Heredity & Variation Test - 1

Key Points

• Thomas Hunt Morgan was an American biologist and embryologist who is known for his contribution towards genetics.
• Morgan's experiments verified the chromosomal theory of inheritance as suggested by Sutton and Boveri.
• Sutton and Boveri had suggested that pairing and separation of a pair of chromosome would lead to the segregation of the pair of alleles that they carried.
• Morgan experimented with Drosophila to establish that the basis of variation was caused by sexual reproduction.
• He further showed that when 2 genes in a dihybrid cross were located on the same chromosome, the proportion of parental gene combinations were much higher than the non-parental type. This was attributed to linkage of genes.
• Morgan found some genes to be tightly linked and some to be loosely linked.
• He received the Nobel Prize for Physiology or Medicine in 1933 for his discovery of 'hereditary transmission mechanisms in Drosophila'.

Important Points

• Drosophila melanogaster or fruit-fly was used as a model organism because of the following reasons:
  • Their size (3mm) is small enough to keep millions in a laboratory, but not so small that cannot be seen without a microscope.
  • They can be grown on simple synthetic media in the laboratory.
  • They complete their life cycles in about 2 weeks, which allows to study multiple generations.
  • A single mating produces a large number of offspring.
  • Male and female individuals are morphologically distinct.
  • Many of the hereditary variations can be studied under low power microscopes.
  • About 75% of the genes causing diseases in humans are also found in Drosophila.

Concept:

• Central dogma of molecular biology states that flow of genetic information takes place from DNA to RNA to proteins.
• Transcription of DNA to RNA is based on the complementarity of their strands.
• However, no such complementarity exists between the RNA and the proteins for translation to take place.
• Evidences show that changes in nucleic acids also caused changes in the amino acids of the proteins.
• This suggested the existence of a Genetic Code that directs the synthesis of amino acids that form the proteins.
• It was George Gamow who suggested that if the 4 nitrogen bases of nucleic acids have to code for the 20 non-essential amino acids produced in the body, the code should constitute a combination of bases.
• The nitrogen bases include Adenine (A), Guanine (G), Cytosine (C) and Uracil (U) for an RNA.
• A permutation combination of 2 bases would give: 4² = 16 amino acids only. Thus, it has to be a combination of 3 bases giving: 4³ = 64 codons.
• Thus it was concluded that 64 codons would code for the 20 amino acids with some codons being signal codons too.

Important Points

• From the above table we can see that:
  • GAG - Glutamic acid (Glu)
  • GAU - Aspartic acid (Asp)
  • GUG - Valine (Val)
  • GGG - Glycine (Gly)
• Therefore, GUG is the triplet codon that codes for Valine.

Additional Information

• Sickle cell anaemia is a genetic disorder that affects the red blood cells (RBCs) of the body.
• The shape of the RBCs changes from biconcave disc to an elongated sickle-like shape.
• Sickle-cell anaemia is caused by a point mutation that causes a substitution of amino acid.
• This Glutamic acid (Glu) is substituted by Valine (Val) due to the point mutation.

Concept-

• Gregor Mendel conducted hybridization experiments on garden peas for seven years (1856-1863).
• A dihybrid cross describes a mating experiment between two organisms that are identically hybrid for two traits.
• A hybrid organism is one that is heterozygous, which means that carries two different alleles at a particular genetic position.
• Therefore, a dihybrid organism is one that is heterozygous at two different genetic loci.
• Gregor Mendel performed dihybrid crosses on pea plants and discovered a fundamental law of genetics called the Law of Independent Assortment.

Explanation-

• Mendel selected traits for dihybrid cross for his experiments-
  • Colour of cotyledons- Yellow (Y) and Green (y).
  • Seed shape - Round (R) and Wrinkled (r).

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• The phenotypic ratio produced is 9: 3: 3: 1.
• The genotypic ratio produced is 1: 2: 2: 4: 1: 2: 1: 2: 1.

Therefore the 9 types of genotype and 4 types of the phenotype are present in the mendelian dihybrid cross.

Additional Information

• Monohybrid Cross
  • Phenotypic ratio- 3: 1
  • Genotypic ratio- 1: 2: 1

Key Points

• Sex determination is the mechanism which determines the biological sex of an individual.
• Sex determination in most of the organisms is determined by the sex chromosomes.
• However, in some organisms sex determination is based on the number of sets of chromosomes in an individual.
• One such example is honeybee, where males are haploid (n) and females are diploid (2n).
• This is known as the haplodiploid system of sex determination.
• The female honeybees have 32 chromosomes, while males have 16 chromosomes.

