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7.1 Chromosomes and Phenotype
• The chromosome theory of inheritance follows
Mendel’s laws of segregation and independent
assortment.
• We give much credit to Thomas Hunt Morgan
and his work with fruit flies- Drosophilia
melanogaster in the early 20th century
– Wild vs mutant types
– Sex linked genes (on X chromosome)
– Linkage between genes (possibly due to
crossing over/recombination)
7.1 Chromosomes and Phenotype
Males and females can differ in sex-linked traits.
• Genes on sex chromosomes are called sex-linked genes.
– Y chromosome genes in mammals are responsible for
male characteristics. About 78 genes (code for about 25
proteins)
– X chromosome genes in mammals affect many traits.
7.1 Chromosomes and Phenotype
– Fathers pass sexlinked alleles to all
daughters, but no
sons.
– Mothers can pass
sex-linked alleles
to daughter and
sons.
– More males than
females have sexlinked recessive
disorders
7.1 Chromosomes and Phenotype
• Sex-linked recessive disorders:
– Color blindness
– Duchenne muscular dystrophy
– Hemophilia
7.1 Chromosomes and Phenotype
X Inactivation
– One X chromosome in each cell in females becomes
almost completely inactivated during embryonic
development. (males=females)
– X in each cell of a female condenses to make a Barr
body (later reactivated to make eggs)
7.1 Chromosomes and Phenotype
• X-inactivation is not the same as genomic imprinting.
• Genomic imprinting occurs during formation of gametes and
results in the “silencing” of one allele of a certain gene.
• This means that certain traits will depend on which parent
passed down silent alleles.
• Geneticists have identified about 2000-3000 of these traits
in mammals.