Download 7.1 Chromosomes and Phenotype

7.1 Chromosomes and Phenotype
KEY CONCEPT
The chromosomes on which genes are located can
affect the expression of traits.
7.1 Chromosomes and Phenotype
Review-Two copies of each autosomal gene affect
phenotype.
• Mendel studied autosomal
gene traits, like hair texture.
• BUT Patterns of inheritance
are often complex!
7.1 Chromosomes and Phenotype
• Mendel’s rules of inheritance apply to autosomal genetic
disorders.
– A heterozygote for a recessive disorder is a carrier.
– Disorders caused by dominant alleles are uncommon.
(dominant)
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.
– X chromosome genes in mammals affect many traits.
7.1 Chromosomes and Phenotype
• Male mammals have an XY genotype.
– All of a male’s sexlinked genes are
expressed.
– Males have no
second copies of
sex-linked genes.
7.1 Chromosomes and Phenotype
(1).X-linked traits or sex-linked traits. Studied by American
Scientist-Thomas Hunt Morgan studied eye color of
Drosophila melanogasta –fruit fly.
• A pattern of inheritance when traits are controlled on the
sex chromosome, not on the autosomes (#1-22).
• If a male inherits the recessive trait on the x
chromosome, the recessive trait will always be
expressed.
• The Y chromosome has no corresponding allele for an xlinked trait. ( It is not there!)
• Two examples in humans are : Hemophilia and color
blindness
7.1 Chromosomes and Phenotype
7.1 Chromosomes and Phenotype
7.1 Chromosomes and Phenotype
• Female mammals have an XX genotype.
(2) X chromosome inactivation randomly “turns off” one
X chromosome which results in two types of cellsfemales can be black, orange, and white.
X-white (both males and females), X0, orange, Xo- black