(3.6) Polydactyly

Know and understand that some disorders like polydactyly are inherited.

You need to be able to evaluate the outcomes of pedigree analysis when screening for genetic disorders e.g. polydactyly.

The genetically inherited disorder called polydactyly is described and genetically explained below, but there are no serious medical consequences.

 

Genomics and polydactyly inherited disease

It is now known most of our characteristics are controlled by more than one gene and this is also true for the genetically inherited disease polydactyly.

Single-gene disorders like polydactyly comply with what is called 'Mendelian inheritance' and genetic diagrams and Punnett squares are quite easy to work out - as I hope you will see below.

(Most diseases with a 'genetic connection' like diabetes, obesity and cardiovascular diseases (heart disease) involve the interaction of many genes including non-coding sections of the genome's DNA and environmental factors e.g. lifestyle choice - diet and exercise.)

 

Polydactyly/polydactyl – having extra fingers or toes – is caused by a dominant allele of a gene and can therefore be passed on by only one parent who has the disorder.

Polydactyly is a physical condition in which a person has more than five fingers per hand or more than five toes per foot. Having an abnormal number of digits (6 or more) can occur on its own, without any other symptoms or disease.

See photographs on https://en.wikipedia.org/wiki/Polydactyly

The frequency of polydactyly varies from 5 to 19 per 10,000 population.

 

Polydactyly may be passed down (inherited) in families and this trait involves only one gene that can cause several variations.

Polydactyly is caused by the dominant allele P (so doesn't need genotype PP, can be Pp too).

The parent that has the defective allele (P) will be affected by the condition of polydactyly.

Note that someone affected by polydactyly only has to inherit one dominant gene (P) from either parent.

The genetic diagrams below shows that there is a 50% chance of a child suffering from polydactyly if just one of the parents is a carrier Pp.

 

Genetic diagram and Punnett squares for polydactyly

Genetic table 1. for polydactyly
Genotypes of parents: Pp x pp affected and normal
Gametes: P, p, p and p (alleles)
Genotypes of children	P	p
p	Pp	pp
p	Pp	pp

The analysis of the parental cross between a non-carrier (homozygous recessive alleles pp) and someone affected by polydactyly (heterozygous alleles Pp).

PP = homozygous alleles (dominant)

Pp = heterozygous alleles

pp = homozygous alleles (recessive)

Genetic diagram for an affected carrier and a non-carrier of the polydactyly gene (allele)

 

Five other possible parental crosses involving the dominant allele P for polydactyly.

I've shown below the analyses for polydactyly using a basic Punnett square of the two pairs of gametes of the parents and the four possible genotypes of offspring (children).

2. genotypes of parents: PP x Pp			Comments on cross 2. Crossing two parents affected by polydactyly. All the offspring will be carriers and all affected by the dominant allele P.
genotypes of children	P	P
P	PP	PP
p	Pp	pp

 

3. genotypes of parents: Pp x Pp			Comments on cross 3. Crossing two parents affected by polydactyly due to dominant allele P. All of the offspring will be carriers and all affected by polydactyly.
genotypes of children	P	p
P	PP	PP
p	Pp	Pp

 

4. genotypes of parents: PP x pp			Comments on cross 4. A non-carrier crossed with someone suffering from polydactyly. 3 in 4 chance (75%) of the offspring will be carriers AND affected by allele P. 1 in 4 chance (25%) will neither be a carrier of, or affected by, polydactyly.
genotypes of children	P	P
p	Pp	Pp
p	Pp	pp

 

Extra note on polydactyly: (no need for Punnett squares for 5. and 6. genetic permutations)

(i) For 5. PP x PP, all offspring will be PP, so all offspring are all carriers and affected

(ii) For 6. pp x pp, all offspring will be pp, non are carriers, so non of offspring can be affected - normal.

(iii) As far as I know, there are no serious harmful effects of polydactyly, but the situation can be dealt with by surgery, but this always carries its own risks.

For lots more examples of genetic analysis of offspring see the index with lots of diagrams and explanations.

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Key points Source of information is based on textbooks & syllabus-specifications for students taking the AQA GCSE, Edexcel GCSE and OCR GCSE level biology examinations (~US grades 9-10).

Key points in understanding the genetic disorder of polydactyly

Polydactyly: Inherited Genetic Disorder and Genetic Outcomes

Polydactyly is a dominant genetic disorder where an individual is born with extra fingers or toes.

It occurs due to mutations in specific genes responsible for limb development during fetal growth.

Understanding polydactyly provides insights into genetic inheritance patterns and developmental biology.

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1. Causes of Polydactyly

Polydactyly is caused by mutations in genes involved in limb formation, such as the GLI3 gene.

The mutation leads to extra digits forming alongside normal fingers or toes.

Types of Polydactyly:

• Preaxial Polydactyly: Extra digits appear on the thumb or big toe side.

• Postaxial Polydactyly: Extra digits form on the pinky finger or little toe side (most common).

• Central Polydactyly: Additional fingers appear between normal digits (least common).

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2. Genetic Inheritance and Possible Outcomes

Polydactyly follows a dominant inheritance pattern, meaning only one copy of the mutated allele is required for the condition to appear.

Punnett Square Example – One Parent Affected (Heterozygous, Pp):

Genetic Outcomes:

• 75% (PP or Pp) – Will have polydactyly

• 25% (pp) – Will have normal fingers/toes

Key Terms:

• Homozygous dominant (PP): Individual has polydactyly and may pass it on to all offspring.

• Heterozygous (Pp): Individual has polydactyly and has a 50% chance of passing it on.

• Homozygous recessive (pp): Individual does not have polydactyly.

If both parents have polydactyly (Pp × Pp), 25% of children will inherit PP (severe cases), 50% Pp (mild cases), and 25% will be unaffected (pp).

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3. Importance in Understanding Human Genetics

Polydactyly provides insights into:

• Dominant Genetic Disorders: Unlike recessive conditions like cystic fibrosis, polydactyly can appear even when only one mutated allele is present.

• Embryonic Development: Studying limb formation genes helps understand growth abnormalities.

• Genetic Testing & Medical Screening: Identifying mutations to assess inheritance risks.

• Evolution & Variation: Examining how genetic mutations influence diversity in human traits.

Polydactyly is a valuable example of dominant inheritance, demonstrating how genetic mutations can shape human development and variation.

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Summary of learning objectives and key words or phrases

Know how to explain the genetics cause of polydactyly.

Explaining the genetics of the inherited genetic disorder polydactyly from a faulty gene, allele, working out possible genetic outcomes using Punnett squares and drawing cross bred diagrams.

For polydactyly you need to be able to construct genetic diagrams of monohybrid crosses and predict the outcomes of monohybrid crosses and be able to use the terms homozygous (same alleles eg PP, pp) genes, heterozygous (different alleles eg Pp), explaining the phenotype (gene expression - the outcome!) and the original genotypes (gene alleles).

Know how do you draw monohybrid genetic diagrams for polydactyly.

Know how to construct Punnett squares for monohybrid crosses for polydactyly.

Be able to predict and/or explain the outcome of crosses between individuals for each possible combination of dominant and recessive alleles of the same gene for the genetic disorder polydactyly and explain the observed phenotypes from the genotypes carriers of the genetic disorder polydactyly.

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