Patterns of Inheritance Beyond Simple Dominance
Beyond Mendel
Nonclassic Inheritance
Not all traits follow simple dominant-recessive patterns. Some show incomplete dominance, codominance, multiple alleles, sex linkage, maternal effects, or lethal allele behavior.
Incomplete Dominance vs Codominance
Incomplete Dominance
- Heterozygote shows intermediate phenotype
- Example: red and white flowers producing pink
Codominance
- Both alleles are fully expressed
- Example: AB blood type
Sex-Linked Traits
Sex-linked genes are on sex chromosomes, often the X chromosome. Because males have one X chromosome, recessive X-linked alleles are more likely to be expressed in males.
Lethal Alleles
Some alleles are lethal in homozygous form, which can distort expected ratios such as the classic surviving phenotype ratio.
Which blood type illustrates codominance?
In AB individuals, both A and B antigens are expressed.
Correct answer: AB
Why are X-linked recessive traits more common in males?
Hemizygosity makes recessive X-linked alleles more visible in males.
Correct answer: Because males have only one X chromosome, so a single recessive allele is expressed.
A heterozygote with an intermediate phenotype is an example of
Incomplete dominance produces an intermediate heterozygous phenotype.
Correct answer: Incomplete dominance
Multiple Alleles
A population can contain more than two alleles for a gene, even though each individual carries at most two. Human ABO blood groups are a classic example.
What is the difference between genotype frequency and phenotype frequency?
These frequencies may differ when dominance affects appearance.
Correct answer: Genotype frequency counts allele combinations, while phenotype frequency counts observable trait classes.