ch 13: modern understandings of inheritance Flashcards
x-linked / sex-linked genes
the genes resides on the X chromosome
males only have 1 X chromosome, so if a disease is coded on that chromosome…
100% of male offspring will have the disease
females have 2 X chromosomes, so if a disease is coded on that chromosome…
the offspring could be dominant for the disease, or be a carrier
barr body
early in development, in each female cell, one X chromosome is inactivated and is highly condensed, meaning that the genes on this homologue are not expressed
because of the barr body, female heterozygous for genes on the X chromosome are…
genetic mosaics
genetic mosaics
1) some cells in body inactivate one X chromosome, other cells inactivate the other (random)
2) if female is heterozygous, some cells will express one allele of a trait, others will express the other
how come female “carriers” are still not as susceptible as males to sex-linked diseases
because the inactivation is random, so in most cases each organ/tissue has some cells expressing the “good” copy
when a zygote is formed, egg and sperm…
equally contribute nuclear DNA but do not equally contribute organelles
mitochondrial genes are inherited from the
female parent (maternal inheritance)
genetic recombination
crossing over during meiosis 1
recombinant gametes
if crossing over occurs, parental alleles are recombined
genes that are farther apart on a chromosome have a —– chance of crossing over
greater
larger % recombinants =
farther genes are apart
if homologues undergo two crossovers between loci, then the parental combination is
restored; leads to an underestimation of the true genetic distance
relationship between true distance on a chromosome and the recombination frequency is
not linear
odd number of crossovers produce
recombinant gametes (offspring)
even number of crossovers produce
parental gametes (parents)
how can you tell if two genes are on the same chromosome (inherited together)
the genes would be inherited more than 50% of the time together
in three point crosses, loci with the lowest frequency of recombination must be
in the middle because it would need two crossovers
if there is no linkage between three genes being crossed, which results would you get
(1/2) * (1/2) * (1/2) = (1/8)
if there is a linkage between the genes being crossed, which results would you get
no set ratio; values will not follow a pattern
single-nucleotide polymorphisms (SNPs)
affect a single base (letter) of a gene locus
nondisjunction
failure of homologues or sister chromatids to separate properly during meiosis
aneuploidy
gain or loss of a chromosome
monosomy-loss
trisomy-gain
what syndrome is caused by XXY
klinefelter syndrome (can live)
what happens to an XXX female
usually nothing; still functional (can live)
XO females
turner syndrome; sterile female (can live)
OY males
nonviable zygotes (cannot survive)
XYY males
jacob syndrome (can live)
genomic imprinting
phenotype exhibited by a particular allele depends on which parent contributed the allele to the offspring
what happens if DNA is methylated (methylation, imprinted)
the DNA will be inactivated and not be able to transcribe
if you only have a chromosome with an imprinted gene, what happens
you get that disease/syndrome
pedigree analysis
used to determine the probability of genetic disorders in the offspring
amniocentesis
collects fetal cells from the amniotic fluid for examination (needle goes through stomach)
