Basic Genetics: Your Ultimate Study Guide
Hello there! Welcome to the fascinating world of genetics. Ever wonder why you have your mum's eyes or your dad's smile? Or why some traits seem to run in families? This chapter is all about answering those questions. We're going to unravel the secrets of heredity – the passing of traits from parents to offspring.
Don't worry if this sounds complicated. We'll break it down step-by-step with clear explanations, genetic diagrams, and real-life examples. Let's get started on your journey to mastering HKDSE genetics!
1. Mendel's Laws & The Nature of Science
Gregor Mendel's Scientific Approach
Gregor Mendel was an Austrian monk who lived in the 1800s, widely regarded as the "Father of Genetics". His groundbreaking success came from rigorous scientific methodology:
Choice of experimental organism: Garden pea plants (Pisum sativum) were easy to cultivate, had a short life cycle, self-fertilised naturally, and possessed distinct, easily observable contrasting traits (e.g. tall vs. short, yellow vs. green seeds).
Controlled cross-breeding: Mendel prevented self-pollination by removing anthers and artificially transferring pollen between pure-breeding parental lines.
Quantitative analysis & large sample sizes: Instead of just descriptive observations, he counted offspring systematically and applied statistical analysis to deduce numerical inheritance ratios.
Hypothesis testing: He formulated testable hypotheses and verified them with test crosses.
Key Genetic Terms You MUST Know
Before analysing genetic crosses, understanding these fundamental terms is essential:
Gene: The basic unit of inheritance; a segment of DNA on a chromosome containing nucleotide sequences that code for a specific polypeptide or functional protein.
Allele: Alternative forms of the same gene occupying the same gene locus on homologous chromosomes (e.g. allele for tallness and allele for shortness).
Genotype: The genetic constitution of an organism, represented by allele symbols (e.g. TT, Tt, or tt).
Phenotype: The observable physical traits or biological characteristics of an organism, determined by genotype and environmental interaction (e.g. tall or short).
Homozygous: Having two identical alleles at a particular gene locus (e.g. TT for homozygous dominant or tt for homozygous recessive).
Heterozygous: Having two different alleles at a particular gene locus (e.g. Tt).
Dominant Allele: An allele that is expressed in the phenotype when present in either homozygous or heterozygous state (e.g. T).
Recessive Allele: An allele that is only expressed in the phenotype when present in the homozygous state (e.g. tt), masked by the dominant allele in heterozygotes.
Mendel's First Law: The Law of Segregation
During gamete formation (meiosis), the pair of alleles for a given gene separate (segregate) from each other so that each gamete carries only one allele for that gene.
Monohybrid Cross Genetic Diagram
A monohybrid cross investigates the inheritance of a single trait. Here is the formal genetic cross layout required for HKDSE:
Parents' phenotypes: Heterozygous tall \(\times\) Heterozygous tall
Parents' genotypes: Tt \(\times\) Tt
Gametes: T, t \(\times\) T, t
Punnett Square:
Gametes | T | t
---------------------------
T | TT (Tall) | Tt (Tall)
---------------------------
t | Tt (Tall) | tt (Short)
Offspring genotypic ratio: 1 TT : 2 Tt : 1 tt
Offspring phenotypic ratio: 3 Tall : 1 Short
Mendel's Second Law: The Law of Independent Assortment
During gamete formation, alleles of two or more different genes on non-homologous chromosomes assort independently into gametes.
Dihybrid Cross Genetic Diagram
Let \(R\) = round seed (dominant), \(r\) = wrinkled seed (recessive); \(Y\) = yellow seed (dominant), \(y\) = green seed (recessive).
Parents' phenotypes: Heterozygous round & yellow \(\times\) Heterozygous round & yellow
Parents' genotypes: RrYy \(\times\) RrYy
Gametes: RY, Ry, rY, ry \(\times\) RY, Ry, rY, ry
16-Cell Punnett Square:
- 9 Round Yellow (R_Y_)
- 3 Round Green (R_yy)
- 3 Wrinkled Yellow (rrY_)
- 1 Wrinkled Green (rryy)
Expected phenotypic ratio: 9 : 3 : 3 : 1
2. Inheritance in Humans
Sex Determination
Humans possess 23 pairs of chromosomes. Pair 23 represents the sex chromosomes:
Females: Homogametic (XX), producing eggs that all carry an X chromosome.
