the F1flies with one another, he found that all the female F2 flies’
possessed red eyes but that half the male F2 flies had red eyes and the other half
had white eyes. This finding was clearly not the expected result for a simple
recessive trait, which should appear in ¼ of both male and female F2offspring.
To explain this unexpected result, Morgan proposed that the locus affecting eye
color is on the X chromosome (i.e., eye color is X linked). He recognized that the
eye-color allelesare present only on the X chromosome; no homologous allele is
present on the Y chromosome. Because the cells of females possess two X
chromosomes, females can be homozygous or heterozygous for the eye-color
alleles.
The cells of males, on the other hand, possess only a single X chromosome and can
carry only a single eye-colorallele.
Males therefore cannot be either homozygous or heterozygous but are said to be
hemizygous for X-linked loci. After a series of experiments morgon realized that
white eyes in flies are x linked.

19. GENETICS
X. linked recessive disorder Red Green Colour Blindness
• Inability to distinguish between red and green
• A red green colour blind person does not see the number 29 on the
right
• In humans normal vision (C) is completely dominant to red-green
colour blindness (c).

20. GENETICS
Genetics of Colour Blindness
• Normal visionC
• Red-green colour blindness c
• These are the alleles are sex-linked because...
• Heterozygous females are called carriers (Cc)
Although they are unaffected themselves there is a 1 in 2 chance
(50%) chance that they will pass the allele on to each of the
offspring.
Genotype Phenotype
C C
X X
Female with normal colour vision
C c
X X
Female (carrier) with normal colour vision.
c c
X X
Female with colour blindness (very rare e.g. 0.5%)
C
X Y
Male with normal colour vision
c
X Y
Male with colour blindness more common (8%)

21. GENETICS
Y-Linked Characteristics
Y-linked traits exhibit a distinct pattern of inheritance and are present only in males
because only males possess a Y chromosome. All male offspring of a male with a
Y-linked trait will display the trait, because every male inherits the Y chromosome
from his father.
Characteristics of the human Ychromosome.
The genetic sequence of most of the human Y chromosome has nowbeen
determined as part of the Human Genome Project
This work reveals that about two-thirds of the Y chromosome consists of short
DNA sequences that are repeated many times and contain no active genes.
The other third consists of just a few genes. Only about 350 genes have been
identified on the human Y chromosome, compared with thousands on most
chromosomes, and only about half of those identified on the Y chromosome
encode proteins.
The function of most Y-linked genes is poorly understood many appear to
influence male sexual development and fertility. Some are expressed throughout
the body, but many are expressed predominately or exclusively in thetestes.
Although the Y chromosome has relatively few genes, recent research in
Drosophila suggests that the Y chromosome car-ries genetic elements that affect
the expression of numerous genes on autosomal and X chromosome.
Inheritance of y linkedgenes;
The genes on the non-homologous chromosome is inherited directly from male to
male progeny.
For example in humans (ichthyosis hystrix gravis hypertrichosis) excessive
development of hair on pinna of ear are transmitted as y linked.
In some fish their body pigments are transmitted as y linked.

22. GENETICS
Inheritance of X Y linkedgenes
The genes which occur in homologous section x and y chromosomes have
inherited like autosomal genes. The X Y linked genes are partially or incompletely
sex linked because some time, the crossing over may occur in the section of both x
and y chromosomes.
In human many diseases are x ylinked.
E.g. Retinitis pigmentosa and complete colorblindness.
Nondisjunction and the Chromosome Theory of Inheritance
When Morgan crossed his original white-eyed male with homozygous red-eyed
females, all 1237 of the progeny had red eyes, except for 3 white-eyed males. As
already mentioned, Morgan attributed these white-eyed F1males to the occurrence
of further random mutations. However, flies with these unexpected phenotypes
continued to appear in his crosses.
Although uncommon, they appeared far too often to be due to spontaneous
mutation. Calvin Bridges, who was one of Morgan’s students, set out to investigate
the genetic basis of theseexceptions.
Bridges found that the exceptions arose only in certain strains of white-eyed flies.
When he crossed one of these exceptional white-eyed females with a red-eyed
male, about 5% of the male offspring had red eyes and about 5% of the female
offspring had white eyes. In this cross, the expected result is that every male fly
should inherit its mother’s X chromosome and should have the genotype 𝑋 𝑤Y and
white eyes.
Every female fly should inherit a dominant red-eye allele on its father’s X
chromosome, along with a white-eye allele on its mother’s X chromo-some; thus,
all the female progeny should be X+ XW and have redeyes.
The continual appearance of red-eyed males and white-eyed females in this cross
was therefore unexpected.
Bridges’explanation

23. GENETICS
Explain the appearance of red-eyed males and white-eyed females in hiscross,
Bridges hypothesized that the exceptional white-eyed females of this strain actually
possessed two X chromosomes and a Y chromosome (XwXwY). Sex in
Drosophilae’s determined by the X: A for XXY females, the X: A ratio is 1.0, and
so these flies developed as females in spite of possessing a Y chromosome.
About 90% of the time, the two X chromosomes of the XwXw Y females separate
from each other in anaphase I of meiosis, with an X and a Y chromosome going
into one gamete and a single X going into another othergamete.
When these gametes are fertilized by sperm from a normal red-eyed male, white-
eyed males and red-eyed females are produced. About 10% of the time, the two X
chromosomes in the females fail to separate in anaphase I of meiosis, a
phenomenon known as nondisjunction.
When nondisjunction of the Xs occurs, half of the eggs receive two copies of the X
chromosome and the other half receive only a Y chromosome.
When these eggs are fertilized by sperm from a normal red-eyed male, four
combinations of sex chromosomes are produced.
An egg with two X chromosomes that is fertilized by an X-bearing sperm produces
an X+ XwXw zygote, which usually dies. When an egg carrying two X
chromosomes is fertilized by a Y-bearing sperm, the resulting inwhite-eyed
female and vice versa.
Confirmation of Bridges’hypothesis.
Bridges’hypothesis predicted that the white-eyed females from these crosses
would possess two X chromosomes and one Y chromosomeand that the red-eyed
males would possess a single X chromosome.
To verify his hypothesis, Bridges examined the chromosomes of his flies and
found precisely what he had predicted. The significance of Bridges’study is not
that it explained the appearance of an occasional odd fly in his culture but that he
was able link the inheritance of a specific gene (w) to the presence of a specific
chromosome(X).

24. GENETICS
This association between genotype and chromosomes gave unequivocal evidence
that sex-linked genes are located on the X chromosome and confirmed the
chromosome the0ry of inheritance.
Colour blindness. A particular traits in human beings renders them unable to
differentiate between red colour and green colour. The gene for this red green
color blindness is located on X-chromosome. Colour blindness is recessive to
normal vision so that if colour blind man marries a girl who is normal
(homozygous) for this character, sons will be normal, but daughters will be
heterozygous (normal phenotype), which means that these daughters would be
carriers of this trait. If such a carrier girl marries a colour blind man, 50% of
female progeny and 50% of male progeny would be colour blind
Hemophilia. Hemophilia is another popular example of sex linked inheritance in
human beings. It is recessive character and is, therefore, masked in heterozygous
condition. Individuals suffering with this disease lack a factor responsible for
clotting of blood. Consequently, even a minor cut may cause prolonged bleeding
leading to death. Since it is a recessive character, a lady may carry the disease and
would transmit the disease to 50% of her sons, even if the father is normal

25. GENETICS