CHAPTER 22 INHERITANCE

 CHAPTER 22

INHERITANCE

Basic Definitions

Inheritance:

Inheritance is the process of transmission of character from individuals of one generation to the next generation. It is regulated by the genes present on chromosomes.

Genetics:

            The branch of biology concerned with the study of inheritance, or the transmission and expression of genetic information.

Character:

            Any heritable feature or characteristic of an organism is known as character e.g. Height, Intelligence, blood group, and hair colour.

Trait:

            A Variant of a character is called as a trait e.g. black or brown hair colour, and short or tall stature. These are characteristic that an organism displays. Also include organism’s behavior (behavioral traits).

Gene: Gene are the unit of heredity. It is a segment of DNA that contains the information to make a functional protein.

Allele:

            Alleles are the alternative form of a specific gene. Alleles Control the same characteristics (e.g. hair colour) but producing different effects (e.g black or red)

Back cross:

            Back cross refers to a cross of F 1 hybrids to individuals that have genotypes of the parental generation or with its parents.

Test cross:

            A cross between a recessive individual and a dominant individual whose genotype is unknown (Homozygous or heterozygous) is known as test cross.

Pure breeding:

An individual that has two identical alleles of a particular gene or an individual that when self-crossed produces progeny of its own type.

Hybrid:

            An individual that has two different alleles of a particular gene A progeny individual from any cross involving parents of different alleles.

Dominant Allele:

An allele that determines the phenotype in the heterozygous condition. For example, if a plant is Tt and has a tall phenotype, the T (tall) allele is dominant over the t (dwarf) allele.

Recessive allele:  

A gene that is masked by the presence of a dominant gene is called recessive.

Reciprocal Crosses:

A pair of crosses in which the traits of the two parents differ with regard to gender. For example, one cross could be a red eyed female fly and a white-eyed male fly, and the reciprocal cross would be a red-eyed male fly and a white-eyed female fly

 

22.1: Mendelian – inheritance  / Origin of Genetics:

The Modern genetics originated in the mid 1800 with a monk named Gregor Mendel, who discovered the basic principles of heredity by breeding garden peas in carefully planned experiments. The rediscovery of his article led to the explanation of basic principles of genetics and he was titled as the “Father of Genetics “.

22.1.1: Gregor John Mendel and hsis work:

Biogeography:

Gregor John Mendel (born on 22 July 1822 in Czech Republic and died on 6 January 1884 in Brunn) was a German-speaking scientist. He conducted planed experiments of nearly 28,000 garden pea plants between 1856 and 1863. The finding are published in 1866 in his paper “experiment on plant hybridization”. These experiments established many rules of heredity later called as the laws of Mendelian inheritance.

 

His work was neglected:

Mendel’s published paper got no impression for nearly 34 years. Later on in 1900 a Hugo de Vries (Dutch), De Correns (Germany) and Tschmarck (Austria) recognized his work.  The most common reasons Mendel’s work got no impressions were:

1. Most of biologists were preoccupied with Darwin’s theory of organic evolution.
2. The journal in which the Mendel’s work was published was not recognized.
3. Most biologist were unfamiliar with the statistical analysis used by Mendel.

 

22.1.2: Mendel’s Experimental model:

Mendel first very carefully selected a suitable plant for his hybridization experiments. He selected the garden pea (Pisum sativum) for the following reasons:

a.      presence of Self-pollination: Garden pea plants are self-pollinating.

b.      Cross pollination: Cross pollination and fertilization can also be achieved easily.

c.       Easy to grow: The pea plant was easy to grow in pots and open garden.

d.      Distinct heritable characters: Peas had many sharply defined inherited characters.

e.       Hermaphrodite flowers: The flowers are bisexual and hermaphrodite.

f.        Short life cycle: It had a short life cycle and germinates in upto7 days.

g.      Large Number of seeds: Its plant produces a large number of seeds.

Contrasting traits of pea plant:

Mendel performed hundreds of crosses using the following Seven pairs of contrasting characters in garden pea plant.

S. No.	Characters	Contrasting pairs (Allelic pairs)
		Dominant	Recessive
1.	Flower color	Purple (P)	White (p)
2.	Seed color	Yellow (Y)	Green (y)
3.	Seed shape	Round (R)	Wrinkled (r)
4.	Pod color	Green (G)	Yellow (g)
5.	Pod shape	Inflated (I)	Constricted (i)
6.	Flower position	Axial (A)	Terminal (a)
7.	Plant height	Tall (T)	Dwarf (t)

Experimental approach:

First, Mendel cross-pollinated two contrasting, true-breeding pea varieties e.g. Round-seeded plants and wrinkled-seeded plants (P generation or parental generation). Hybrid offspring were produced (F1 generation or first filial generation). Second, he allowed these F1 hybrids to self-pollinate and produces an F2 generation (second filial generation). He followed a single character e.g. shape or color to study dominance relation and two characters in a single cross like color and shape to study independent assortment of genes.

