sex linked genes and recombinants, linkage, etc Flashcards
autosomal genes
inherited through recessive alleles or dominant alleles
n=22
non-sex related
x-linked genes
recessive and dominant inheritance
n=1
genes found on x-chromosome
(more genes are x-linked)
y-linked genes
passed down by male lineage only
genes found on y chromosomes
(less genes are known to be y linked)
recessively inherited disorders
only show up in homozygous recessive individuals (aa)
need to get both or 2 copies of the gene to be affected
both affected parents —> affected offspring
what does a autosomal recessively inherited disorder look like on pedigree
half filled in squares/circles; equal chance for affected males/females
affected offspring often from unaffected parents
skipping between affected offspring generation could occur
2 heterozygotes = 1/4 probability of child being affected
autosomal dominant traits on pedigree
affected children have at least one affected parent
doesn’t skip generations
equal # of affected males and females
unaffected parents dont produce affected children (other than by mutation)
autosomal dominant disorders
associated with the dominant allele
affected have Aa or AA genotype
just need one copy of the gene to be affected
autosomes vs sex chromosomes
chromosomes 1-22 are autosomes
if an allele of a gene on autosomes specify a trait, you can inherit those alleles from either parent
last pair of chromosomes #23 = sex chromosome
z-w chromosomal system
birds snakes butterflies and fish
z=male
z w=female
x-o chromosomal system
insects
x = male
xx= females
haploid-diploid chromosomal system
bees ants wasps
diploid = female (develop from a fertilized egg)
haploid= male
(develop from an unfertilized egg)
which sex chromosome determines sex of a human child
mainly sperm
sex linked genes
genes found on one sex-chromosome, but not the other one
many ___ cause diseases in humans
x-linked alleles
these diseases occur more frequently in male
^ only need one X to have the gene
x-linked recessive pedigree
more affected males than females
all sons of afffected females will be affected
affected daughters have affected fathers
can skip generations
x-linked dominant pedigree
affected fathers have 100% affected daughters
for an x-linked recessive trait to be expressed what must occur
a person with XX needs two copies of the allele(homozygous)
a person with XY only needs 1 copy of the allele (hemizygous)
why are males more likely to have x-linked recessive genes
they only need 1 allele whereas females need two
hemophilia A
caused by x-linked recessive genes
dominant allele = X^H
recessive = X^h-A
encodes defective version of a clotting protein
x-activation in female mammals
in early embryo, all X chromosomes are initially active and cells have both alleles
one of the two X chromosomes is randomly inactivated in each somatic cell, which becomes the Barr body
heterozygous females for a sex-linked trait will have
a mosaic for that character
ex: cat with black and orange fur
mendel’s law of independent assortment
separation of homologous chromosomes during metaphase I
explains why he always observed a 9:3:3:1 phenotypic ratio in the F2 generation when he followed 2 characters at a time
linkage / linked genes
when two or more genres are close together on the same chromosome, then they tend to be transmitted as a unit
linkage groups
group of genes that tend to stay together during meiosis
these genes don’t follow the law of independent assortment
genetic recombination
production of offspring with combination of traits that differ from those found in either P generation parent
unlinked genes on the same chromosome can undergo crossing over with other chromosomes to produce new allele combinations
nonrecombinatn vs recombinant
nonrecombinant offspring match P generation parents
recombinants have new combinations of alleles/traits that don;t match the parents
crossing over with linked vs unlinked genes
unlinked genes on the same chromosome can do more crossing over and increases the likelihood or recombinants
linked genes are less likely to have crossing over lead to new allele combinations
how do we explain the occurrence of recombinant when genes are linked
crossing over still does occur but at a much lesser rate
how does the distance between two genes impact the likelihood of crossing over?
if genes are very close together, a crossover is unlikely to occur in the region between them
if genes are very far apart, a crossover is more likely to occur in the region between them, leading to more recombinants
what allows us to determine the relative distance between linked genes based on the likelihood that a crossover will occur between them
genetic mapping
arrangement of genes along a chromosome (genetic map)
map distance
recombinant frequency of test crosses
units of map distance —> mu
how to calculate map distance
number of recombinants
———————————. * 100
total number of offspring
