PRINCIPLES OF INHERITANCE AND VARIATION (NOTES)

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REVISION NOTES

 PRINCIPLES OF INHERITANCE AND VARIATION

Genetics: the branch of biology that deals with the study of genes, genetic variations and heredity in living organisms. 

Heredity : The transmission of characters from parents to offsprings. 

Inheritance: it is the process by which characters are passed on from parent to offsprings and it forms the basis of heredity. 

Variation: it is the degree by which a progeny differs from their parents. 

The cause of variations are:

 o Recombination 

o Reshuffling of genes

 o Mutation 

Mendel’s Law of Inheritance:

 Mendel conducted seven years of hybridization research on garden pea (Pisum sativum) and postulated the rule of inheritance in living beings.

 Selection of Pea plants: Mendel used garden pea (Pisum sativum) for his research for the following reasons:

 o Pea has numerous unique and opposing personalities.

 o The pea plant has a short life cycle.

 o Flowers exhibit self-pollination, with reproductive whorls surrounded by the corolla.

 o It is simple to cross-pollinate pea flowers artificially. The resulting hybrids are fertile. 

 His Methodology: he was successful in his experiments because: 

o He just studied one character at a time.

o He employed every known means to avoid crosspollination from unwanted pollen grains. 

o He used mathematics and statistics to analyses the data he acquired. 

o Mendel chose seven opposing garden pea characteristics for his hybridization experiments. 

o Mendel Investigated Contrasting Characters in Pea. 

Mendel used true-breeding pea lines to perform artificial hybridization/cross pollination. True breeding lines are those that exhibit consistent trait inheritance and undertake continuous self-pollination.

 The hybridization experiment comprises emasculation (anther removal) and pollen transfer (pollination). 

Genotype is the ratio of genes present in progeny.

Phenotype is the external features shown by progeny.

Monohybrid Cross (Inheritance by one Gene) 

✓ Mendel crossed tall and dwarf pea plants and gathered all of the seeds. 

✓ He nurtured all of the seeds to produce plants of the first hybrid generation, known as the F1 generation. 

✓ He saw that all of the plants are tall. A similar discovery was made in another pair of features. 

✓ Mendel self-pollinated the F1 plants and discovered that some plants in the F2 generation are similarly small.

✓ Dwarf plants account for ¼ of the total, while tall plants account for the ¾ of the total. 

✓ Mendel called that 'factors' were responsible for the transfer of gametes from generation to generation. It is now referred to as genes (unit of inheritance). 

✓ Alleles are genes that code for a pair of opposing traits. 

✓ Each gene is represented by an alphabetical symbol, with a capital letter (TT)for genes expressed in the F1 generation and a tiny letter (tt) for other genes.

 ✓ Mendel also claimed that in true breeding tall and dwarf varieties, the height allelic pair is homozygous (TT or tt). The genotype is TT, Tt, or tt, represents the phenotypic trait, tall or dwarf.

 ✓ Heterozygous hybrids are those that have alleles that display opposing features (Tt).

 ✓ The F2 hybrid's monohybrid ratio is 3:1 (phenotypic) and 1:2:1. (genotypic). 

Test Cross: it is a cross between an individual with dominant trait and a recessive organism in order to know whether the dominant trait is homozygous or heterozygous.

Law of Dominance:

 the law states that: 

o Characters are controlled by separate units known as factors. 

o Factors are always found in pairs. 

o In a dissimilar pair of factors, one dominates the other. 

Dominance: Dominance occurs when a factor (allele) manifests itself in the presence or absence of its dominant factor. It generates a fully functioning enzyme that expresses it precisely. 

Recessive: It can only manifest in the absence of or recessive factor allele. When present with its dominant allele, i.e., in a heterozygous state, it generates an incomplete deficient enzyme that fails to express itself.

 Law of segregation: the law states that: 

o Alleles do not mix, and both characters are recovered after gamete creation, as in the F2 generation. 

o Traits segregate (separate) from one another during gamete formation and transmit to various gametes.

o Homozygous individuals generate identical types of gametes, whereas heterozygous individuals create gametes with distinct characteristics. 

Incomplete Dominance: It is a post-Mendelian finding, incomplete dominance occurs when none of the two alleles is dominant, resulting in expression in the hybrid that is a fine mixture or intermediate between the expressions of the two alleles.

 There are two varieties of pure breeding plants in snapdragon (Mirabilis Jalapa), red flowered and white flowered. Pink flowers are produced by crossing the two. When selfed, the F2 generation has one red, two pinks, and one white. The pink flowers is the result of incomplete dominance.

