Inheritance

Inheritance is the passing of traits from parents to their offspring, either through asexual reproduction or sexual reproduction. This is the process by which an offspring cell or organism acquires or becomes predisposed to the characteristics of its parent cell or organism.

Important Terms: 

Term Meaning
Inheritance  The transmission of genetic information from generation to generation.
Chromosome A thread of DNA, made up of a string of genes.
Gene A length of DNA that codes for a protein.
Haploid nucleus A nucleus containing a single set of unpaired chromosomes

Examples:

·       Sperm cells

·       Egg cells

Diploid nucleus A nucleus containing 2 sets of chromosomes

Examples:

·       Zygote

·       Heart cells

·       Liver cells

Mitosis Nuclear division that gives rise to genetically identical cells
Meiosis Reduction division in which the chromosome number is halved from diploid to haploid, resulting in genetically different cells
Allele Any of the alternative versions of a gene

Such as TT, Tt, tt are alternative forms of the gene for height)

Genotype Is the genetic make-up of an organism in terms of the alleles present.

Example: To describe a person with gene for height, ‘T’ can be used.

Phenotype Is the feature of an organism (in terms of words)
Homozygous Means having two identical alleles of a particular gene (such as TT and tt)

·       Two identical homozygous individuals that breed together will be pure breeding (for example if TT breeds with TT, an offspring with TT will be born)

Heterozygous Having two different alleles of a particular gene (such as Tt), not pure breeding.
Dominant alleles An allele that is expressed if it is present (such as T)
Recessive alleles An allele that is expressed only if the dominant allele of the gene is absent (such as t)
Codominance A condition where both the alleles (dominant and recessive have an effect on the phenotype of that plant

For example,

·       C­­WCW produces white flowers in a plant.

·       C­­RCR produces red flowers in a plant.

·       C­­WCR produces pink flowers in a plant (and not white!)

Important:

Dominant and recessive expressions

  • TT = Homozygous dominant
  • tt = Homozygous recessive
  • Tt = Heterozygous 

Mitosis

Nuclear division that gives rise to genetically identical cells

 <Image for the process of mitosis here>

 Meiosis

Reduction division in which the chromosome number is halved from diploid to haploid, resulting in genetically different cells

<Image for the process of meiosis here>

 A comparison between mitosis and meiosis

 

Mitosis Meiosis
Type of reproduction carried on is asexual Type of reproduction carried on is sexual
Genetically identical chromosomes are formed Genetically different chromosomes are formed
Two diploid cells formed Four haploid cells formed
All cells except sex cells are made by this kind of division Only sex cells (sperm and egg cells) are made by this kind of division.

 

Stem cells

When a zygote forms, it begins to divide by mitosis; forming genetically identical cells. This soon forms an embryo.

 

After this period, the cells from the embryo (known as embryonic stem cells) begin to take up different functions; some of them will become skin cells, some liver, some brain, etc.

This is known as differentiation.

Though every cell in our body has the same genes, only some of them specifically, are expressed.

Inherited diseases

Inherited disease Notes
Cystic fibrosis In cystic fibrosis, the cells in the lungs make mucus more thicker than usual

It decreases the efficiency of the gas exchange surfaces due to the trapped mucus

The mucus made blocks the pancreatic duct and prevents digestive juices from flowing, affecting digestion

Haemophilia In haemophilia, the blood in a person’s body fails to clot when there is a cut or wound.

It is caused by the inheritance of a defective gene that prevents blood clotting

Sickle cell anaemia Is a genetic disease where the red blood cells become sickle shaped in the absence of oxygen
Downs syndrome Is another genetic disorder where the chromosomes 21s fail to separate during meiosis in a woman’s ovaries.

This means that an extra chromosome 21 will be in the egg cell; if it gets fertilized, then the zygote will have 47 chromosomes instead of 46!

Children with downs syndrome are usually extremely friendly people and have heart diseases when they grow up

Genetic diagrams

There is a standard way of writing information such as an organism’s and its parent’s genotypes and phenotypes. Moreover, these genetic crosses help us to predict the probabilities of an offspring inheriting a particular characteristic of its parents.

For example, a tall pea plant is bred with a dwarf one. What do you think will its offspring be?

Parental phenotype Tall plant        Dwarf plant
Parental genotype TT t t
Gametes T t

Genetic Inheritance Diagram

‘Tt’ means that all the offsprings born would be tall, and they would be having the ‘hidden’ or non expressed recessive gene in them. This means that the plant offspring is a ‘carrier’ of the gene for height.

