Principles of Inheritance and Variation

1. Chapter at a Glance

Have you ever wondered why an elephant always gives birth only to a baby elephant and not some other animal? Or why a mango seed forms only a mango plant and not any other plant? Given that they do, are the offspring identical to their parents? Or do they show differences in some of their characteristics? Genetics is the branch of biology that deals with inheritance as well as the variation of characters from parents to offspring. Inheritance is the process by which characters are passed on from parent to progeny; it is the basis of heredity. Variation is the degree by which progeny differ from their parents. This chapter covers Mendel's experiments and laws, deviations from Mendelism (incomplete dominance, co-dominance, multiple alleles), the chromosomal theory, linkage, polygenic inheritance, pleiotropy, sex determination, mutation and genetic disorders. It is one of the most important Class 12 chapters: NEET asks four to five questions every year, including ratio-based problems, blood-group crosses, pedigree analysis and disorders.

2. Mendel's Laws of Inheritance

It was during the mid-nineteenth century that headway was made in the understanding of inheritance. Gregor Mendel conducted hybridisation experiments on garden peas for seven years (1856–1863) and proposed the laws of inheritance in living organisms. During Mendel's investigations into inheritance patterns, it was for the first time that statistical analysis and mathematical logic were applied to problems in biology. His experiments had a large sampling size, which gave greater credibility to the data that he collected. Also, the confirmation of his inferences from experiments on successive generations of his test plants proved that his results pointed to general rules of inheritance rather than being unsubstantiated ideas.

Mendel investigated characters in the garden pea plant that were manifested as two opposing traits, e.g. tall or dwarf plants, yellow or green seeds. This allowed him to set up a basic framework of rules governing inheritance, which was expanded on by later scientists to account for all the diverse natural observations and the complexity inherent in them. Mendel conducted such artificial pollination/cross-pollination experiments using several true-breeding pea lines. A true-breeding line is one that, having undergone continuous self-pollination, shows the stable trait inheritance and expression for several generations. Mendel selected 14 true-breeding pea plant varieties, as pairs which were similar except for one character with contrasting traits.

3. Inheritance of One Gene (Monohybrid Cross)

Let us take the example of one such hybridisation experiment carried out by Mendel where he crossed tall and dwarf pea plants to study the inheritance of one gene. He collected the seeds produced as a result of this cross and grew them to generate plants of the first hybrid generation. This generation is also called the Filial₁ progeny or the F₁. Mendel observed that all the F₁ progeny plants were tall, like one of its parents; none were dwarf. He made similar observations for the other pairs of traits — he found that the F₁ always resembled either one of the parents, and that the trait of the other parent was not seen in them.

Mendel then self-pollinated the tall F₁ plants and to his surprise found that in the Filial₂ generation some of the offspring were "dwarf"; the character that was not seen in the F₁ generation was now expressed. The proportion of plants that were dwarf were 1/4th of the F₂ plants, while 3/4th of the F₂ plants were tall. The tall and dwarf traits were identical to their parental type and did not show any blending, that is, all the offspring were either tall or dwarf; none were of in-between height.

Based on these observations, Mendel proposed that something was being stably passed down, unchanged, from parent to offspring through the gametes, over successive generations. He called these things "factors". Now we call them genes. Genes, therefore, are the units of inheritance. They contain the information that is required to express a particular trait in an organism. Genes which code for a pair of contrasting traits are known as alleles, i.e. they are slightly different forms of the same gene. If we use alphabetical symbols for each gene, then the capital letter is used for the trait expressed at the F₁ stage and the small alphabet for the other trait. For example, in case of the character of height, T is used for the tall trait and t for the "dwarf", and T and t are alleles of each other. Hence, in plants the pair of alleles for height would be TT, Tt or tt.

Mendel also proposed that in a true-breeding, tall or dwarf pea variety the allelic pair of genes for height are identical or homozygous, TT and tt, respectively. TT and tt are called the genotype of the plant, while the descriptive terms tall and dwarf are the phenotype. The hybrid with Tt is called heterozygous. Since the hybrids contained alleles which expressed contrasting traits, the plants were heterozygous. In a heterozygous plant, of the two alleles, one is dominant (T) and the other recessive (t).