Explanation:

• As females are diploid, they produce haploid gametes (eggs) by meiosis.
• Males are haploid and so they produce haploid gametes (sperms) by mitosis.
• Fusion of a sperm and egg forms a diploid zygote.
• This zygote develops into a female honeybee.
• Male honeybee develops from a haploid egg by parthenogenesis, without any fertilization.
• Therefore, males have 16 chromosomes and females have 32 chromosomes in honeybees.
• This haplodiploid sex determination system also indicates some peculiar features:
  • Male gametes are produced by mitosis.
  • Males do not have a father and thus cannot have sons.
  • But males do have a grandfather and can have grandsons.

Additional Information

• Humans -
  • They have XX/XY type of sex determination.
  • The females possess a pair of X-chromosomes, while the males possess one X-chromosome and one Y-chromosome.
• Grasshoppers -
  • They have XX/XO type of sex determination.
  • The females possess a pair of X-chromosomes, but the males posses only one X-chromosome.
  • Thus males have one less chromosome than the females.
• Birds -
  • They have ZW/ZZ type of sex determination.
  • The females possess two types of sex chromosomes, one Z and one W chromosome.
  • Males possess a pair of Z-chromosomes.

Key Points

• According to Mendelian genetics, the expression of a character is determined by a pair of alleles.
• The alleles can either be dominant or recessive.
• These alleles can occur in 2 ways:
  • Homozygous - It is the condition in which the pair of alleles determining a phenotype is identical, either both dominant or both recessive.
  • Heterozygous - It is the condition where 2 different types of alleles are present, one dominant and one recessive. Here, the character expressed is dominant.
• Genotype - It is the allelic constitution of a gene determining a particular character.
• Phenotype - It is the expressed character.
• Selfing - It is the process of self-fertilization of a plant, meaning both male and female gametes are used from the same plant.

Important Points

• In the given question, the plant with genotype AABbCC has 3 pairs of alleles -
  • AA - It is homozygous dominant
  • Bb - It is heterozygous
  • CC - It is homozygous dominant
• Thus, the gametes produced by this plant can be of 2 types only - ABC and AbC.
• We can find the phenotypic and genotypic ratios of F₂ generation from the following diagram.

Explanation:

• The genotypes AABBCC and AABbCC will give the same phenotype because the pair BB and Bb both will express the dominant character.
• The genotype AAbbCC will have a different phenotype because the pair 'bb' will express the recessive character.
• Thus the phenotypic ratio of F₂ generation will be 3 : 1.

Key Points

 Gregor Mendel gave the Laws of Inheritance based on his experiments on Pisum sativum (garden pea):

• Law of Dominance -
  • It states that characters are controlled by discrete units called factors (alleles) which occur in pairs.
  • It also states that in a dissimilar pair of alleles, one of the alleles is expressed (dominant) over the other (recessive).
• Law of Segregation - 
  • It states that alleles do not show any blending and both parental characters are recovered in the F2 generation.
  • It also states that during gamete formation, alleles of a pair separate in such a way that one gamete receives only one of the two alleles of a pair.
  • It is also known as the Law of purity of gametes.
  • It can be proved with a monohybrid cross.
• Law of Independent Assortment -
  • It states that when 2 pairs of traits are combined in a hybrid, segregation of one pair is independent of the other pair.
  • It can be proved with a dihybrid cross.

Additional Information

•  Law of Natural Selection -
  • It was given by Charles Darwin.
  • It states that individuals with more favourable characters survive better and leave more progeny that again survives better and are hence selected by nature.

Concept: 

• A normal Human has 23 pairs of chromosomes (46 chromosomes) out of which 22 pairs are autosomes or allosomes  (Non - sex chromosomes), while the 23rd pair is the sex chromosome which is responsible for the hereditary changes.
• One - One chromosome each in the sex chromosome pair comes from the maternal and paternal side respectively.
• Any deviation from this normal arrangement may result in a disorder or syndrome.
• These disorders may arise either by addition, deletion, or nonfunctioning of a single or multiple chromosomes.
• Examples of such disorders are Turner syndrome, Edward's syndrome, Patau's syndrome, Klinefelter's syndrome, etc.

Explanation:

• Turner's syndrome: It is caused due to the absence of one X-chromosome in the females i.e. Karyotype 45, (44 + XO).
• Such females are sterile with underdeveloped breasts, short stature, reduced ovaries & absence of menstrual cycle.

