Understanding Genetics: A Complete Guide to Heredity and Inheritance
Genetics is the study of genes and how traits are passed down from parents to offspring. Genes, which are made of DNA, contain the instructions for building and operating living organisms. Understanding genetics involves exploring how genes function, how they are inherited, and how they contribute to an organism's characteristics and potential for disease.
Genetics is the branch of biology concerned with how traits are passed from parents to offspring. It explains why children resemble their parents and how certain traits like height, eye color, or genetic disorders show up in individuals. This article covers key genetic concepts, Mendel’s foundational experiments, human inheritance patterns, sex-linked traits, and real-world applications of genetics.
What Is Genetics?
Genetics is the scientific study of heredity (how traits are passed from parent to offspring) and variation (differences among individuals). Inheritance refers to the process through which these traits—like skin color, intelligence, or height—are transmitted across generations.
The field of genetics began with the work of Gregor Mendel, a monk who experimented on garden peas to uncover the basic principles of heredity.
Key Terms in Genetics
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Gene: A segment of DNA on a chromosome responsible for a specific trait.
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Allele: An alternative form of a gene. For example, alleles for height could be 'T' (tall) and 't' (short).
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Genotype: The genetic makeup of an organism (e.g., TT, Tt, or tt).
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Phenotype: The physical expression of a trait (e.g., tall or short).
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Dominant allele: An allele whose trait appears even when paired with a different allele (e.g., T in Tt).
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Recessive allele: An allele whose trait only appears when paired with an identical allele (e.g., t in tt).
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Homozygous: Having two identical alleles (TT or tt).
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Heterozygous: Having two different alleles (Tt).
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Hybrid: Offspring from parents with different traits.
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Back cross/Test cross: Mating an individual showing a dominant trait with a recessive parent to determine the genotype.
Mendel’s Experiments: The Foundation of Modern Genetics
Gregor Mendel studied monohybrid inheritance—the inheritance of a single trait—using pea plants. He chose peas because they grow quickly and show clear trait differences (e.g., tall vs. short).
His Key Experiments:
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P1 Generation: Crossed pure tall plants (TT) with pure short plants (tt).
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F1 Generation: All offspring were tall (Tt) – tallness is dominant.
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F1 Cross: He self-pollinated the F1 plants (Tt × Tt).
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F2 Generation: Produced tall and short plants in a 3:1 ratio.
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Mendel’s Conclusions:
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Traits are controlled by pairs of alleles.
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Alleles separate during gamete formation (law of segregation).
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Traits are inherited independently of each other (law of independent assortment).
Mendel’s Laws of Inheritance
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Law of Segregation: Each parent contributes one allele per trait to their offspring.
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Law of Independent Assortment: Alleles of different traits are distributed to gametes independently.
Monohybrid Inheritance in Humans
Albinism:
A genetic condition caused by a recessive gene resulting in lack of melanin. Symptoms include white hair, light skin, and pink eyes.
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Genotypes and Phenotypes:
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AA – Normal
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Aa – Carrier
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aa – Albino
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Test Cross in Practice
Used to determine if an individual showing a dominant trait is homozygous or heterozygous by crossing it with a recessive individual.
Example: Cross a tall plant (Tt or TT) with a short plant (tt). If some offspring are short, the tall parent is heterozygous.
Sex Determination in Humans
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Females have XX chromosomes and produce only X gametes.
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Males have XY chromosomes and produce both X and Y gametes.
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The male determines the sex of the offspring:
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X sperm + X egg → Female (XX)
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Y sperm + X egg → Male (XY)
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Interesting fact: Though more boys are born (due to faster Y sperm), male infants have slightly higher mortality rates.
Sex-Linked Traits
Sex-linked traits are carried on sex chromosomes, especially the X chromosome, and are more common in males since they only have one X.
Common Sex-Linked Conditions:
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Colour Blindness
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Haemophilia (bleeder disease)
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Sickle Cell Anaemia
Colour Blindness
Caused by a recessive gene on the X chromosome.
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XBXB – Normal female
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XBXb – Carrier female
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XbXb – Colour blind female
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XBY – Normal male
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XbY – Colour blind male
Haemophilia
A disorder where blood clots slowly. It's also caused by a recessive gene on the X chromosome.
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XHXH – Normal female
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XHXh – Carrier
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XhXh – Haemophiliac
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XHY – Normal male
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XhY – Haemophiliac male
Sickle Cell Anaemia
A condition caused by a recessive mutation in the gene responsible for haemoglobin.
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BB – Normal
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Bb – Carrier (no symptoms)
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bb – Has sickle cell disease
Sickle cells carry less oxygen and block capillaries, leading to health complications.
Exceptions to Mendelian Inheritance
Incomplete Dominance:
Neither allele is fully dominant. The offspring’s phenotype is intermediate.
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Example: Red × White flower → Pink flowers
Co-dominance:
Both alleles are expressed equally in heterozygotes.
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Example: Red cow × White cow → Roan cow (red + white hairs)
Multiple Alleles:
More than two alleles control a trait.
Example: Blood Groups in Humans
Controlled by three alleles: IA, IB, IO
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IA and IB are co-dominant
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IO is recessive
| Genotype | Blood Group |
|---|---|
| IAIA / IAIO | A |
| IBIB / IBIO | B |
| IAIB | AB |
| IOIO | O |
