Lecture 20 Basics: Chromosomal inheritance
20.1 Laws of segregation and independent assortment
Definition: Law of segregation:
Monohybrid cross:
P
p
P
p
`
Genotypes:
Phenotypes:
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Definition: Law of independent assortment:
Dihybrid crosses:
Genotypes:
Phenotypes:
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20.2 Sex-linked genes
Book definition:
Female
Male
Explanations:
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A man with hemophilia (a recessive , sex-linked condition) has a daughter of normal phenotype.
She marries a man who is normal for the trait.
1. What is the probability that a daughter of this mating will be a hemophiliac?
2. A son?
3. If the couple has four sons, what is the probability that all four will be born with
hemophilia?
20.3 Pedigrees
Determining a dominant trait:
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Determining a recessive trait:
20.4 Barr bodies
Xa – active X chromosome
Xi – inactive X chromosome
Two causes of Xi inactivation:
1. Methylation of
2. Activation of Xist gene
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20.5 Recombination frequencies
Fruit fly genetics:
Crossing over can mix genes
Recombination common:
Recombination rare:
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Calculating a recombination frequency
Few recombinants –
Many recombinants –
Recombination frequency:
Textbook question:
A wild-type fruit fly (heterozygous for gray body color and normal wings was mated with a black
fly with vestigial wings. The offspring had the following phenotypic distribution: wild type, 778;
black-vestigial, 785; black-normal, 158; gray-vestigial, 162.
What is the recombination frequency between these genes for body color and wing type?
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