22.1.3: Inheritance of single trait:

Monohybrids cross:

A monohybrid cross is a cross between the two individuals that are hybrid (heterozygous) for one particular character. Mendel derived the Law of Segregation from these crosses.

Parental generation:

Mendel started with a cross between a true breeding (Homozygous) dominant variety (e.g. Round seeded plant) and a recessive variety (e.g. Wrinkled seeded plant). These two varieties he referred as parental generation (P₁).

Gametes:

The pure breeding round seeded plant has both the dominant alleles i.e. RR and wrinkled seeded plants has both recessive alleles i.e. rr, therefore, gametes produced by these plants have dominant allele R and recessive allele respectively.  

F1 progeny:

First filial generation showed all dominant phenotypes that were hybrid for the seed shape.

P2:

Mendel self-crossed the individuals of F1 generation.

Gametes:

The F1 individuals were Heterozygous dominant, Therefore, they process to types of gametes i.e. with a dominant allele R and recessive allele r.

F2 progeny:

In the Second filial generation 3:1 ratio was observed. Three out of four were dominant while one was found to have recessive character. The genotypic ratio was 1:2:1 i.e. one homozygous dominant, two heterozygous dominant, and one recessive.

Mendel’s predictions:

Mendel predicted that:

a.       Characters are controlled by some agents. He called these agents as factors or elements.

b.      Each parent has a pair of these factors for one character.

c.       Each parent inherits only one of these factors to their offspring (Assort independently during gamete formation).

d.      Factors retain their individuality from generation to generation.

 

Mendelian factors were later named as gene in 1900 by Johnson.

22.1.4: Mendel’s principles of inheritance:

 

The principles of inheritance that arose from the Mendel careful experiments on the pea plants are the:

 

a.       Law of dominance,

b.      Law of segregation and

c.       Law of independent assortment.

Symbols used to represent Mendel’s Laws:

Alleles for particular characters were represented by the upper case letters (Dominant) and lower case letters (recessive). For example, the representation for the height would be:

• Dominant homozygous (tall):  TT
• Dominant heterozygous (tall):  Tt.
• Recessive trait (dwarf) :  tt

 

Along with the letter presentation a + sign was also use for Wild type phenotype.

i. Law of dominance:

Mendels this law states that, in a heterozygous condition, the dominant allele, determines the organism’s appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance.

Complete dominance:

In complete dominance, one of the allele for a particular character completely dominates over the other and masks its phenotypic expression in the heterozygous condition. In this type of dominance relation, both homozygous dominant and heterozygous dominant have the same phenotype.

Example:

a.       Accordingly, Mendel’s F1 plants had purple flowers because the allele for that trait is dominant and the allele for white flowers is recessive.

b.      Round yellow seed is dominant over wrinkled green seed shape plant.

Mendel crossed a true breed  purple flower plant with true bred white color flower plant. He observed that all the off spring in the  F1 generation were purple color flowering plants. He supposed purple color as “dominant.”

 

 

22.1.4.ii Law of segregation:

Statement:

Law of segregation states that the two alleles for a heritable character segregate (separate from each other) during gamete formation and end up in different gametes. Thus, an egg or a

sperm gets only one of the two alleles that are present in the diploid cells.

This law is applicable to the genes present on the same locus (alleles). Alleles are located on the homologous chromosomes. This law is also called as “law of Purity”.

Explanation:

According to law of dominance or law of purity of gametes, in a heterozygous condition the dominance and recessive allele remain together without mixing with each other. The allele separate or segregates from each other during gametogenesis, so that each gamete receives only one allele, either it is dominant or recessive.

Law of segregation can be easily demonstrated in a monohybrid cross.

Monohybrid cross:  It is a cross between the two individual that are hybrid (heterozygous) for only one character.

 

22.1.4: iii. LAW OF INDEPENDENT ASORTMENT:

Statement:

Law of independent assortment states that two or more genes assort independently that is, each pair of alleles segregates independently of any other pair of alleles during gamete formation. This law applies only to genes (allele pairs) located on different chromosomes (non-homologous) or, alternatively, to genes that are very far apart on the same chromosome. This principle can be easily demonstrated in a dihybrid cross.

Dihybrid crosses: It is a cross between the two individual that are hybrid (heterozygous) for two characters e.g. seed shape and seed color (Round Yellow Seed – RrYy)            

How genes get separated?

Independet assortment of genes occurs because of the presence of two different genes i.e. gene for seed color and gene color either on non-homologous chromosomes or due to crossing over (exchange of chromosomal segments in meiosis) incase the genes are present on homologous chromosomes.

Example:

P1 Generation: Mendel crossed true bred plants for round seed shape and yellow seed color (genotype RRYY) with plants true bred for the recessive condition, wrinkled seed shape and green seed color. (genotype rryy).

