5.6 Chromosomal Inheritance

Genes on chromosomes

The chromosome theory of inheritance says genes are located on chromosomes, which explains Mendel's laws. Sex chromosomes (XX in females, XY in males) carry sex-linked genes. Because males have only one X, X-linked recessive traits (like color blindness and hemophilia) are more common in males.

Linkage and errors

Genes on the same chromosome are linked and tend to be inherited together (unless crossing over separates them). Sometimes chromosomes fail to separate properly — nondisjunction — producing gametes with the wrong chromosome number. This can cause conditions like Down syndrome (trisomy 21).

Key terms

Chromosome theory of inheritance

Genes are located on chromosomes, which accounts for Mendel's laws.

Sex-linked (X-linked)

Genes on the sex chromosomes; X-linked recessive traits are more common in males.

Linked genes

Genes on the same chromosome that tend to be inherited together.

Nondisjunction

Failure of chromosomes to separate in meiosis, giving abnormal chromosome numbers.

Trisomy 21

An extra copy of chromosome 21, which causes Down syndrome.

Try an AP-style question

A karyotype shows 47 chromosomes with three copies of chromosome 21. What event most likely caused this?

1. A point mutation on chromosome 21
2. Crossing over between chromosome 21 homologs
3. Mitosis producing an extra cell
4. Nondisjunction during meiosis in a parent