Inherited Disorders

Define and explain Inherited Disorders in your own words
Use key terms such as gene accurately
Apply what you have learned to new examples and questions
Avoid the common mistakes learners make with this topic

This lesson focuses on Inherited Disorders: trace cystic fibrosis and polydactyly through family trees.

Definition: Inherited Disorders

Trace cystic fibrosis and polydactyly through family trees.

Key ideas

Some disorders are caused by single genes. Cystic fibrosis is recessive: a child must inherit the faulty allele from both parents to have the disorder, so two carrier parents have a 25% chance of an affected child. Polydactyly (extra fingers or toes) is dominant: only one faulty allele is needed, so an affected parent can pass it on even when the other parent is unaffected.

From DNA to characteristics

DNA is a double-helix molecule found in chromosomes in the nucleus. A gene is a section of DNA coding for one protein, and different versions of a gene are called alleles. Humans have two copies of each gene, one inherited from each parent. The genotype is the pair of alleles; the phenotype is what you see, shaped by genotype plus environment.

Key term — gene: A section of DNA that codes for a particular protein and therefore influences a characteristic.

Worked example: Inherited Disorders

Two carriers of cystic fibrosis (Ff × Ff) have a child. What is the probability the child has cystic fibrosis (ff)? Show your reasoning.

The Punnett square gives FF, Ff, Ff, ff. Only ff is affected: 1 in 4, or 25%.

Answer: The Punnett square gives FF, Ff, Ff, ff. Only ff is affected: 1 in 4, or 25%.

Common mistakes

Saying dominant means common Dominant describes how an allele behaves in a heterozygote, not how frequent it is. A dominant allele can be rare in a population.
Confusing genotype with phenotype The genotype is the allele combination (Tt); the phenotype is the observed trait (tall). Two organisms can share a phenotype with different genotypes.

Practice

Give one genetic and one environmental cause of variation in humans.

Think alleles and lifestyle.

Genetic: different alleles inherited from parents, e.g. eye colour. Environmental: e.g. scars, or muscle size affected by exercise and diet.

What is the relationship between DNA, a gene and a chromosome?

Think of them as nested structures.

DNA is the molecule; a gene is a section of DNA coding for a protein; chromosomes are long DNA molecules, each carrying many genes.

Define the terms homozygous and heterozygous.

Think about whether the two alleles match.

Homozygous means carrying two identical alleles (e.g. TT or tt); heterozygous means carrying two different alleles (e.g. Tt).

In humans, brown eyes (B) are dominant to blue eyes (b). What eye colour will a Bb person have?

Which allele is expressed?

Brown eyes, because the dominant B allele is expressed even with only one copy.

Quick check

Inherited Disorders — quick check

Which of these best defines "gene"?

A section of DNA that codes for a particular protein and therefore influences a characteristic.

Why can two brown-eyed parents have a blue-eyed child?

Both parents can be heterozygous (Bb). Each can pass on the recessive b allele, and a child receiving b from both (bb) will have blue eyes — a 25% chance per child.

Key takeaways

Inherited Disorders: trace cystic fibrosis and polydactyly through family trees.
Inherited disorders: Some disorders are caused by single genes.
allele: One version of a gene; for example, a flower-colour gene might have a red allele and a white allele.
Watch out for: saying dominant means common

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