F1 generation:

All Individual of the F1 generation were hybrid (Heterozygous) for the dominant traits i.e. round seed shape and yellow seeds color.

Self-fertilization of F1:

Mendel self-fertilized F1 plants (RrYy). The F2 progeny included both parental phenotypes and also new varieties were obtained (recombinants), proving that genes for the shape and color have assorted independently.

Result: Phenotypic ratio:

• Round yellow: 9
• Round green: 3
• Wrinkled yellow: 3
• Wrinkled green: 1

Limitations of Mendelian Principles:

A.      Mendelian principles cannot explain incomplete dominance, co-dominance, over dominance, and other complex inheritance patterns.

B.      Mendelian principles can only be applied to diploid organisms.

C.      Genes on the same chromosomes could not be assorted independently until crossing over.

D.     X linked inheritance patterns may also vary from the Mendel’s inheritance patterns

 

STATISTICS AND PROBABILITY RELEVANT TO GENETICS

Probability is the extent to which an event is likely to occur. The probability scale ranges from 0 to 1. An event that is certain to occur has a probability of 1, while an event that is certain not to occur has a probability of 0. Mendel’s laws also follow the same rules of tossing coins. There are two basic rules of probability that help us predict the outcome of the fusion of gametes.

EXCEPTIONS TO MENDALIAN INHERITANCE:

In Mendel’s classic pea crosses, the F1 offspring always looked like one of the two parental varieties because one allele in a pair showed complete dominance over the other. There are some other dominance relations between the two alleles that do not follow the Mendel’s law of dominance. Following are the few exceptions to Mendelian inheritance.

1)      Incomplete dominance

2)      Co-dominance

3)      Over-dominance

1.      INCOMPLETE DMINANCE:

In incomplete dominance, none of the allele for a character completely dominates over the other. In this type the phenotypic expression in the heterozygous condition is intermediate between both parents.

 

Intermediate phenotype:

The F1 phenotype did not resemble either of the two parents. The inheritance of flower colour in snapdragon or Antirrhinum sp. and Japanese 4’Oclok pant are good examples. In a cross between true-breeding red-flowered (RR) and truebreeding white-flowered plants(rr), the F1 (Rr) was found pink.

 

F1 Self-fertilization When the F1 was self-pollinated the F2 resulted in the ratio 1 (RR) Red: 2 (Rr) Pink: 1 (rr) White

 

 

 

2.      CO-DOMINANCE:

In a do-dominance relation, the effect of both alleles is equally visible in the phenotype of the heterozygote without being diluted by the presence of the other allele (as in incomplete dominance) or being suppressed by a dominant allele (as in complete dominance).

Example The ABO blood group system provides an example. Three alleles, I^A,  I^B , and I determine a person’s blood type. The two of these alleles, I^A,  I^B are codominant to each other, producing an AB blood type in the heterozygote.

 

4. Over Dominance:

It is and inter-allelic dominance relation in which the phenotypic expression of a dominant allele is more in a heterozygote than in a homozygote.

Example:

The expression of florescent pigment in drosophila eyes is an example of over dominance. The florescence is more in heterozygote fruit fly (W⁺/W) than wild (W⁺ /W⁺) or white eye (W/W) homozygote.

MULTIPLE ALLELES:

Multiple Allele:

An allele is the alternative form of a gene. When more than two forms of a gene exist on a single locus of a chromosome, the alleles are then called as multiple alleles.

Production of multiple alleles:

Multiple alleles are produced by gene mutation. A slight change in the nucleotide sequence of a gene results in the formation of alleles.

Number of multiple alleles: The number of alleles controlling a character varies. The ABO blood group system is controlled by three alleles of gene I. Some genes may have as many as 300 alleles for a character. Multiples alleles exist in the individual of a population but individuals have only two of those alleles. It is because most of organisms are diploid having two homologs of each chromosome.

 

 

22.A:  ABO BLOOD GROUP:

History:

ABO blood group system was discovered in 1901 by Karl Landsteiner of the University of Vienna. Later he was awarded a Nobel Prize. ABO blood groups are found in all humans and in many primates such as apes, chimpanzees, baboons and gorillas.

Genetic Basis:

The ABO blood groups are controlled by Gene I located on the chromosome 9. Gene I has three allelic forms i.e. I^A ,I^B and i. The I^A and I^B alleles each encode a glycosyltransferase that catalyzes the synthesis of the A and B antigen, respectively. The O or i allele encodes an inactive glycosyltransferase that leaves the ABO antigen precursor (the H antigen) unmodified.

