11.4 - Inheritance in Humans

Two types chromosomes can be found in humans which are autosomes and sex chromosomes. Humans have 44 autosomes and 2 sex chromosomes. A karyotype refers to the number and structure of chromosomes arranged based on the size, location of the centromeres and banding patterns of the chromosomes.

Differences between autosomes and sex chromosomes

• Autosomes

  • Characteristics: Refers to the paired homologous chromosomes 1 to 22
  • Functions: Control all characteristics of somatic cells
  • Examples: Height, skin colour, types of blood groups

• Sex chromosomes

  • Characteristics: Paired homologous chromosomes 23
  • Functions: Has genes that determine an individual's gender type
  • Examples: Male sex chromosome is XY, whereas female sex chromosome is XX

Failure of the homologous chromosomes to separate during anaphase I or sister chromatids to separate during anaphase II will lead to abnormal number of chromosomes. This condition is known as nondisjunction. If nondisjunction happens in the gametes, the number of chromosomes will become less or more than 23. Consequently, the product of fertilization of the gametes will produce a zygote with 45 or 47 chromosomes. Down syndrome, Turner syndrome and Klinefelter syndrome are genetic diseases most commonly seen.

Figure 8 - Karyotypes of individuals with Down syndrome (top), Turner syndrome (middle) and Klinefelter syndrome (bottom).

Human Inheritance

ABO blood groups

• As you have learnt previously, the human blood groups can be divided into 4 types – A, B, AB and O blood.
• Human ABO blood groups are controlled by a gene that consists of three alleles - I A, I B and I O.
• These alleles will determine which antigens to be present on the surface of the red blood cells.
• I A and I B are dominant alleles, whereas I O is a recessive allele.

Phenotypes (blood groups) and Genotypes in Alleles

I A and I B are codominant to one another.Therefore, when two of these alleles exist together, both of them will be expressed and form the AB blood group.

• Blood group A: Genotypes- IA IA or IAIO
• Blood group B: Genotypes- IB IB or IBIO
• Blood group AB: Genotypes- IAIB
• Blood group O: Genotypes- IOIO

Figure 9 - Schematic diagram of ABO blood group inheritance.

Rhesus factor (Rh)

• One other antigen found on the surface of the red blood cells that you have learnt is the D antigen or the rhesus factor (Rh).
• Individuals who possess the Rh factor are called Rh positive (Rh+) , whereas those who do not have this factor are called Rh negative (Rh-).
• The Rh factor is controlled by a gene that consists of a pair of alleles which are Rh+ (dominant) and Rh- (recessive).

Figure 10 - Schematic diagram of Rhesus factor inheritance.

Thalassemia

• Thalassemia is a type of genetic disease caused by formation of abnormal and reduced numbers of hemoglobins.
• Thalassemia patients seem tired and pale, suffer shortness of breath and changes in facial bones since the age of 3 to 18 months.
• Caused by gene mutation at the autosomes , which are chromosome 11 or 16.
• Thalassemia carriers are said to have a thalassemia minor condition where the individual has recessive allele for thalassemia but do not express those signs of thalassemia.
• Thalassemia patients are said to have a thalassemia major condition where the individual carries both recessive alleles for thalassemia.

Figure 11 - An example of thalassemia inheritance.

Sex determination

• Males have 44 + XY chromosomes whereas females have 44 + XX chromosomes.
• For males, the gametes formed consist of 22 + X and 22 + Y , whereas for females, the gametes formed consist of 22 + X.

Figure 12 - An example of sex inheritance.

Ability to roll tongue and types of earlobe

• Having the ability to roll tongue is dominant trait.
• Free earlobe is a dominant trait compared to attached earlobe.

Figure 13 - Ability to roll tongue (above) and types of earlobe (bottom).

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Sex-linked genes refer to the genes that control specific characteristics present on the sex chromosomes but are not involved in sex determination. For instance, color blindness and hemophilia genes are located on the X chromosomes. These genes are known as X-linked genes. Both of these are caused by recessive genes on the X chromosomes. For males, any traits caused by a dominant or recessive allele on the X chromosome will be exhibited.

Color blindness

• A condition where an individual cannot differentiate a certain color, especially red and green.
• Most of the color-blind victims are males.

Phenotypes (blood groups) and Genotypes in Sex-inherited inherintance

In sex-inherited inheritance, X and Y chromosomes must be shown when writing the genotypes.

• Male

  • Normal: XBY
  • Carrier: -
  • Color blind: XbY

• Female

  • Normal: XBXB
  • Carrier: XBXb
  • Color blind: XbXb

Figure 14 - An example of schematic diagram of color blindness inheritance.

Hemophilia

• A condition where the blood cannot clot normally due to lacking of blood clotting factors.
• As a result, the patients can lose excessive blood and die.

Phenotypes (blood groups) and Genotypes in Hemophilia

Hemophilia is caused by the presence of recessive alleles on X chromosomes.

• Male

  • Normal: XHY
  • Carrier: -
  • Hemophilic: XhY

• Female

  • Normal: XHXH
  • Carrier: XHXh
  • Hemophilic: XhXh

Figure 15 - An example of a schematic diagram for hemophilia.

Family Pedigree

Family pedigree or lineage is a flowchart that shows the ancestral relationships and inheritance of characteristics from one generation to the next. This pedigree is used to investigate an inherited human characteristic. For example, a recessive gene hidden in a certain generation can be discovered and identified.

Figure 16 - Hemophilic inheritance in a family.