The production of gametes by the parents, the formation of the zygotes, the F₁ and F₂ plants can be understood from a diagram called the Punnett square, developed by a British geneticist, Reginald C. Punnett. It is a graphical representation to calculate the probability of all possible genotypes of offspring in a genetic cross. The possible gametes are written on two sides, usually the top row and left columns. All possible combinations are represented in boxes below in the squares, which generate a square output form.

Test cross

Given a tall plant in F₂, how can we determine its genotype — whether it is TT or Tt? Mendel crossed the tall plant from F₂ with a dwarf plant. This is called a test cross. In a typical test cross an organism (pea plants here) showing a dominant phenotype (whose genotype is to be determined) is crossed with the recessive parent instead of self-crossing. The progenies of such a cross can easily be analysed to predict the genotype of the test organism. If all progeny are tall, the plant is TT; if half are tall and half dwarf (1:1), the plant is Tt.

Mendel's first two laws

• Law of Dominance: (i) characters are controlled by discrete units called factors; (ii) factors occur in pairs; (iii) in a dissimilar pair of factors one member of the pair dominates (dominant) the other (recessive). The law of dominance is used to explain the expression of only one of the parental characters in a monohybrid cross in the F₁ and the expression of both in the F₂. It also explains the proportion of 3:1 obtained at the F₂.
• Law of Segregation: this law is based on the fact that the alleles do not show any blending and that both the characters are recovered as such in the F₂ generation, though one of these is not seen at the F₁ stage. Though the parents contain two alleles during gamete formation, the factors or alleles of a pair segregate from each other such that a gamete receives only one of the two factors. Of course, a homozygous parent produces all gametes that are similar, while a heterozygous one produces two kinds of gametes, each having one allele with equal proportion.

4. Deviations from Mendelian Ratios

Incomplete dominance

When experiments on peas were repeated using other traits in other plants, it was found that sometimes the F₁ had a phenotype that did not resemble either of the two parents and was in between the two. The inheritance of flower colour in the dog flower (snapdragon or Antirrhinum sp.) is a good example. In a cross between true-breeding red-flowered (RR) and true-breeding white-flowered plants (rr), the F₁ (Rr) was pink. When the F₁ was self-pollinated, the F₂ resulted in the following ratio: 1 (RR) red : 2 (Rr) pink : 1 (rr) white. Here the genotype ratios were exactly as we would expect in any Mendelian monohybrid cross, but the phenotype ratios had changed from 3:1 dominant:recessive ratio to 1:2:1. What happened was that R was not completely dominant over r, and this made it possible to distinguish Rr as pink from RR (red) and rr (white).

Co-dominance

Till now we were discussing crosses where the F₁ resembled either of the two parents (dominance) or was in-between (incomplete dominance). But in the case of co-dominance, the F₁ generation resembles both parents. A good example is the different types of red blood cells that determine ABO blood grouping in human beings. ABO blood groups are controlled by the gene I. The plasma membrane of the red blood cell has sugar polymers that protrude from its surface, and the kind of sugar is controlled by the gene. The gene (I) has three alleles: Iᴬ, Iᴮ and i. The alleles Iᴬ and Iᴮ produce a slightly different form of the sugar, while allele i does not produce any sugar. Because humans are diploid organisms, each person possesses any two of the three I gene alleles. Iᴬ and Iᴮ are completely dominant over i; in other words, when Iᴬ and i are present only Iᴬ expresses (because i does not produce any sugar), and when Iᴮ and i are present, Iᴮ expresses. But when Iᴬ and Iᴮ are present together, they both express their own types of sugars: this is because of co-dominance. Hence red blood cells have both A and B types of sugars. Since there are three different alleles, there are six different combinations of these three alleles possible, and therefore a total of six different genotypes of the human ABO blood types. How many phenotypes are possible? Four.

The ABO blood group also provides a good example of multiple alleles. Here you can see that there are more than two, i.e. three alleles, governing the same character. Since in an individual only two alleles can be present, multiple alleles can be found only when population studies are made.

5. Inheritance of Two Genes (Dihybrid Cross)

Mendel also worked with and crossed pea plants that differed in two characters, as is seen in the cross between a pea plant that has seeds with yellow colour and round shape and one that had seeds of green colour and wrinkled shape. Mendel found that the seeds resulting from the crossing of the parents had yellow-coloured and round-shaped seeds. Here can you tell which of the characters in the pairs yellow/green colour and round/wrinkled shape was dominant? Thus, yellow colour was dominant over green and round shape dominant over wrinkled. These results were identical to those that he got when he made separate monohybrid crosses between yellow- and green-seeded plants and between round- and wrinkled-seeded plants.