 

Blood type	Antigen on RBC	Allele for Antigen	Possible Genotypes	Dominance relation
A	A	IA	IA IA ,  IAi	IA Dominant to i
B	B	IB	IB IB , IBi	IB Dominant to i
AB	A & B	IA and IB	IA IB	IA and IB  Co-Dominant
O	None	I	ii	i is recessive

 

Blood group types:

A person’s blood group may be one of four types: A, B, AB, or O. These blood group types are due to the presence of antigen A and antigen B on the surface of RBC. A person having antigen a on the surface of RBC will have blood group A, having antigen B on the surface of RBC will have blood group B, having both A and B antigens will have blood group AB. When no antigen is expressed (nor A neither B), then the blood group is said to be O blood group.

 

Ability to produce Antibodies against antigen A and Antigen B

Blood group is determined by antigens present on the surface of RBCs. The immune system has the ability to produce antibodies against the foreign antigens. In normal conditions, the body do not produce antibodies against body own cells, but can produce against any foreign agent that enters in to the body.

Blood group A: An individual with the blood groups A has antigen A on the RBC surface, therefore, it only produce Antibodies against the antigen B.

Blood group B: A person having blood group B produces antibodies against the antigen A.

Blood group O: A person having none of antigen on the surface of RBC (Blood group O) produces anti-A and Anti-B antibodies.

Blood group AB: Those have AB blood group do not produce antibodies.

Universal donor:

Blood type O has no antigen therefore; a person with blood group O can donate the blood to all blood types.

Universal recipient: Person with blood type AB do not produce anti-A and anti-B antibodies, therefore can transfuse blood from all blood types.

Other blood group systems:  Beside the ABO blood group system there are also some other blood groups systems i.e. Rh, MNS, P, Lutheran, Kell, Lewis, secretor, Duffy, kidd, Diago, etc.

 

22.4 Rh Blood Group System:

            The Rh Blood group system is one of the clinically most important blood group system. The term Rh in the name of this system refers to the Rhesus monkey. The antigen determining this blood system was first discovered in Rhesus monkey by Landsteiner in 1930s.

Basis of Rh blood group system

            The Rh Blood group system is determined by upto 50 known antigens among which D, C, E, c, and e are the most significant. Rh blood group system is mainly controlled by D gene which determines the formation of D-antigen/Rh factor, while its alternative allele d inhibits the formation of Rh factor. The allele D is completely dominant over the allele d.

Types of blood groups

The two types of blood groups in this system are RH positive and Rh negative.

a.      Rh positive: The person having “D” antigen on the RBC membrane will have the Rh positive blood group.

b.      Rh negative: The person without “D” antigen on the RBC membrane will have the Rh negative blood group.

 

Table 22.4  Rh-Blood group system
Blood group	Rh antigen	Genotypes	Anti Rh-Antibody	Can receive blood from	Can donate blood to
Rh Positive	Present	DD or Dd	Not Produced	Rh Positive and Rh Negative	Rh Positive
Rh Negative	Absent	dd	Produced (if stimulated)	Rh negative	Rh Positive and Rh Negative

 

Maternal-Foetal Rh incompatibility – Erythroblastosis Foetalis:

Erythroblastosis fetalis, is a hemolytic disease of the newborn. In this disease the red blood cells (erythrocytes) of a fetus are destroyed from a blood group incompatibility between the fetus and its mother. This incompatibility arises when the fetus inherits Rh factor from the father that is absent in the mother.

Causes: Rh factor inherits as a dominant trait. When a male with Rh positive blood (having Rh antigen) marries a female with Rh negative blood (not having Rh antigen), then the fetus will have the Rh positive blood group. When, fetal red blood cells with the Rh factor enter the mother’s bloodstream. They stimulate the production of antibodies, some of which pass across the placenta into fetal circulation resulting in the destruction of red blood cells of the fetus. The anaemic fetus then starts to produce many immature erythroblasts into his blood stream Thus called erythroblastosis fetalis. This may leads to abortion or still birth.

22.5 Gene Interactions:

            The phenomenon of two or more genes affecting the expression of each other in various ways in the development of a single character of an organism is known as gene interaction.

a.       Intra-genic Interactions – Interaction between two allelomorphs of a single type of gene.

b.      Inter-genic interaction – interaction between the alleles of different gene pairs located on different loci of same or different chromosomes.

 

Epistasis:

The term epistasis is Greek word mean “standing upon”. It is a type of intergenic interaction in which a gene masks or modifies the expression of another gene present at a different locus.

Genes:

Epistatic Gene: The gene which modifies or mask the phenotypic expression of another gene called epistatic gene.

Hypostatic Gene: It is a gene whose phenotypic expression is affected by a epistatic gene.

Example:

Epistatsis can be best demonstrated in Bombay phenotype and coat color of Labrador retrievers.

 

Bombay phenotype:

Bombay phenotype is a rare blood group phenotype in which individuals are genetically of type A, B, or AB blood group but phenotypically O type blood group. This blood phenotype was first discovered in Bombay (Mumbai) in India by Dr. Y.M. Bhende in 1952.

