Inheritance in Humans

11.4

Inheritance in Humans

Types of human chromosomes:

	Autosome	Sex chromosome
Feature	Consists of chromosome pairs from 1 to 22	Consists of one chromosome pair,that is number 23
Function	Controls all characteristics of somatic cells	Consists of genes which determine gender
Example	Types of blood groups,height and skin colour	Male has XY chromosomes whereas female has XX chromosomes

Total chromosome number is 47,which is 45+XY. There is an extra chromosome for chromosome pair number is 21
Down syndrome is also known as trisomy 21.Down syndrome can occur both males and females
In Turner syndrome, total number of chromosomes is 45,which is 44+XO
There is a missing X chromosome in the pair of sex chromosomes
The gender of individual with turner syndrome is a female
Karyotype of Klinefelter syndrome has a total of 47 chromosomes, that is 44+XXY
There is an extra X chromosome in the pair of sex chromosomes.The gender of individual with Klinefelter syndrome is male
However,his secondary sex characteristics are not well-developed

Human inheritance:

ABO blood groups:

Controlled by three different alleles of a single gene called \(I\) gene; \(I^A\)(dominant), \(I^B\) (dominant) and \(I^O\)(recessive).
There are six possible genotypes and four possible phenotypes.
Blood group AB is a universal recipient because it does not contain any antibodies.
Blood group O as universal donor because it does not contain antigen on the surface of the cell.
Antibodies are unable to bind on the surface of the cell without antigen and vice versa, which prevents from blood agglutination.

Blood group	Antigen	Can donate blood to blood group	Can receive blood from blood group
A	anti-B	A and AB	A and O
B	anti-A	B and AB	B and O
AB	-	AB only	All blood type
O	anti-A and anti-B	All blood type	O only

Rhesus factor (Rh):

Another antigen that presents on the surface of the red blood cell that affects blood compatibility.
Blood agglutination occurs when an individual with antigen reacts with the antibodies from individuals without this antigen.
Controlled by a pair of allele; Rh-allele (dominant) and rh-allele (recessive).
Rhesus positive (Rh+) individuals: Rh-Rh or Rh-rh.
Rhesus negative (Rh-) individuals: rh-rh.
Rhesus factor can be a problem when:

A Rh- person receives Rh+ blood during blood transfusion because the recipient’s blood reacts by producing rhesus antibodies-caused blood agglutination.
Rh- mother carries another Rh+ baby in the second pregnancy can cause miscarriage because the mother already contains rhesus antibodies that attack the Rh+ baby from the first pregnancy.

Sex determination:

Normal autosomes (somatic cells) in male and female are 44 (22 pairs).
Normal sex chromosomes in male and female are XY and XX respectively.
It makes the number of chromosomes in humans are 44 +XY (46) or 44+XX (46).
However, the cells of an individual with the genetic disease show a different number of chromosomes than a normal being.

Sex-linked inheritance:

Sex chromosomes also carry genes for other traits other than to determine sex.
Sex-linked genes refer to the genes carried on the X chromosome (contain a longer segment than the Y chromosome).
Thus, in a male, any trait caused by dominant or recessive allele present on the X chromosome will be manifested fully. This is because male contains one X chromosome.
Examples:

Haemophilia (blood unable to clot normally): caused by recessive allele on the X chromosome.
Colour blindness (unable to differentiate between certain colours): caused by recessive allele on the X chromosome.

Haemophilia:

Colour blindness: