Concept of Sex chromosomes and autosomes, Inheritance of X- linked genes – eye colour in Drosophila, Inheritance of colour blindness in humans, Inheritance of Y-linked Genes -Holandric genes in humans, Sex influenced genes – baldness in humans Sex-limited genes - feathering in domestic fowl

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December 5, 2020
Sex Linked Inheritance
Dr. A. S. Wabale
Dr. A. S. Wabale
Assistant Professor and Research Guide
Assistant Professor and Research Guide
Post Graduate Department of Botany and Research Centre,
Post Graduate Department of Botany and Research Centre,
Padmashri Vikhe Patil College of Arts, Science and Commerce,
Padmashri Vikhe Patil College of Arts, Science and Commerce,
Pravaranagar
Pravaranagar-
- 413 713
413 713
dranilwabale78@gmail.com
dranilwabale78@gmail.com

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CONTENTS
CONTENTS
Ò Concept of Sex chromosomes and autosomes,
Ò Inheritance of X- linked genes – eye colour in Drosophila,
Ò Inheritance of colour blindness in humans,
Ò Inheritance of Y-linked Genes -Holandric genes in humans,
Ò Sex influenced genes – baldness in humans
Ò Sex-limited genes - feathering in domestic fowl
Dr. A. S. Wabale
December 5, 2020

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Dr. A. S. Wabale
WHAT AUTOSOMES AND SEX CHROMOSOMES?
WHAT AUTOSOMES AND SEX CHROMOSOMES?
Autosomes are the non-sex chromosomes present in organisms. In humans, there are 22
paris of autosomes, referred numerically from 1 to 22, like chromosome 1, chromosomes 2,
chromosomes 3, etc. labelled according to their shape, size and other properties.
Except for the sex-linked traits, autosomes are known for regulating all other inheritance of
an organism’s characteristics. It means, autosomes are responsible for transferring the
genetic information from parents to their offspring.
In human body, there are numerous cells, each having 22 pairs of autosomes. The
chromosomes in each pair are of the same length, and even the centromere is placed in the
same location. During mitosis, the chromosomes double themselves and then transferred
to the daughter cells. Hence, the new daughter cells receive a full complete copy of
chromosome, containingthe genetic information of their parent cell.
Sex Chromosomes or allosomes play a vital role in determining the gender or sex of
humans or other species of animals. These are named ‘X’ and ‘Y’ by scientists. In humans
out of the 23 pairs of chromosomes, one pair is sex chromosomes. The one with the ‘XX’
pair of chromosomes is the female while the individual having ‘XY’ pair of the chromosome
is the male.
December 5, 2020

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Dr. A. S. Wabale
9 10 9 10
17 17
1 2 3 4 5
6 7 8 11 12
13 14 15 16 18
19 20 21 22 Y
X
1 2 3 4 5
6 7 8 11 12
13 14 15 16 18
19 20 21 22 X
X
HUMAN AUTOSOMES AND SEX CHROMOSOMES
HUMAN AUTOSOMES AND SEX CHROMOSOMES
December 5, 2020
Human Genome (Male) Human Genome (Female)

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KEY DIFFERENCES BETWEEN AUTOSOMES
KEY DIFFERENCES BETWEEN AUTOSOMES
AND SEX CHROMOSOMES
AND SEX CHROMOSOMES
Dr. A. S. Wabale
The pair of chromosomes that regulate the somatic characters of the body are known
as autosomes, whereas the pair of chromosomes that determines the sex of an organism,
as they regulate the sex-linked traits are known as sex chromosomes or allosomes.
Autosomes are named or numbered from 1 to 22, while sex chromosomes are
recognizedby the letters as XO, XY, ZW and ZO.
The maximum number of chromosomes in our genome are the autosomes, and there are a
few numbers of sex chromosomes in our genome.
Autosomes (22 pairs) are homologous in humans, whereas male sex chromosomes (XY)
are non-homologous, while female sex chromosomes (XX) are homologous.
In autosomes the centromere position is identical but in sex chromosomes the centromere
positionis not identical.
Autosomes prove the Mendelian inheritance and sex chromosomes show Non-Mendelian
inheritance.
The number of genes carried by autosomes varies from 200 to 2000, but in case of sex
chromosomes, the Y chromosomes contain only a few genes, while X chromosome has
more than 300 genes.
December 5, 2020

