Models of single-gene inheritance: family tree diagrams | Biology

Starter quiz

Starting with the smallest, put these in order of increasing size.

1

⇔

nucleotide

2

⇔

gene

3

⇔

chromosome

4

⇔

nucleus

5

⇔

cell
Which is the correct end for the sentence? All humans have the same ...

alleles.

genes. ✓

genotype.

phenotype.
A single gene with two alleles controls the wetness of earwax. The E allele is dominant and causes wet ear wax. The e allele is recessive. What is the genotype of a person with dry earwax?

Ee

ee ✓

EE
The shape of peas is controlled by a single gene with two alleles: R for round and r for wrinkled. R is dominant. Which of the numbered boxes in the Punnett square contains a mistake?

1

2 ✓

3

4
PKU is a disorder in humans caused by a recessive allele, p. Two carriers of PKU are shown this Punnett square. What can they learn from the Punnett square?

The probability that they will have a child with PKU is 0.

Only if they have four children, will one inherit PKU.

The probability that they will have a child who carries PKU is 0.75.

The probability that any child they have will inherit PKU is 0.25 ✓
A dominant allele R codes for red flower colour, and the allele r codes for white flowers. A RR plant is crossed with a rr plant. What colour flowers will their offspring have?

'Red' ✓

Exit quiz

For a recessive genetic disorder, what does it mean when an individual is referred to as a “carrier”? It is someone who ...

has the disorder but only has mild symptoms.

hasn’t got the disorder and who can’t pass it on.

has the disorder, and can pass it on.

hasn’t got the disorder, but can pass it on. ✓
Match the genotype to its correct description.

EE

⇔

homozygous dominant ✓

Ee

⇔

heterozygous ✓

ee

⇔

homozygous recessive ✓
Here is a family tree. How many generations are shown?

1

2

3 ✓

4

5
Here is a family tree. Which individuals are brother and sister?

1 and 2

3 and 4

5 and 6

8 and 9

10 and 11 ✓
Here is a family tree. The shaded individuals have a genetic disorder caused by a recessive allele. Which two parents must be carriers?

3 and 4 ✓

5 and 6

1 and 2
Marfan syndrome is a genetic disorder caused by dominant allele M. A father is Mm. The mother is mm. Their 1st child has Marfan. What is the probability that a 2nd child will have Marfan.

'0.5' ✓

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Lesson Details

Key learning points

Offspring inherit two alleles, one from each parent, the combination of these alleles is the genotype.
The phenotypes of members of a family can be shown in a diagram such as a family tree (pedigree).
A family tree diagram is a model of inheritance, showing whether a characteristic was passed from parents to offspring.
A family tree can be used to work out an individual’s genotype if their phenotype and relatives’ genotypes are known.
A family tree can be used to determine the history and probability of genetic disorders: cystic fibrosis and polydactyly

Common misconception

The dominant allele is stronger than the recessive allele preventing its expression or that the recessive allele is an absence of the dominant allele, also that all genetic disorders are recessive.

Linking the allele to change in protein structure, showing that proteins are produced in the case of dominant and recessive alleles. Examples of dominant and recessive disorders provided.

Keywords

Family tree - A family tree diagram is a model of inheritance, showing whether a characteristic was passed from parents to offspring.
Genotype - The genotype is the combination of alleles that an individual has for each gene.
Phenotype - The phenotype is the physical characteristics of an organism.
Carrier - A carrier of a recessive genetic disorder does not express the disorder in their phenotype, but has one recessive allele in their genotype that can be passed to offspring.
Inheritance - Inheritance is the passing of genetic information from parents to offspring by reproduction.