- Define and explain DNA, Genes and Chromosomes in your own words
- Use key terms such as genotype accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
Why do children resemble their parents but are never identical to them? In this chapter you will connect DNA, genes and chromosomes, use Punnett squares to predict inheritance, trace genetic disorders through families, and explain where the variation that fuels evolution comes from.
This lesson focuses on DNA, Genes and Chromosomes: connect DNA, genes, chromosomes and alleles into one clear picture.
Connect DNA, genes, chromosomes and alleles into one clear picture.
Key ideas
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.
Inherited disorders
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.
Key term — genotype: The combination of alleles an organism carries (e.g. Tt), while the phenotype is the observable characteristic (e.g. tall).
What is the relationship between DNA, a gene and a chromosome?
DNA is the molecule; a gene is a section of DNA coding for a protein; chromosomes are long DNA molecules, each carrying many genes.
Answer: DNA is the molecule; a gene is a section of DNA coding for a protein; chromosomes are long DNA molecules, each carrying many genes.
- Putting two alleles in each gamete box Gametes are haploid and carry only one allele of each gene. Each row and column heading of a Punnett square holds a single allele.
- 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.
Practice
Homozygous means carrying two identical alleles (e.g. TT or tt); heterozygous means carrying two different alleles (e.g. Tt).
Genetic: different alleles inherited from parents, e.g. eye colour. Environmental: e.g. scars, or muscle size affected by exercise and diet.
Brown eyes, because the dominant B allele is expressed even with only one copy.
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.
Quick check
Which of these best defines "genotype"?
Two carriers of cystic fibrosis (Ff × Ff) have a child. What is the probability the child has cystic fibrosis (ff)? Show your reasoning.
- DNA, Genes and Chromosomes: connect DNA, genes, chromosomes and alleles into one clear picture.
- From DNA to characteristics: DNA is a double-helix molecule found in chromosomes in the nucleus.
- allele: One version of a gene; for example, a flower-colour gene might have a red allele and a white allele.
- Watch out for: putting two alleles in each gamete box