(UNIT 5) Chapter 15 - The Chromosomal Basis of Inheritance

Why choose fruit flies?

• Prolific breeders (single mating will produce hundreds of offspring)

• New generation can be bred every two weeks

• Only have four pair of chromosomes (easily distinguishable, three pairs of autosomes and one pair of sex chromosomes)

• Same sex chromosomes to humans

Wild vs Mutant Types

• Wild Type: The most commonly observed phenotype.

• Mutant Type: Less common phenotypes, due to alleles assumed to have originated as changes or mutations in the wild type allele.

How do sex-linked traits work?

A trait is passed on from its X or Y chromosome. If it’s X-linked the male will either have or not have the trait they cannot be carriers as they only need one X chromosome (XY). While, a female who has two X chromosomes needs two recessive X-linked alleles to display the trait and can be carriers.

	Xr	Y
XR	XRXr	XRY
Xr	XRXr	XrY

When are genes linked and not?

• Genes that are close together on a single chromosome are linked and do not assort independently (likely to be inherited together). Example: Sex-linked genes. 

• Genes that are on different chromosomes or far from each other are not linked.

X and Y Chromosomes (In mammals & humans)

• Mammals have two types of sex chromosomes, designated X and Y. The Y chromosome is much smaller than the X chromosome. (XX is female, XY is male). 

  • Short segments at either end of the Y chromosome are the only regions that are homologous with regions on the X. 

  • The egg carries only X chromosomes while a sperm can carry a X or Y chromosome. 

How are sex characteristics formed?

• WNT4: A gene on chromosome 1 encodes a protein that promotes ovary development.The signs of sex emerge when the embryo is about 2 months old.

• Sex is determined by the interactions of a network of gene products and is much more complicated than simple sex chromosomes. (Certain chromosomes other than the sex chromosomes can be involved with producing “sex” characteristics.)

Sex-linked genes + Types

• Sex-linked gene: A gene located on either sex chromosome. 

  • X-linked genes: The genes on the X chromosome. 

  • Y-linked genes: The genes on the Y chromosome. 

    • There are little Y-linked genes so fathers often do not pass disorders to their sons.

    • SRY: A gene on the Y chromosome (sex-determoining region of Y) is required for the development of testes. (In the absence it develops ovaries). 

How are X-linked alleles passed?

• Fathers pass X-linked alleles to all of their daughters but none to their sons while mothers can pass X-linked alleles to both sons and daughters.

X Inactivation in Female Mammals

• One of the X chromosomes in each cell in female mammals become inactivated during early embryonic development so the cells of females and males have the same effective copy of most X-linked genes.

  • Barr Body: The inactive X condenses into this compact object. Lies along the inside of the nuclear envelope. Most genes on it are not expressed, however in the ovaries the Barr body chromosomes are reactivated in the cells that give rise to eggs, resulting in every female gamete having an active X after meiosis. 

  • The selection of which X chromosome will form the Barr body occurs randomly and independently in each cell. Females consist of a mosaic of the two types of cells: ones with active X from the father and ones with active X from the mother. (The phenotype will be like codominance.) 

    • Thus if a female is heterozygous for a sex-linked trait, about half of her cells will express one allele,while the others will express the other.