Lecture for Thursday September 25, 2003

What’s due?

CH3 problem set

Assignments:

Study for Exam!

Today’s lecture:

Review material from 9/23

Genes on the X Chromsome

Sex-linked, -limited, -influenced Inheritance

Review:*Pedigree- a family tree that shows the phenotype of a particular trait for each family member

*Modification of Mendelian Ratios:

Incomplete dominance- expression of a heterozygous phenotype which is distinct from, and often intermediate to, that of either parent

P: Red Flower CRCR

White Flower CWCW

X

F1: Pink Flower CRCW

F2:¼ Red CRCR ½ Pink CRCW ¼ White CWCW

Review con’t:*Codominance- The effects of two alleles are equally and simultaneously expressed in the heterozygote

*Multiple Alleles- three or more alleles of the same gene

*ABO Blood group in humans

Genotype IAIA IAIO IBIB IBIO IAIB

IOIO

Antigen A A B B A,B

Neither

Phenotype A A

B B AB

O

simple dominancesimple dominancecodominance

A=B>O

*Lethal Allele- an allele whose expression results in the death of the individual expressing it

*Can be either dominant or recessive

Review con’t:Gene Interactions:

*Epistasis: situation in which one gene masks the expression of another*Novel phenotypes: interaction between genes yields new phenotype

Example: shape of summer squash

In the F2 genotypes: A_B_ disc phenotype A_bb or aaB_ sphere phenotype aabb long phenotype

Genes on the X Chromosome:

Sex chromosomes- chromosomes involved in sex determinationAutosomes- non sex determining chromosomes

Example: In Drosophila and all mammals

sex chromosomes designated as X and Y

XX=female

XY=male

*1909 Thomas Hunt Morgan determined that there were differences in chromosomes of male and female Drosophila

II III IV

XX

XY

or

X-linkage- transmission and expression of genes located on the X chromosome

X-linkage in Drosophila: white mutation (eyes)

Genes on the X Chromosome con’t:

½ red

½ white

Genes on the X Chromosome con’t:

*Morgan proposed that the white eyed mutation was X-linked

*Reciprocal cross did not yield identical results

*Males were XY and females were XX

*Since females could be white eyed, the trait was not determined by a locus on Y

½ red

Symbols:

red eye = w+

white eye = w

w+w+ X wY

w+w

w+Y

Gametes from mother

:½ w+ ½ w

Gametes from father:

½ w+ ½ Y

¼ w+w+ ¼ w+Y ¼ ww+ ¼ wY

½ red female ¼ red male ¼ white male

Sex-limited Inheritance:

Sex-limited trait: A trait that is expressed in only one sex even though the trait may not be X-linked

*holandric genes: genes on the Y chromsome

Example: hypertrichosis (ear hair)

P: XX x XYh note: h is only on Y

F1: ½ XX + ½ XYh

*autosomal genes

Example: milk production in mammals; L=lots, l=little

Genotype Phenotypefemale

male LL lotsnone Lllots

none ll littlenone

Sex-influenced trait: Phenotypic expression that is conditioned by the sex of the individual

Sex-influenced Inheritance:

Examples:

*cleft palate in humans*horns in sheep *pattern baldness in

humansGenotype Phenotype

female male BB bald bald Bb not bald bald bbnot bald not bald

B=bald b=not bald

Summary:

Sex-linked on X or Y sex-chromosome

Sex-limited all or none expression by sex

Sex-influenced genotype + sex determines phenotype

Phenotypic Expression:

Penetrance: (quantitative term) the frequency with which individuals with a given genotype, exhibit some degree of a phenotype associated with that trait

Expressivity: (qualitative term) the degree or range in which a phenotype for a given trait is expressed

*Gene expression often governed by genotype and environment

Example:

if 9/10 of individuals carrying an allele express the trait, the trait is said to be 90% penetrant

*Onset of genetic expression (when is a gene expressed?)

Tay-Sachs autosomal rec. lipid metab death by 3 yearsLesch-Nyhan X-linked rec. nucleic acid metab onset >6mo.

DMD X-linked rec. muscular degeneration diagnosed at 3-5yrsHuntingtons autosomal dom. brain deterioration onset >30yrs