Introduction
In somatic cells of most animals the chromosomes occur in homologous pairs. The two chromosomes of a homologous pair exactly match up and are morphologically similar. However, in many cases, there is an exception to this rule. The two chromosomes of one of the pairs differ morphologically from each other and are not homologous. This non-homologous pair of chromosomes is responsible for determining the sex an individual and is, therefore, called “sex chromosomes” or “heterosomes”. All the other chromosomes pairs are known as autosomes.
Example
In man, there are 22 pairs of autosomes and one pair of sex chromsomes.
Pattern of Sex Determination
The exact pattern of differences of chromosomes between the male and female of different species vary a great deal. Some patterns are discussed below:
1. Sex Determination in Grass Hopper (X-O System)
2. Sex Determination in Drosphila (X-Y System)
3. Sex Determination in Humans (x-Y System)
4. Sex Determination in Birds and Fishes (Z-W System)
1. Sex Determination in Grass Hopper (X-0 System)
The simplest pattern of chromosomal difference between the sexes is found in grass hopper. in these insects the male has one less chromosome than the female. The female has 24 chromosomes (11 pairs of autosomes and one pair of sex chromosomes) and the male has 23 chromosomes (11 pairs of autosomes and single sex chromosomes). The genotype of female is designated as XX and that of male as XO; the O stands for absence of sex chromosome. The male forms two types of sperms, one type containing 12 chromosomes (11 + X chromosomes) and the other type with 11 chromosomes (without any sex chromosome). Thus the male is said to be heterogametic. On the other hand the female produces one type of ova only containing 12 chromosomes (11 + X chromosomes). Therefore the female is termed as homogametic. If a sperm with 12 chromosomes fertilizes the ovum, a female is produced and if a sperm with 11 chromosomes fertilizes the ovum, a male is produced.
2. Sex Determination Drosphila (X-Y System)
T.H. Morgan, during his genetic experiments on fruit fly Drosphila, noted that male and female flies show difference in the chromosomes. Drosphila contains 4 pairs of chromosomes, out which three pairs are same in male and female and are homologous. These chromosomes are called autosomes. The fourth pair shows difference in male and female and is termed as the sex chromosomes, because it determines the sex. In female both sex chromosomes are rod shaped (morphologically similar). In male one sex chromosomes is rod shaped and the other is hook shaped (morphologically different). The rod shaped chromosomes, which are similar in both male and female are called X chromosomes. The hook shaped chromosomes present in male only is known as the Y chromosomes. Thus the genotype of female Drosphila is XX and that of male is XY. In other words in Drosphila the female is homogametic and the male is heterogametic.
3. Sex Determination in Humans (X-Y System)
In humans there are 23 pairs of chromosomes in a somatic cell out of which 22 pairs are similar both in males as well as females and are known as the autosomes. The 23rd pair constitutes the sex chromosomes and differ in the male and female. In female the two sex chromosomes are similar but in male the two sex chromosomes differ in size and shape. They are termed as X and Y chromosomes. The X chromsomes in humans is long and contains full compliment of the genes, but the Y chromosomes is short and carries lesser number of genes. It does not bear alleles of most of the genes present on the X chromosomes. An individual which inherits two X chromosomes becomes female (XX), while the one which inherits one X and one Y chromosomes becomes male (XY). Thus human female possesses genotype of 44 A + XX and the male has genotype of 44 A + XY.
It is obvious that the female produces one type of gametes only each containing one X chromosomes. Therefore, the human female is said to be homogametic. In contrast the male produces two types of gametes and is heterogametic. Half of the sperms formed contain X chromosomes and other half contain Y chromosomes. If a sperm containing X chromosomes fertilizes with the ovum, the zygote contains two X chromosomes and the resulting child would be female (XX). On the other hand if the fertilizing sperm contains Y chromosomes the zygote contains one Y and one Y chromosomes and the offspring would be male (XY). All mammals show X-Y system of sex determination and therefore the sex is determined by the fertilizing sperm.
4. Sex Determination in Birds and Fishes (Z-W System)
In birds, moths and some fishes it is the female which contains two different sex chromosomes and is heterogametic. The male is homogametic and contain two similar sex chromosomes. As a result two types of ova and only one type of sperms are produced and the ovum determines the sex of the individual. The sex chromosomes in these animals are designated as Z and W to avoid confusion with X-Y system. The genotype of male is ZZ and that of female is ZW.
