Genetics & Heredity
Introduction
Why do children resemble their parents? Why do some traits -- like attached earlobes or a widow's peak -- seem to skip generations? The answers lie in genetics, the study of how biological information is stored and passed from one generation to the next. Understanding genetics helps us explain inheritance patterns, predict the likelihood of inherited diseases, and appreciate the remarkable similarity and diversity of life on Earth.
DNA: The Molecule of Heredity
Every living cell contains a molecule called DNA (deoxyribonucleic acid). DNA is the master instruction manual for building and running an organism. It is made of two long strands twisted around each other in a shape called a double helix, discovered by Watson and Crick in 1953 (building on X-ray work by Rosalind Franklin).
Each strand is made of repeating units called nucleotides. Every nucleotide contains one of four nitrogen bases: Adenine (A), Thymine (T), Guanine (G), and Cytosine (C). The bases pair up across the two strands in a specific way: A pairs with T, and G pairs with C. This complementary base pairing is what allows DNA to be copied accurately during cell division.
The sequence of bases along a strand of DNA is the genetic code -- different sequences spell out different instructions.
Genes and Chromosomes
DNA in the cell nucleus is coiled and packaged into structures called chromosomes. Humans have 46 chromosomes, arranged in 23 pairs. One chromosome of each pair comes from the mother (via the egg) and one from the father (via the sperm).
A gene is a specific segment of DNA on a chromosome that codes for a particular protein (or controls the production of one). That protein in turn influences a biological trait -- like eye colour, blood type, or whether an enzyme is produced. Humans have roughly 20,000 to 25,000 protein-coding genes.
The two copies of each chromosome in a pair are called homologous chromosomes. They carry genes for the same traits in the same positions, but the exact versions of those genes may differ between the two chromosomes.
Alleles
Different versions of the same gene are called alleles. For example, the gene for pea-plant seed colour has a "yellow" allele and a "green" allele. An organism that has two identical alleles for a gene is homozygous (e.g. YY or yy). An organism with two different alleles is heterozygous (e.g. Yy).
Dominant and Recessive Inheritance
Gregor Mendel, an Austrian monk working in the 1860s, was the first to work out the mathematical rules of inheritance by carefully crossing pea plants and counting thousands of offspring. His experiments revealed a key pattern: some alleles are dominant and others are recessive.
- A dominant allele is expressed in the phenotype (observable appearance) whenever it is present -- even if only one copy exists. Dominant alleles are written as capital letters (e.g. B).
- A recessive allele is only expressed when the organism has two copies of it (homozygous recessive). A single recessive allele is hidden if a dominant allele is also present. Recessive alleles are written in lowercase (e.g. b).
An organism's genotype is its actual genetic make-up (e.g. Bb). Its phenotype is the physical trait that results from that genotype (e.g. brown eyes).
Punnett Squares
Biologists use a Punnett square to predict the probability of offspring genotypes. Each parent contributes one allele per gene to the offspring, chosen at random during the formation of sex cells (gametes).
For a cross between two heterozygous brown-eyed parents (Bb x Bb):
| B | b | |
|---|---|---|
| B | BB | Bb |
| b | Bb | bb |
The expected outcomes are: 1 BB (dominant/dominant), 2 Bb (dominant/recessive), 1 bb (recessive/recessive) -- a 3 : 1 ratio of dominant phenotype to recessive phenotype. This is exactly the ratio Mendel observed in his pea plants.
Sex Determination
Twenty-two of the 23 human chromosome pairs are autosomes (non-sex chromosomes). The 23rd pair are the sex chromosomes. Females have two X chromosomes (XX); males have one X and one Y chromosome (XY).
Because fathers contribute either an X or a Y chromosome to offspring (while mothers always contribute an X), the father's sperm determines the biological sex of a child: an X-carrying sperm produces a daughter (XX) and a Y-carrying sperm produces a son (XY).
Codominance and Incomplete Dominance
Not all inheritance follows the simple dominant-recessive pattern.
Codominance occurs when both alleles are expressed equally in the phenotype. A classic example is the ABO blood group system, where the I^A and I^B alleles are codominant -- a person with genotype I^A I^B has type AB blood, showing characteristics of both alleles simultaneously.
Incomplete dominance occurs when the heterozygous phenotype is a blend of the two homozygous phenotypes. Red (R) and white (W) snapdragon flowers crossed together (R x W) produce pink (RW) offspring -- neither allele is fully dominant.
Sex-Linked Traits
Some genes are located on the sex chromosomes. Because males only have one X chromosome (and the Y chromosome carries very few genes), males only need one recessive allele on their single X to express a sex-linked trait, whereas females need two.
This is why conditions like red-green colour blindness and haemophilia are far more common in males. A female can be a carrier -- she has one normal and one affected allele on her two X chromosomes -- but does not show the condition because the normal allele masks the recessive one.
Inherited Conditions
Mutations -- changes to the DNA sequence -- can alter how a gene functions. Some mutations are harmless; others lead to inherited medical conditions.
- Cystic fibrosis is caused by a recessive mutation in a single gene on chromosome 7. Both parents must carry the faulty allele for a child to be affected.
- Sickle cell disease is caused by a recessive mutation that alters the shape of red blood cells, impairing their ability to carry oxygen.
- Huntington's disease is caused by a dominant mutation, so only one copy of the altered allele is needed for the condition to develop.
Genetic counselling helps families understand the probability of passing an inherited condition to their children, using pedigree analysis and DNA testing.
Why Variation Exists
The shuffling of alleles during meiosis -- the cell division that produces gametes -- and the random union of sperm and egg create enormous genetic variation among offspring. Additional variation arises from:
- Crossing over: homologous chromosomes exchange segments of DNA during meiosis, creating new combinations of alleles.
- Independent assortment: pairs of chromosomes separate independently of each other, so each gamete gets a random combination of maternal and paternal chromosomes.
- Mutation: rare, spontaneous changes to the DNA sequence.
This variation is the raw material on which natural selection acts, driving the long-term evolution of species.
Summary
Genetics explains how traits are inherited from generation to generation. DNA, packaged into chromosomes inside the nucleus, contains genes -- segments that code for proteins controlling traits. Alleles are different versions of a gene; dominant alleles are expressed over recessive ones, but inheritance patterns also include codominance, incomplete dominance, and sex-linked traits. Punnett squares allow us to predict the probability of offspring genotypes. Mutations in genes can cause inherited conditions. The variation produced by meiosis and random fertilisation is fundamental to the diversity of life.