Heredity
Genes and alleles, Mendelian crosses, inheritance patterns, sex-linked traits and chromosomal disorders
By the end of this unit you can…
- ✓Define gene, allele, genotype, phenotype, homozygous and heterozygous
- ✓Explain segregation and independent assortment, and how meiosis creates variation
- ✓Use Punnett squares to predict monohybrid crosses
- ✓Describe dominant–recessive, incomplete dominance, codominance, multiple-allele and polygenic inheritance
- ✓Explain X-linked inheritance and describe common chromosomal disorders
- ✓Describe environmental influences, epigenetics and genetic screening
Key terms
Practice →1Genes, alleles & chromosomes
Genetics studies how traits are passed on. A gene is a segment of DNA coding for a protein (or RNA); its location on a chromosome is its locus. Humans have 23 pairs of chromosomes: 22 pairs of autosomes and 1 pair of sex chromosomes (XX female, XY male).
A karyotype is a display of a cell's chromosomes arranged in homologous pairs by size — used to detect chromosomal abnormalities.
- Homologous chromosomes
- A matched pair — one from the mother, one from the father — carrying genes for the same traits at the same loci.
- Alleles
- Different versions of the same gene (e.g. A and a).
- Genotype
- The genetic makeup — the actual alleles (e.g. Aa).
- Phenotype
- The observable trait produced by the genotype (e.g. dimples).
- Homozygous
- Two identical alleles (AA or aa).
- Heterozygous
- Two different alleles (Aa) — a carrier if the recessive allele causes disease.
- Dominant vs recessive
- A dominant allele masks a recessive one; recessive traits appear only when homozygous recessive.
2Sources of genetic variation
| Source | How it creates variety |
|---|---|
| Segregation | The two alleles of each gene separate during meiosis — each gamete gets one allele |
| Independent assortment | Homologous pairs line up randomly at metaphase I — 2²³ (~8.5 million) possible combinations of chromosomes per gamete |
| Crossing over | Homologous chromosomes exchange segments during prophase I, creating new combinations on the same chromosome (recombinant chromosomes) |
| Random fertilization | Any sperm can fertilize any egg → ~72 trillion zygote combinations from one couple, before crossing over |
3Mendelian crosses
A Punnett square predicts the possible genotypes of offspring: write each parent's gametes along the sides and fill in the combinations. Each box is an equally likely outcome — probabilities apply to each child independently.
| Cross | Genotype ratio | Phenotype ratio |
|---|---|---|
| Aa × Aa | 1 AA : 2 Aa : 1 aa | 3 dominant : 1 recessive (25% chance of recessive trait) |
| AA × aa | All Aa | All dominant |
| Aa × aa | 1 Aa : 1 aa | 1 dominant : 1 recessive (50%) |
4Patterns of inheritance
| Pattern | How it works | Examples |
|---|---|---|
| Autosomal dominant | One copy causes the trait; affected parent × unaffected → 50% chance per child | Huntington disease, achondroplasia, Marfan syndrome; dimples, widow's peak |
| Autosomal recessive | Two copies needed; carriers unaffected; carrier × carrier → 25% affected | Cystic fibrosis, sickle-cell anemia, Tay–Sachs, PKU, albinism |
| Incomplete dominance | Heterozygote shows an intermediate phenotype | Sickle-cell trait (Ss — mild symptoms under low O₂); familial hypercholesterolemia |
| Codominance | Both alleles fully expressed in the heterozygote | AB blood type (Iᴬ Iᴮ) |
| Multiple alleles | More than two alleles exist in the population for a gene | ABO blood groups: Iᴬ, Iᴮ, i |
| Polygenic | Many genes add up → continuous range of phenotypes (bell curve) | Height, skin color, eye color, intelligence |
| ABO blood type | Possible genotypes |
|---|---|
| A | IᴬIᴬ or Iᴬi |
| B | IᴮIᴮ or Iᴮi |
| AB | IᴬIᴮ (codominant) |
| O | ii |
5Sex-linked inheritance
Genes on the X chromosome are X-linked; the small Y chromosome carries few genes (including SRY, which triggers male development). Because males have only one X, a single recessive X-linked allele is always expressed in males — they can't be carriers.
| Parents | Sons | Daughters |
|---|---|---|
| Carrier mother (XᴴXʰ) × normal father (XᴴY) | 50% affected | 50% carriers, none affected |
| Normal mother × affected father (XʰY) | None affected (get Y from father) | All carriers |
X-linked recessive examples: red–green color blindness, hemophilia A, Duchenne muscular dystrophy. They're far more common in males; a son's X always comes from his mother.
X-linked dominant traits are rare (e.g. vitamin D–resistant rickets): an affected father passes the trait to all daughters and no sons.
6Chromosomal disorders
Nondisjunction — failure of homologous chromosomes or sister chromatids to separate during meiosis — produces gametes with an extra or missing chromosome. Fertilization then gives aneuploidy: trisomy (three copies, 2n + 1) or monosomy (one copy, 2n − 1). Most autosomal aneuploidies are lethal; the risk rises with maternal age.
| Disorder | Karyotype | Features |
|---|---|---|
| Down syndrome | Trisomy 21 (47, +21) | Intellectual disability, characteristic facial features, heart defects, early Alzheimer-type changes; most common viable autosomal trisomy |
| Turner syndrome | 45, XO (monosomy X) | Female; short stature, webbed neck, underdeveloped ovaries — infertile |
| Klinefelter syndrome | 47, XXY | Male; small testes, infertility, some breast development, tall |
| Patau / Edwards syndromes | Trisomy 13 / trisomy 18 | Severe defects; most die in infancy |
7Environment, epigenetics & screening
Genes aren't destiny: environmental factors (nutrition, drugs, infections, maternal health) can change gene expression. Phenocopies are environmentally caused traits that mimic genetic ones (e.g. thalidomide limb defects). Epigenetic marks — DNA methylation and histone modification — switch genes on or off without changing the DNA sequence; some marks can be passed to offspring. Genomic imprinting silences one parent's allele (e.g. Prader–Willi vs Angelman syndrome on chromosome 15).
- Pedigree
- A family tree showing a trait across generations — used to work out the inheritance pattern and carrier risk.
- Carrier screening
- Blood tests to identify carriers of recessive disorders (e.g. Tay–Sachs, CF, sickle-cell).
- Amniocentesis
- Sampling amniotic fluid (~week 15+) to karyotype fetal cells.
- Chorionic villus sampling (CVS)
- Sampling chorionic villi (~week 10–12) — earlier results; slightly higher risk.
- Noninvasive prenatal testing (NIPT)
- Analyzes fetal DNA fragments in the mother's blood — screens for trisomies.