Chapter 13 chromosomal inheritance

What is chromosomal theory of inheritance?

Genes are located on chromosomes, which are passed from parents to offspring

• Walter Sutton and Theodor Boveri proposed this theory in the early 1900s

  • Explains how Mendel’s law relate to chromosome behavior during meiosis

What is gene linkage, and how does it violate Mendel’s laws

linked genes are located close together on the same chromosome and do not assort independently.

• Violates Mendel’s law of Independent Assortment, linked genes are inherited TOGETHER more often unless crossing over occurs

What is recombination, and what are recombinants?

Recombination occurs when homologous chromosomes exchange genetic material during crossing over in Prophase I.

Recombinants are offspring with new allele combinations different from their parents

How do we calculate recombination frequency?

Recombination frequency (%) = (number of recombinants / total offspring) * 100

Higher frequency = genes are farther apart on the chromosome

How do we map genes using recombination frequency?

• genes father apart have HIGHER recombination frequency

• 1% recombination frequency = 1 map unit (centimorgan, cM)

• Arrange genes based on recombination percentages to determine their relative positions

What is X-inactivation?

• One X chromosome in female is randomly inactivated to balance gene dosage

• Forms a Barr Body, which remains inactive in all future cell divisions

• Example: Calico cats have patchy due to X-inactivation

What is nondisjuction, and when is it worse?

Nondisjunction = failure of chromosomes to separate properly during meiosis

Meiosis I nondisjunction is worse - all gametes are affected.

Meiosis II nondisjunction affects HALF of the gametes (extra or missing chromosomes)

How do you analyze a karyotype?

Count chromosome (Normal = 46)

Look for missing or extra chromosomes (trisomy = extra chromosome)

sex chromosomes (XX = female, XY = male)

Identify structural abnormalities (eg. deletions, duplication)