Lesson 1: Heredity and Variation of Traits

1.1 Why are family members alike but not identical?

• Look at a family photograph.
• Children resemble their parents in some features and differ in others.
• Genetics is the science that explains these similarities and differences by studying how genetic information passes from parents to offspring.

• The genetic information is stored in a chemical substance called the genetic material, deoxyribonucleic acid (DNA).
• In a cell, the genetic material is in the nucleus.
• A DNA molecule is extremely long, so it is coiled around protein molecules to form thread-like chromosomes.
• The genetic material of a chromosome is divided into sections; each section is a gene.

A gene is a segment of the genetic material (DNA) that carries the information responsible for a specific inherited trait, such as eye colour, hair texture or height.

So: cell → nucleus → chromosomes → genes (DNA).

1.2 How are traits passed on?

• Inherited traits are transferred from parents to children through genes carried by the gametes: the sperm (male gamete) and the ovum or egg (female gamete).
• Gametes are produced in the gonads (testes and ovaries) by a type of cell division called meiosis.
• Each gamete contains only half of the genetic material, so it carries only one copy of each gene.
• At fertilisation, the nucleus of the sperm fuses with the nucleus of the ovum, so the new individual has two copies of every gene: one from the father and one from the mother.

1.3 Alleles, dominance and recessiveness

Look at a table of eye colour in four people:

• When the two copies are identical (persons A and D) the trait is clearly brown or blue.
• When the two copies are different (B and C), the eyes are still brown.
• So the brown copy is dominant over the blue copy, and the blue copy is recessive.

Definitions:

• Allele: each of the different forms of the same gene. A gene can have more than two alleles (for example eye colour: brown, blue, green; hair colour: blond, red, brown, black).
• Each individual usually has two alleles for each trait, one from each parent.
• Dominant allele: the allele whose effect appears in the organism when two different alleles are present.
• Recessive allele: the allele whose effect is completely masked in the presence of a dominant allele; it appears only when two copies are present.
• Principle of complete dominance: when the dominant trait appears and completely masks the contrasting trait.
• Homozygous (pure): two identical alleles (BB or bb). Heterozygous (hybrid): two different alleles (Bb).
• Phenotype: the observable inherited trait (e.g. brown eyes).
• Genotype: the genetic makeup, i.e. the alleles carried (e.g. Bb).

Symbols. The dominant allele is written as a capital letter (first letter of the trait), the recessive allele as the same small letter. For eye colour: B = brown (dominant), b = blue (recessive).

• Notice: two people with the same phenotype (brown eyes) can have different genotypes (BB or Bb).
• A recessive trait appears only when the genotype is bb.
• A difference in genotype (cause) can produce a difference in phenotype (effect).

1.4 Mendel and the discovery of the laws of heredity

In the nineteenth century the scientist Gregor Mendel (1822–1884) carried out systematic experiments on pea plants. Why peas?

• They show clear, contrasting traits that are easy to tell apart: seed colour (yellow/green), seed shape (round/wrinkled), plant height (tall/short), flower colour (purple/white), flower position (axial/terminal), pod shape (inflated/constricted), pod colour (green/yellow).
• Pollination can be controlled easily.

• Mendel crossed (hybridised) plants that differ in one trait after making sure each parent was purebred (always gave the same trait).
• Analysing his results he concluded that inherited traits do not appear at random but follow regular patterns that can be explained scientifically and used to predict the probability of traits in offspring.
• This was the foundation of modern genetics.

Pairs of contrasting traits in pea plants (dominant / recessive):

Genetic notation: P = parents; ♂ = male; ♀ = female; × = cross; G = gametes; F₁ = first generation; F₂ = second generation.

1.5 Mendel's first law (law of segregation)

When two purebred individuals that differ in one pair of contrasting traits are crossed:

• all individuals of the first generation (F₁) show only one of the parents' traits, the dominant one (100%);
• when F₁ individuals are crossed with each other, both traits appear in F₂ in the ratio 3 (dominant) : 1 (recessive).

Explanation.

• Each trait is controlled by a pair of alleles.
• During gamete formation the two alleles of the pair separate (segregate), so each gamete carries only one allele.
• At fertilisation, an allele from the father and one from the mother come together to form a new pair.

Example: seed colour (Y = yellow, y = green)

P: pure yellow (YY) × green (yy) Gametes: Y and y F₁: all Yy (yellow), 100% F₁ × F₁: Yy × Yy; gametes Y, y (each parent)

Punnett square:

F₂ genotypes: 1 YY : 2 Yy : 1 yy. F₂ phenotypes: 3 yellow : 1 green. Probability of green seeds = 1/4 = 25%.

1.6 The Punnett square

A Punnett square is a tool for predicting the possible genotypes and phenotypes of the offspring of a particular cross.

How to build one:

1. Work out the gametes each parent can make; the number of boxes depends on the number of gamete types.
2. Draw a grid of rows and columns.
3. Write the gametes of one parent across the top, and the gametes of the other parent down the left side.
4. Fill each box by combining the allele above with the allele at the side.
5. Count genotypes and phenotypes and give the ratio or percentage.

Worked example 1: purple × white (Check your understanding)

Purple (P) is dominant over white (p). Cross pure purple PP × white pp. Gametes: P and p. F₁: all Pp, purple (100%). F₁ × F₁: Pp × Pp

F₂: 1 PP : 2 Pp : 1 pp, i.e. 3 purple : 1 white (75% purple, 25% white).

Worked example 2: a heterozygous axial plant × a terminal plant

A = axial (dominant), a = terminal. Cross Aa × aa. Gametes: A and a (from Aa); a only (from aa).

