Meiosis and Sex-linked traitsCh. 10.4-10.5

Objectives

1. Explain chromosome theory of inheritance

2. Understand genetic linkage

3. Explain sex-linked genes and why more common in males

Chromosome Theory of Inheritance

• Mendel’s findings were not widely accepted at first• During the 1900’s, parallels discovered between

genes and chromosomes• This is referred to as the chromosome theory of

inheritance:• Chromosome Theory of Inheritance:

– Genes located on chromosomes

– accounts for inheritance patterns

Chromosome Theory of Inheritance

• Remember, diploid individuals have 2 sets of homologous chromosomes – One from mom, one

from dad

• Gene locus:– Alleles of a gene reside

at same location

Genetic Linkage and Crossing Over

• Genetic Linkage:– Tendency for alleles on

one chromosome to be inherited together

• Closer the genes, the greater the genetic linkage

• Farther apart, greater chance of separation

Sex Linked Genes

• What two sex chromosomes make up a male? Female?– Male = XY, Female = XX

• Which chromosome is physically larger?– The X. It can hold a lot more genetic info

• Sex linked gene:– Any gene on a sex chromosome

Sex-linked Disorders• Many sex-linked disorders

are recessive alleles – Let’s say that white eyes in

fruit fly is an example

• Disorders always carried on X chromosome (Y doesn’t have any room, too small)

• What does all this mean?

Sex-linked Disorders• If disease is recessive and always carried on X chromosome, then

females (XX), will most likely have dominant form of allele on other chromosome --> don’t get disorder.

• If males (XY) have allele for disorder on their X chromosome, they will get disorder because Y chromosome doesn’t carry alleles.

Rare!