BIOLOGY OF HEREDITY: GENETICS

WEEK 7

TOPIC: BIOLOGY OF HEREDITY

CONTENT:

1. Transmission and expression of characters in organisms
2. Chromosomes the basis of heredity.
3. Probability in genetics.
4. Applications of principles of heredity
5. Explanation on cross and self fertilization

BIOLOGY OF HEREDITY: GENETICS

Genetics is the scientific study of heredity and variation in living organism. Scientists who study genetics are known as geneticists.

The laws of genetics were laid down by Gregor Mendel, an Austrian monk, in 1866 although the

work was not credited to him until 1900.

Johannsen, a Danish botanist called the ‘factors’ that transmitted Mendel’s characters, genes in 1909.

Thomas Morgan, an American geneticist showed that genes were on chromosomes, in 1912.

SUB-TOPIC 1: TRANSMISSION AND EXPRESSION OF CHARACTERS IN ORGANISMS

Character is a distinct structural or functional feature of an organism. Heredity is the transmission of inherited characters from parent to offspring through genes.

Common experience has shown that plants and animals produce offspring which look like them but are still not exactly like the parents.

Every member of a species shares in common a set of traits. These traits or characters distinguish one species from the other.

Hereditary Variations

Hereditary variations refer to differences among individuals which can be passed from parents to their offspring (progeny). Variations are due to a new combination of genes. If variation makes an offspring more suited to the environment more suited to the environment, it stands a better chance of surviving and reproducing to pass on its genes to the next generation.

Characters that can be Transmitted

Only characters controlled by genes can be transmitted. A gene (or genes) controlling a character direct the development of one or more proteins. These proteins lead to the visible expression of the character/trait.

These characters include:

1. Colour of the skin,
2. Colour Eye

iii.Shape and colour of teeth

1. Hair texture,
2. Length of neck,
3. Voice,
4. Intelligence,
5. Composure and
6. sickle cell anaemia in animals.

While in plants variations is characterized by

1. Height of plants,

1. Colour of leaves and flowers,

1. Size of seed and fruits and

1. Pigmentation may be observed.

The sum total of genes that an offspring inherits from its parents is referred to as its genotype or genetic make-up. The actual physical expression of the character is the organism’s phenotype. The phenotype is due to the interaction between an organism’s genotype and its environment. For instance, a person may inherit genes for growing tall, but malnutrition may result in the individual becoming stunted. This is a modification of the inherited character brought about by the environment and cannot be transmitted by the individual to its offspring. Such traits are called acquired traits and they do not change the structure of genes. A change in the structure of genes is called a mutation and can only be inherited if it occurs in the gamete, gamete- producing cells or in the zygote (germline mutations).

These variations may be described as;

1. Discontinuous hereditary variations
2. Continuous hereditary variations

• Discontinuous Hereditary Variations: In this type of variation there is no in-between feature of the trait or character e.g.

(a) Sex is either male or female.

(b) A person is either a tongue roller or not.

(c) A person either has sickle cell anaemia, is a carrier or has no sickle cell trait.

(d) An individual may only have one blood group i.e. A, B, AB or O

1. Continuous Hereditary Variations: In this form of variation there is a range of ever changing intermediate values of a given trait e.g.

(a) The height of a plant or person at a given period of time

(b) Size of leaves, fruits and roots of plants.

How Characters Get Transmitted

All the body (somatic) cells of plants and animals have a characteristic number of chromosomes fixed for a particular species. These are referred to as the diploid (2n) number. Humans have a diploid number of forty six (46) chromosomes, there are twenty three kinds of chromosomes meaning there are two of each kind present in the diploid set. These paired chromosomes are alike and are said to be homologous.

Each chromosome is made up of genes and so controls the major features of heredity. A gene for a particular character e.g. colour of the eye exist at the same location or locus on two homologous chromosomes. Genes that occupy the same relative position or loci on homologous chromosomes but separate during meiosis are referred to as alleles; the pair is known as allelic pair.

Alleles produce contrasting characters e.g. a tall or short pea plant, have alleles T and t respectively. A gene may affect more than one character and some characters may be affected by more than one gene e.g. intelligence, hair texture and hair colour.

When gametes are formed during meiosis, the homologous chromosomes separate so that each gamete will contain one allele or allelomorphic gene. During sexual reproduction, gametes of the male and female parent fuse (fertilization) to form the zygote. The zygote is diploid (2n) because one haploid set of chromosomes is contributed by each parent. Thus the offspring that develops from the zygote combine characters inherited from parents.

The male and female individual contributing gametes are referred to as the parent generation.