Let us use the genotypic symbols Y for dominant yellow seed colour and y for recessive green seed colour, R for round-shaped seeds and r for wrinkled seed shape. The genotype of the parents can then be written as RRYY and rryy. The cross between the two plants can be written down, showing the gametes RY and ry and the F₁ hybrid RrYy. When Mendel self-hybridised the F₁ plants he found that 3/4th of F₂ plants had yellow seeds and 1/4th had green. The yellow and green colour segregated in a 3:1 ratio. Round and wrinkled seed shape also segregated in a 3:1 ratio, just like in a monohybrid cross.

Law of independent assortment

In the dihybrid cross, the phenotypes round, yellow; wrinkled, yellow; round, green and wrinkled, green appeared in the ratio 9:3:3:1. Such a ratio was observed for several pairs of characters that Mendel studied. The ratio of 9:3:3:1 can be derived as a combination series of 3 yellow : 1 green, with 3 round : 1 wrinkled. This derivation can be written as follows: (3 round : 1 wrinkled)(3 yellow : 1 green) = 9 round, yellow : 3 wrinkled, yellow : 3 round, green : 1 wrinkled, green. Based upon such observations on dihybrid crosses (crosses between plants differing in two traits), Mendel proposed a second set of generalisations that we call Mendel's Law of Independent Assortment. The law states that "when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters".

The Punnett square can be effectively used to understand the independent segregation of the two pairs of genes during meiosis and the production of eggs and pollen in the F₁ RrYy plant. Consider the segregation of one pair of genes R and r. Fifty per cent of the gametes have the gene R and the other 50 per cent have r. Now, besides each gamete having either R or r, it should also have the allele Y or y. The important thing to remember here is that segregation of 50 per cent R and 50 per cent r is independent from the segregation of 50 per cent Y and 50 per cent y. Therefore, 50 per cent of the R-bearing gametes have Y and the other 50 per cent have y. Similarly, 50 per cent of the r-bearing gametes have Y and the other 50 per cent have y. Thus there are four genotypes of gametes (four types of pollen and four types of eggs): RY, Ry, rY and ry, each with a frequency of 25 per cent or 1/4th of the total gametes produced.

6. Chromosomal Theory of Inheritance

Mendel published his work on inheritance of characters in 1865, but for several reasons it remained unrecognised till 1900. Firstly, communication was not easy in those days and his work could not be widely publicised. Secondly, his concept of genes (or factors, in Mendel's words) as stable and discrete units that controlled the expression of traits, and of the pair of alleles which did not "blend" with each other, was not accepted by his contemporaries as an explanation for the apparently continuous variation seen in nature. Thirdly, Mendel's approach of using mathematics to explain biological phenomena was totally new and unacceptable to many of the biologists of his time. Finally, though Mendel's work suggested that factors (genes) were discrete units, he could not provide any physical proof for the existence of factors or say what they were made of.

In 1900, three scientists — de Vries, Correns and von Tschermak — independently rediscovered Mendel's results on the inheritance of characters. Also by this time, due to advancements in microscopy, scientists were able to carefully observe cell division. This led to the discovery of structures in the nucleus that appeared to double and divide just before each cell division. These were called chromosomes (coloured bodies, as they were visualised by staining). By 1902, the chromosome movement during meiosis had been worked out. Walter Sutton and Theodore Boveri noted that the behaviour of chromosomes was parallel to the behaviour of genes and used chromosome movement to explain Mendel's laws. Sutton united the knowledge of chromosomal segregation with Mendelian principles and called it the chromosomal theory of inheritance.

Experimental verification of the chromosomal theory of inheritance was done by Thomas Hunt Morgan and his colleagues, which led to discovering the basis for the variation that sexual reproduction produced. Morgan worked with the tiny fruit flies, Drosophila melanogaster, which were found very suitable for such studies. They could be grown on simple synthetic medium in the laboratory. They complete their life cycle in about two weeks, and a single mating could produce a large number of progeny flies. Also, there was a clear differentiation of the sexes — the male and female flies are easily distinguishable. Also, it has many types of hereditary variations that can be seen with low-power microscopes.

