Did you know that indentation on people cheeks, also known as dimples, are actually a genetic defect? In fact, it is a genetic defect of a facial muscle called the zygomaticus major.
People with zygomaticus major has a shorter muscle than normal people. This makes the cheeks' skin to be pulled to form an indentation. Dimples are dominant traits that can be inherited.
So, what is this all about inheritance?
The inheritance topic may confuse you a bit, especially with the genetic terms in bio-life. Let’s take a look into a few important genetics terms to help you understand this Biology form 5 topic better.
- A gene is the basic unit of inheritance which consists of a DNA segment located on a specific locus of a chromosome. It controls a specific characteristic of an organism.
- An allele is an alternative form of the gene for a specific trait that is located on the same locus of a pair of the homologous chromosome.
- A dominant allele is an allele that always shows its trait when it is present. It suppresses the effect of the recessive allele. It is always presented in capital letters, for example, B.
- A recessive allele is an allele that shows its trait when both alleles are recessive alleles. The effect of the recessive allele is suppressed by the presence of the dominant allele. It is represented in small letters, for example, b.
- A characteristic is a heritable feature, such as height, eye colour, blood group, and presence of dimples. Each of the characteristics is a feature of a different organism on Earth.
- A trait is a variation of specific characteristics. Each of the inherited characteristics consists of specific traits.
- A dominant trait is expressed when both alleles are dominants or one dominant allele is paired with a recessive allele.
- A recessive trait is expressed if a recessive allele is a pair with another recessive allele.
- Phenotype is the observable characteristics of an organism, such as weight.
- Genotype is the genetic composition of an organism that cannot be seen.
- Both alleles at the loci of a pair of homologous chromosomes are the same.
- Alleles at the loci of a pair of homologous chromosomes are different.
- The parental generation is the first generation of two individuals that are mated to predict or analyse the genotypes of their offspring.
- Filial generation is a successive generation as a result of mating between individuals of purebreds parental generation.
- Purebreed is the individual that carries two identical alleles for a trait. Self-cross always produces offsprings with the same characteristics in every generation.
- Hybrid is the product of mating between two purebred varieties.
- Locus is a specific location of a gene in a chromosome.
What is monohybrid inheritance?
Monohybrid inheritance involves the inheritance of one characteristic and contrasting trait controlled by a gene in bio-life.
Mendel’s First Law (Law of Segregation)
A characteristic of an organism is controlled by a pair of alleles, only one of the allelic pairs is inherited in a gamete.
What is dinohybrid inheritance?
Dinohybrid inheritance involves the inheritance of two characteristics. Each of the characteristics is controlled by a different gene located at a different locus.
Mendel’s Second Law (Law of Independent Assortment)
During gamete formation, each allele from a pair of alleles can combine randomly with any allele from another pair of alleles.
What is the relationship between allele, gene and locus?
Each chromosome carries many genes. The number of genes that code for proteins in a set of haploid chromosomes for humans is estimated to be 25,000. The allele that represents a gene is located at the same locus as the gene.
Inheritance in humans
The human chromosomes can be divided into two types: autosomes and sex chromosomes.
The human somatic cell consists of 44 autosomes and 2 sex chromosomes. The autosome varies in size and length.
- Consists of chromosome pairs from 1 to 22.
- Controls all characteristics of somatic cells.
- Type of blood group, height and skin colour.
- Consists of one chromosome pair that is number 23.
- Consists of genes that determine gender.
- Male has XY chromosome and female has XX chromosome.
The number and structure of chromosomes present in a cell nucleus are known as a karyotype. Chromosomes are arranged in pairs based on homologous chromosomes in terms of their size, centromere locations and banding pattern of chromosomes.
Changes in the number of chromosomes may take place due to the failure of homologous chromosomes to separate during anaphase I. It may also cause the failure of sister chromatids to separate during anaphase II.
This disorder is known as nondisjunction. When this takes place in humans, either male gametes or female gametes can possess chromosome numbers of less than 23 which is 22. This can also mean that the gametes can possess chromosome numbers more than 23, which is 24.
The fertilization of this abnormal gamete will produce a zygote with 45 or 47 chromosomes. This will lead to incurable genetic diseases. You may refer to the form 2 science textbook to learn about human health and body defense.
What Is Human Inheritance?
Each offspring inherit certain characteristics from their parents due to inheritance. You can refer to the form 4 Science textbook for extra reference.
ABO Blood Group
ABO blood group in humans is an example of multiple alleles. The blood group is controlled by a gene that consists of three different alleles. They are called IA, IB and IO. These alleles determine the types of antigen present on the surface membrane of the red blood cell.
A person only possesses two alleles to determine their blood group. A blood test is used to identify a person’s blood group.
Apart from antigen A and antigen B on the surface of human red blood cells, there is another antigen which is called antigen D. Antigen D is known as Rhesus factor (Rh).
A person who has Rhesus factor in their red blood cell is Rhesus positive (Rh+) and those who do not have Rhesus factor is called Rhesus negative (Rh-).
Rhesus factor is inherited from parents to children based on the principle of Mendel’s Law. it is controlled by genes that consist of a pair of alleles Rh+ dominant and Rh- recessive.
The genotype of an RH positive individual is either homozygous dominant (Rh+Rh+) or heterozygous (Rh+Rh-). An Rh-negative individual is homozygous recessive (Rh-Rh-).
Thalassemia is an inherited disease. It can be passed down from one generation to another generation. The disease occurs because of the gene mutation on an autosome that is on chromosome 11 or 16.
Thalassemia takes place because of the abnormality and the low number of haemoglobin. The red blood cells tend to be smaller and paler.
There are two thalassemia conditions that can affect people:
1. Thalassemia Minor
Thalassemia minor is a condition where the individual possesses a recessive allele of thalassemia, but they do not show any symptoms of the disease. Thalassemia can only be detected by a blood test.
2. Thalassemia Major
Thalassemia major is a condition where the individual has both the recessive allele. They show symptoms like tiredness, paleness, breathing difficulties, and changes in the facial bone formation in infant stages.
There is a chapter in textbook science form 2 that emphasises on human immunity system and diseases. Basically, you have already learned briefly about thalassemia in science form 2, as well as during science form 3 and biology form 4.
We recommend you to keep your science form 2 textbook or your previous Science form 2 all chapters' notes for revision.
A male has 44 + XY chromosomes. A female has 44 + XX chromosomes.
Sperma that is produced in the testis is a haploid. Each of the sperm has either 22 + X or 22 + Y chromosomes.
The secondary oocyte that is produced in the ovary is a haploid. It consists of 22 + X chromosomes.
Did you know that the sex or gender of a child is determined during fertilization?
Genes located on sex chromosomes control specific characteristics, but they do not get involved in the sex determination. These genes are called sex-linked genes.
Genes of colour blindness and hemophilia are located in the X chromosomes. Characteristics of colour blindness and hemophilia are caused by recessive genes linked to X chromosomes.
Y chromosomes are shorter than the X chromosome. It does not contain as many alleles as the X chromosome. As a result, any traits in males caused by either the dominant allele or recessive allele in chromosome X is observed.
What is family pedigree?
Family pedigree or lineage is analysed to investigate the inheritance of human characteristics. The form 3 English textbook mentions genealogy in one of its chapters about inheritance.
It is a flowchart through a few generations to show ancestral relationships and inheritance of characteristics from ancestor to individual in the present generation.
This family pedigree’s analysis allows the geneticists to predict inherited characteristics of interest. It also enables them to identify the features of a dominant or recessive gene.
A dominant gene normally appears in every generation, meanwhile the recessive gene is probably hidden in certain generations.
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