Co-dominance: It is caused by two alleles that do not have a dominant-recessive connection and both express themselves in the organism.

 ✓ ABO blood grouping in humans is governed by gene I. There are three alleles in the gene: I A, I B, and i. Any two of the three alleles I A, I B are dominant over I in any individual. 

✓ The sugar polymers that protrude from the surface of the plasma membrane of red blood cells are regulated by the gene. 

✓ Because of co-dominance, when I A and I B are present together, they both express their own forms of sugars.

 Multiple alleles: They are several variants of a Mendelian factor or gene that occur on the same gene locus and are dispersed in the gene pool in various animals, with an organism having only two alleles and a gamete bearing just one allele. ABO blood categorization is another example of multiple alleles.

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 segregation of the other pair of characters." Due to the separate assortment of characteristics for seed shape (round, wrinkled) and seed colour (yellow and green) in Dihybrid crosses, two novel combinations, round green & wrinkled yellow, are produced. The 9:3:3:1 ratio may be calculated by combining 3 yellow: 1 green and 3 round: 1 wrinkled. This is how the derivation is written:(1 Wrinkled: 3 Round) (3 yellow: 1 green) = 9 round, yellow: 3 wrinkled, yellow: 3 round, green: 1 wrinkled, green: 1 wrinkled, green: 1 wrinkled, green: 1 wrinkled, green: 1

Chromosomal theory of inheritance: 

In 1902, Theodor Boveri observed that proper embryonic development of sea urchins does not occur unless chromosomes are present. That same year, Walter Sutton observed the separation of chromosomes into daughter cells during meiosis. Together, these observations led to the development of the Chromosomal Theory of Inheritance, which identified chromosomes as the genetic material responsible for Mendelian inheritance. The Chromosomal Theory of Inheritance was consistent with Mendel's laws and was supported by the following observations:
a) During meiosis, homologous chromosome pairs migrate as discrete structures that are independent of other chromosome pairs.
b) The sorting of chromosomes from each homologous pair into pre-gametes appears to be random.
c) Each parent synthesizes gametes that contain only half of their chromosomal complement.

d) Even though male and female gametes (sperm and egg) differ in size and morphology, they have the same number of chromosomes, suggesting equal genetic contributions from each parent.

 Linkage and Recombination 

When two genes in a Dihybrid cross were located on the same chromosome, the fraction of parental gene combination was much greater than the non-parental kind. Morgan described these to the two genes' physical relationship or linkage, and developed the term linkage to characterise the physical association of genes on the same chromosome.

 Recombination is the process through which non-parental gene combinations are generated during a Dihybrid cross. When genes are found on the same chromosome, they are closely connected and exhibit relatively little recombination.

Polygenic inheritance:

1. Polygenic inheritance is defined as quantitative inheritance, where all the multiple independent genes have an additive or similar effect on a single quantitative trait.
2. Polygene refers to the gene that shows a slight effect on a phenotype along with the other genes.
3. Inheritance of skin pigmentation is an example of polygenic inheritance.
4. Skin color in humans is controlled by at least three genes.
5. The genes determine the amount of pigment melanin produced.
6. AABBCC are genes for dark colour in humans and aabbcc are genes for light colour. Blending of these two genes gives  AaBbCc genes which have intermediate colour.

Pleiotropy:

1. Pleiotropy occurs when one gene affects multiple unrelated phenotypic characters.
2. It was first observed by Gregor Mendel.

Examples of pleiotropy:

1. Sickle cell anemia: Single mutation in the beta-globin chain results in changes in blood shape and function.
2. Phenylketonuria: Single mutation in the gene responsible for the production of an enzyme that leads to hypopigmentation of hair , skin, and mental retardation.

Sex Determination

 In 1891, Henking discovered a trace of a unique nuclear structure in a few insects. He also discovered that this unique nuclear structure is only seen in 50% of sperms. He referred to this as an X body. He couldn't understand its relevance. Later, it was discovered that the ovum that receives sperms with an X body becomes female, while those that do not receive an X body become males, hence this X body was called the sex chromosome, and the other chromosomes were called autosomes. 

Humans and other species exhibit XY sex determination, whereas some insects, such as Drosophila, exhibit XO sex determination.

 Males produce two types of gametes in both methods of sex determination, either with or without an X chromosome, or some with an X chromosome and some with Y chromosomes. This is called male heterogamety.

 In birds, the ZW form of sex determination is present; females produce two different types of gametes in terms of sex chromosomes; this type of sex determination is known as female heterogamety. 

