Punnet Squares and Pedigrees

What is a dominant allele?

An allele that is always expressed, even if only one copy is present

What is a recessive allele?

An allele that is only expressed in the homozygous state (when both copies are recessive)

What does homozygous dominant mean?

Having two dominant alleles (e.g. BB, DD, AA)

What does homozygous recessive mean?

Having two recessive alleles (e.g. bb,dd,aa)

What is a homozygote

An individual who is homozygous (has two identical alleles for a gene)

What is a heterozygote?

An individual who is heterozygous (has one dominant and one recessive allele for a gene)

What is mendelian inheritance?

A pattern of inheritance based on Mendel’s laws, where traits are controlled by a single gene locus and follow set ratios

What are the three laws of Mendelian inheritance?

1. Law of dominance and segregation: Dominant alleles are expressed; allele pairs separate during gamete formation

2. Law of independent assortmnet: Alleles for different traits sort independently

3. Autosomes & sex chromosomes: Traits can be inherited via autosomes or sex chromosomes

What is an autosomal recessive inheritance pattern?

• Affected individuals inherit one recessive allele from each parent

• Recessive traits are only expressed in homozygous individuals

• Alleles segregate randomly during gamete formation

What is a punnet square?

A diagram used to predict genotype and phenotype ratios in offspring form a genetic cross

What type of crosses can punnet squares be used for?

• Monohybrid crosses (one gene)

• Dihybrid crosses (two genes)

What is incomplete dominance?

A form of inheritance where the phenotype if a blend of two alleles (e.g. Red + white snapdragons = pink snapdragons)

How are alleles written in incomplete dominance?

As superscripts next to the gene symbol (e.g. Cᴿ for red, Cʷ for white).

What is codominance?

A form of inheritance where both alleles are fully expressed in the phenotype (e.g. Red + white cattle= roan cattle)

How are alleles written in codominance?

Each allele gets a capital letter (e.g. Hᴿ for red, Hʷ for white).

What is a pedigree chart?

A diagram used to track traits through multiple generations in a family

How do symbols in a pedigree chart represent individuals?

• Circle (○) = Female

• Square (□) = Male

• Coloured shape = Affected individual

What is an autosomal dominant trait?

A trait that is expressed when an individual carries atleast one dominant allele (AA or Aa)

What are the key features of Autosomal dominant inheritance?

✅ Both males & females affected equally
✅ At least one affected parent
✅ Does not skip generations

What is an autosomal recessive trait?

A trait that is only expressed when an individual has two recessive alleles (aa)

What are the key features of autosomal recessive inheritance?

✅ Both males & females affected equally
✅ Can skip generations
✅ Two unaffected parents can have an affected child (Aa × Aa → aa)