| |
| 11.4 |
Inheritance in Humans |
|
| |
 |
| |
| Types of Human Chromosomes |
| Aspect |
Autosom |
Sex Chromosomosomes |
| 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. |
|
| |
| Changes in Number of Chromosomes |
- Changes in number of chromosome can occur due to failure of homologous chromosomes to separate during anaphase I or failure of sister chromatids to separate during anaphase II.
- This disorder is known as nondisjuction which can occur in some chromosomes. When nondisjunction occurs in humans, either male gamete (sperm) or female gamete (ovum) can possess chromosome number of less than 23, that is 22 or more than 23, which is 24.
- Therefore, fertilisation that involves the abnormal gamete with a normal gamete produces a zygote with 45 chromosomes or 47 chromosomes.
- Examples of genetic diseases caused by nondisjunction are Down syndrome, Turner syndrome and Klinefelter syndrome.
- Down Syndrome:
- Down syndrome is also known as trisomy 21.
- Total chromosome number is 47,which is 45+XY.
- There is an extra chromosome for chromosome pair number is 21
- Down syndrome can occur both males and females.
- 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.
- Klinefelter Syndrome:
- 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 |
- 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.
| Characteristics of Phenotype, Genotype and Antigen of Human Blood Group |
| Phenotype (Blood Group) |
Genotype |
Antigen |
| A |
\(I^AI^A, I^AI^O\) |
A |
| B |
\(I^BI^B, I^BI^O\) |
B |
| AB (universal recipient) |
\(I^AI^B\) (codominat) |
AB |
| O (universal donor) |
\(I^OI^O\) |
- |
| 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 |
|
- 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.
|
- 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 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.
| Hemophilia |
| |
Female |
Male |
| Homozygous dominant |
\(X^HX^H\)(normal) |
\(X^HY\) (normal) |
| Homozygous recessive |
\(X^hX^h\) (haemophiliac) |
\(X^hY\) (haemophiliac) |
| Heterozygous |
\(X^HX^h\) (carrier) |
- |
| Colour Blindness |
| |
Female |
Male |
| Homozygous dominant |
\(X^BX^B\) (normal) |
\(X^BY\) (normal) |
| Homozygous recessive |
\(X^bX^b\) (colour blind) |
\(X^bY\) (colour blind) |
| Heterozygous |
\(X^BX^b\) (carrier) |
- |
|
| Ability to Roll Tongue and Types of Earlobe |
- The ability to roll tongue and the types of earlobes are two characteristics that can be inherited from parents to children according to Mendel’s Law.
- Ability to roll tongue is a dominant trait.
- Free earlobe is a dominant trait whereas attached earlobe is a recessive trait.
|
|
| |
| Family Pedigree |
- Family pedigree or lineage can be analysed to investigate inheritance of human characteristics.
- Family pedigree is a flowchart through a few generations to show ancestral relationship and inheritance of characteristics from ancestors to individuals in the present generation.
- Analysis of family pedigree enables the geneticist to predict an inherited characteristic of interest and also to identify the features of dominant or recessive gene.
- Normally a dominant gene appears in every generation whereas a recessive gene is probably hidden in certain generations.
|
|
| |