Sex Linked Inheritance
Definition
All genes located on the sex chromosomes are said to be sex-linked. Inheritance of sex linked genes or any genetic trait which is transmitted through sex chromosomes is called the sex linked inheritance.
Sex Linked Inheritance in Drosphila
The sex linked inheritance was first discovered by Morgan (1910) in Drosphila during breeding experiments. In Drosphila the normal wild type has real eyes and a eye colour is dominant over white eye colour. When Morgan crossed a white eyed male with a red eyed female, the F1 and F2 generation showed simple Mendelian ratios. However when a red eyed male was crossed with white eyed female the results were different. From his experiments Morgan concluded that the gene controlling the eye colour in Drosphila is located on the X chromosomes and the Y chromosomes does not carry any allele for the eye colour.
Sex Linked Inheritance in Man
In humans there are about 200 genes present on the X chromosomes and therefore many sex linked traits are found. The two important sex linked traits are colour blindness and haemophilia. Pattern of their inheritance can be found by studying family histories (pedigrees) only because breeding experiments cannot be conducted on humans. The alleles for these traits are located on the x chromosomes and have no corresponding alleles on the Y chromosomes. Consequently, in the male even a single recessive X-linked allele can express itself in the phenotype, while in females such alleles are expressed only in double dose. Therefore, recessive sex-linked traits, such as colour blindness and haemophilia are relatively more common in male.
Inheritance of Colour Blindness
Colour blind persons cannot distinguish between red and green colours. This is a sex linked trait and is controlled by a recessive allele, which is present on the X chromosomes only the Y chromosomes does not bear any allele for colour blindness. The gene for normal vision (N) is dominant over the gene for colour blindness (n). Therefore a female can be colour blind only in homozygous condition (nn) and for that she needs to inherit two recessive alleles. A male needs to inherit only one recessive allele to be colour blind because the Y chromosome does not bear any allele for this trait. This explains why colour blindness is more common in men than in women. A bout 4% males are colour blind against less than 1% females. Heterozygote (Nn) are normal but are called carriers because they carry the defective allele and can transmit it to the next generation. The pattern of inheritance of colour blindness can be easily studied by making some theoretical crosses.
Haemophilia
Haemophilia is a human genetic disease caused by a sex-linked recessive allele, located on the X chromosomes. Haemophiliacs lack a blood clotting factor and their blood does not clot easily. Therefore haemophiliacs bleed excessively even after a minor injury. They may also suffer from internal bleeding and the severely affected individuals may bleed to death after relatively minor injuries. This disease is also called the bleeder’s disease and was common in royal families of the Europe. About 1 in 15000 males are born with haemophilia, but the disease is very rare in women. The reason is that a single recessive allele for haemophilia can express itself in male because the Y chromosomes is inert for this trait. However, a women will need to have two recessive alleles to suffer from the disease.
Pattern of Inheritance of Haemophilia
It is same as that of other sex-linked diseases controlled by recessive allele, such as colour blindness. However, the occurrence of haemophilia is rare, because the haemophilia usually die before marriageable age and therefore cannot transmit their defective alleles to next generations.
Linkage
Definition
The genes located on the same chromosome are said to be linked, and are expected to be distributed to gametes together during meiosis. The tendency of two or more genes in a chromosome to remain together during inheritance is called linkage.
However linkage is not absolute and in most cases, linkage is broken due to crossing over and some some recombinant forms appear. The linkage in such cases is termed as incomplete.
Crossing Over
The crossing over is defined as mutual exchange of chromosome parts between two non-sister chromatids of a homologous pair of chromosomes. After crossing over, chromosomes carry some genes that were earlier located on the other member of the pair of homologous chromosome. Due to crossing over lined genes are separated and enter different gametes. Therefore, in the next generation, some new phenotypes can be formed which are different from the two parents and are called recombinant.
Crossing over takes place during gamete formation at diplotene stage of meiosis and occurs randomly at one or more than one points called chiasmata along the whole length of homologous pair of chromosomes.
Example
The most through studies of linkage and crossing over have been made in fruit fly Drosophila. The wild type of Drosophila flies show following pairs of contrasting characters.
The genes for grey colour (B) and normal (V) are dominant over black colour (b) and vestigial wings (v). When a homozygous grey fly with normal wings (BBVV) is crossed with black fly with vestigial wings (bbvv), all F1 individuals are grey with normal wings, as expected. When the F1 heterozygous fly (BbVv) is test crossed with double recessive (bbvv), following results are obtained.