Offspring: 2 Aa : 2 aa → 50% axial (Aa), 50% terminal (aa); phenotype ratio 1 : 1. Such a cross (with a homozygous recessive) is useful for finding out whether a dominant-looking plant is pure or hybrid.

Worked example 3: unknown purity

A tall pea plant (T = tall, t = short) is crossed with a short plant. Half of the offspring are short. What is the genotype of the tall parent? Short offspring are tt, so each parent gave a t; so the tall parent must be Tt (heterozygous). If the tall parent had been TT, all offspring would have been tall.

1.7 Mendel's second law (law of independent assortment)

• When two purebred individuals that differ in two or more pairs of contrasting traits are crossed, each pair of traits is inherited independently of the others.
• Analysed separately, each trait appears in F₂ in the ratio 3 : 1.

The dihybrid cross: seed shape and seed colour

R = round (dominant), r = wrinkled; Y = yellow (dominant), y = green. P: pure round yellow (RRYY) × wrinkled green (rryy) Gametes: RY and ry F₁: all RrYy, round yellow (100%) — both dominant traits appear.

F₁ × F₁: RrYy × RrYy. Each parent produces four types of gametes with equal probability: RY, Ry, rY, ry. Fusion gives 16 combinations:

Count of phenotypes in F₂ (out of 16):

• round yellow (both dominant): 9
• round green (first dominant, second recessive): 3
• wrinkled yellow (first recessive, second dominant): 3
• wrinkled green (both recessive): 1

Phenotype ratio 9 : 3 : 3 : 1.

Check each trait alone: round = 9 + 3 = 12, wrinkled = 3 + 1 = 4 → 12 : 4 = 3 : 1. Yellow = 9 + 3 = 12, green = 3 + 1 = 4 → 3 : 1. So shape is inherited independently of colour.

Worked example 4 (check your understanding)

T = tall, t = short; G = green pod, g = yellow pod. Two plants heterozygous for both traits (TtGg × TtGg) are crossed. What fraction are tall with green pods? From the 9 : 3 : 3 : 1 pattern, tall green = 9/16 (56.25%). You can also multiply independent probabilities: P(tall) = 3/4 and P(green) = 3/4, so 3/4 × 3/4 = 9/16.

Worked example 5: a test cross in humans

• Curly hair (H) is dominant over straight (h); brown eyes (B) dominant over blue (b).
• A man with curly hair and brown eyes (HhBb) marries a woman with straight hair and blue eyes (hhbb).
• Man's gametes: HB, Hb, hB, hb.
• Woman's gametes: only hb.

Phenotypes: curly brown : curly blue : straight brown : straight blue = 1 : 1 : 1 : 1 (25% each).

1.8 Genetic diversity

Every person has a unique genetic makeup (except identical twins). This variation arises because:

• there are different alleles for the same gene;
• the alleles are distributed randomly among gametes when they are formed;
• gametes from the two parents combine by chance at fertilisation.

• So even brothers and sisters differ in some traits.
• This genetic diversity helps a species adapt to different environmental conditions and survive over time.

1.9 Genetic (inherited) diseases

• Sometimes a gene carries a mutation (a change) that causes a disease.
• Such a disease is a genetic disease; it can appear from birth or later in life, and it can be passed to the next generation.

In many inherited diseases the disease-causing allele is recessive. Let N = normal allele and n = disease allele:

• NN: healthy, not a carrier.
• Nn: healthy-looking, but a carrier: has one copy of the disease allele but shows no symptoms; can pass it on.
• nn: has the disease (two disease alleles, one from each parent).

Worked example 6: two carrier parents

Both parents are Nn (carriers). Punnett square:

• Healthy, not carrier (NN): 25%
• Healthy carriers (Nn): 50%
• Affected (nn): 25% So each child of two carriers has a 1 in 4 chance of the disease, even though both parents look healthy. This answers "Why can a child be affected though both parents appear healthy?"

Worked example 7: one carrier, one normal

Nn × NN → 50% NN and 50% Nn. No child is affected, but half are carriers.

Worked example 8: carrier × affected

Nn × nn → 50% Nn (carrier) and 50% nn (affected).

Examples of recessive inherited diseases:

• Thalassaemia (Mediterranean anaemia): affects the production of haemoglobin and causes anaemia of varying severity.
• Galactosaemia: the body cannot break down the sugar galactose; babies suffer vomiting, yellowing of skin and eyes, and poor growth.

1.10 Marriage between close relatives

• Relatives share some genes they inherited from common ancestors.
• If one of these genes causes a disease, the probability that both partners are carriers of the same disease allele is higher in relatives than in unrelated couples.
• When both carry the same recessive allele, the chance of an affected child (1 in 4) increases.
• For this reason health authorities recommend:

• pre-marital medical screening for certain inherited diseases;
• genetic counselling: specialists study the family's medical history, run genetic tests and advise couples about the risks, which protects families and reduces the spread of diseases;
• comprehensive genetic screening: modern techniques that examine thousands of genes at the same time, helping early detection of rare diseases.

1.11 Summary of Lesson 1

• DNA → chromosome → gene; gametes carry one copy, zygote two.
• Alleles, dominant/recessive, homozygous/heterozygous, genotype/phenotype.
• Mendel's first law: segregation; 100% dominant in F₁ and 3 : 1 in F₂.
• Mendel's second law: independent assortment; 9 : 3 : 3 : 1 in F₂.
• The Punnett square predicts offspring.
• Recessive genetic diseases appear when both alleles are the disease allele; carriers look healthy; relatives' marriage raises risk; screening and counselling reduce it.