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Dr. A. S. Wabale
When a gene is present on the X- chromosome, but not on the Y chromosome, it is said to
be X-linked. X-linked genes have different inheritance patterns than genes on non-sex
chromosomes (autosomes).
Since a female has two X chromosomes, she will have two copies of each X-linked gene. For
instance, in the fruit fly Drosophila (which, like humans, has XX females and XY males), there
is a eye color gene called white that's found on the X-chromosome, and a female fly has two
copies of this gene. If the gene comes in two different alleles, such as XW(dominant, normal
red eyes) and Xw (recessive, white eyes), the female fly may have any of three genotypes: XW
XW(red eyes), XW Xw (red eyes), and Xw Xw (white eyes).
A male has different genotype possibilities than a female. Since it has only one X-
chromosome (paired with a Y), it will have only one copy of any X-linked genes. For instance,
in the fly eye color example, the two genotypes a male can have are XW Y (red eyes) and Xw Y
(white eyes). Whatever allele the male fly inherits for an X-linked gene will determine his
appearance, because it has no other gene copy, even if the allele is recessive in females.
Rather than homozygous or heterozygous, males are said to be hemizygous for X-linked
genes.
We can see how sex linkage affects inheritance patterns by considering a cross between two
flies, a white-eyed female (Xw Xw) and a red-eyed male (XW Y). If this gene were on a non-sex
chromosome, or autosome, we would expect all of the offspring to be red-eyed, because the
red allele is dominant to the white allele. What we actually see is the following:
December 5, 2020
INHERITANCE OF X
INHERITANCE OF X-
- LINKED GENES
LINKED GENES –
– EYE COLOR IN
EYE COLOR IN DROSOPHILA
DROSOPHILA

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Dr. A. S. Wabale
December 5, 2020
However, because the gene is
X-linked, and because it was
the female parent who had
the recessive phenotype
(white eyes), all the male
offspring who get only X
chromosome from their
mother will have white eyes
(XwY). All the female
offspring have red eyes
because they received two Xs,
with the XW from the father
concealing the recessive Xw
from the mother.
INHERITANCE OF X
INHERITANCE OF X-
- LINKED GENES
LINKED GENES –
– EYE COLOR IN
EYE COLOR IN DROSOPHILA
DROSOPHILA

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Dr. A. S. Wabale
December 5, 2020
INHERITANCE OF COLOR BLINDNESS IN HUMANS
INHERITANCE OF COLOR BLINDNESS IN HUMANS
Color blindness (color vision deficiency) is the decreased ability to see color or differences in color. It
is a common hereditary (inherited) condition which means it is usually passed from parents. Red/green
color blindness is passed from mother to son on the 23rd chromosome, which is known as the sex
chromosome because it also determines sex.
The color blind ‘gene’ is carried on one of the X-chromosomes. Since men have only one X-chromosome,
if his X-chromosome carries the color blind ‘gene’ (Xc) he will be color blind (XcY). A woman can have
either:-
(i) two normal X-chromosomes, so that she will not be color blind or be a carrier (XcX),
(ii) or, one normal X and one color blind carrying X-chromosome, in which case she will be a carrier
(XcX), or rarely
(iii) she will inherit a color blind X from her father and a color blind X from her mother and be color blind
herself (XcXc). She will pass on color blindness to all of her sons if this is the case.
FATHER
Normal (XY)
MOTHER
Colour Blind (XcXc)
SON
Colour Blind (XcY)
SON
Colour Blind (XcY)
Daughter
Carrier (XcX)
Daughter
Carrier (XcX)

9. Man with
hairy pinnae
Women
(Normal)
Son with
hairy pinnae
Daughter
(Normal)
Son with
hairy pinnae
Daughter
(Normal)
Women
(Normal)
Son with
hairy pinnae
Daughter
(Normal)
Son with
hairy pinnae
Daughter
(Normal)
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INHERITANCE OF Y
INHERITANCE OF Y-
-LINKED GENES :
LINKED GENES :
HOLANDRIC GENES IN HUMANS
HOLANDRIC GENES IN HUMANS
Dr. A. S. Wabale
December 5, 2020
The genes that are carried on the Y-
chromosome are called holandric genes.
Holandric genes can only be passed by males
onto their sons; they code for 'maleness' but
sometimes cause rare conditions like
hypertrichosis pinnae (hairy ears) and color
blindness.
The differential region of Y- chromosome
carries only Y-linked or holandric genes, e.g.
testis determining factor (TDF). It is perhaps
the smallest gene occupyingonly 14 pairs.
When a man with hairy ears marries a normal
women, what percentage of their daughters
would be expected to have hairy ears can be
explained with this diagram

