Heredity

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Introduction

Some Important Terms

• Chromosomes:  These are the highly coiled and condensed fibres.

• Chromatin: Thread-like structure present inside the nucleus which constitute DNA (40%) and histone proteins (60%).

• DNA is composed of :

1. Phosphate
2. Pentose sugar
3. Nitrogenous bases: adenine , guanine , cytosine , thymine

• Genes are specific sequences of nucleotides on a chromosome that encode particular proteins which express in the form of some particular feature of the body.
• Genetics :  A branch of biology that deals with the heredity and variation of organisms the genetic makeup and phenomena of an organism, type, group, or condition.

• Alleles: Alternative forms of a gene, occupying the same position (locus) on homologous chromosomes and affecting the same characteristic but in different ways.
• Dominant allele : The kind of allele which expresses itself regardless of the presence of another allele for a given gene.
• Recessive allele : The kind of allele whose expression is suppressed in presence of a dominant allele for a given gene.
• Variations: Small differences among the individuals of the same species.
• Character: Any inheritable feature of an organism.
• Traits: Alternative forms of a character.

Mendel’s Laws Of Inheritance

• Father of genetics : Gregor Mendel
• He selected pea plant for his experiments.
• The plant is technically termed as Pisum sativum.

Characters	Dominant trait	Recessive trait
Flower colour	Purple	White
Seed colour	Yellow	Green
Seed shape	Round	Wrinkled
Pod colour	Green	Yellow
Pod shape	Inflated	Constricted
Flower position	Axillary	Terminal
Plant height	Tall	Dwarf

Monohybrid Cross

• Phenotypic Ratio – 3 : 1

• Genotypic Ratio – 1 : 2 : 1

• Why was Pea Plant Selected for Mendel’s Experiments?

• He selected a pea plant for his experiments for the following reasons:

1. The pea plant can be easily grown and maintained.
2. They are naturally self-pollinating but can also be cross-pollinated.
3. It is an annual plant, therefore, many generations can be studied within a short period of time.
4. It has several contrasting characters.

• He cross-pollinated two pure lines for contrasting characters and the resultant offspring’s were called F1 generation (also called the first filial generation). The F1 generations were then self-pollinated which gave rise to the F2 generation of second filial generation.

• Results of Mendel’s Experiments

1. Let us look at the results of Mendel’s experiments on crossing a pure tall pea plant with a pure short pea plant.
2. In the F1 generation, Mendel observed that all plants were tall. there were no dwarf plants.
3. In the F2 generation, Mendel observed that 3 of the offspring’s were tall whereas 1 was dwarf.
4. Similar results were found when Mendel studied other characters.
5. Mendel observed that in the F1 generation, the characters of only one parent appeared whereas, in the F2 generation, the characters of the other parent also appeared.
6. The characters that appear in the F1 generation are called dominant traits and those that appear for the first time in the F2 generation are called recessive traits.

• Conclusions :

1. The genes that are passed from the parents to the offsprings exist in pairs. These pairs are called alleles.
2. When the two alleles are the same, they are called homozygous. When both the alleles are different, they are called as heterozygous.
3. Dominant characters are described using capital letters and recessive using small letters. For example, the dominant genes for tallness in a pea plant are written as TT and recessive genes as tt. The heterozygous genes are written as Tt where the plant appears tall has the recessive gene which might express itself in the future generations.
4. The appearance of the plant is known as the phenotype whereas the genetic makeup of the plant is called the genotype. So, a plant with Tt genes appears tall phenotypically but has a recessive gene.
5. During gametogenesis, when the chromosomes become half in the gametes, there is a 50% chance of either of the alleles to fuse with that of the other parent to form a zygote.

Dihybrid Cross Of Mendel

• Phenotypic ratio for dihybrid cross – 9 : 3 : 3 : 1

Acquired And Inherited Characters

1. A trait (or characteristic) of an organism which is ‘not inherited’ but develops in response to the environment is called an acquired trait.
2. Eg– a beetle if it does not get sufficient food for a considerable time, its weight will be reduced due to starvation. The ‘low weight’ of this beetle is an acquired trait of the beetle which has been acquired in response to the environment which contained insufficient food.
3. We have already studied that the traits (or characteristics) of parents are passed to their offspring’s through genes in reproductive cells (or gametes) during the process of reproduction. So, for the trait of an organism to be passed on, it must have been caused by a change in the genes ( or DNA ) present in the reproductive cells of the organism.
4. Note – Only those traits can be transmitted to future generations in which changes have occurred in the genes (or DNA) present in the reproductive cells ( or gametes) of parent organisms.
5. Note – The changes in the non-reproductive body cells of an organism cannot be inherited by its offspring’s.

Discussion On Tailless Rats

• Let us discuss the other example now, if we breed some mice, all the progeny of mice will have tails, just like their parents.
• Now, if we cut the tails of these first generation mice surgically and breed them, we will get new mice, all with full tails.
• It has been observed that even after cutting the tails of mice for a number of generations, a tail-less mouse is never born.
• Actually, the cut tail of mice is an acquired trait which is never passed on to their progeny.
• This is because cutting the tails of mice does not change the genes of their reproductive cells (or gametes).
• And since, the acquired trait of ‘cut tails’ does not bring about a change in the genes of mice, this trait cannot be passed on to their next generations.
• From this discussion we conclude that the experiences of an individual during its life time( acquired traits )  and cannot be passed on to its progeny, and hence cannot lead to evolution ( because they are not caused by the change in genes).

Sex Determination

• Some animal’s sex is determined entirely on environmental cues. Eg – some reptiles.
• Some individuals can change their sex. Eg – snails.

• In humans :

1. XX – Females
2. XY – Males

• Human beings have two kinds of chromosomes :
• Autosomes / somatic chromosomes : The chromosomes which determine the general body features of an organism, like- height, hair colour , skin colour , body structure etc.
• Our body has 22^nd  pair of autosomes.
• Sex – chromosomes : The chromosomes which determine the sex/ gender of an individual.
• Our body has 1 pair of sex- chromosomes (ie. 23^rdpair).

Human Sex-Determination

Temperature Controlling Sex

• In some animals, sex determination is also controlled by the environmental factors.

• Eg–

1. In some reptiles, the temperature at which the fertilized egg is incubated before hatching plays a role in determining the sex of the offspring.
2. A turtle ( Chrysemapicta) – high incubation temperature leads to the development of female offspring ( or female progeny ).
3. In case of a lizard ( Agama agama) – high incubation temperature results in male offsprings ( or male progeny ).
4. In some snails, individuals can change sex, indicating that sex is not determined genetically in such animals.