How is genetic information stored in DNA, and how are characteristics inherited from parents?
DNA, genes and chromosomes, sexual and asexual reproduction, meiosis producing genetically varied gametes, dominant and recessive alleles, genotype and phenotype, Punnett squares and inheritance ratios, and inherited disorders.
A focused answer to the OCR Gateway GCSE Combined Science A topic B5 on genes and inheritance, covering DNA, genes and chromosomes, sexual reproduction and meiosis, dominant and recessive alleles, genotype and phenotype, Punnett squares and ratios, and inherited disorders.
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What this topic is asking
OCR wants you to describe DNA, genes and chromosomes, compare sexual and asexual reproduction, explain meiosis and variation, define alleles, genotype and phenotype, complete Punnett squares to predict ratios, and explain inherited disorders.
DNA, genes and chromosomes
The DNA of an organism is its genome. DNA is a double helix made of two strands. A gene is a section of DNA that codes for the order of amino acids in a protein, and proteins determine the organism's characteristics. Chromosomes come in pairs (one from each parent), so genes also come in pairs, with one copy on each chromosome of a pair. Most characteristics are controlled by several genes interacting, though some (such as the disorders below) are controlled by a single gene.
Reproduction, meiosis and variation
There are two kinds of reproduction:
- Sexual reproduction involves two parents and the fusion of a male gamete (sperm or pollen) with a female gamete (egg). Gametes are made by meiosis, which halves the chromosome number (so the full number is restored at fertilisation) and shuffles the alleles, producing four genetically different gametes. The offspring inherit a mixture of alleles, so they show variation.
- Asexual reproduction involves one parent and no gametes. The offspring are produced by mitosis and are genetically identical clones of the parent, so there is no genetic variation.
Sexual reproduction's variation is an advantage when the environment changes, because some offspring may be better suited to the new conditions, which is the raw material for natural selection.
Alleles, genotype, phenotype and crosses
An allele is a version of a gene. A dominant allele (written as a capital letter) is expressed if at least one copy is present; a recessive allele (lower case) is only expressed when both copies are recessive. The genotype is the combination of alleles an organism has (for example BB, Bb or bb), and the phenotype is the characteristic that results. An organism with two identical alleles (BB or bb) is homozygous; one with two different alleles (Bb) is heterozygous.
Some disorders are caused by a single gene. Cystic fibrosis is caused by a recessive allele, so a person must inherit two recessive alleles to have the disorder, and carriers (heterozygous) show no symptoms. Polydactyly (extra fingers or toes) is caused by a dominant allele, so one copy is enough to cause it. Punnett squares are used to work out the probability that a child will inherit such a disorder.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20184 marksA characteristic is controlled by a single gene with a dominant allele B and a recessive allele b. Two heterozygous parents (Bb) have children. Draw a Punnett square and state the ratio of offspring with the dominant characteristic to those with the recessive characteristic.Show worked answer →
A Biology Paper 2 genetic-cross calculation. Method: draw a Punnett square with B and b along the top (one parent's gametes) and B and b down the side (the other parent's gametes). The four offspring genotypes are BB, Bb, Bb and bb. Three of the four have at least one dominant allele (BB, Bb, Bb), so they show the dominant characteristic; one (bb) shows the recessive characteristic. The ratio of dominant to recessive is therefore . Markers award correct gametes, the four genotypes, and the ratio. A common slip is to give the genotype ratio () when the phenotype ratio () was asked for.
OCR 20214 marksExplain the difference between sexual and asexual reproduction, including how each affects the genetic variation of the offspring.Show worked answer →
A B5 structured comparison. Reward: sexual reproduction involves two parents and the fusion of gametes (made by meiosis); the offspring inherit a mixture of alleles from both parents, so they are genetically different from each other and from the parents, giving variation. Asexual reproduction involves only one parent and no gametes; the offspring are produced by mitosis and are genetically identical clones of the parent, so there is no genetic variation. Markers credit the number of parents, the role of gametes/meiosis versus mitosis, and the link to variation (varied versus identical offspring).
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