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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
Two copies of each autosomal gene affect phenotype.
• Mendel studied autosomal
gene traits, like hair texture.
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
Work the following problem:
• Two parents, who both know their genotype, are carriers
for albinism. They are both heterozygous. What is the
chance that their children have albinism?
7.1 Chromosomes and Phenotype
Work the following problem:
• Huntington’s disease is a rare, but not uncommon,
disease that is caused by a dominant allele. Suppose
that two parents are crossed one that is heterozygous for
Huntington’s and one that is homozygous recessive.
What is the chance that their offspring will have
Huntington’s disease?
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
• A male (XX) and a female (XY) mate – what is the
chance that they have a male offspring?
7.1 Chromosomes and Phenotype
Weird combinations have happened…..
• Any combination (up to XXXXY) produces maleness.
Males with more than one X are usually underdeveloped
and sterile.
• XXX and XO women are known, although in most cases
they are sterile.
7.1 Chromosomes and Phenotype
7.1 Chromosomes and Phenotype
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
• Female mammals have an XX genotype.
– Expression of sex-linked genes is similar to autosomal
genes in females.
– X chromosome inactivation randomly “turns off” one X
chromosome.