To get dwarf plants, the offsprings in the above example can be breeded amongst themselves; this would give rise to half of the offsprings being dwarf.

Sex determination

Similarly, using genetic diagrams can be useful to determine the sex (gender) of an offspring.

Sex Determination

Sex linkage

  • A sex-linked characteristic is one in which the gene responsible for a particular function is located on a sex chromosome, which makes it more common in one sex than the other.
  • The sex chromosomes X and Y not only determine your gender; they contain other genes as well.
  • The X chromosome has a larger chromatid than the Y chromosome. Thus the genes present on the X chromosome (apart from the gene for your sex) will not be present on the Y chromosome.
  • This means that if an X chromosome has a gene for colour blindness, The Y chromosome will not have the gene; this can lead to a male having colour blindness being born as the X chromosome is dominant and there is no recessive allele on the Y chromosome.
  • Similarly when an X chromosome (from a female) with a gene for colour blindness fuses with another X chromosome (from a male) with a recessive allele for the same gene, an offspring who is the carrier of colour blindness is born.
  • Hence colour blindness is more common due to these reasons in males rather than in females as:
    • There is less chance of a recessive allele being expressed in a female (XX) because the other X chromosome may carry the dominant form of the allele.
    • The male chromosome doesn’t have a recessive allele.

The 5 possible phenotypes and their genotypes for red-green colour blindness are:

  1. XBXB : Woman with normal vision
  2. XBXb: Woman with normal vision (who is a carrier)
  3. XbXb: Woman with red-green colour blindness
  4. XBY: Man with normal vision
  5. XbY: Man with red-green colour blindness

Inheritance of sex linked characteristics

Let us see a much more simplified genetic diagram that will help explain the inheritance of sex linked characteristics.

In the following case, a male without red-green colour blindness (XBY) mates with a female with its carrier gene (XBXb). Let’s see what will happen now!

Inheritance of Characteristics

The genetic diagram predicts that about half of their male children will have red-green colour blindness; whereas all of their female children will have normal vision.

DNA and Protein synthesis

  • A DNA molecule is made up of 2 strands of nucleotides.
  • Proteins are made up of long chains of amino acids.
  • The sequence of these amino acids in a protein molecule determines the final shape of the molecule.
  • This shape affects the way the protein works.
  • This can control metabolic reactions in the body.
  • This then affects the way the organism’s body works.

DNA and the genetic code

 The genetic code is carried by a long and complicated molecule called Deoxyribonucleic acid or DNA. DNA is responsible in the production of proteins (see protein synthesis)

  • The DNA is found in the nucleus of all cells. It is formed into X-shaped structures called In human diploid cells (except for eggs and sperm), there are 46 chromosomes. These are divided into 23 pairs.
  • A chromosome is made up of two chromatids held together in the middle by a centromere.
  • This DNA strand looks a bit like a ladder twisted into a double helix structure. The rungs of the ladder are made up of pairs of base molecules with shapes complimentary to each other.
  • It is the order of the bases that carry the actual genetic code.
  • There are only 4 different types of bases:
    1. Adenine (A),
    2. Cytosine (C),
    3. Guanine (G)
    4. Thymine (T)
  • As mentioned above, the pairs of bases have shapes complimentary to each other (like an enzyme and its substrate!)
  • Adenine and Thymine always join together (AT or TA)
  • Cytosine and Guanine always join together (CG or GC)
  • The genetic code contained in our chromosomes is used to make new cells by making proteins.

Passing on the code

The code is passed on to the new cells using either of two processes, mitosis or meiosis.

(See mitosis and meiosis)

Protein synthesis

Protein synthesis is all about how a protein is made in reality with the ‘synthesis’ (joining) of amino acid molecules.

Here is the process:

  • DNA is found in the nucleus
  • Protein synthesis happens on the ribosomes, in the cytoplasm
  • An mRNA (Messenger RNA) is used as a mode of transport from the DNA to the ribosomes.
  • The DNA in the nucleus unwinds
  • This produces a length of mRNA which has bases complementary to the DNA’s length.
  • When the mRNA finishes copying the base sequence, it moves out of the nucleus through a nucleus pore.
  • It then attaches itself to a ribosome
  • As the mRNA molecule passes through the ribosome, a strand of amino acid begins to join simultaneously.
  • The ribosome links amino acids exactly in the right order to make the desired protein.

Note: when the mRNA passes through the ribosome, it doesn’t ‘make’ the amino acids but simply join them to form a protein.

(it is like joining a necklace of fallen beads!)

External Links

  1. Heredity

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