What are Chromosomal Inheritance Patterns?
Chromosomal inheritance patterns describe how traits pass from parents to offspring based on where genes sit on chromosomes. Some traits follow simple dominant–recessive rules, others are sex-linked, and some involve multiple alleles—all shaping the visible variation in human and animal populations.
Inheritance patterns depend on chromosome location and allele interactions. Autosomal traits (on non-sex chromosomes) can be dominant or recessive; sex-linked traits (on X chromosome) show different frequencies in males vs females; multiple-allele systems (e.g. ABO blood) involve more than two allelic forms.
- •Gene on autosomes (chromosomes 1–22)
- •Dominant allele shown in both sexes equally
- •Example: cystic fibrosis (recessive), Huntington (dominant)
- •Gene on X chromosome (X-linked)
- •Males (XY) show trait if one recessive allele
- •Females (XX) need two recessive alleles
- •Example: color blindness, haemophilia
Step-by-step worked examples
A woman is heterozygous for cystic fibrosis (Cc), a recessive autosomal trait. Her partner is homozygous normal (CC). What is the probability their child has CF?
Cross: Cc × CC Offspring: 50% CC (normal), 50% Cc (normal carrier) 0% express cystic fibrosis
A woman is a carrier of hemophilia (XᴴXʰ). Her partner has normal blood (XᴴY). What fraction of male children will have hemophilia?
Cross: XᴴXʰ (carrier) × XᴴY (normal) Sons: 50% XᴴY (normal), 50% XʰY (hemophilia) 50% of males affected
Human ABO blood type involves three alleles (Iᴬ, Iᴮ, i). A person with blood type AB has which genotype?
AB phenotype = two dominant alleles Genotype = IᴬIᴮ This person can donate to AB, receive from O
Flashcards
Quick quiz
Q1.A trait appears only in males in a pedigree, never in females. This suggests:
Q2.In ABO blood typing, a person with type B blood could have genotype:
Q3.If both parents are heterozygous for a dominant autosomal trait (Aa), what % of children show the trait?
Q4.Haemophilia is X-linked recessive. A carrier female (XᴴXʰ) has a normal male (XᴴY). Their daughter's probability of being affected:
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Common mistakes
Sex-linked traits affect males and females equally. — Correct: X-linked recessive traits are much more common in males (one X, no masking).
A carrier never shows the trait. — Correct: Carriers of dominant traits show the trait; carriers of recessive traits don't.
Multiple alleles means multiple genes. — Correct: Multiple alleles are different versions of one gene.
Colour blindness is equally common in males and females. — Correct: Red–green colour blindness is X-linked, so ~8× more common in males.
FAQ
What is the difference between autosomal and sex-linked inheritance?
Autosomal genes are on non-sex chromosomes; sex-linked are on X or Y. Sex-linked traits show different patterns in males vs females.
Why do recessive traits skip generations?
Heterozygous carriers have the dominant allele (normal phenotype) but carry the recessive. When two carriers mate, recessive homozygotes appear.
What is a carrier?
A person heterozygous for a recessive allele—phenotypically normal but can pass the trait to offspring.
How do blood type inheritance and Punnett squares relate?
Punnett squares show all possible offspring genotypes from two parental crosses, predicting trait frequencies.