Inheritance is the transmission of genetic information from parents to offspring. It explains why you resemble your parents yet are not identical to them. In this topic you will zoom in from chromosomes to genes to the DNA molecule itself, learn how cells divide by mitosis and meiosis, and use genetic diagrams to predict the offspring of crosses, including tricky cases such as ABO blood groups and conditions carried on the sex chromosomes.
Inside the nucleus of a cell are chromosomes, which are long threads of DNA. A gene is a length of DNA that codes for a particular protein and so controls a feature. DNA (deoxyribonucleic acid) is the chemical that makes up genes; its sequence of bases is the genetic code. Different forms of the same gene are called alleles.
It is important to separate the genotype (the alleles an organism has) from the phenotype (the observable features). Two organisms can share a phenotype but have different genotypes.
Mitosis is nuclear division giving rise to genetically identical cells. The chromosome number is kept the same, so each new cell has a full set. Mitosis is used for growth, repair of damaged tissues, replacement of cells and asexual reproduction. Before division the DNA is copied (replicated) exactly so each new cell receives a complete copy.
Meiosis is reduction division in which the chromosome number is halved from diploid to haploid, resulting in genetically different cells. It happens in the reproductive organs to make gametes. Because meiosis produces variation and halves the chromosome number, fertilisation can then restore the diploid number without it doubling each generation.
A monohybrid cross follows the inheritance of a single gene. By convention a dominant allele is given a capital letter and the recessive allele the same letter in lower case, for example T for tall and t for short. To work out a cross, write the parents’ genotypes, show the gametes in circles, and use a Punnett square to combine them.
Crossing two heterozygous tall plants (Tt × Tt) gives offspring in the ratio 3 tall to 1 short. A test cross (crossing an unknown dominant phenotype with a homozygous recessive) can reveal whether the unknown is homozygous or heterozygous: any recessive offspring show it must have been heterozygous.
Codominance is when both alleles in a heterozygote are expressed and both affect the phenotype, so neither is recessive. The ABO blood group system is the key example. There are three alleles: IA, IB and IO. IA and IB are codominant with each other and both are dominant to IO. This gives four blood groups: A (IAIA or IAIO), B (IBIB or IBIO), AB (IAIB) and O (IOIO).
Sex is determined by chromosomes: females are XX and males are XY. Because the Y chromosome is small, some genes on the X chromosome have no matching allele on the Y. These are sex-linked genes. Red-green colour blindness is sex-linked and recessive, so it is more common in males, who need only one recessive allele on their single X chromosome to show the condition.
Practise exam-style questions on this topic.