Heredity and Variation

Heredity

• Passing down similar characteristics across generations is heredity.
• Genes control heredity.
• Gene combination differences lead to variations among family members.
• Genetics involves the study of heredity and variation.

Objectives after the lesson

• Define heredity and variation.
• Explain Mendelian inheritance patterns.
• Describe chromosome and gene location, structure, and function.
• Briefly explain DNA fingerprinting with its significance.
• Outline DNA replication.
• Provide the four human blood groups and their inheritance patterns.
• Explain the chromosomal basis of human sex determination.
• List hereditary disorders and dangers of consanguineous marriage.
• Stress genetic counseling relevance.
• Briefly describe the human genome.
• Outline genetic engineering's prominent points.

Heredity and Variation

• Heredity passes characters from parents to offspring, controlled by genes.
• Variations are differences among individuals (e.g., rose color, puppy fur).
• Variations arise from genes or the environment.
• Ear lobe attachment is a hereditary trait.

Contribution of Gregor Johann Mendel

• Gregor Johann Mendel (1822-1884), an Austrian monk, studied heredity using pea plants.
• Mendel compiled data, analyzed it, and proposed inheritance laws.
• Correns, Tschermak, and Hugo de Vries recognized Mendel's work after his death.
• Each came to similar conclusions independently.

Mendel’s Laws of Inheritance

• Each trait is controlled by a pair of factors.
• During gamete formation, factors segregate, with fertilization combining factors to express the trait through segregation.
• A dominant factor expresses itself despite the presence of a recessive one.
• A recessive factor expresses itself only without a dominant factor (2nd law).
• Mendel also proposed the 'law of parental equivalence' and the 'law of independent assortment.'
• Scientists later found deviations from Mendel’s other laws.
• Sutton (1902) found Mendelian factors on grasshopper chromosomes.
• The term "gene" replaced "factor," meaning genes reside on chromosomes.

Chromosomes and Genes

• Genes, responsible for heredity, are present at fixed points on chromosomes.

Chromosome Structure and Function

• Chromosomes are present in the nucleus of every cell (except mammalian red blood cells)
• Chromosomes are present in pairs, one from each parent
• They are visible only during cell division
• In nondividing cells, they form a network called chromatin.
• Paired chromosomes exist in a fixed diploid (2n) number
• Each chromosome consists of a DNA molecule and proteins.
• Before cell division, a chromosome's DNA replicates into two DNA molecules called chromatids.
• Chromatids attach at the centromere and separate into two chromosomes during cell division.
• Bacteria have one circular DNA chromosome in a nucleoid region.
• Human cells contain 46 chromosomes, and can be written as 2n=46.
• Gametes (sperm/egg) contain a haploid number of 23 chromosomes.
• Chromosomes are distinct with two identical chromatids joined by the centromere.
• Chromatids become independent chromosomes when they acquire a centromere.

Genes

• Chromosomes organize in pairs in dividing cells during metaphase.
• Similar chromosomes from each parent are homologous.
• Of 23 pairs, 22 are autosomes; the 23rd pair (XX in females, XY in males) are sex chromosomes.
• The X chromosome has survival genes, whereas the Y chromosome has maleness genes.
• Testes-determining factor exists on the Y chromosome.
• Genes are Mendel's factors paired on chromosomes and dictate heredity.
• One member of a gene pair has a corresponding location on the homologous chromosome.
• The chromosome contains one DNA molecule; genes are DNA segments.
• The DNA is identical across all cells of an individual.

DNA Fingerprinting

• DNA fingerprinting identifies criminals using DNA.
• An individual's DNA is consistent across every bodily cell.
• Children inherit DNA from the parents, resembling their DNA.
• DNA of every single person is unique.

DNA Molecule

• A DNA molecule is a polynucleotide made of nucleotide units.
• Nucleotides contain a nitrogenous base, deoxyribose sugar, and phosphate group.
• Adenine, guanine, thymine, and cytosine are the four nitrogenous bases.
• These nucleotide combinations form the different genes.
• Physically, DNA is a double helix with two polynucleotide strands.

DNA Replication

• Cell division yields two identical cells with identical chromosomes.
• Before division, chromosomes must duplicate through DNA replication with two identical DNA molecules.
• The double-stranded DNA unwinds using enzymes, producing two DNA strands.
• DNA polymerase catalyzes a new daughter strand.
• It forms a double helix with a parental DNA strand.
• Each new DNA has a parental and new strand.
• Both identical DNAs turn into two chromatids attached by a centromere.
• Post-replication, each chromosome comprises two identical DNA molecules inside two chromatids.
• During cell division, chromatids separate into two chromosomes for daughter cells.

