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'Tall' is an example of a phenotype.

# Unit 2 - Construct Genetic Diagrams (H)

In GCSE Biology students will spend some time studying genetics. Learning how to construct and interpret genetic diagrams, such as the Punnet square, will help them to predict the outcomes of genetic crosses.

Scientists in the 19th Century were fascinated by the living world, but there were many things that they still didn't understand. It took Charles Darwin many years to work out and publish his book about evolution by natural selection (titled The Origin of Species) but had he known about genetics, it would have been much easier for him. At the time when Darwin published his book, an Augustinian friar called Gregor Mendel was carrying out a series of experiments with pea plants. He cross-pollinated plants with different characteristics and observed which of these were passed on to subsequent generations. He eventually noticed patterns appearing that could only be explained if some of the 'factors' were dominant and others were recessive. His work was re-discovered in the early part of the 20th Century and from that the science of genetics was born.

1.
Which word describes TT?
Heterozygous
Heterogametic
Homozygous
Homogametic
Since the alleles are the same, it could only be one of the last two alternatives
2.
Which of the following is an example of a heterozygous genotype?
TT
CC
Tt
Tc
The same letter of the alphabet is used for each feature (gene) - T in this case - so Tc is incorrect
3.
What is the ratio of Tall:short if Tt is crossed with tt? Use a genetic diagram to predict the outcome.
2:1
3:1
4:1
1:1
Using a Punnett square:
T          t
t        Tt         Tt
t        tt           tt      2 Tall Tt and 2 short tt cancels to 1:1, Equal numbers of Tall to short in the offspring
4.
Using a Punnett square, predict the ratio of tall to short when Tt is crossed with Tt. The allele for tall is T and the allele for short is t.
2:1
4:1
3:1
1:1
The gametes are T or t. The Punnett Square looks like this:
T          t
T       TT      Tt
t        Tt         tt      3 Tall (either TT or Tt) and 1 short (tt), hence a 3:1 ratio Tall:short
5.
Genotype describes which of the following?
All alleles present in a population
One allele
Alleles present in a single organism, for a particular feature
Four genes
It can also refer to an individual's collection of genes
6.
What does homozygous mean?
That the alleles are the same
That the alleles are different
That the alleles are varied
That the alleles are mutated
The prefix homo always means the same e.g. homologous, homogenous
7.
Phenotype is the expression of what?
The face
The genotype
The eyes
The zygote
Phenotype is usually described as 'what you look like'! Blue eyes or brown hair, tall or short. The genes (alleles) in the genotype dictate what we look like
8.
Which word describes Tt?
Heterozygous
Homozygous
Heterogametic
Homogametic
Since the alleles are different, you can immediately rule out the two options beginning with 'homo'
9.
'Tall' is an example of which of the following?
A genotype
An allele
A gene
A phenotype
Phenotypes are descriptions of the effects of alleles
10.
Alleles are different forms of the same...
allele
gene
code
egg
In some inherited diseases, like cystic fibrosis or Huntingtons disease, one of the alleles is faulty