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If Mary’s dad developed male
patterned baldness, but she did not
have the trait, and she married Joe,
who does not have male pattern
baldness, what would be their
chances of having a son with male
pattern baldness?
Total
marks 4: 1 mark for ratio value, 3 marks for punnett squarea
Since Mary’s dad had the trait but she did
not, it must be sex-linked recessive
XB
XB
Y
XB XB
XBY
Xb
XBXb
XbY
If Mary and Joe
had a baby, it
would have a
25% chance of
being a male
with male pattern
baldness
Multiple Alleles
18.4
Page 608
Beyond Mendel
Multiple Alleles
 Trifolium repens (Clover)
• thus far there has been only three types of genotypes
(homozygous recessive or dominant and heterozygous)
• but in Clover one gene is responsible for all the
patterns on the leaves.
• in most organisms many genes have more than two
alleles.
• a gene with more than two alleles is said to have
multiple alleles.
Beyond Mendel
Polygenic Inheritance
 Mendel selected characteristics that were distinct so there would be no
question of phenotypes.
• since then people have looked at continuous traits
• traits that gradually change from one extreme to another.
• examples: ears, length in corn, weight of beans
• continuous traits are usually controlled by more than one gene.
• traits that are controlled by many genes are called polygenetic
traits.
• a group of genes that all contribute to the same trait is
called a polygene
Beyond Mendel
Polygenic Inheritance
 example: corn length
• an ear of corn is controlled by two genes, A and B
• each dominant allele contributes to length, recessive
alleles do not contribute
AABB is the genotype with the largest length
phenotype
aabb is a genotype with the smallest length phenotype
Beyond Mendel
Polygenic Inheritance
 a true-breeding lines for longest and shortest ear lengths
are crossed
P generation AABB x aabb
F1 generation AaBb
 with four genes you start to see a range of lengths.
• continuous phenotypic traits