The offspring are the first filial (f1) generation 

How Characters manifest from Generation to Generation

To understand how characters behave when transmitted from generation to generation, the inheritance pattern of a character controlled by a single pair of genes can be investigated. This is referred to as a single factor or monohybrid inheritance. An example is the flower colour of pea plants which is determined by two alleles; one for red colour (R) and the other for white colour (r).

If a red flowered pea plant (RR) and the cross produces only red-flowered offspring (RR), the red flowered pa rent is said to breed true. The plant is said to produce a pure stock or pure line. The red flowered parent plant has two genes, RR for red colour. Similarly, the white flowered pea plant that breeds true has two genes for white colour (rr).

Parent Generation

(a)    RR

x

RR

(b)    rr

x

rr

↓

↓

↓

↓

R

Sperm (n)

R

egg cell (n)

r

sperm (n)

r

egg cell (n)

F1 generation

↘↙

↓

RR

↘↙

↓

rr

(Offspring 2n)

All these pea plants that breed true are said to be homologous for flower colour because the genes controlling the flower colour in either case are identical i.e. RR or rr.

When a pure stock of red flowered pea plants is crossed with a pure stock of white flowered plants (monohybrid cross), the offspring are all red flowered pea plants (Rr) and they form the F1 generation. The character for red colour is said to be dominant. The white colour which does not appear in the F1 generation is said to be recessive.

(a) Parental gametes

RR    x

rr

↓

↓

R

Sperm (n)

r

egg cell (n)

↘↙

↓

F1 generation    Rr

(Offspring 2n, all red flowered)

The result of this type of cross shows that the dominant form of a character masks the recessive form in the F1 generation. The white flowered colour can only appear when the plant has identical alleles (rr) for that colour i.e. the dominant allele R, for red colour is absent.

(b) F1 Parents

Rr

x

Rr

↓

↓

Gametes

R

r

R

r

F2 generation

RR

Rr

Rr

rr

In the F2 generation the phenotype shows three red flowered and one white flowered pea plant. The genotype however reveals one homozygous and two heterozygous red flowered pea plants and one homozygous white flowered pea plant.

In the F2 generation, there are two types of red flowered pea plants;

1. one in which the two alleles for red flower colour are identical (RR) i.e. it is homozygous for red colour.
2. one in which the two alleles for red flower colour are different (Rr), i.e. it is heterozygous for red flower colour.

An individual is said to be heterozygous for a character that has more than one form of expression, if the two copies of the gene controlling that character are different.

The homozygous and heterozygous red-flowered pea plants are said to show the same

phenotype but different genotypes. These heterozygous plants are referred to as hybrids.

Crossing the hybrids will produce a mixture of red and white flowered pea plants with roughly three quarters bearing red flowers and one quarter bearing white flowers. These results show that;

1. Heterozygotes do not breed through.

1. The recessive form of the flower colour masked in the F1 generation can appear in the F2 generation (a recessive gene can skip some generations and appear in a latter one).
2. The majority of members of the F1 and F2 generations exhibit the dominant form of flower colour (more members of a population exhibit the dominant form of a character).

EVALUATION

1. How are characters transmitted in living organisms?
2. What is heredity?
3. Describe variation.
4. What determines the characters inherited by an individual?
5. How are characters transmitted from parents to offspring?

ASSIGNMENT

Making use of a punnet square, show how seed colour and seed shape are transmitted in two generations assuming you cross a pure stock of pea plants bearing round (RR), yellow (YY) seeds with a pure stock of pea plants bearing wrinkled (rr), green (yy) seeds.

MENDELIAN LAWS

Gregor Mendel (1882 – 1884) was an Austrian monk who carried out simple experiments on heredity for nine years (1856 – 1865) using the common garden pea. He published his research findings “Experiments on Hybridization” in the journal of Natural History in Austria in 1866/67.

He formulated two principles of inheritance;

(i) First Mendelian law

The law of segregation; states that “ the two factors segregate or separate from one another unaltered and unblended as they pass from one generation to the next”. The pair of factors (genes) segregate during the formation of gametes (meiosis). Only one passes into a single gamete. During fertilization genes pair up in new ways.

(ii) Second Mendelian law

The law of Independent assortment; states that “two pairs of factors in the same cross assort or separate independently of each other”. In other words, a member of a pair of genes can combine separately with any other member of another pair. It acts randomly and it is thus inherited.

Mendel arrived at this law from his findings on the experiment he carried out with two pairs of contrasting characters (dihybridization). For example, in the crossing of round yellow seeds with wrinkled green seeds, the F1 generation showed the dominant character of round and yellow (RRYY), but the F2 generation showed a proportion that was the square of 3: 1.