Klinefelter's syndrome:

• It occurs due to the trisomy of the X- chromosome in a male, resulting in a karyotype of 47, (44+ XXY). 
• Individuals have long legs, sparse body hair, small prostate gland, small testes, reduced mental intelligence, and enlarged breasts (Gynaecomastia).
• Such individuals are sterile.

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Concept:

• Broadly, genetic disorders may be grouped into two categories – Mendelian disorders and Chromosomal 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 seen in the principle of inheritance.
• The pattern of inheritance of such Mendelian disorders can be traced in a family by the pedigree analysis.
• The most common and prevalent Mendelian disorders are Haemophilia, Cystic fibrosis, Sickle-cell anaemia, Colour blindness, Phenylketonuria, Thalassemia, etc.
• The Mendelian disorders may be linked with sex chromosomes or autosomes & can be dominant or recessive.

Explanation:

• Colour blindness is a sex-linked recessive disorder due to a defect in either 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 percent of males and only about 0.4 percent of females.
• This is because the genes that lead to red-green colour blindness are on the X chromosome.
• Males have  X and Y chromosomes and females have two.
• The son of a woman who carries the gene has a 50 percent chance of being colour blind.
• The mother is not 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.

Additional Information 

Key Points

• According to Mendelian genetics, the expression of a character is determined by a pair of alleles.
• The alleles can either be dominant or recessive.
• These alleles can occur in 2 ways:
  • Homozygous - It is the condition in which the pair of alleles determining a phenotype is identical, either both dominant or both recessive.
  • Heterozygous - It is the condition where 2 different types of alleles are present, one dominant and one recessive. Here, the character expressed is dominant.
• Genotype - It is the allelic constitution of a gene determining a particular character.
• Phenotype - It is the expressed character.
• According to Mendel's Law of Dominance:
  • In a dissimilar pair of alleles, one allele dominates (dominant) over the other allele (recessive).
  • Thus each trait has 2 distinct phenotypic characters - one dominant and one recessive.
• However, some traits were found to be not consistent with the Law of Dominance.
• One such trait is the flower colour in snapdragon plant (Antirrhinum sp.) which shows incomplete dominance.

Important Points

• Incomplete dominance - It is the condition in which the dominant allele is not completely dominant over the recessive allele.
• This results in an intermediate phenotype when in heterozygous condition.
• For the flower colour in snapdragon plant, red flower colour (RR) is dominant over white (rr).
• When plants with red flower is crossed with white flower plants, we obtain pink (Rr) flowers in F1 generation.
• When these pink flowers are self-crossed, we get the parental phenotypes red and white along with pink flowers in F2 generation.

Explanation:

• From the above diagram we can see that:
  • Genotypic ratio = RR : Rr : rr = 1:2:1
  • Phenotypic ratio = Red : Pink : White = 1:2:1
• Thus, the phenotypic ratio is a deviation from the ratio of Mendelian monohybrid cross.
• The heterozygous condition produces a hybrid phenotype, which is different from the parent phenotypes.

The correct answer is Sutton.

Key Points

• Sutton's chromosome theory of inheritance states that genes are found at specific locations on chromosomes and that the behaviour of chromosomes during meiosis can explain Mendel's laws of inheritance. 

Important Points

• Chromosomes are thread-like structure of nucleic acids and protein located within the nucleus of the living cells and are mainly involved in carrying genetic information in the form of genes.
• The Chromosomal Theory of Inheritance:
  • The chromosomal theory of inheritance was given by Boveri and Sutton in the early 1900s.
  • It is the fundamental theory of genetics.
  • According to this theory, genes are the units of heredity and are found in the chromosomes.
  • Chromosomal Theory of Inheritance came into existence long after Mendelian genetics.
  • Sutton and Boveri observed the behaviour of the chromosomes when the cells were divided.

Explanation:

• They noted some similarities in the behaviour of genes and chromosomes:
  • Both occur in pairs.
  • Both segregate during gamete formation in such a way that only one of each pair is transmitted to a gamete.
  • For both genes and chromosomes, one pair segregates independent of another pair.
• These similarities led to the conclusion that genes are units of heredity and are located on the chromosomes.
• It also helped in explaining the Mendelian laws of genetics.

Additional Information

• Experimental verification for chromosomal theory of inheritance was given by T. H. Morgan.
• Morgan's experiments on Drosophila showed that variation is caused by sexual reproduction.