Genetic basis of Bombay Phenotype:

The ABO blood groups are determined by antigen A and antigen B present on the surface of RBC. The production of antigen A and antigen B antigen is controlled by gene I^A and I^B and is also dependent upon gene H (present on chromosome 19). The dominant H gene allele produces H substance, a precursor for the antigen A and antigen B. Antigen A and antigen B will only be produced if the H substance is present. The allele I^A and I^B modifies the H substance to antigen A and antigen B respectively.

 

Genotype		H Substance	Type of Antigen	Blood group
I gene	H gene
IA IA ,  IAi	HH, Hh	Produced	A	A
IB IB , IBi	HH, Hh	Produced	B	B
IA IB	HH, Hh	Produced	A and B	AB
IA IA ,  IAi	hh	Not produced	None	O (Bombay)
IB IB , IBi	hh	Not produced	None	O (Bombay)
IA IB	hh	Not produced	None	O (Bombay)

 

22.5.3 polygenic inheritance

Polygenic inheritance: The inheritance of those traits that are controlled by more than two gene is called as polygenic inheritance.

Polygenic traits: Those traits which are encoded by more than two gene found at different loci are called pohygenic traits.

Polygenes: All the genes that control a polygenic trait or a quantitative trait are called polygenes.

Quantitative traits: Those traits, which have various numbers of phenotypes, are called quantitative traits. These traits show continuous variations between the two extreme phenotypes. Common examples in human beings are height, body weight, intelligence, and skin color.

Qualitative Traits: Those traits which have few phenotypes and show discontinuous variations are called qualitative variations e.g: Pea seed shape, human blood group types.

Properties of polygenic traits:

Following are the common properties of polygenic traits.

a.       Vary in the population in gradations along a continuum (show continuous variations).

b.      There is an additive effect of two or more genes on a single phenotypic character.

c.       More than two gene control the expression of these traits (may be up to more than a hundred).

d.      The recessive gene also expresses in the presence of a dominant.

 

Human skin color:

Human skin pigmentation is a good example of polygenic trait. It is controlled by many separately inherited genes, up to 378 at latest count. Most of these are involved in the production of melanin (a skin pigment). To simplify and easily study the inherence let’s consider three genes, with a dark-skin allele A, B, or C and light-skin allele a, b, or c.

Shades in skin color: A person with AABBCC would be very dark, whereas an aabbcc individual would be very light. An AaBbCc person would have skin of an intermediate shade. Due to the cumulative effect, the genotypes AaBbCc, AABbcc, aaBbCC, aaBBCc and AAbbCc would make the same genetic contribution. There are seven skin color phenotypes that could result from a mating between AaBbCc heterozygotes.

No. of dominant allele	6	5	4	3	2	1	0
Phenotype	Dark brown	Moderately dark brown	Brown	Light Brown	Pinkish brown	Light brown	Pure white
Ratio	1	6	15	20	15	6	1

 

Genetic control:

Gene A: Gene A controls the survival, proliferation and migration of melanocytes.

Gene B: Gene B encodes an enzyme named tyrosinase which produces melanin from tyrosine.

Gene C: Gene C determines whether pheomelanin or eumelanin will be produced.

 

Wheat grain color

Wheat grain color is also a good example of polygenic trait. It is controlled by many separately inherited genes. The genetics of wheat grain color was studied by Nilsson Elle.

Shades in grain color: A grain with AABBCC would be dark red, whereas an aabbcc would be white. An AaBbCc would have an intermediate shade (light red). Due to the cumulative effect, the genotypes AaBbCc, AABbcc, aaBbCC, aaBBCc and AAbbCc would make the same genetic contribution. There are seven skin color phenotypes that could result from a mating between AaBbCc heterozygotes.

No. of dominant allele	6	5	4	3	2	1	0
Phenotype	Dark red	Moderately dark red	Red	Light red	Pink	Light pink	white
Ratio	1	6	15	20	15	6	1

 

22.6.1 GENE LINKAGE:

According to the chromosome theory of inheritance, the Mendelian factors now called genes reside on the chromosomes. The number of genes in a cell is greater than the number of chromosomes. In human beings for examples the chromosome number is 46 and genes in thousands. Therefore, each chromosome has hundreds and thousands of genes.

Linked genes: Genes that are located on the same chromosome and tend to be inherited together in genetic crosses are said to be linked genes.

Linkage group: The group of all the linked genes found on the same homologous pair of chromosome is known as linkage group. Therefore, the number of linkage groups in an organism is equal to the number of homologous pair of chromosomes.

Gene linkage: The phenomenon in which the linked genes stay together on the same chromosome is called gene linkage.

Autosomal linkage: It is linkage phenomena, in which many genes are linked on autosomes,

Sex linkage: When genes are linked on sex chromosome, their linkage is called sex linkage.