The result was 16 combinations with four phenotypes in the ratio 9:3:3:1 i. Nine round yellow

1. Three round yellow
2. Three wrinkled green
3. One wrinkled green.

There were nine genotypes which include four homozygous and five heterozygous conditions.

SUB-TOPIC 2: CHROMOSOMES; THE BASIS OF HEREDITY Chromosomes are located in the nucleus of living cells of plants and animals. All plants and animals possess two types of cells;

1. somatic or vegetative cells and
2. gametes or sex cells

Somatic cells are the body cells which are not concerned with reproduction. They carry the diploid (2n) set of chromosomes. They are obtained as a result of fertilization, i.e. the contribution of a set of chromosomes from a male and a female individual. Nuclear division in somatic cells is referred to as mitosis.

1. Sex cells (gametes) are cells which are found in the reproductive organs of male and female organisms. The gametes (e.g. sperm, Ovum, pollen grain) are haploid i.e. they carry half the number of chromosomes of somatic cells. Nuclear division which results in their formation is referred to as meiosis (reduction division).

Each organism has a specific number of chromosomes in its somatic cells i.e. the number of chromosomes in a species is constant. Every human has 23 pairs (46) of chromosomes in each somatic cell, while each gamete has half i.e. 23 chromosomes. Drosophila (fruit fly) has 4 pairs and tomato plant has 10 pairs of chromosomes each.

Structure of Chromosomes

Each chromosome is thread like in appearance and made up of two nuclear threads called chromatids held in the middle by a centromere. Homologous or identical chromosomes occur in pairs. Each chromosome has several transverse bands along its length. Numerous hereditary materials (genes) are located on these bands. Genes are DNA (deoxyribonucleic acid) molecules. It is estimated that there are 2-3million genes in every human cell.

The DNA consists of a double chain formed by repeating small chemical units called nucleotides. Each nucleotide is composed of a deoxyribose sugar (S), a phosphate group (P) and a nitrogenous compound base which may be Adenine (A), Guanine (G), Cytosine (C) or Thymine (T).These nucleotides are arranged in a structure depicting a ladder. Each pair of the ladder consists of a pair of nitrogenous bases linked together by a hydrogen bond. Adenine pairs only with Thymine (A-T), and Guanine with Cytosine (G-C). The two chains are coiled like a spring to give a double helix structure.

The structure of the chromosome

DNA double helix structure

During cell division, the chromosomes reproduce themselves. This is achieved by the replication of DNA molecules. As the two strands unwind, free nucleotides in the cell are joined to each strand appropriately to form two new double helices of DNA. When T is exposed, only A is added from the nuclear fluids (cytoplasm). Similarly when C is exposed, only G is added.

Heredity information exists in coded form in the DNA. The DNA determines the makeup of proteins, enzymes and other substances in a cell. It controls the physical and chemical activities of each cell as well as the entire organism. The DNA molecule is structurally the same in all organisms but instruction or genetic codes are arranged in different sequences for every species. It is these codes that determine the pattern of growth and behaviour of every member of a species.

Role of Chromosomes in Transmission of Hereditary Characters

There is a segregation of genes at meiosis to produce haploid gametes. These genes (from the parents) recombine during sexual reproduction (fertilization) to produce a zygote that develops into an offspring.

The role of chromosomes in making this possible is as follows;

(a) Formation of gametes

As a result of the meiotic division of cells, gametes are formed.

During meiosis, members of a homologous pair of chromosomes separate first, then the sister chromatids of each chromosome separate. The result is one diploid cell giving rise to four haploid cells (gametes), thus meiosis is a reduction division. Each gamete has one set of chromosomes and hence one copy of genes.

(b) Crossing Over

During the process of meiosis, exchange of genetic materials takes place between chromatids of a homologous pair of chromosomes; this is referred to as crossing over. This process gives rise to a new combination of alleles on a chromosome and hence more types of allele combinations in gametes.

(c) Fertilization

The process of fertilization also shows the role of chromosomes in transmitting hereditary characters. When a sperm fertilizes an egg to form a zygote, only the nuclei of the two cells fuse. This shows that chromosomes are the actual structural materials that transmit genes from the parents to the offspring.

Fertilization restores the homologous pair of chromosomes. Fertilization occurs randomly and brings about new chromosome combinations, hence new allele combinations.

EVALUATION

1. What causes the variations among living organisms of the same species?
2. Give five examples of hereditary variations that can be passed down from parents to offspring.
3. What are chromosomes and what role do they play in transmission of characters from parents to their offspring?
4. State and explain the two laws of Mendel

WEEKEND ASSIGNMENT

Discuss the following:

1. single-factor inheritance
2. albinism
3. gamete formation