Ch 15: The Chromosomal Basis of Inheritance

chromosome theory of inheritance

• Mendelian genes have specific loci along chromosomes

• chromosomes undergo segregation and independent assortment

why fruit flies are good candidates for genetic research

• many offspring

• new generation every two weeks

• only four chromosome pairs

disorders caused by recessive alleles on x chromosomes

• color blindness

• Duchene muscular dystrophy

• hemophilia

types of changes in chromosome structure

• deletion

• duplication

• inversion

• translocation

wild type

phenotype for the character most often observed in nature

sex-linked gene

• a gene located on either sex chromosome

• few y-liked disorders due to small number of genes on the y chromosome

• y genes are inherited nearly unchanged from father to son

x-linked genes

• the around 1100 genes on the x chromosome

• males and females inherit different numbers of x-chromosomes, leading to a different pattern of inheritance than autosomes

Duchenne muscular dystrophy

a missing muscle protein causes progressive weakening and loss of coordination

hemophilia

the absence of one or more blood clotting proteins

barr body

a condensed, inactive x chromosome in females

linked genes

• genes located near each other on the same chromosome

• tend to be inherited together

• do not follow the law of independent assortment

genetic recombination

production of offspring with combinations of traits that differ from those found in either parent

parental types

offspring with a phenotype that matches one of the parental phenotypes

recombination types/recombinants

offspring with a nonparental phenotype (new combination of traits)

crossing over

a set of proteins organizes an orchestrated exchange of corresponding segments on homologous chromosomes

genetic map

an ordered list of the genetic loci along a particular chromosome

likage map

a genetic map based on recombination frequency

map units

• measures the distance between genes

• one unit represents 1% recombination frequency

nondisjunction

• members of a pair of homologous chromosomes do not separate in meiosis I or sister chromosomes do not separate in meiosis II

• one gamete has two of one chromosome, and another has none

aneuploidy

• result of nondisjunction

• a zygote with an abnormal chromosome number

monosomic

a zygote which is missing a chromosome

trisomic

a zygote with three copies of a chromosome

polyploidy

organisms with more than two complete chromosome sets in all somatic cells

deletion

• a chromosome segment is lost

• the entire chromosome is lost if the centromere is deleted

duplication

an extra segment is added to a chromosome

inversion

a chromosome fragment is attached to the original chromosome in reverse orientation

translocation

a chromosome fragment attaches to a different chromosome

Down Syndrome

• distinct facial features, short stature, heart defects, developmental delays, sterility

• decreased instance of high blood pressure, arteriosclerosis, stroke, tumors

• increased risk of Alzheimers and leukemia

• trisomy 21

• risk increases with age of mother

genomic imprinting

• variation in phenotype depending on whether the allele is inherited from the mother or father

• occurs during gamete formation

• results in silencing of a particular allele

• the imprint is erased in each subsequent generaction and reimprinted