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BIOL 2000 Lecture Notes - Sex Linkage, Zygosity, Y Chromosome

Course Code
BIOL 2000
Joseph Rasmussen

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Chapter 2
Sex linkage and pedigrees
Autosome- a chromosome that is not a sex chromosome
Sex linkage- the location of a gene on a sex chromosome
Hemizygous- a gene present in only one copy in a diploid organism
Sex-linked single- gene inheritance patterns
Gene is on the sex chromosome
Human sex chromosomes- X and Y chromosomes
Differential region- accounts for sex linkage
Pseudo autosomal region- sequence similarity allows chromosome pairing
In Mendel’s monohybrid crosses, reciprocal crosses yielded identical results
o In the case of the re and white eyed flies reciprocal crosses yielded
different results
o Red x white and white x red gave different F1 results
o Red x white and white x red gave different F 2 results
Pedigree analysis
Applying Mendelian principles to human inheritance patterns
Is the trait shown in generation IV autosomal or sex linked? Dominant or
recessive? (page 67)
o No tendency of phenotype with particular sex (small sample size);
suggest autosomal
o Unaffected individuals give rise to affected individuals: recessive
If affected individuals in generation IV are homozygous recessive, what
genotype must their parents have been?
o Heterozygous in order to produce affected offspring
Phenotype of siblings= dominant phenotype
o Genotype of siblings in generation IV
Must have A; may be AA or Aa; 2/3 Aa, 1/3 AA
What about genotypes of grandparents?
o Both are unaffected, there fore have a dominant allele
o As least one must carry recessive allele
o If condition Is rare, they both likely carry the recessive allele
Siblings in generations II and III
o Unaffected, therefore must be A
o Other allele is unknown (ether A or A); designate as A/-
In generation III, individuals have 50% chance of being AA and
50% of being Aa
Genotypes of great-grandparents (generation I)
o Unaffected- must have A
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