Males: Heterogametic (XY), producing sperm that carry either an X or a Y chromosome (1:1 ratio).
Therefore, the sex of an offspring is determined by the sex chromosome carried by the fertilising sperm, resulting in a 1:1 sex ratio (50% male : 50% female).
Sex Linkage (X-Linked Inheritance)
Genes located on the non-homologous region of the X chromosome are called sex-linked genes. Common examples include red-green colour blindness and haemophilia.
Because males (XY) have only one X chromosome, they are hemizygous for X-linked genes. A single recessive allele on the X chromosome will be expressed in males. Females (XX) have two copies, so a recessive allele is masked in heterozygous carriers (\(X^B X^b\)). Hence, sex-linked recessive conditions occur far more frequently in males than in females.
Multiple Alleles: ABO Blood Groups
A gene with more than two alternative alleles in a population demonstrates multiple alleles. The ABO blood group gene has three alleles: \(I^A\), \(I^B\), and \(i\).
\(I^A\) and \(I^B\) are both dominant to \(i\).
When \(I^A\) and \(I^B\) are present together (\(I^A I^B\)), both antigens are produced, resulting in blood group AB.
Allele \(i\) produces neither antigen and is recessive.
Blood Group Genotypes and Phenotypes:
- Group A: \(I^A I^A\) or \(I^A i\)
- Group B: \(I^B I^B\) or \(I^B i\)
- Group AB: \(I^A I^B\)
- Group O: \(i i\)
3. Pedigree Analysis & Genetic Counselling
Reading Pedigree Charts
A pedigree chart traces the transmission of a specific trait through successive generations in a family:
Square: Male; Circle: Female
Shaded shape: Affected individual; Unshaded shape: Unaffected individual
Horizontal line: Marriage/mating; Vertical line: Offspring
Determining Inheritance Patterns
Autosomal Recessive: Unaffected parents can produce an affected child (both parents are heterozygous carriers). The trait often skips generations.
Autosomal Dominant: Every affected individual has at least one affected parent. Two affected parents can have an unaffected child if both parents are heterozygous.
X-linked Recessive: Affected mothers pass the condition to all their sons; carrier mothers pass it to 50% of sons; affected fathers cannot pass the allele to sons.
Application in Genetic Counselling
Pedigree analysis enables genetic counsellors to determine parental genotypes and calculate the probability of prospective parents passing on hereditary diseases (e.g. cystic fibrosis, thalassaemia, haemophilia) to their future children, helping families make informed reproductive decisions.
4. Variation: Continuous vs. Discontinuous
Variation refers to phenotypic differences among individuals of the same species.
Discontinuous Variation
Characteristics: Distinct, qualitative categories with no intermediate forms (e.g. ABO blood groups, tongue rolling, presence of earlobes).
Genetic control: Controlled by one or a few major genes (monogenic); largely unaffected by environmental conditions.
Graphical presentation: Bar chart with discrete, non-touching bars.
Continuous Variation
Characteristics: Quantitative traits showing a continuous gradient of values across a range (e.g. human height, body mass, hand span, foot length).
Genetic control: Polygenic (controlled by many additive genes) and significantly modified by environmental factors (e.g. nutrition, climate).
Graphical presentation: Histogram that forms a bell-shaped normal distribution curve.
Sources of Genetic Variation
Meiotic Recombination: Crossing over between non-sister chromatids in prophase I and independent assortment of homologous chromosomes in metaphase I generate novel combinations of alleles.
Random Fertilisation: Random fusion of genetically unique male and female gametes creates diverse offspring genotypes.
Mutation: Random, spontaneous changes in DNA nucleotide sequence (gene mutation) or chromosome number/structure (chromosome mutation). Mutation is the primary, ultimate source of entirely new alleles.