Linkage and recombination

Morgan carried out several dihybrid crosses in Drosophila to study genes that were sex-linked. The crosses were similar to the dihybrid crosses carried out by Mendel in peas. For example, Morgan hybridised yellow-bodied, white-eyed females to brown-bodied, red-eyed males and intercrossed their F₁ progeny. He observed that the two genes did not segregate independently of each other and the F₂ ratio deviated very significantly from the 9:3:3:1 ratio (expected when the two genes are independent).

Morgan and his group knew that the genes were located on the X chromosome and saw quickly that when the two genes in a dihybrid cross were situated on the same chromosome, the proportion of parental gene combinations was much higher than the non-parental type. Morgan attributed this due to the physical association or linkage of the two genes and coined the term linkage to describe this physical association of genes on a chromosome, and the term recombination to describe the generation of non-parental gene combinations. Morgan and his group also found that even when genes were grouped on the same chromosome, some genes were very tightly linked (showed very low recombination) while others were loosely linked (showed higher recombination). For example, he found that the genes for white and yellow were very tightly linked and showed only 1.3 per cent recombination, while white and miniature wing showed 37.2 per cent recombination. His student Alfred Sturtevant used the frequency of recombination between gene pairs on the same chromosome as a measure of the distance between genes and "mapped" their position on the chromosome. Today genetic maps are extensively used as a starting point in the sequencing of whole genomes, as was done in the case of the Human Genome Sequencing Project.

7. Polygenic Inheritance and Pleiotropy

Mendel's studies mainly described those traits that have distinct alternate forms such as flower colour, which are either purple or white. But if you look around you will find that there are many traits which are not so obvious in their spread and are spread across a gradient. For example, in humans we don't just have tall or short people as two distinct alternatives but a whole range of possible heights. Such traits are generally controlled by three or more genes and are thus called polygenic traits. Besides the involvement of multiple genes, polygenic inheritance also takes into account the influence of the environment. Human skin colour is another classic example. In a polygenic trait the phenotype reflects the contribution of each allele, i.e. the effect of each allele is additive. To understand this better, let us assume that three genes A, B, C control skin colour in humans, with the dominant forms A, B and C responsible for dark skin colour and the recessive forms a, b and c for light skin colour. The genotype with all the dominant alleles (AABBCC) will have the darkest skin colour and that with all the recessive alleles (aabbcc) will have the lightest skin colour. As expected, the genotype with three dominant alleles and three recessive alleles will have an intermediate skin colour. In this manner the number of each type of alleles in the genotype would determine the darkness or lightness of the skin in an individual.

We have so far seen the effect of a gene on a single phenotype or trait. There are however instances where a single gene can exhibit multiple phenotypic expressions. Such a gene is called a pleiotropic gene. The underlying mechanism of pleiotropy in most cases is the effect of a gene on metabolic pathways, which contribute towards different phenotypes. An example of this is the disease phenylketonuria, which occurs in humans. The disease is caused by mutation in the gene that codes for the enzyme phenylalanine hydroxylase (single gene mutation). This manifests itself through phenotypic expression characterised by mental retardation and a reduction in hair and skin pigmentation.

8. Sex Determination

The mechanism of sex determination has always been a puzzle before the geneticists. The initial clue about the genetic/chromosomal mechanism of sex determination can be traced back to some of the experiments carried out in insects. In fact, the cytological observations made in a number of insects led to the development of the concept of the genetic/chromosomal basis of sex determination. Henking (1891) could trace a specific nuclear structure all through spermatogenesis in a few insects, and it was also observed by him that 50 per cent of the sperm received this structure after spermatogenesis, whereas the other 50 per cent sperm did not receive it. Henking gave a name to this structure as the X body, but he could not explain its significance. Further investigations by other scientists led to the conclusion that the "X body" of Henking was in fact a chromosome, and that is why it was given the name X-chromosome.

In humans, the sex determining mechanism is of the XY type. Out of 23 pairs of chromosomes present, 22 pairs are exactly the same in both males and females; these are the autosomes. A pair of X-chromosomes is present in the female, whereas the presence of an X and a Y chromosome is determinant of the male characteristic. During spermatogenesis among males, two types of gametes are produced. 50 per cent of the total sperm produced carry the X-chromosome and the rest 50 per cent have the Y-chromosome besides the autosomes. Females, however, produce only one type of ovum with an X-chromosome. There is an equal probability of fertilisation of the ovum with the sperm carrying either the X or Y chromosome. In case the ovum fertilises with a sperm carrying the X-chromosome the zygote develops into a female (XX), and fertilisation of the ovum with a Y-chromosome-carrying sperm results in a male offspring. Thus, it is evident that it is the genetic makeup of the sperm that determines the sex of the child. It is also evident that in each pregnancy there is always a 50 per cent probability of either a male or a female child.