Sex Determination in Honeybee 

Sex determination in Honey Bee is based on the number of set of chromosomes an individual receives. An offspring formed from the union of a sperm and an egg develops as a female (Queen or worker) and an unfertilized egg develops as a male (drone) by means of parthenogenesis, this means that the males will have half the number of chromosomes than that of female, the females have 32 chromosomes and males are haploid having 16 chromosome, this is called haploid diploid sex determination.

Sex Determination in Human Beings 

Humans show XY type of sex determination. Autosomes are 22 pairs of chromosomes that are identical in male and female. Females have two X chromosomes, whereas males have XY chromosomes. During spermatogenesis, males produce two types of gametes (sperms), with half carrying the Y chromosome and the other half carrying the X chromosome. Females produce only one type of gamete (ovum) with X chromosomes. When Y chromosome sperm fertilises an egg, the child is male; when X chromosome sperm fertilises an egg, the child is female.

Mutation: is a phenomenon that causes the DNA sequence of an organism to change, resulting in a change in genotype and phenotype.

Point mutations are those that occur as a result of a change in a single base pair of DNA, such as Sickle cell anaemia.

A frameshift mutation (also called a framing error or a reading frame shift) is a genetic mutation caused by addition and deletion of a number of nucleotides in a DNA sequence that is not divisible by three. Due to the triplet nature of gene expression by codons, the insertion or deletion can change the reading frame (the grouping of the codons), resulting in a completely different translation from the original.

 Pedigree Analysis: Pedigree analysis refers to the study of traits in several generations of a family. A family tree depicts the inheritance of a certain trait over numerous generations. It is used to trace the ancestry of a certain trait, aberration, or illness.

 Genetic Disorders: They are transmitted as the affected individual is sterile. This is always dominant in nature. Broadly, genetic disorders may be grouped into two categories – 

• Mendelian disorders: These are the result of a single gene change. They are passed down via generations using Mendelian inheritance principles. E.g. 

o Colour Blindness : It is a sex-linked recessive disorder due to defect in either red or green cone of 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.

o Haemophilia: A sex-linked recessive condition in which a tiny injury causes non-stop bleeding in an affected individual. Females who are heterozygous (carriers) for the condition can pass it on to their son. The likelihood of a female becoming a haemophilic is exceedingly unlikely since the mother must be at least a carrier and the father must be a haemophilic (unviable in the later stage of life). 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 mother of such a female has to be at least 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 descendents as she was a carrier of the disease.

• Sickle cell anaemia: it is an autosomal recessive condition in which mutant haemoglobin molecules polymerize under low oxygen tension, causing the form of the RBC to shift from a biconvex disc to an elongated sickle-like structure. The deficiency is produced by the replacement of Glutamic acid (Glu) for Valine (Val) at the sixth position of the haemoglobin molecule's beta-globin chain. The amino acid change in the globin protein is caused by a single base substitution from GAG to GUG at the sixth codon of the beta globin gene. Diagram representing sickle cell anaemia.

 o Phenylketonuria: Inborn error of metabolism inherited as an autosomal recessive trait. The afflicted person is deficient in an enzyme that transforms the amino acids phenylalanine to tyrosine. As a result, phenylalanine accumulates and is transformed into phenyl pyruvic acid and other derivatives, which causes mental retardation.

 o Thalassemia: it is an autosome linked recessive blood disease that is transmitted to parents to offspring when both the partners are an unaffected carrier for the gene (or heterozygous). The defect is either due to mutation or deletion that results in the reduced rate of synthesis of one of the globin chains (alpha or beta) that make up haemoglobin. The cause of the formation of abnormal haemoglobin molecules resulting into anaemia is a characteristic of the disease. 

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 into 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.

Chromosomal disorders: These are caused by the absence or presence of one or more chromosomes, or by an aberrant arrangement of one or more chromosomes. In nature, they might be recessive or dominant.

 Aneuploidy occurs when chromatids fail to segregate during cell division, resulting in chromosomal loss or gain. The failure of cytokinesis results in two sets of chromosomes, known as polyploidy. 

o Down’s Syndrome: it is due to the existence of an extra copy of chromosome 21. The afflicted person is short and stocky, with a tiny rounded head, wrinkled tongue, and half parted mouth. Mental growth is slowed.

o Klinefelter’s syndrome: because of the existence of an extra copy of the X-chromosome (XXY). Such people have overall male growth, but they also exhibit feminine development (breast development, i.e., Gynaecomastia). They are sterile. 

o Turner’s syndrome: caused by the lack of one of the X chromosomes 45 with XO, are infertile because their ovaries are underdeveloped. They don't have any secondary sexual characteristics.