Example of gene linkage: In human beings, the genes for sickle cell anaemia, leukemia, and albinism are found on chromosome 11. Thus these genes are linked genes and the type of linkage is autosomal linkage. These genes tend to be inherited together in the offspring.

22.6.2 Detection of gene linkage

A dihybrid test cross (between two gene pairs) can detect gene linkage. In a dihybrid test cross, a heterozygous individual for two traits is crossed with a recessive parent for two traits. If only parental variety is produced then a tight linkage exists between the genes for the two traits. When both parental and recombinants (four phenotypic combinations) are produced in equal 1:1:1:1 ratio, then there would be no linkage between the genes. When this ratio is deviated i.e. more parental types and less recombinant types, this indicates the incomplete or partial linkage.

 

Test cross experiment

T. H. Morgan performed a dihybrid test cross to see how linkage between genes affects the inheritance of two different characters in Drosophila. In Drosophila, the normal shape of wings is dominant over vestigial wing. Similarly, grey body color is dominant over black body color.

 

Character	Dominant trait	Recessive trait
Wing shape	Normal wing	Vestigial wing
Body color	Grey color	Black color

 

Morgan made a cross between the individual having grey body and normal wings with another individual having black body and vestigial wings, all the F1 progeny inherited grey body and normal wing phenotypes, When F1 flies were test crossed with their P1 recessive, following results were observed:

a.       Grey body and normal wings (parental type) = 965

b.      Black body and vestigial wings (parental types) =944

c.       Grey body and vestigial wings (recombinant types) =206

d.      Black body and normal wings (recombinant types) =185

From the above parental and recombinants ratio, Morgan concluded that the genes for body color and wing size are located on the same chromosome (linkage exist). However, small number of recombinants indicated that occasionally this linkage breaks (due to crossing over).

 

Crossing over

Crossing over is an exchange of maternal and paternal chromatid parts between homologous chromosomes.

Occurrence: This exchange of chromosomal segments occurs during prophase of meiosis I.

Consequences: This recombination brings alleles together in new combinations, resulting in a variety of gametes. Crossing over results in the breaking of gene linkage and formation of recombinants. The frequency of recombination is based on the percentage of meiotic divisions. Sometimes a second crossing over may cancel the effect of first crossing over by bringing the separated allele back together.

The farther apart two genes are, the higher the probability that a crossover will occur and therefore the higher the recombination frequency.

Recombination frequency and genetic map of chromosomes

Recombination frequency A recombination frequency is the percentage of recombinant in the total pool of offspring. The farther apart two genes are, the higher the probability that a crossover will occur and therefore the higher the recombination frequency.

Calculating recombination frequency: let take an example, if the number of recombinant individuals in a dihybrid test cross are 391 and the total number of individuals are 2300 then recombination frequency can be calculated using the following formula.

Chromosome mapping:

Genetic map: A genetic map, an ordered list of the genetic loci along a particular chromosome.

Linkage map: A genetic map based on recombination frequencies is called a linkage map.

Map unit: One map unit as equivalent to a 1% recombination frequency. Sturtevant expressed the distances between genes in map units.

 

Constructing genetic map:

 Sturtevant predicted that a genetic map can be constructed by using recombination frequencies. The farther apart two genes are, the higher will crossover will occur between them and therefore the higher the recombination frequency. According to him,  the greater the distance between two genes, the more points there are between them where crossing over can occur. Thus using recombination frequencies Sturtevant assigned relative positions to genes on the same chromosomes.

Linkage map of drosophila b, vg, and cn genes:

In Drosophila, grey body color is dominant over black body color, normal shape of wings is dominant over vestigial wings, and normal eye color is dominant over mutant cinnabar brighter red eyes. The genes for these three characters are b, vg, and cn respectively. The recombination frequency between cn and b is 9%; that between cn and vg, 9.5%; and that between b and vg, 17%. In other words, crossovers between cn and b and between cn and vg are about half as frequent as crossovers between b and vg. From this information cn locates about midway between b and vg.

22.6 SEX-DETERMINATION:

 

Sex (Gender) in many species is determined largely by the combination sex chromosomes. Human beings and other mammals have two types of sex chromosomes, designated X and Y. A person who inherits two X chromosomes, one from each parent, usually develops female anatomy, while male properties are associated with the inheritance of one X chromosome and one Y chromosome.

SRY gene:

A gene on the Y chromosome, called SRY (sex-determining region of Y) is required for the development of testes. In the absence of SRY, the gonads develop into ovaries (even in an XY embryo).

 

Usually gender is determined by a heterogametic individual. These are various systems of sex determination.

 

a.        X-Y system:

 

Occurrence: This system is dominant in mammals.

Mechanism of determination: Sex of the offspring is determined by male. The sex of an offspring depends on whether the sperm cell contains an X chromosome or a Y. If a sperm cell bearing an X chromosome fertilizes an egg, the zygote is XX, a female; if a sperm cell containing a Y chromosome fertilizes an egg, the zygote is XY, a male.  

