Chapter 3: Principles of Genetics_Sex Chromosomes in Humans and Sex Link Diseases n Summary

8/6/2019 Chapter 3: Principles of Genetics_Sex Chromosomes in Humans and Sex Link Diseases n Summary

1/19

Sex Chromosomes in Humans The chromosome number in Man is 46 i.e. 23

pairs.

Out of 23 pairs, 22 pairs are common in both

males and females. These are calledAutosomes.

23rd pair of chromosomes is a small pair and itdetermine the sex in the man.

In male, 22 pairs are Autosomes and 23rd pairare sex chromosomes that are denoted by X &Y (22pairs of A + XY).

Similarly in females, 22 pairs are Autosomesand 23rd pair are sex chromosomes but theyare identical i.e. there are 2 X chromosomes(22 pairs of A + XX).


2/19

Sex Chromosomes in Humans

The X and Y chromosomes are different in

shape and size.

Y chromosome is J shaped and is smallerthan X chromosome but they pair up during

. From the given information it is clear that

maleness is due to presence of one X and

one Y chromosome and femaleness is

presence of two X chromosomes.


3/19

Determination of Sex

During meiosis, the sex chromosomes arealso reduced to half like Autosomes forming

a haploid gamete. In female, ova will have one X chromosome +

22 Autosomes makin the ha loid

chromosome number 23.

But in male, sperms half will have one X

chromosome + 22 Autosomes and the otherhalf will have one Y chromosome + 22Autosomes.


4/19

In humans and many other species males produce

two types of gametes i.e. X and Y, hence they are

Heterogametic.

Females produce only one type of gametes i.e. X,hence they are Homogametic.


5/19

Sex determination in Human


6/19

Sex Determination in Human


7/19

Sex Linkage

Sex chromosomes (X & Y) carry sex determininggenes i.e. male & female.

But they also carry certain other genes that controlsome important traits.

genes occurring on sex chromosomes are calledsex-linked characters.

The genes controlling these characters are referredto as sex-linked genes and the inheritance of thesegenes is called sex-linkage or more commonly sex-linked inheritance.


8/19

SEX-Linked inheritance of Diseases

Certain genes which occur on the X

chromosome are more likely to affect a male

than a female.

This is clearl evident in the case of a certain

form of colour blindness and haemophilia,two important sex-linked inherited diseases

of human.


9/19

It is a sex-linked inherited human disease.

People suffering from this disease are not able to

distinguish between the colour red and green.

The gene for red-green colour blindness is carried

on e -c romosome. Normal vision is dominant over red-green colour

blindness.

the condition was often called daltonism, although

this term is now used for a type of colour blindness

called deuteranopia.


10/19


11/19


12/19

Haemophilia

Haemophilia is an another X-linked inherited disease.

The people suffering from haemophilia are unable to

synthesize a normal blood protein called Anti-Haemophilic Globulin (AHG) that helps in clotting.

As a result haemo hiliacs can bleed for a lon time

even from a very small cut.

That is why haemophilia is also called Bleeders

disease.

The gene causing Haemophilia is also a recessive gene

located on the X chromosome similar to that of red-

green color blindness.


13/19

Haemophilia chart


14/19

Haemophilia chart


15/19

Other X-linked Factors

It include Brown Enamel on teeth.

Sexual characters like beard and muscularphysique in men, mammary glands, wider pelvis inwomen are not due to sex-linked genes.

y xgenes.

It is due to the presence of the X or Y chromosome

that creates an environment of maleness orfemaleness causing to the sex chromosomespresent in an individual.


16/19

A very few abnormalities are thought to be linkedto the Y chromosome but now even they are open

to more research.

It should be remembered that the X-chromosomeof a man always comes from his mother while a

mother and father.

Thus, a man can never transfer a sex-linked gene

directly to his son because the son inherits onlythe Y-chromosome from his father.


17/19

Father of Genetics: Gregor Mendel

Mendels law: 1). Law of Dominance 2). Law of

segregation 3). Law of independent assortment.

According to Mendel each characters has two alleles:

Dominant and Recessive

Law of Segregation: it states that when a pair of contrasting alleles come together in an organism they

remain pure and separate again in the gametes.

Law of independent assortment: it states that whenthe parents differ in two or more alleles then

inheritance of a pair of allele is independent of others.


18/19

There are exceptions to Mendels law e.g.

Incomplete Dominance, linkage, MultipleAllelism and Gene Interactions.

Mendels law are important because

genotypes and phenotypes of nextgeneration can be predicted even before

Chromosomes common to male and femaleindividuals are called Autosomes.

Chromosomes that determine the sex of adiploid individual are called sexchromosomes.


19/19

In humans, there are 46 chromosomes (23 pairs of

chromosomes: 22 pairs are Autosomes and 23

rd

pair is Sex chromosome, XY in male and XX in

female.)

Characters controlled by genes on sexchromosomes are called sex-linked characters.

diseases are haemophilia and colour blindness.

They are both due to a recessive gene present on

X-chromosome. X-chromosome linked recessive diseases are less

common in female because of their heterozygosity.

Date post:	07-Apr-2018
Category:	Documents
Upload:	ankurbiology
View:	220 times
Download:	0 times

Chapter 3: Principles of Genetics_Sex Chromosomes in Humans and Sex Link Diseases n Summary

Documents