Gray normal …….. 40%
Black Vestigial …….. 40%
Gray Vestigial …….. 10%
Black Normal …….. 10%
Following conclusions can be drawn from above results
In case of complete linkage, no recombinants are expected and only the parental types would have been obtained.
In case of independent assortment, a ratio of 1:1:1:1 or 50% parental forms and 50% recombinants would have been obtained.
The cross does not show the above two results. It is therefore concluded that the two pairs of genes in question are linked together but get separated due to crossing over. Because crossing over takes place in about 20% of the cells undergoing meiosis, nearly 20% offspring, known as recombinants, show recombined characters in phenotpe.
Mechanism of Crossing Over
During their meiotic division, the homologous chromosomes pair together and this pairing process is called synapsis. Later on, during diplotene stage, the two non-sister chromatids of the homologous pair mutually exchange chromosomal parts and the crossing over takes place. The sites on the chromosomes where crossing over occurs are called chiasmata. After separation, the chromosomes carry genes that were previously located on the other chromosome of the homologous pair.
Terms Used in Genetics
Gene
Gene is a basic unit of hereditary information and consists of a sequence of DNA bases. Each gene occupies a specific position called locus on a chromosomes.
Gene Locus
It is the specific location of a particular gene along the length of a certain chromosome. In other words it is the position of alleles on homologous chromosomes.
Alleles
Alleles are alternative forms of teh same gene, controlling a give trait. Alleles of a gene are present on the same relative locus on a homologous pair of chromsomes. Each individual inherits two alleles for a given trait, one form each parent.
Example
in pea plant, the gene for seed coat has two alleles; one for smooth seed and the other for wrinkled seeds.
Homozygous
When a diploid organism contains two identical alleles for a particular character, it is said to be homozygous for that character.
Example
Genes TT and tt are homozygous for length of stem.
Heterozygous
When a diploid organism contains two different alleles for a particular character, it is termed as heterozygous for that character.
Example
Genes Tt are heterozygous for length of stem.
Dominant
In a heterozygous condition, the allele which expresses itself morphologically and masks the appearance of other allele is said to be dominant.
Example
In pea plant allele for tall stem is dominant over allele for dwarf stem.
Recessive
In a heterozygous condition, the allele which does not express itself morphologically and remains masked by the other allele is called recessive.
Example
Allele for dwarf stem is recessive in pea plant.
Genotype
The description of genetic make up of an organism for a particular trait.
Example
In pea plant genotype for tall stem can be TT (homozygous) or Tt (heterozygous).
Phenotype
It is the visible expression of genes, both physical and physiological.
Example
Phenotype for both TT and Tt genotype will be tall stem.
Hybrid
The product obtained as a result of crossing two pure breeding varieties of an organism with contrasting characters is known as hybrid.
Example
Individuals obtained as a result of crossing a pure breeding tall pea plant with dwarf pea plant will be a hybrid.
Test Cross
It is a cross which is used to determine the genotype that is homozygosity or heterozygosity of an organism. In this cross a phenotypically dominant individual, whose genotype is to be determined is crossed with a recessive homozygous individual (known genotype). If F1 shows all dominant individual the tested individual is homozygous. On the other hand, if F1 produces half dominant and half recessive offspring the tested organism is heterozygous.
Multiple Alleles
A gene controlling a trait may have more than two alleles in a population. In such cases the various allele forms are together called as multiple alleles. Since only two alleles can occupy the same locus in a homologous pair of chromosomes, a diploid individual can posses only two out of a set of multiple alleles.
Examples of Multiple Alleles
A well known example of multiple alleles in humans is the inheritance of blood groups. The classification of ABO blood group system is based on presence or absence of a certain substance known as antigen on the surface of the RBCs. There are two types of these antigens; antigen A and antigen B. A person with antigen “A” has blood group A, with antigen “B” has blood group B with both antigens has blood group “AB” and if there is none of the two antigens, the blood group will be O.
The production of these antigens is controlled by a gene known as “I” gene. The gene has three alleles represented by the symbols I(A), I(B) and i. The allele I(A) is responsible for production of antigen A; the allele I(B) produces antigen B, while the third allele i does not produce any of the antigens. The allele I(A) and I(B) are equally dominant (co-dominant) while allele i is recessive to both.
As only two of the three alleles can be present in any one individual, a person may posses on of the combination of the alleles and the resulting blood group.