 

b.       Z-W system.

 

Occurrence: This system is present in birds, some fishes, and some insects (Butterfly and moths).

Mechanism of determination: The sex of an offspring depends on whether the ovum cell contains an X chromosome or a Y. If an ovum cell bearing an X chromosome fertilizes with a sperm, the zygote is XX, a male; if an ovum cell containing a Y chromosome fertilizes with a sperm, the zygote is XY, a female. The sex chromosomes are designated Z and W. Females are ZW and males are ZZ.

 

 

c.        The X-0 system.

 

Occurrence: In grasshoppers, cockroaches, and some other insects.

Mechanism of determination: There is only one type of sex chromosome, the X. Females are XX; males have only one sex chromosome (X0). Sex of the offspring is determined by whether the sperm cell contains an X chromosome or no sex chromosome. If a sperm cell bearing an X chromosome fertilizes an egg, the zygote is XX, a female; if a sperm cell with no sex chromosome fertilizes an egg, the zygote is Xo, a male. 

 

d.       Haplo-diploid system.

 

There are no sex chromosomes in most species of bees and ants. Females develop from fertilized eggs and are thus diploid. Males develop from unfertilized eggs and are haploid; they have no fathers.

 

e.        Sex determination in drosophila:

 

In Drosophila, males are XY, but sex not only depends on the presence of a Y chromosome.  Drosophila is a diploid organism with a pair of sex chromosome (XX or XY) and three autosomal pairs of chromosomes (two autosomal sets). Sex depends on the ratio of the number of X chromosomes to the number of autosome sets.

 

Sex chromosomes combination	Autosomal sets	Total chromosomes	X:A	Sexual phenotype
XX	AA	8		Female
XY	AA	8		Male
Xo	AA	7		Male
XXY	AA	9		Female
XX	AAA	11		Intersex

 

An X:A ratio of 1.00 or higher produces a female whereas an X:A ratio of 0.5 or lower produces a male. The X:A between 0.5 and 1.0 produces intersex.

 

Comparison of chromosoml determination of sex between human beings and Drosophila:

 

Sex (Gender) in many species is determined largely by the combination sex chromosomes. Human beings and other mammals have X and Y sex chromosomes. A person, who inherits two X chromosomes, develops female anatomy, while male properties are associated with the inheritance of one X chromosome and one Y chromosome. In this determination the SRY (sex-determining region of Y) gene on the Y chromosome, is required for the development of testes. In the absence of SRY, the gonads develop into ovaries (even in an XY embryo). In drosophila on the other hand, sex determination is not associated SRY gene. The Sex depends on the ratio of the number of X chromosomes to the number of autosome sets.

 

Species	Chromosomal combinations and Gender
	XX	XY	Xo	XXY
Drosophila	Female	Male	Male	Female
Human	Female	Male	Female	Male

 

 

Sex linkage in drosophila

History: In 1860, Gregor Mendel proposed the existence of “hereditary factors” but their location was unclear. Later the study of mitosis and meiosis showed that Mendel’s factors (genes) and chromosomes have same behavior. Chromosomes and genes are both present in pairs in diploid cells, and homologous chromosomes separate and alleles segregate during the process of meiosis. After meiosis, fertilization restores the paired condition for both chromosomes and genes. This discovery led to the development of the chromosome theory of inheritance in 1902 by Walter S. Sutton and Theodor Boveri. According to this theory, Mendelian genes have specific loci (sites) along chromosomes and therefore, these are chromosomes that undergo segregation and independent assortment. Thomas Hunt Morgan, an experimental embryologist at Columbia University was initially doubtful about Mendelian genetics and the chromosome theory. His early experiments provided convincing evidence that chromosomes are indeed the location of Mendel’s heritable factors.

 

Mendel’s model organism

Morgan selected a species of fruit fly, Drosophila melanogaster, a common insect that feeds on the fungi growing on fruit.

Characteristics:

a.       Fruit flies are prolific breeders; a single mating will produce hundreds of offspring.

b.      New generation can be bred every two weeks.

c.       The fruit fly is that it has only four pairs of chromosomes, which are easily distinguishable with a light microscope. There are three pairs of autosomes and

c.one pair of sex chromosomes. Female fruit flies have a pair of homologous X chromosomes, and males have one X chromosome and one Y chromosome.

Symbolizing the alleles: The alleles in Drosophila are symbolized by w. A superscript + identifies the allele for the wild-type trait: w⁺ for the allele for red eyes, and w for allele of white eye trait.

Experimental work: Morgan carried out many matings of fruit flies and then microscopically inspected the offspring in search of naturally occurring variant individuals. After 2 years of work, finally he was rewarded with the discovery of a single male fly with white eyes (mutant) instead of the usual red (wild type).

