Genetics & Inheritance

Heredity

Heredity is the passing on of characteristics from parents to offspring via genes. A species is a group of organisms that can interbreed to produce fertile offspring.

Key Terms

• Locus: the location of a gene on a chromosome
• Allele: alternative forms of a gene (e.g. B = brown eyes, b = blue eyes)
• Homozygous: both alleles the same (BB or bb)
• Heterozygous: alleles are different (Bb)
• Genotype: the genetic make-up (e.g. Bb)
• Phenotype: the physical appearance (e.g. brown eyes)
• Dominant: allele expressed when present (capital letter)
• Recessive: only expressed in homozygous state (lowercase)

Nuclear vs Non-nuclear Inheritance

Nuclear DNA: inherited from both parents via meiosis (46 chromosomes).

Mitochondrial DNA: circular, ~37 genes, inherited only from the mother. Mutations cause mitochondrial diseases.

Chloroplast DNA: ~120 genes controlling photosynthesis, also maternally inherited.

Epigenetics

Epigenetics studies how behaviour and environment affect gene expression without changing the DNA sequence. Mechanisms include:

• DNA methylation: chemical group added to DNA, blocking gene expression
• Histone modification: changes how tightly DNA is packed, affecting gene access

Epigenetic changes are reversible and can be influenced by diet, stress, smoking, and pollution.

Monohybrid Crosses

A monohybrid cross involves parents differing in one trait. Use a Punnet square to predict offspring.

Example: Tall (T) is dominant over short (t).

Tt × tt → 50% Tt (tall), 50% tt (short) → 1:1 ratio

Tt × Tt → 25% TT, 50% Tt, 25% tt → 3 tall : 1 short

Incomplete Dominance

Neither allele is fully dominant. The heterozygous phenotype is a blend. Example: Red (RR) × White (WW) → Pink (RW).

Mendel's Laws (HL)

• Law of Segregation: Each trait is controlled by a pair of alleles that separate during gamete formation
• Law of Independent Assortment: Genes for different traits are inherited independently (unless linked)

Sex Determination

Females = XX, Males = XY. The Y chromosome determines maleness.

Sex-linked Traits

Genes on the X chromosome (e.g. colour blindness, haemophilia). Males are more affected because they have only one X - a single recessive allele will be expressed. Females need two recessive alleles to show the trait; one recessive makes them a carrier.

Every cell in your body carries the same genes, yet a nerve cell and a liver cell behave completely differently. The difference is gene expression: which genes are switched on. Epigenetics is the study of changes in gene expression that do not alter the DNA base sequence.

Two main mechanisms

DNA methylation: methyl groups attach to the DNA, usually silencing that gene so it cannot be transcribed.

Histone modification: DNA is wound around histone proteins. Chemical tags on the histones tighten the coil, hiding the gene, or loosen it so the gene can be read.

What triggers epigenetic change

Diet, stress, exercise, smoking, pollutants and even temperature can add or remove these marks. Studies of people conceived during the Dutch Hunger Winter found methylation differences decades later, and identical twins accumulate different epigenetic marks as their lifestyles diverge, which is why one twin can develop a condition the other does not.

Genetic versus epigenetic

Epigenetics matters in medicine because some cancers involve silenced tumour suppressor genes, and drugs that reverse methylation are already in use.

Benefits

• Provides a predictable mathematical framework (ratios and Punnett squares) for simple inheritance.
• Allows us to calculate the probability of offspring inheriting specific traits or genetic disorders.
• Forms the foundation of modern genetic counselling and selective breeding programmes.

Limitations

• Applies only to single genes with clear dominant and recessive alleles.
• Does not account for linkage, where genes on the same chromosome are inherited together.
• Cannot explain incomplete dominance or codominance.
• Ignores polygenic traits such as height and skin colour, which are controlled by many genes.
• Does not include environmental effects or epigenetics, which alter gene expression without changing the DNA sequence.

Common mistake

Do not state that Mendel's laws are wrong. They are correct for the cases they describe; they are simply incomplete because Mendel worked before chromosomes, linkage and DNA were understood.

Mitochondrial DNA (mtDNA)

Mitochondria contain their own small, circular loop of DNA, separate from the chromosomes in the nucleus. Because the egg cell contributes all the mitochondria to the zygote, mtDNA is inherited exclusively from the mother.

Key points

• Mutations in mtDNA cause disorders of energy-hungry tissues such as muscle and nerve, for example Leber's hereditary optic neuropathy.
• Because it changes very little between generations, mtDNA is used to trace maternal ancestry and in evolutionary studies.
• Chloroplasts in plants also carry their own DNA, supporting the endosymbiotic theory: mitochondria and chloroplasts were once free-living bacteria engulfed by an ancestral cell.

Common mistake

Mitochondrial disorders pass from a mother to all of her children, but only her daughters can pass the condition on. Affected fathers pass it to nobody, because sperm contribute no mitochondria to the zygote.

Chromosomes are structures in the nucleus made of DNA wrapped tightly around histone proteins. Humans have 46 chromosomes (23 pairs) - 22 pairs of autosomes and 1 pair of sex chromosomes (XX or XY).

Structure of a chromosome:
• Each chromosome consists of two chromatids joined at the centromere
• DNA exists as chromatin (loose) during interphase, and condenses into visible chromosomes during division
• DNA contains both coding regions (genes) and non-coding regions (formerly called 'junk DNA')

Key terms:
• Gene: a section of DNA that codes for a specific protein
• Allele: different versions of the same gene (e.g. B = brown eyes, b = blue eyes)
• Genotype: the alleles an organism carries (BB, Bb, bb)
• Phenotype: the observable characteristic (brown eyes, blue eyes)
• Dominant: expressed when one or two copies present (B)
• Recessive: only expressed when two copies present (bb)

Epigenetics: changes in gene expression caused by environment or behaviour without altering the DNA sequence. E.g. identical twins can develop different traits due to different environments affecting which genes are switched on/off.

Gregor Mendel (1822–1884), an Austrian monk, is the 'Father of Genetics'. He discovered the laws of inheritance through careful experiments with pea plants, tracking traits like height, seed colour, and flower colour over multiple generations.

Law of Segregation: Each organism has two alleles per gene. During gamete formation (meiosis), these alleles separate so each gamete receives only one allele.

Law of Independent Assortment: Genes on different chromosomes are inherited independently of each other. (Exception: linked genes on the same chromosome)

Monohybrid cross (one trait):
E.g. Tt × Tt → gametes: T, t × T, t
Punnett square: TT, Tt, Tt, tt → 3 tall : 1 short (phenotypic ratio)

Incomplete dominance: neither allele is fully dominant. Heterozygote shows a blended phenotype. E.g. red × white snapdragons → pink flowers.

Sex-linked traits: genes carried on the X chromosome. Males (XY) are more affected because they only have one X. E.g. colour blindness: X^BX^b (carrier female) × X^BY → 50% sons colour blind.