Blood Group Inheritance

• Blood groups depend on gene pairs inherited from parents.
• Blood groups include A, B, AB, and O.
• Genes IA, IB, and i determine blood group inheritance.
• A foetus inherits two genes from its parents.
• Gene combinations and blood groups are shown in Table 25.1.
• IA and IB genes are dominant; the i gene is recessive.
• Humans also belong to Rhesus positive (Rh+) or negative (Rh-) groups.
• Rh+ gene dominates over the Rh- gene.

Sex Determination

• The combination of sex chromosomes dictates the foetus's sex.
• Foetus develops from a zygote after gamete fusion (male sperm and female egg).
• Gametes are haploid with 'n' chromosomes whereas zygote is diploid (2n).
• Ova/eggs contain 22 autosomes and one X chromosome
• Sperm are of two types: 22 autosomes with an X chromosome, or 22 autosomes with a Y chromosome.
• When X bearing sperm fuses with the egg, a female child results with 44 autosomes and two X chromosome.
• A Y chromosome signifies a male child.
• If a woman does not bear a male child, it is wrong to blame her alone.
• Sex depends solely on chance.
• The Pre-natal Diagnostic Techniques Act (1994) prohibits prenatal sex determination to check female foeticide.
• The person contravening this act is punishable with imprisonment and fine.

Hereditary Disorders

• Genes control organism features, and mutated genes may not remain normal.
• Defective parent genes may be masked by a dominant normal gene and only harmful if inherited from both parents.
• Such a condition creates a hereditary or genetic disorder.
• Some hereditary disorders come from the presence of a single defective dominant gene.
• The presence of two defective recessive genes can also be the cause.
• Such disorders cannot be cured; scientists seek to replace defective genes via gene replacement therapy.

Common Genetic (Hereditary) Disorders

• Common genetic disorders include Thalassemia, Haemophilia, and Colourblindness.

Thalassemia

• Affected cannot produce haemoglobin required to carry oxygen in the red blood cells.
• Defective gene pairs control haemoglobin production.
• Thalassemia sufferers need blood transfusions.
• The thallasemia gene is present on an autosome.

Haemophilia

• There is a defective gene or deficient in genes that control blood clotting, resulting in profuse bleeding.

Color Blindness

• One cannot distinguish different colors.
• There is a defective or an absence of vision.
• Genes for haemophilia and colour-blindness locate on X-chromosomes and pass from mother to son.
• Female carriers mask their X-chromosome defect with another normal X gene.
• The genetic condition manifests if, at the chromosome level, a male, has only one X chromosome with the defective gene.

Genetic Counseling

• Thalassemia is autosomal, haemophilia and colour blindness are X-chromosomal.
• Marriages between close relatives elevate odds of offspring inheriting defective genes.
• Professionals provide genetic counselling and assess genetic disorder probability.

Human Genome

• Numerous genes dictate complex human structure, behaviour, and functions.
• In 2003, a project to identify a lot of human genes was completed.
• The research pinpointed the location of combinations of nucleotides.
• Genome knowledge aids genetic defect therapies, location, and treatment.

Genetic Engineering

• It is also called recombinant DNA technique, this technique allows transferring genes, in the form of organisms, into part of other species.
• In this way, the new organism is genetically modified (GMO).
• The transfer of gene is possible through plasmids, with bacteria.
• Plasmids form circular DNA in bacteria, yet are separate from the bacterias cell.
• Phages carry cells and culture or transmit bacteria transfer through attacking viruses. Genetic engineering aids in gene therapy and offers benefits.

Key Points of the Lesson

• Genetic terms include heredity and genetics.
• Mendel postulated genetics laws, noting heredity due to "factors" or features governed by paired factors.
• Inheritance states genes may be dominant or recessive, wherein dominant masks recessive genes.
• Sutton found Mendelian factors or genes present in chromosomes.
• Chromosomes exist in pairs in nuclei and form of DNA and proteins.
• Human diploid chromosomes number 46 with 22 autosome pairs, and 2 sex chromosomes (X and Y).
• DNA forms genes or segments within the chromosome molecule.
• Each nucleotide consists of nitrogen base, sugar, and phosphate.
• Helic DNA has a repeating unit made of two strands.
• Chromosomes consist of genes, responsible for protein production.
• X genes determine females and XY chromosomes men.
• Lacking/defective genes lead to thalassemia, haemophilia and colour blindness.
• Thalassemia causes haemoglobin damage requires treatment.
• Haemophiliacs inability to produce genes required to blood clot.
• Defective colourblind genes hinder colour discernibility on X chromosomes.
• Genetic engineering transfers species using plasmids.
• DNA uniquely fingerprints individuals identify.