 

Morgan mated the white-eyed male fly with a red-eyed female. All the F1 offspring had red eyes, suggesting that the wild-type allele is dominant.

When Morgan bred the F1 flies to each other, he observed the classical 3:1 phenotypic ratio

among the F2 offspring. However, the white-eye trait showed up only in males. All the F2 females had red eyes, while half the males had red eyes and half had white eyes.

Morgan concluded that a fly’s eye color was linked to its sex. If not, half of the white-eyed flies would have been female. Morgan concluded that the gene involved in his white-eyed mutant was located exclusively on the X chromosome, with no corresponding allele present on the Y chromosome. Therefore, for a male, a single copy of the mutant allele would confer white eyes and a female could have white eyes only if both her X chromosomes carried the recessive mutant allele (w). To test the hypothesis Morgan crossed the F1 females heterozygous for white eye and white-eyed male.

Morgan further crossed a white-eyed female with a red-eyed male to confirm that color gene is located on X chromosome. All the female progeny was red-eyed because they inherited one of their X chromosome from the male parent, while all male individuals were white eyed because they inherited X chromosome from the female parent.

 

Modes of Inheritance

Modes of inheritance are rules that explain the common patterns of single-gene transmission. Knowing the mode of inheritance makes it possible to calculate the probability of inheritance of a particular condition. The inheritance depends on whether the gene that determines it is on an autosome or on a sex chromosome, and whether the allele is recessive or dominant.

Following are the common modes of inheritance.

a.       Autosomal dominant

b.      Autosomal recessive

c.       X-linked dominant

d.      X-linked recessive

e.       Y-linked

 

Autosomal Dominant:

An autosomal dominant trait can appear in either sex because gene is on autosome. If a child has the trait, at least one parent also has it. These traits do not skip generations.

Examples: Huntington disease is an autosomal dominant condition.

 

Autosomal recessive:

An autosomal recessive trait can appear in either sex. Affected individuals have a homozygous recessive genotype, whereas in heterozygotes (normal carriers) the wild type allele masks expression of the mutant allele.

Examples: Cystic fibrosis is an autosomal recessive condition.

 

Comparison of Autosomal Dominant and Autosomal Recessive inheritance
Autosomal dominant	Autosomal recessive
Males and females affected, with equal frequency	Males and females affected, with equal frequency
Can not skip generation (generations affected until no one inherits the mutation	Can skip generations
Affected individual has an affected parent, unless he or she has a de novo mutation.	Affected individual has parents who are affected or are carriers (heterozygotes)
Gene present on autosomes	Gene present on autosomes

 

Sex linked inheritance:

a.       Genes for the X-linked traits are on the X chromosome.

b.      There is no male-to male transmission of X-linked traits.

c.       A man inherits an X-linked trait only from his mother.

d.      The human male is considered hemizygous for X-linked traits, because he has only one set of X-linked genes.

 

X-linked dominant:

a.       Genes are on the X chromosome.

b.      In females, one copy of gene is required for expression of a trait.

c.       High rates of miscarriage due to early lethality in males

d.      Much more severe effects in males

e.       Passed from male to all daughters but to no sons

Examples: Alport’s syndrome, coffin-Lowry syndrome, idiopathic hypoparathyroidism, and vitamin resistant rickets are the common examples.

 

(1). Alport’s Syndrome:

               It is a disease which affects type IV collagen and the tiny blood vessels are damaged in kidneys leadings to renal failure and deafness. It can also cause problems related to eyes.

(2). Coffin-Lowry syndrome:

               Rare genetic disorder characterized by intellectual disability, abnormalities of the head and facial area, large, soft hands with short, thin fingers; short stature; and various skeletal abnormalities.

(3). Idiopathic hyperparathyroidism:

               IHP is a rare endocrine condition, which is frequently represented by neuropsychiatric disorders.

(4). Vitamin D resistant rickets:

               Hypophosphatemia rickets is a disorder in which the bones become painfully soft and bend easily, due to low levels of phosphate in the blood.

 

X-linked recessive:

a.       Genes are on the X chromosome

b.      In females, two copies are required for expression of a recessive allele.

c.       For a female to have trait, both parents should inherit the recessive allele.

d.      Female with one recessive allele are normal carriers (can inherit trait to her offspring).

e.       In males, a single copy of an X-linked allele causes expression of the trait.

f.       More common in male individuals.

g.      Affected male inherits trait from heterozygote or homozygote mother

Examples: Hemophilia and color blindness are the examples of x-linked recessive traits.

 

Y-linked: 

Genes on the Y chromosome are Y-linked. Y-linked traits are rare because the chromosome has few genes, and many of its genes have counterparts on the X chromosome. Y-linked traits are passed from male to male, because a female does not have a Y chromosome. No other Y-linked traits besides infertility (which obviously can’t be passed on) are yet clearly defined.