9. Mutation

Mutation is a phenomenon which results in alteration of DNA sequences and consequently results in changes in the genotype and the phenotype of an organism. In addition to recombination, mutation is another phenomenon that leads to variation in DNA. As you will learn in the next chapter, one DNA helix runs continuously from one end to the other in each chromatid, in a highly supercoiled form. Therefore, loss (deletions) or gain (insertion/duplication) of a segment of DNA results in alteration in chromosomes. Since genes are known to be located on chromosomes, alteration in chromosomes results in abnormalities or aberrations. Chromosomal aberrations are commonly observed in cancer cells. In addition to the above, mutation also arises due to change in a single base pair of DNA. This is known as a point mutation. A classical example of such a mutation is sickle cell anaemia. Deletions and insertions of base pairs of DNA cause frame-shift mutations. There are many chemical and physical factors that induce mutations. These are referred to as mutagens. UV radiations can cause mutations in organisms — it is a mutagen.

10. Genetic Disorders

Pedigree analysis

The idea that disorders are inherited has been prevailing in human society since long. This was based on the heritability of certain characteristic features in families. After the rediscovery of Mendel's work, the practice of analysing inheritance patterns of traits in human beings began. Since it is evident that control crosses that can be performed in pea plants or some other organisms are not possible in the case of human beings, the study of the family history about inheritance of a particular trait in several generations of a family is the alternative. Such an analysis of traits in several generations of a family is called pedigree analysis. In pedigree analysis the inheritance of a particular trait is represented in the family tree over generations.

In human genetics, pedigree study provides a strong tool, which is utilised to trace the inheritance of a specific trait, abnormality or disease. Each and every feature in any organism is controlled by one or the other gene located on the DNA present in the chromosome. DNA is the carrier of genetic information. It is hence transmitted from one generation to the other without any change or alteration. However, changes or alterations do take place occasionally. Such an alteration or change in the genetic material is referred to as mutation. A number of disorders in human beings have been found to be associated with the inheritance of changed or altered genes or chromosomes.

Broadly, genetic disorders may be grouped into two categories — Mendelian disorders and chromosomal disorders.

Mendelian disorders

Mendelian disorders are mainly determined by alteration or mutation in a single gene. These disorders are transmitted to the offspring on the same lines as we have studied in the principle of inheritance. The pattern of inheritance of such Mendelian disorders can be traced in a family by pedigree analysis. Most common and prevalent Mendelian disorders are haemophilia, cystic fibrosis, sickle-cell anaemia, colour blindness, phenylketonuria, thalassemia etc. It is important to mention here that such Mendelian disorders may be dominant or recessive. By pedigree analysis one can easily understand whether the trait in question is dominant or recessive. Similarly, the trait may also be linked to the sex chromosome, as in the case of haemophilia. It is evident that this X-linked recessive trait shows transmission from carrier female to male progeny.

