DNA, genes and inheritanceIB MYP Sciences: Revision notes
Section 1
DNA, chromosomes and genes
DNA is the molecule that carries the genetic information, the instructions for how an organism is built and works. In a cell, long strands of DNA are coiled up into structures called chromosomes in the nucleus. Human body cells have 46 chromosomes, in 23 pairs: one chromosome of each pair came from each parent.
A gene is a short section of DNA that carries the instructions for one characteristic (for example, how to make a particular protein). Each chromosome carries many genes.
Section 2
Alleles
Genes come in different versions called alleles. For example, the gene for hamster fur colour has an allele for black fur and an allele for brown fur. Because chromosomes come in pairs, you have two alleles of every gene, one from each parent. The two alleles can be the same or different.
Section 3
Dominant and recessive alleles
A dominant allele shows its effect even if only one copy is present. A recessive allele only shows its effect when there is no dominant allele, that is, when two copies are present.
We write dominant alleles as capital letters (B) and recessive alleles as lower-case letters (b). A person with one recessive allele and one dominant allele does not show the recessive characteristic, but can pass the recessive allele on. They are a carrier.
Section 4
Genotype and phenotype
The genotype is the combination of alleles an organism has, e.g. Bb. The phenotype is the characteristic that you can see or measure, e.g. black fur.
- Homozygous: two identical alleles (BB or bb)
- Heterozygous: two different alleles (Bb)
So BB and Bb both give a black phenotype, but only bb gives brown.
Genotype is the letters (Bb), phenotype is what you see (black fur). Do not mix them up.
Section 5
Punnett squares for monohybrid crosses
A monohybrid cross follows the inheritance of one gene. A Punnett square shows every possible combination of gametes.
Worked example: cross two heterozygous black hamsters (Bb x Bb).
- Write the gametes from each parent: B or b
- Fill in the square: BB, Bb, Bb, bb
- Read off the ratio: 3 black (BB, Bb, Bb) to 1 brown (bb)
This is a 3 : 1 phenotype ratio, with a probability of 25% (1 in 4) for brown. The genotype ratio is 1 BB : 2 Bb : 1 bb.
Remember that the ratio is a probability. A small number of real offspring may not match it exactly because fertilisation is random.
Always write the gametes first, then fill the square, then state the ratio in words (for example 3 black : 1 brown).
That's the notes covered.
Carry on to the next subtopic.
Exam questions on DNA, genes and inheritance
- A science museum in Seoul has an exhibit about the nucleus of a human body cell. It explains that the nucleus contains 46 chromosomes arranged in 23 pairs, and that each chromosome is made of one very long molecule that is tightly coiled.Explain what is meant by the term allele and why a body cell contains two alleles of each gene.2 marks
- A pet breeder in Brazil crosses two hamsters that both have black fur. Black fur is controlled by a dominant allele (B) and brown fur by a recessive allele (b). Some of the offspring in the litter have brown fur.Use a Punnett square to show the cross between the two black parents, and state the ratio of black to brown offspring that is expected.2 marks
- A genetic counsellor in Toronto meets a couple who do not have cystic fibrosis, but whose first child does. Cystic fibrosis is caused by a recessive allele (f). The dominant allele (F) allows the body to work normally.Explain how two parents who do not have cystic fibrosis can have a child who has the condition.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).