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IB DemystifiedMYP Sciences
DNA, genes and inheritance
Two parents without a genetic condition can have a child who has it. Explaining how comes down to a four-letter code, pairs of chromosomes, and chance.
Recommended for MYP 4 · eAssessment priority · About 4 lessons · Criteria A, B, C and D
Figure 1. Zooming in from a nucleus to a single base pair of DNA.
describe the structure of DNA and how it relates to genes and chromosomes
use the terms allele, dominant, recessive, genotype, phenotype, homozygous and heterozygous
use Punnett squares to predict ratios and probabilities of offspring
interpret family trees and deduce genotypes
explain sex determination and why some conditions are more common in males
distinguish genetic and environmental variation, and explain the effects of mutations
Before you start
You will use these skills. If any feel shaky, review them first.
the nucleus and cell structures (see Cells and microscopy)
fractions, percentages and simple probability
reading bar charts and frequency tables
Key vocabulary
Gene
A section of DNA that codes for a protein, and so helps determine a characteristic.
Allele
A different version of the same gene, such as T (tall) and t (short).
Dominant / recessive
A dominant allele shows its effect with one copy; a recessive allele only with two copies.
Genotype / phenotype
Genotype is the alleles an organism has (Tt); phenotype is the characteristic that results (tall).
Homozygous / heterozygous
Two identical alleles (TT or tt) / two different alleles (Tt).
Carrier
A person with one recessive allele for a condition, who does not have the condition.
Understanding inheritance
What is it?
DNA is a double helix of two strands joined by base pairs: A with T, C with G. The order of bases along a gene is a code for a protein. DNA is packed into chromosomes; human body cells have 23 pairs, one chromosome of each pair from each parent.
Why does it happen?
Because chromosomes come in pairs, you have two copies of most genes, which may be different alleles. Sperm and eggs carry only one chromosome from each pair, chosen at random, so each parent passes on one allele of each gene. Which alleles combine at fertilisation is a matter of chance.
How do we know?
In the 1860s Gregor Mendel counted thousands of pea plants and found regular ratios, such as 3 : 1, long before anyone knew about DNA. In 1953, X-ray images of DNA made by Rosalind Franklin and Maurice Wilkins helped James Watson and Francis Crick work out the double helix, explaining how the code is copied and passed on.
Why does it matter?
Understanding inheritance lets doctors test for genetic conditions and advise families, helps breeders produce better crops and animals, and underpins new technologies such as gene editing, which raise important ethical questions.
What does it connect to?
Inheritance links to cell division, variation and natural selection, and biotechnology. In mathematics it connects to probability, ratios and statistics.
Punnett squares and family trees
A Punnett square shows every way the parents' alleles can combine. For two heterozygous tall pea plants (Tt × Tt), the offspring genotypes are TT, Tt, Tt and tt: a 3 : 1 ratio of tall to short. These are probabilities for each offspring, not a guaranteed pattern.
Figure 2. A family tree for cystic fibrosis, a recessive condition.
Spotting a recessive condition: two unaffected parents with an affected child (1 and 2 with 4).
Affected people with a recessive condition must be homozygous recessive (ff).
Unaffected parents of an affected child must both be carriers (Ff).
Unaffected children of two carriers are carriers with a probability of 2/3, because the ff outcome has already been ruled out.
Genetics in the real world
Many countries test newborn babies for some genetic conditions so that treatment can start early. Plant breeders use genetic markers to choose parents for new crop varieties. Genetic counsellors help families understand test results and the choices open to them.
Worked examples
Example 1: a Punnett square
In guinea pigs, rough fur (R) is dominant to smooth fur (r). What proportion of offspring from Rr × rr will have smooth fur?
Gametes: R or r from the first parent; r only from the second.
Offspring: Rr, Rr, rr, rr.
Two of four are rr, so half (50%) will have smooth fur.
Example 2: deducing a genotype
Two brown-eyed parents (brown dominant, B) have a blue-eyed child (bb). What are the parents' genotypes?
The child is bb, so each parent passed on a b allele.
Both parents have brown eyes, so each also has a B allele.
Both parents must be Bb. (Real eye colour depends on several genes; this is a simplified model.)
In the eAssessment
Genetics questions often combine a family tree or a table of offspring with a chain of linked questions. Expect:
Deduce and determine: work out genotypes from a family tree, showing the evidence (who is affected, and whose child).
Calculate: probabilities and ratios from Punnett squares, as fractions, decimals or percentages, and ratios from real data such as 787 : 277.
Evaluate: compare observed results with predicted ratios, and explain differences using chance and sample size.
Discuss: implications of genetic testing and gene editing, weighing ethical, social and economic points.
Common ways to lose marks: confusing gene and allele; counting alleles instead of genotypes in a Punnett square; assuming earlier children change the odds for later ones; and giving 1/2 instead of 2/3 for an unaffected child being a carrier.
Check your understanding
Quick questions on the ideas above. Try each one before using a hint.
Practice questions
Show
Investigation: is there a link between height and hand span?
Partially guided investigation · about 50 minutes · whole class
Research question
Is there a relationship between the height and the hand span of students aged 15 in our year group?
Scientific background
Height and hand span both show continuous variation, influenced by many genes and by environmental factors such as diet. Growth affects the whole skeleton, so the two may be linked.
Hypothesis
Write your own hypothesis, with a justification.
Variables
Identify which variable you will plot on each axis, and at least three variables you must control (for example, how hand span is measured). Explain how and why.
Apparatus
Height measure or tape measure against a wall, ruler, set square, recording table.
Method
Agree a class method: shoes off, heels against the wall, and hand span from thumb tip to little-finger tip with the right hand fully stretched on a flat surface.
Measure each student's height to the nearest centimetre and hand span to the nearest millimetre.
Take each measurement twice and use the mean.
Collect data from at least 30 students.
Plot a scatter graph and describe any correlation.
Safety and respect. Take part only if you are comfortable; measurements are personal information, so record them without names. No one should feel judged by their results.
Height results from one class are used in the Criterion C questions below. Then evaluate: does a correlation between height and hand span prove that one causes the other?
Criterion-linked questions
Criterion B: inquiring and designing
Criterion C: processing and evaluating
Criterion D: reflecting on the impacts of science
Challenge questions
Harder problems in unfamiliar contexts. Plan before you calculate.
Topic check
Five questions picked at random from the whole topic. Take a new set whenever you like.
Review your mistakes
Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.
Your progress
Tracked separately for each skill, on this device only.