• Haemophilia: this sex-linked recessive disease, which shows its transmission from unaffected carrier female to some of the male progeny, has been widely studied. In this disease, a single protein that is a part of the cascade of proteins involved in the clotting of blood is affected. Due to this, in an affected individual a simple cut will result in non-stop bleeding. The heterozygous female (carrier) for haemophilia may transmit the disease to sons. The possibility of a female becoming a haemophilic is extremely rare, because the mother of such a female has to be at least a carrier and the father should be haemophilic (unviable in the later stage of life). The family pedigree of Queen Victoria shows a number of haemophilic descendants, as she was a carrier of the disease.
• Sickle-cell anaemia: this is an autosome-linked recessive trait that can be transmitted from parents to the offspring when both the partners are carriers for the gene (or heterozygous). The disease is controlled by a single pair of alleles, Hbᴬ and Hbˢ. Out of the three possible genotypes, only homozygous individuals for Hbˢ (HbˢHbˢ) show the diseased phenotype. Heterozygous (HbᴬHbˢ) individuals appear apparently unaffected, but they are carriers of the disease, as there is a 50 per cent probability of transmission of the mutant gene to the progeny, thus exhibiting sickle-cell trait. The defect is caused by the substitution of glutamic acid (Glu) by valine (Val) at the sixth position of the beta globin chain of the haemoglobin molecule. The substitution of amino acid in the globin protein results due to the single base substitution at the sixth codon of the beta globin gene from GAG to GUG. The mutant haemoglobin molecule undergoes polymerisation under low oxygen tension, causing the change in the shape of the RBC from biconcave disc to elongated sickle-like structure.
• Phenylketonuria: this inborn error of metabolism is also inherited as an autosomal recessive trait. The affected individual lacks an enzyme that converts the amino acid phenylalanine into tyrosine. As a result of this, phenylalanine is accumulated and converted into phenylpyruvic acid and other derivatives. Accumulation of these in the brain results in mental retardation. These are also excreted through urine because of poor absorption by the kidney.
• Thalassemia: this is also an autosome-linked recessive blood disease transmitted from parents to the offspring when both the partners are unaffected carriers for the gene (or heterozygous). The defect could be due to either mutation or deletion, which ultimately results in reduced rate of synthesis of one of the globin chains (α and β chains) that make up haemoglobin. This causes the formation of abnormal haemoglobin molecules resulting in anaemia, which is characteristic of the disease. Thalassemia can be classified according to which chain of the haemoglobin molecule is affected. In α thalassemia, production of α globin chain is affected, while in β thalassemia, production of β globin chain is affected. α Thalassemia is controlled by two closely linked genes HBA1 and HBA2 on chromosome 16 of each parent, and it is observed due to mutation or deletion of one or more of the four genes. The more genes affected, the less alpha globin molecules produced. While β Thalassemia is controlled by a single gene HBB on chromosome 11 of each parent and occurs due to mutation of one or both the genes. Thalassemia differs from sickle-cell anaemia in that the former is a quantitative problem of synthesising too few globin molecules, while the latter is a qualitative problem of synthesising an incorrectly functioning globin.
• Colour blindness: this sex-linked recessive disorder is due to a defect in either the red or green cone of the eye, resulting in failure to discriminate between red and green colour. This defect is due to mutation in certain genes present in the X chromosome. It occurs in about 8 per cent of males and only about 0.4 per cent of females. This is because the genes that lead to red-green colour blindness are on the X chromosome. Males have only one X chromosome and females have two. The son of a woman who carries the gene has a 50 per cent chance of being colour blind. The mother is not herself colour blind because the gene is recessive. That means that its effect is suppressed by her matching dominant normal gene. A daughter will not normally be colour blind, unless her mother is a carrier and her father is colour blind.

Chromosomal disorders

The chromosomal disorders, on the other hand, are caused due to absence or excess or abnormal arrangement of one or more chromosomes. Failure of segregation of chromatids during the cell division cycle results in the gain or loss of a chromosome(s), called aneuploidy. For example, Down's syndrome results in the gain of an extra copy of chromosome 21. Similarly, Turner's syndrome results due to loss of an X chromosome in human females. Failure of cytokinesis after the telophase stage of cell division results in an increase in a whole set of chromosomes in an organism; this phenomenon is known as polyploidy. This condition is often seen in plants. The total number of chromosomes in a normal human cell is 46 (23 pairs). Out of these, 22 pairs are autosomes and one pair of chromosomes are sex chromosomes. Sometimes, though rarely, either an additional copy of a chromosome may be included in an individual or an individual may lack any one of any one pair of chromosomes. These situations are known as trisomy or monosomy of a chromosome, respectively. Such a situation leads to very serious consequences in the individual.

11. Master Summary Table

12. Mind Map

13. Tips and Tricks

14. Important One-liners

• Mendel worked on garden pea for seven years (1856–1863).
• Punnett square was developed by Reginald C. Punnett.
• de Vries, Correns and von Tschermak rediscovered Mendel's work in 1900.
• Sutton and Boveri proposed the chromosomal theory of inheritance.
• Morgan used Drosophila melanogaster to prove linkage.
• Sturtevant made the first genetic map using recombination frequencies.
• Henking named the X body in 1891.
• Honey bee males are haploid and develop by parthenogenesis.
• Queen Victoria was a carrier of haemophilia.
• Colour blindness affects about 8% of males and 0.4% of females.
• β-thalassemia gene HBB is on chromosome 11.
• Polyploidy results from failure of cytokinesis.

15. NEET-pattern Questions with Detailed Answers

These questions are original and based on concepts that NEET UG asks again and again.

16. Quick Revision Summary