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BIOL 121 Lecture Notes - Meiosis, Y Chromosome, Allosome

Course Code
BIOL 121

of 2
Biol 121 225
Freeman 276-281, 286-289 Feb 8, 10
Ch. 13, Linkage
Wild type
-individuals with the most common phenotype
-change in a gene (mutant = individuals with mutated traits)
Reciprocal cross
-set of matings where mother’s phenotype in first cross is father’s
phenotype in second cross; and father’s phenotype in first cross is mother’s
phenotype in second cross
Sex chromosomes
-sex chromosomes refer to the X and Y chromosomes in organisms
-X and Y chromosomes are different in size and shape the Y chromosome is
-they also contain different genes
-however, they have regions that are similar enough to lead to proper
pairing during prophase I of meiosis I
-male: XY, female: XX
X-linkage or X-linked inheritance
-the gene for a trait is located on the X chromosome and the Y chromosome
does not carry an allele for the gene
-Y-linkage is the opposite
-though X and Y chromosomes synapse during prophase I, they differ in size,
shape and gene content
Autosomal inheritance
-genes on non-sex chromosomes show autosomal inheritance
-the physical association of genes that are found on the same chromosome
-if two or more genes are linked, it means they are located on the same
-linked genes violate the principle of independent assortment
-two alleles for two different genes located on one chromosome will be
inherited together
-an exception to this is due to recombination because of crossing over in
prophase I of meiosis
Physical distance between genes (b/t
-greater physical distance between genes increases the chance that crossing
over will take place between them
-then, it is possible to figure out where genes are in relation to each other
based on the frequency of recombinants between various pairs
Genetic map
-a diagram that shows the relative positions of genes along a particular
chromosome (genes on the same single chromosome)
-it is predicted by examining the frequency of recombinants between
various pairs of genes since physical distance affects the frequency
-a pedigree records the genetic relationships among the individuals in a
family along with each person’s sex and phenotype with respect to the trait
in question
-if a trait is due to a single gene, analyzing the pedigree may reveal whether
the trait is due to a dominant (D) or recessive (R) allele and whether it is
located on a sex chromosome (sex-linkage) or not
If phenotype is due to an autosomal
recessive allele
-individuals with the trait must be homozygous
If parents of an affected individual do
not have the trait
-they must be heterozygous
-carriers are heterozygous individuals who carry a recessive allele for an
inherited disease
-these individuals carry the allele and transmit it even though they do not
exhibit signs of the disease (have genotype, do not have phenotype)
-when two carriers mate, 25% chance of offspring with recessive phenotype
Biol 121 225
Freeman 276-281, 286-289 Feb 8, 10
For autosomal dominant traits,
individuals with a single copy of the
-must have the dominant phenotype
One parent is heterozygous, other is
homozygous recessive
-even in this case, the dominant phenotype will show up in the offspring
50% of the time
-also, unless a new mutation occurs, any child with the trait must have a
parent with the trait
When it is not possible to arrange
reciprocal crosses, can data in a
pedigree indicate whether a trait is
autosomal or sex-linked?
-simple premise: if trait appears about equally often in males and females,
likely to be autosomal, and vice versa
Y-linked inheritance
-gene is found on the Y sex chromosome
-Y-linked inheritance is rare b/c the Y chromosome has few genes