Incomplete Dominance
Mendel’s experiment with Garden pea showed that in heterozygous condition, one allele is completely dominant over the other and expressed itself morphologically. Therefore, in Mendel’s crosses with pea plant, F1 off spring always looked like one of the two parental types. In other words heterozygous and homozygous dominants exhibited the same phenotype.
However, post-Mendelian studies have shown that this mode of inheritance is present in many organism but not in all the organisms. There are many instances where the dominance is partial and the heterozygous show phenotype which is intermediate between the two homozygous parents. An inheritance where neither of the two alleles is completely dominant over the other and the heterozygous show phenotype which is intermediate between the two homozygous parents is called incomplete dominance. It can also be said that in incomplete dominance blending of the characters occur.
Examples of Incomplete Dominance
Four O’clock plant (Mirabilis jalapa) shows two true breeding varieties for flower colour; red flower and white flowers. A cross between the true breeding red and white flowers produces pink flowers in F1 generation. On selfing the F2 generation produces Red, pink and white flowers in the ratio of 1:2:1.
Co-Dominance
It is a type of inheritance where in a heterozygous condition, the two alleles are equally dominant and both express themselves and separately in the phenotype. As a result the heterozygous offspring displays the phenotype of both the homozygous parents and none of the two alleles masks the appearance of the other.
Examples of Co-Dominance
In short horned cattle there are two true breeding varieties red and white for skin colour. When the two are crossed, the heterozygous offspring shows roan colour. However there is no hair of roan colour or any intermediate shade; only red hair and white hair are present on the skin. The appearance of roan colour is due to mixture of red and white hair. It is evident that both the alleles (for red and white hair) are equally dominant and have equally and separately expressed themselves in phenotype.
Epistasis
(Genetic Interaction)
Epistasis is a condition in which the expression of a gene depends on expression of another gene located at a different locus. The second gene, termed as epistatic gene, suppresses or modifies the expression of the first gene.
Epistasis is different from dominance in which one allele (dominant) masks the expression of other allele (recessive) located at the same locus. The term epistasis was coined by Bateson.
Inheritance of fur colour in mice is an example of epistasis. Normal wild fur colour is gray, but black and white colours are also found. There are two genes (two pairs of alleles) involved in determining the colour of the fur. One gene controls the synthesis of the pigment; the dominant allele “B” produces gray colour and the recessive allele “b” forms black colour. The second gene at a different locus, is responsible for deposition of the pigment. It has a dominant allele “C” and a recessive allele “c”. Expression of dominant allele is necessary for deposition of the pigment. Therefore, if a mouse has genotype cc for this gene, it will become white regardless of the genotype of the other gene (gray/black). It is evident that the gene responsible for deposition of the pigment controls the expression of the gene for synthesis of the pigment.
To illustrate further let us study results of a dihybrid cross between two gray coloured mice, both heterozygous for the two genes.
Pleiotropy
It is a condition in which a single gene can affect many phenotype characteristics of an individual. Such a gene with multiple effects is called pleitropic gene.
Example of Pleiotropy
- In Drosophila the gene for white eye also influences the colour of testes and shape of spermatheca.
- In cats the gene responsible for white fur and blue eyes also causes deafness.
- Another example of pleiotropy is a human genetic disease called phenylketonuria which produces severs mental retardation. However the affected children also have light hair and light skin colour. This shows that same gene is responsible for more than one phenotypic characters.
(Polygenic Inheritance)
Some traits, such as colour of human skin are controlled by two or more than two separate pairs of genes (alleles), located on different gene loci. These genes express themselves in additive manner producing continuous variations in phenotype. Such traits are known as continuously varying traits (polygenic traits) and their inheritance is called polygenic inheritance.
Polygenic traits show a range of intermediate phenotypes between the two parental phenotypes. These intermediate phenotypes appear because each allele produces a small effect on the phenotype and each additional allele increases the effect gradually. Thus the final phenotype is determined by the additive or cumulative effect of all the genes controlling that trait.
Example
The simplest polygenic situation occurs when there are at least two gene pairs which affect the same trait. In a certain wheat variety colour of the kernel is a polygenic trait and is determined by two pairs of genes (alleles). There is a dominant allele for red and a recessive allele for white. Each dominant allele produces a dose of red pigment, while the recessive allele does not produce any pigment. Therefore, the colour of kernel depends on the number of dominant alleles present.
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