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SEX INFLUENCED GENES: BALDNESS IN HUMANS
SEX INFLUENCED GENES: BALDNESS IN HUMANS
Dr. A. S. Wabale
December 5, 2020
Sex linked traits are controlled by alleles found on the sex chromosomes, whereas Sex
influenced traits are controlled by alleles on autosomal chromosomes .
Sex influenced traits are controlled by a pair of alleles found on the autosomal
chromosomes but its phenotypic expression is influenced by the presence of certain
hormones – estrogen, progesterone, testosterone etc. It can be seen in both sexes, but will
vary in frequency between the sexes or in the degree of the phenotypic expression.
Example: Pattern baldness can occur in both males and females, however it is much more
common in males because the trait is influenced by the hormone testosterone. In presence
of high level of this hormone, the baldness causing allele has a very powerful influence,
whereas in low levels it has less effect. Males have much more levels of this hormone than
the females, as a result, in males the baldness causing allele behaves like a dominant allele,
whereas in females it behaves like a recessive allele.
The combination of alleles for pattern baldness leads to different phenotypic expressions
depending on the sex . Example: If B represents the non bald allele, BB genotype- Non-
bald in both sexes, bb genotype- Bald in both sexes and Bb genotype- Bald in men and
non-bald in females. The B-allele acts as a dominant allele in the heterozygous genotype in
females, but acts as a recessive allele in the heterozygous genotype of the male.

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In cases of sex influenced traits, same allele had different expressions in male and female individuals,
but both alternative forms of trait are known in each sex. For example, both horned and hornless
sheep are known as ♀ and ♂. In still other cases, we find, that a particular trait may not express at all
in one of the two sexes. In poultry, for example, plumage is one such character, where cock
feathering is never seen in female, although hen feathering is common among males. In most
breeds of domestic poultry, male is cock feathered and female is hen feathered, so that plumage of
two sexes is strikingly different. However, in some cases (e.g. some Sebright bantams), both sexes
are hen feathered while in still other cases (e.g. Hamburgh breed), males may be either hen
feathered or cock feathered. It has been shown through appropriate crosses, that hen feathering (h+)
is a dominant character, so that both male as well as female with genotypes h+h+ or h+h will be hen
feathered. However, homozygous recessive (hh) is cock feathered in male, but hen feathered in
female, suggesting that in the presence of female hormone, hh can not express its cock feathering
character as shown in the table. This was confirmed through experiments of gonadectomies (removal
of gonad). When ovary of hh female was removed, it resulted into cock feathering. Similarly in
castrated males (festes removed), expression of h+ is inhibited.
Dr. A. S. Wabale
December 5, 2020
SEX
SEX-
-LIMITED GENES
LIMITED GENES -
- FEATHERING IN
FEATHERING IN
DOMESTIC FOWL
DOMESTIC FOWL
Genotype Males Females
h+h+ Hen feathered Hen feathered
h+h Hen feathered Hen feathered
hh Cock feathered Hen feathered

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ASSIGNMENT
Dr. A. S. Wabale
December 5, 2020
Define the following
1. Autosomes
2. Sex Chromosomes
3. X-linked genes
4. Y-linked genes
5. Colour blindness
Short Questions
1. What are the key differences between Sex chromosomes and autosomes?
2. Explain the pattern of inheritance of X- linked genes w.r.t eye colour in Drosophila
3. Explain the pattern of inheritance of colour blindness in humans
4. Explain the pattern of inheritance of Y-linked/Holandric genes in humans
5. Explain the pattern of inheritance w.r.t sex influenced genes (baldness in humans)
6. Explain the pattern of inheritance w.r.t sex-limited genes (feathering in domestic fowl)