Genetic terms and common inherited disorders
Inheritance, variation and evolution • Reproduction
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Gamete
Gametes are sex cells such as sperm and ova that contain one set of chromosomes (haploid) produced by meiosis. Gametes fuse during fertilisation to form a diploid zygote with the normal chromosome number, so inheritance involves one allele from each parent . Limiting factor: gametes carry only one allele per gene, so they cannot show dominant/recessive interactions until after fertilisation.
Chromosome
Chromosomes are long, thin structures of coiled DNA located in the nucleus that contain many genes. Chromosome number varies between species; humans have 23 pairs in body cells (diploid) and 23 single chromosomes in gametes (haploid) . Cause → effect: gene presence and arrangement on chromosomes determine which proteins can be coded and thus which characteristics may develop.
Gene and Allele
A gene is a defined section of DNA on a chromosome that provides the code to make a specific protein. Alleles are different versions of the same gene; each individual inherits two alleles for most genes (one from each parent) . Limiting factor: some characteristics are controlled by multiple genes, so single-gene allele examples are simplifications for basic inheritance.
Dominant and Recessive
A dominant allele produces its trait when present in one or two copies; a recessive allele produces its trait only when present as two copies (homozygous recessive) . Cause → effect: inheritance of a dominant allele from a single parent is sufficient for the trait to appear; inheritance of a recessive allele from both parents is required for the trait to appear.
Homozygous and Heterozygous
Homozygous describes two identical alleles for a gene (either two dominant or two recessive). Heterozygous describes one dominant and one recessive allele. Cause → effect: homozygous genotypes produce consistent phenotypes for that gene, while heterozygous genotypes express the dominant phenotype but can pass the recessive allele to offspring .
Genotype and Phenotype
Genotype is the genetic makeup represented by allele letters (for example BB, Bb, bb). Phenotype is the observable characteristic described in words (for example brown eyes or blue eyes) . Cause → effect: the genotype provides the potential (alleles present) and the phenotype is the realised trait after gene expression and environmental influences.
Polydactyly (dominant inherited disorder)
Polydactyly causes extra fingers or toes and results from a dominant allele. Inheritance of a single dominant allele from one parent is sufficient to produce the extra digit(s) in the offspring. Population effect: the condition is rare; the dominant allele can appear in family pedigrees across successive generations because affected heterozygotes display the trait and can pass it on .
Cystic fibrosis (recessive inherited disorder)
Cystic fibrosis (CF) is a genetic disorder caused by inheriting two copies of a recessive allele. Heterozygous carriers (one normal dominant allele and one recessive allele) do not show symptoms but can pass the recessive allele to children. Cause → effect: defective CF alleles cause thick, sticky mucus that reduces gas exchange and increases lung infections; long-term effects include reduced exercise tolerance and shorter life expectancy unless managed clinically .
Key notes
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